more idiot-proof new-day script

.gitignore

@@ -1 +1,2 @@
-input
+input
+testinput

01/main.go

@@ -5,11 +5,16 @@ import (
 	"fmt"
 	"os"
 	"strconv"
+	"time"
 )
 
 func main() {
+	start := time.Now()
 	partOne()
+	duration := time.Since(start)
 	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
 }
 
 // [...] they need you to find the two entries that sum to 2020 and then multiply those two numbers together.

02/main.go

@@ -6,12 +6,17 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"time"
 	"unicode"
 )
 
 func main() {
+	start := time.Now()
 	partOne()
+	duration := time.Since(start)
 	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
 }
 
 //To try to debug the problem, they have created a list (your puzzle input) of passwords (according to the corrupted database) and the corporate policy when that password was set.

03/main.go

@@ -5,11 +5,16 @@ import (
 	"fmt"
 	"os"
 	"strings"
+	"time"
 )
 
 func main() {
+	start := time.Now()
 	partOne()
+	duration := time.Since(start)
 	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
 }
 
 type slope struct {

04/main.go

@@ -6,11 +6,16 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"time"
 )
 
 func main() {
+	start := time.Now()
 	partOne()
+	duration := time.Since(start)
 	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
 }
 
 type passport struct {

05/main.go

@@ -5,11 +5,16 @@ import (
 	"fmt"
 	"os"
 	"sort"
+	"time"
 )
 
 func main() {
+	start := time.Now()
 	partOne()
+	duration := time.Since(start)
 	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
 }
 
 type boardingPass struct {

06/main.go

@@ -5,11 +5,16 @@ import (
 	"fmt"
 	"os"
 	"strings"
+	"time"
 )
 
 func main() {
+	start := time.Now()
 	partOne()
+	duration := time.Since(start)
 	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
 }
 
 func union(s string) string {

07/main.go

@@ -6,11 +6,16 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"time"
 )
 
 func main() {
+	start := time.Now()
 	partOne()
+	duration := time.Since(start)
 	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
 }
 
 // Bag is a bag that contains other bags

08/main.go

@@ -6,11 +6,16 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"time"
 )
 
 func main() {
+	start := time.Now()
 	partOne()
+	duration := time.Since(start)
 	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
 }
 
 type opCode struct {

09/main.go

@@ -6,11 +6,16 @@ import (
 	"os"
 	"sort"
 	"strconv"
+	"time"
 )
 
 func main() {
+	start := time.Now()
 	partOne()
+	duration := time.Since(start)
 	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
 }
 
 func checkNotSum(t int, s []int) bool {

10/main.go

@@ -0,0 +1,122 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"sort"
+	"strconv"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+// Find a chain that uses all of your adapters to connect the charging outlet to your device's built-in adapter
+// and count the joltage differences between the charging outlet, the adapters, and your device.
+// What is the number of 1-jolt differences multiplied by the number of 3-jolt differences?
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	// wall
+	adapters := []int{0}
+
+	// load
+	for scanner.Scan() {
+		line := scanner.Text()
+		i, _ := strconv.Atoi(line)
+		adapters = append(adapters, i)
+	}
+
+	// sort
+	sort.Slice(adapters, func(i, j int) bool {
+		return adapters[i] < adapters[j]
+	})
+
+	// device
+	adapters = append(adapters, adapters[len(adapters)-1]+3)
+
+	one := 0
+	three := 0
+
+	// count gaps
+	for i, j := range adapters {
+		if i < len(adapters)-1 {
+			if adapters[i+1]-j == 3 {
+				three = three + 1
+			} else if adapters[i+1]-j == 1 {
+				one = one + 1
+			}
+		}
+	}
+
+	fmt.Println(three * one)
+}
+
+// You glance back down at your bag and try to remember why you brought so many adapters; there must be more than a trillion valid ways to arrange them!
+// Surely, there must be an efficient way to count the arrangements.
+// What is the total number of distinct ways you can arrange the adapters to connect the charging outlet to your device?
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	// wall
+	adapters := []int{0}
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		i, _ := strconv.Atoi(line)
+		adapters = append(adapters, i)
+	}
+
+	// sort
+	sort.Slice(adapters, func(i, j int) bool {
+		return adapters[i] < adapters[j]
+	})
+
+	// device
+	adapters = append(adapters, adapters[len(adapters)-1]+3)
+
+	combinations := 1
+	ones := 0
+	three := 0
+
+	// count gaps
+	for i, j := range adapters {
+		if i < len(adapters)-1 {
+			if adapters[i+1]-j == 1 {
+				// small steps...
+				ones = ones + 1
+			} else if adapters[i+1]-j == 3 {
+				combinations = combinations + 1
+				if ones == 2 {
+					combinations = combinations * 1
+				}
+				if ones == 3 {
+					combinations = combinations * 2
+				}
+				if ones == 4 {
+					combinations = combinations * 4
+				}
+				if ones == 5 {
+					combinations = combinations * 7
+				}
+				ones = 0
+				three = three + 1
+			}
+		}
+	}
+
+	fmt.Println(adapters, combinations)
+}

11/main.go

@@ -0,0 +1,303 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+type cell struct {
+	taken int  // 1: someone there; 0: empty
+	seat  bool // t: seat available; f: open floor
+}
+
+type waitingRoom struct {
+	seats  [][]cell
+	maxCol int
+	maxRow int
+}
+
+func (w *waitingRoom) parseRow(s string) {
+	row := []cell{}
+
+	for _, r := range s {
+		switch {
+		case r == 'L':
+			row = append(row, cell{taken: 0, seat: true})
+		case r == '.':
+			row = append(row, cell{taken: 0, seat: false})
+		}
+	}
+	w.seats = append(w.seats, row)
+}
+
+func (w *waitingRoom) countNeighborsPartOne(row, col int) int {
+	c := 0
+	if row-1 > -1 {
+		c = c + w.seats[row-1][col].taken // north
+		if col-1 > -1 {
+			c = c + w.seats[row-1][col-1].taken // northwest
+		}
+		if col+1 < w.maxCol {
+			c = c + w.seats[row-1][col+1].taken // northheast
+		}
+	}
+	if row+1 < w.maxRow {
+		c = c + w.seats[row+1][col].taken // south
+		if col+1 < w.maxCol {
+			c = c + w.seats[row+1][col+1].taken // southeast
+		}
+		if col-1 > -1 {
+			c = c + w.seats[row+1][col-1].taken // southwest
+		}
+	}
+	if col-1 > -1 {
+		c = c + w.seats[row][col-1].taken // west
+	}
+	if col+1 < w.maxCol {
+		c = c + w.seats[row][col+1].taken // east
+	}
+	return c
+}
+
+func (w *waitingRoom) makeNextStepPartOne() *waitingRoom {
+	newRoom := &waitingRoom{
+		seats:  make([][]cell, w.maxRow),
+		maxCol: w.maxCol,
+		maxRow: w.maxRow,
+	}
+
+	for r := range w.seats {
+		newRoom.seats[r] = make([]cell, newRoom.maxCol)
+		for c := range w.seats[r] {
+			if w.seats[r][c].seat && w.seats[r][c].taken == 0 && w.countNeighborsPartOne(r, c) == 0 {
+				newRoom.seats[r][c].seat = w.seats[r][c].seat
+				newRoom.seats[r][c].taken = 1
+			} else if w.seats[r][c].seat && w.seats[r][c].taken == 1 && w.countNeighborsPartOne(r, c) >= 4 {
+				newRoom.seats[r][c].seat = w.seats[r][c].seat
+				newRoom.seats[r][c].taken = 0
+			} else {
+				newRoom.seats[r][c].seat = w.seats[r][c].seat
+				newRoom.seats[r][c].taken = w.seats[r][c].taken
+			}
+		}
+	}
+
+	return newRoom
+}
+
+func (w *waitingRoom) countNeighborsPartTwo(row, col int) int {
+	c := 0
+	trow := row
+	tcol := col
+
+	// south
+	for trow < w.maxRow-1 {
+		trow = trow + 1
+		if w.seats[trow][col].seat {
+			c = c + w.seats[trow][col].taken
+			break
+		}
+	}
+	trow = row
+
+	// north
+	for trow > 0 {
+		trow = trow - 1
+		if w.seats[trow][col].seat {
+			c = c + w.seats[trow][col].taken
+			break
+		}
+	}
+	trow = row
+
+	// west
+	for tcol > 0 {
+		tcol = tcol - 1
+		if w.seats[row][tcol].seat {
+			c = c + w.seats[row][tcol].taken
+			break
+		}
+	}
+	tcol = col
+
+	// east
+	for tcol < w.maxCol-1 {
+		tcol = tcol + 1
+		if w.seats[row][tcol].seat {
+			c = c + w.seats[row][tcol].taken
+			break
+		}
+	}
+	tcol = col
+
+	// southeast
+	for trow < w.maxRow-1 && tcol < w.maxCol-1 {
+		trow = trow + 1
+		tcol = tcol + 1
+		if w.seats[trow][tcol].seat {
+			c = c + w.seats[trow][tcol].taken
+			break
+		}
+	}
+	trow = row
+	tcol = col
+
+	// southwest
+	for trow < w.maxRow-1 && tcol > 0 {
+		trow = trow + 1
+		tcol = tcol - 1
+		if w.seats[trow][tcol].seat {
+			c = c + w.seats[trow][tcol].taken
+			break
+		}
+	}
+	trow = row
+	tcol = col
+
+	// northeast
+	for trow > 0 && tcol < w.maxCol-1 {
+		trow = trow - 1
+		tcol = tcol + 1
+		if w.seats[trow][tcol].seat {
+			c = c + w.seats[trow][tcol].taken
+			break
+		}
+	}
+	trow = row
+	tcol = col
+
+	// northwest
+	for trow > 0 && tcol > 0 {
+		trow = trow - 1
+		tcol = tcol - 1
+		if w.seats[trow][tcol].seat {
+			c = c + w.seats[trow][tcol].taken
+			break
+		}
+	}
+
+	return c
+}
+
+func (w *waitingRoom) makeNextStepPartTwo() *waitingRoom {
+	newRoom := &waitingRoom{
+		seats:  make([][]cell, w.maxRow),
+		maxCol: w.maxCol,
+		maxRow: w.maxRow,
+	}
+
+	for r := range w.seats {
+		newRoom.seats[r] = make([]cell, newRoom.maxCol)
+		for c := range w.seats[r] {
+			if w.seats[r][c].seat && w.seats[r][c].taken == 0 && w.countNeighborsPartTwo(r, c) == 0 {
+				newRoom.seats[r][c].seat = w.seats[r][c].seat
+				newRoom.seats[r][c].taken = 1
+			} else if w.seats[r][c].seat && w.seats[r][c].taken == 1 && w.countNeighborsPartTwo(r, c) >= 5 {
+				newRoom.seats[r][c].seat = w.seats[r][c].seat
+				newRoom.seats[r][c].taken = 0
+			} else {
+				newRoom.seats[r][c].seat = w.seats[r][c].seat
+				newRoom.seats[r][c].taken = w.seats[r][c].taken
+			}
+		}
+	}
+
+	return newRoom
+}
+
+func (w *waitingRoom) countFilledSeats() int {
+	count := 0
+	for r := range w.seats {
+		for c := range w.seats[r] {
+			count = count + w.seats[r][c].taken
+		}
+	}
+	return count
+}
+
+func (w *waitingRoom) print() {
+	for r := range w.seats {
+		s := ""
+		for c := range w.seats[r] {
+			if !w.seats[r][c].seat {
+				s = s + "."
+			} else if w.seats[r][c].taken == 0 {
+				s = s + "L"
+			} else if w.seats[r][c].taken == 1 {
+				s = s + "#"
+			}
+		}
+		fmt.Println(s)
+	}
+}
+
+func checkStasis(old, new *waitingRoom) bool {
+	for r := range old.seats {
+		for c := range old.seats[r] {
+			if old.seats[r][c].seat && old.seats[r][c].taken != new.seats[r][c].taken {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	oldRoom := &waitingRoom{}
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		oldRoom.parseRow(line)
+	}
+
+	oldRoom.maxCol = len(oldRoom.seats[0])
+	oldRoom.maxRow = len(oldRoom.seats)
+
+	for {
+		newRoom := oldRoom.makeNextStepPartOne()
+		if checkStasis(newRoom, oldRoom) {
+			fmt.Println(newRoom.countFilledSeats())
+			return
+		}
+		oldRoom = newRoom
+	}
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+	oldRoom := &waitingRoom{}
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		oldRoom.parseRow(line)
+	}
+
+	oldRoom.maxCol = len(oldRoom.seats[0])
+	oldRoom.maxRow = len(oldRoom.seats)
+
+	for {
+		newRoom := oldRoom.makeNextStepPartTwo()
+		if checkStasis(newRoom, oldRoom) {
+			fmt.Println(newRoom.countFilledSeats())
+			return
+		}
+		oldRoom = newRoom
+	}
+}

12/main.go

@@ -0,0 +1,197 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+type ship struct {
+	horizontal int
+	vertical   int
+	direction  rune // E W N S
+}
+
+// both horizontal/vertical are relative to ship
+type waypoint struct {
+	horizontal int
+	vertical   int
+}
+
+func parseLine(s string) (rune, int) {
+	i, _ := strconv.Atoi(s[1:])
+	return []rune(s)[0], i
+}
+
+func movePartOne(s ship, action rune, vector int) ship {
+	switch action {
+	case 'R', 'L':
+		s.direction = rotateShip(s.direction, vector, action)
+	case 'N':
+		s.vertical = s.vertical + vector
+	case 'S':
+		s.vertical = s.vertical - vector
+	case 'E':
+		s.horizontal = s.horizontal + vector
+	case 'W':
+		s.horizontal = s.horizontal - vector
+	case 'F':
+		s = movePartOne(s, s.direction, vector)
+	}
+	return s
+}
+
+func movePartTwo(s ship, w waypoint, action rune, vector int) (ship, waypoint) {
+	switch action {
+	case 'R', 'L':
+		w = rotateWaypoint(w, action, vector)
+	case 'N':
+		w.vertical = w.vertical + vector
+	case 'S':
+		w.vertical = w.vertical - vector
+	case 'E':
+		w.horizontal = w.horizontal + vector
+	case 'W':
+		w.horizontal = w.horizontal - vector
+	case 'F':
+		s.horizontal = s.horizontal + (w.horizontal * vector)
+		s.vertical = s.vertical + (w.vertical * vector)
+	}
+	return s, w
+}
+
+// for example E, 90, L => N; E, 180, R => W; E, 90, R => S
+func rotateShip(current rune, degrees int, rl rune) rune {
+	// easy
+	if degrees == 180 {
+		switch current {
+		case 'N':
+			return 'S'
+		case 'S':
+			return 'N'
+		case 'E':
+			return 'W'
+		case 'W':
+			return 'E'
+		}
+	}
+	// translate L to R
+	if rl == 'L' {
+		if degrees == 90 {
+			degrees = 270
+		} else {
+			degrees = 90
+		}
+	}
+	// the R set
+	if degrees == 90 {
+		switch current {
+		case 'N':
+			return 'E'
+		case 'S':
+			return 'W'
+		case 'E':
+			return 'S'
+		case 'W':
+			return 'N'
+		}
+	} else if degrees == 270 {
+		switch current {
+		case 'N':
+			return 'W'
+		case 'S':
+			return 'E'
+		case 'E':
+			return 'N'
+		case 'W':
+			return 'S'
+		}
+	}
+
+	return current // we're made a terrible mistake somewhere
+}
+
+func rotateWaypoint(w waypoint, action rune, vector int) waypoint {
+	if vector == 180 {
+		w.horizontal, w.vertical = -w.horizontal, -w.vertical
+	} else if action == 'R' {
+		if vector == 90 {
+			w.horizontal, w.vertical = w.vertical, -w.horizontal
+		}
+		if vector == 270 {
+			w.horizontal, w.vertical = -w.vertical, w.horizontal
+		}
+	} else if action == 'L' {
+		if vector == 90 {
+			w.horizontal, w.vertical = -w.vertical, w.horizontal
+		}
+		if vector == 270 {
+			w.horizontal, w.vertical = w.vertical, -w.horizontal
+		}
+	}
+	return w
+}
+
+func manhattan(i, j int) int {
+	return abs(i) + abs(j)
+}
+
+func abs(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return x
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	s := ship{
+		horizontal: 0,
+		vertical:   0,
+		direction:  'E',
+	}
+
+	for scanner.Scan() {
+		dir, vec := parseLine(scanner.Text())
+		s = movePartOne(s, dir, vec)
+	}
+
+	fmt.Println(manhattan(s.horizontal, s.vertical))
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	s := ship{
+		horizontal: 0,
+		vertical:   0,
+		// no longer care about ship direction
+	}
+	w := waypoint{
+		horizontal: 10,
+		vertical:   1,
+	}
+
+	for scanner.Scan() {
+		dir, vec := parseLine(scanner.Text())
+		s, w = movePartTwo(s, w, dir, vec)
+	}
+
+	fmt.Println(manhattan(s.horizontal, s.vertical))
+}

13/main.go

@@ -0,0 +1,89 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"math"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	busWaitTimes := map[int]int{}
+	shortestWait := -1
+
+	// don't actually need loop logic for a two-line input
+	scanner.Scan()
+	ts := scanner.Text()
+	currentTimestamp, _ := strconv.Atoi(ts)
+
+	scanner.Scan()
+	buses := strings.Split(scanner.Text(), ",")
+	for _, b := range buses {
+		if b != "x" {
+			j, _ := strconv.Atoi(b)
+			busWaitTimes[j] = (((currentTimestamp / j) + 1) * j) - currentTimestamp
+			if shortestWait == -1 || busWaitTimes[shortestWait] > busWaitTimes[j] {
+				shortestWait = j
+			}
+		}
+	}
+
+	fmt.Println(busWaitTimes[shortestWait] * shortestWait)
+}
+
+type bus struct {
+	route  float64
+	offset float64
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	buses := []bus{}
+
+	scanner.Scan() // discard first line
+	scanner.Scan()
+	line := strings.Split(scanner.Text(), ",")
+	for i, b := range line {
+		if b != "x" {
+			j, _ := strconv.Atoi(b)
+			buses = append(buses, bus{
+				route:  float64(j),
+				offset: float64(i),
+			})
+		}
+	}
+
+	t := float64(0)
+	offset := buses[0].route
+loop:
+	t = t + offset
+	offset = 1
+	for i := range buses {
+		if math.Remainder(t+buses[i].offset, buses[i].route) != 0 {
+			goto loop
+		} else {
+			offset = offset * buses[i].route
+		}
+	}
+
+	fmt.Println(int(t))
+}

14/main.go

@@ -0,0 +1,148 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+type bitset struct {
+	bits []string
+}
+
+func newBitset(i int) bitset {
+	b := strings.Split(fmt.Sprintf("%036b", i), "")
+	bs := bitset{
+		bits: strings.Split("000000000000000000000000000000000000", ""),
+	}
+	for i, bt := range b {
+		if bt != "0" {
+			bs.bits[i] = bt
+		}
+	}
+	return bs
+}
+
+func (b bitset) ToInt() int {
+	i, _ := strconv.ParseInt(strings.Join(b.bits, ""), 2, 64)
+	return int(i)
+}
+
+func parseMask(s string) string {
+	return strings.Split(s, " = ")[1]
+}
+
+func parseMemoryToAddrAndBitset(s string) (int, bitset) {
+	a := strings.Split(s, " = ")
+	addr, _ := strconv.Atoi(strings.TrimFunc(a[0], func(r rune) bool {
+		if r == 'm' || r == 'e' || r == '[' || r == ']' {
+			return true
+		}
+		return false
+	}))
+	i, _ := strconv.Atoi(a[1])
+	return addr, newBitset(i)
+}
+
+func parseMemoryToBitsetAndValue(s string) (bitset, int) {
+	a := strings.Split(s, " = ")
+	addr, _ := strconv.Atoi(strings.TrimFunc(a[0], func(r rune) bool {
+		if r == 'm' || r == 'e' || r == '[' || r == ']' {
+			return true
+		}
+		return false
+	}))
+	i, _ := strconv.Atoi(a[1])
+	return newBitset(addr), i
+}
+
+func applyMaskToValue(m string, b bitset) int {
+	for i, r := range m {
+		if r == '0' {
+			b.bits[i] = "0"
+		}
+		if r == '1' {
+			b.bits[i] = "1"
+		}
+	}
+	return b.ToInt()
+}
+
+func applyMaskToAddr(prefix []string, m string, attr bitset) []bitset {
+	if len(m) == 0 {
+		return []bitset{{bits: prefix}}
+	}
+	res := []bitset{}
+	for _, r := range m {
+		if r == '0' || r == 'X' {
+			res = append(res, applyMaskToAddr(append(prefix[:], "0"), m[1:], attr)...)
+		}
+		if r == '1' || r == 'X' {
+			res = append(res, applyMaskToAddr(append(prefix[:], "1"), m[1:], attr)...)
+		}
+	}
+	return res
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	mem := map[int]int{}
+	var currentMask string
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		if strings.HasPrefix(line, "mask") {
+			currentMask = parseMask(line)
+		} else {
+			addr, value := parseMemoryToAddrAndBitset(line)
+			mem[addr] = applyMaskToValue(currentMask, value)
+		}
+	}
+	sum := 0
+	for _, v := range mem {
+		sum = sum + v
+	}
+	fmt.Println(sum)
+}
+
+func partTwo() {
+	f, _ := os.Open("testinput")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	mem := map[int]int{}
+	var currentMask string
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		if strings.HasPrefix(line, "mask") {
+			currentMask = parseMask(line)
+		} else {
+			addr, value := parseMemoryToBitsetAndValue(line)
+			for _, b := range applyMaskToAddr([]string{}, currentMask, addr) {
+				mem[b.ToInt()] = value
+			}
+		}
+	}
+
+	sum := 0
+	for _, v := range mem {
+		sum = sum + v
+	}
+	fmt.Println(sum)
+}

15/main.go

@@ -0,0 +1,85 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	// naive implementation
+	scanner.Scan()
+	numbers := strings.Split(scanner.Text(), ",")
+	for {
+		i := len(numbers) - 1
+		lastNumber := numbers[i]
+		// walk backward through the list, looking for a match
+		for j := range numbers {
+			if len(numbers) == 2020 {
+				fmt.Println(numbers[2019])
+				return
+			}
+			if i-j == 0 {
+				// made it to the head of the list, no matches; append 0
+				numbers = append(numbers, "0")
+				break
+			} else if numbers[i-j-1] == lastNumber {
+				// a number matched; insert the difference between the last number and when we last saw it
+				numbers = append(numbers, fmt.Sprintf("%d", len(numbers)-1-(i-j-1)))
+				break
+			}
+		}
+	}
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	// caching implementation. It's still slow, but it at least finishes...!
+	scanner.Scan()
+	numbers := strings.Split(scanner.Text(), ",")
+	seen := make(map[int]int)
+
+	// load in numbers
+	for i, v := range numbers[:len(numbers)-1] {
+		j, _ := strconv.Atoi(v)
+		seen[j] = i
+	}
+
+	i := len(numbers) - 1
+	lastNumber, _ := strconv.Atoi(numbers[i])
+	for {
+		if seen[lastNumber] == 30000000-1 {
+			fmt.Println(lastNumber)
+			return
+		}
+		if v, ok := seen[lastNumber]; ok {
+			// a number matched; insert the difference between the last number and when we last saw it
+			seen[lastNumber] = i
+			lastNumber = i - v
+		} else {
+			// never seen that before
+			seen[lastNumber] = i
+			lastNumber = 0
+		}
+		i = i + 1
+	}
+}

16/main.go

@@ -0,0 +1,232 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+type valuerange struct {
+	min int
+	max int
+}
+
+type rule struct {
+	name   string
+	ranges []valuerange
+	field  int
+}
+
+type ticket struct {
+	values []int
+	valid  bool
+}
+
+func parseRule(s string) rule {
+	q := strings.Split(s, ": ")
+	ru := rule{
+		name:   q[0],
+		ranges: make([]valuerange, 0, 2),
+	}
+	r := strings.Split(q[1], " or ")
+	for _, rr := range r {
+		mm := strings.Split(rr, "-")
+		min, _ := strconv.Atoi(mm[0])
+		max, _ := strconv.Atoi(mm[1])
+		ru.ranges = append(ru.ranges, valuerange{min: min, max: max})
+	}
+	return ru
+}
+
+func parseTicket(s string) ticket {
+	t := ticket{
+		values: []int{},
+	}
+	vs := strings.Split(s, ",")
+	for _, v := range vs {
+		vi, _ := strconv.Atoi(v)
+		t.values = append(t.values, vi)
+	}
+	return t
+}
+
+// return valid/invalid and the invalid value, if found
+func validateTicket(r []rule, t ticket) (bool, int) {
+	for _, ta := range t.values {
+		ok := false
+		for _, ru := range r {
+			outsideRule := (ta < ru.ranges[0].min || (ta > ru.ranges[0].max && ta < ru.ranges[1].min) || ta > ru.ranges[1].max)
+			if ok || !outsideRule {
+				ok = true
+			}
+		}
+		if !ok {
+			return false, ta
+		}
+	}
+	return true, 0
+}
+
+// returns a mapping of fields to possible rules
+func guessFields(rules []rule, tickets []ticket) map[int][]int {
+	guesses := map[int][]int{}
+	for ri, r := range rules {
+		for i := range tickets[0].values { // loop over the fields on tickets, and...
+			ok := true
+			for _, t := range tickets { // loop over all the tickets...
+				ta := t.values[i] // checking the current field value...
+				outsideRule := (ta < r.ranges[0].min || (ta > r.ranges[0].max && ta < r.ranges[1].min) || ta > r.ranges[1].max)
+				if !ok || outsideRule { // checking if any of the tickets fall outside our rule's range.
+					ok = false
+				}
+			}
+			if ok { // if all tickets pass, we've got a hit
+				guesses[i] = append(guesses[i], ri)
+			}
+		}
+	}
+	return guesses
+}
+
+func reduce(g map[int][]int) (int, int) {
+	for i, j := range g {
+		if len(j) == 1 {
+			return i, j[0]
+		}
+	}
+	return 0, 0
+}
+
+func remove(a []int, rule int) []int {
+	for i, j := range a {
+		if j == rule {
+			a[i] = a[len(a)-1]
+			a[len(a)-1] = 0
+			return a[:len(a)-1]
+		}
+	}
+	return a
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	rules := []rule{}
+	tickets := []ticket{}
+	invalidityRate := 0
+
+	seenAllRules := false
+	seenMyTicket := false
+	for scanner.Scan() {
+		line := scanner.Text()
+		// check for end of rules list
+		if line == "" && !seenAllRules {
+			seenAllRules = true
+			scanner.Scan() // clear out the following line
+			continue
+		}
+		// parse rules until we run out
+		if !seenAllRules {
+			rules = append(rules, parseRule(line))
+			continue
+		}
+		// parse own ticket, then clear for remaining tickets
+		if !seenMyTicket {
+			tickets = append(tickets, parseTicket(scanner.Text()))
+			seenMyTicket = true
+			scanner.Scan() // clear out the next two lines
+			scanner.Scan()
+			continue
+		}
+		// parse and validate other people's tickets
+		t := parseTicket(line)
+		ok, i := validateTicket(rules, t)
+		if !ok {
+			t.valid = false
+			invalidityRate = invalidityRate + i
+		} else {
+			t.valid = true
+			tickets = append(tickets, t)
+		}
+	}
+	fmt.Println(invalidityRate)
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	rules := []rule{}
+	myTicket := ticket{}
+	tickets := []ticket{}
+
+	seenAllRules := false
+	seenMyTicket := false
+	for scanner.Scan() {
+		line := scanner.Text()
+		// check for end of rules list
+		if line == "" && !seenAllRules {
+			seenAllRules = true
+			scanner.Scan() // clear out the following line
+			continue
+		}
+		// parse rules until we run out
+		if !seenAllRules {
+			rules = append(rules, parseRule(line))
+			continue
+		}
+		// parse own ticket, then clear for remaining tickets
+		if !seenMyTicket {
+			myTicket = parseTicket(scanner.Text())
+			tickets = append(tickets, myTicket)
+			seenMyTicket = true
+			scanner.Scan() // clear out the next two lines
+			scanner.Scan()
+			continue
+		}
+		// parse and validate other people's tickets
+		t := parseTicket(line)
+		ok, _ := validateTicket(rules, t)
+		if ok {
+			t.valid = true
+			tickets = append(tickets, t)
+		}
+	}
+	guesses := guessFields(rules, tickets)
+	var lastRuleSolved int
+	// iterate over the guesses list. on every iteration, only one field has only one rule remaining that's valid
+	// save it off into the rules list, then remove that rule from all fields.
+	// Luckily, we don't need any higher-order elimination logic.
+	for i := 1; i < len(guesses); i++ {
+		for i, g := range guesses {
+			if len(g) == 1 {
+				rules[g[0]].field = i
+				lastRuleSolved = g[0]
+			}
+		}
+		for i := range guesses {
+			guesses[i] = remove(guesses[i], lastRuleSolved)
+		}
+	}
+	result := 1
+	// first 6 rules are our "departure" rules
+	for _, r := range rules[:6] {
+		result = result * myTicket.values[r.field]
+	}
+	fmt.Println(result)
+}

17/main.go

@@ -0,0 +1,235 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+type cell struct {
+	x int // length
+	y int // width
+	z int // depth
+	w int // spissitude
+}
+
+type universe struct {
+	cells map[cell]bool
+	hyper bool // true: 4-dimensional; false: 3-dimensional
+}
+
+func makeNeighborsList(c cell, hyper bool) []cell {
+	cells := []cell{}
+	if hyper {
+		cells = append(cells, []cell{
+			// outward...?
+			// above
+			{x: c.x + 1, y: c.y + 1, z: c.z + 1, w: c.w + 1},
+			{x: c.x + 1, y: c.y, z: c.z + 1, w: c.w + 1},
+			{x: c.x + 1, y: c.y - 1, z: c.z + 1, w: c.w + 1},
+			{x: c.x, y: c.y + 1, z: c.z + 1, w: c.w + 1},
+			{x: c.x, y: c.y, z: c.z + 1, w: c.w + 1},
+			{x: c.x, y: c.y - 1, z: c.z + 1, w: c.w + 1},
+			{x: c.x - 1, y: c.y + 1, z: c.z + 1, w: c.w + 1},
+			{x: c.x - 1, y: c.y, z: c.z + 1, w: c.w + 1},
+			{x: c.x - 1, y: c.y - 1, z: c.z + 1, w: c.w + 1},
+
+			// same layer
+			{x: c.x + 1, y: c.y + 1, z: c.z, w: c.w + 1},
+			{x: c.x + 1, y: c.y, z: c.z, w: c.w + 1},
+			{x: c.x + 1, y: c.y - 1, z: c.z, w: c.w + 1},
+			{x: c.x, y: c.y + 1, z: c.z, w: c.w + 1},
+			{x: c.x, y: c.y, z: c.z, w: c.w + 1},
+			{x: c.x, y: c.y - 1, z: c.z, w: c.w + 1},
+			{x: c.x - 1, y: c.y + 1, z: c.z, w: c.w + 1},
+			{x: c.x - 1, y: c.y, z: c.z, w: c.w + 1},
+			{x: c.x - 1, y: c.y - 1, z: c.z, w: c.w + 1},
+
+			// below
+			{x: c.x + 1, y: c.y + 1, z: c.z - 1, w: c.w + 1},
+			{x: c.x + 1, y: c.y, z: c.z - 1, w: c.w + 1},
+			{x: c.x + 1, y: c.y - 1, z: c.z - 1, w: c.w + 1},
+			{x: c.x, y: c.y + 1, z: c.z - 1, w: c.w + 1},
+			{x: c.x, y: c.y, z: c.z - 1, w: c.w + 1},
+			{x: c.x, y: c.y - 1, z: c.z - 1, w: c.w + 1},
+			{x: c.x - 1, y: c.y + 1, z: c.z - 1, w: c.w + 1},
+			{x: c.x - 1, y: c.y, z: c.z - 1, w: c.w + 1},
+			{x: c.x - 1, y: c.y - 1, z: c.z - 1, w: c.w + 1},
+			// inward...?
+			// above
+			{x: c.x + 1, y: c.y + 1, z: c.z + 1, w: c.w - 1},
+			{x: c.x + 1, y: c.y, z: c.z + 1, w: c.w - 1},
+			{x: c.x + 1, y: c.y - 1, z: c.z + 1, w: c.w - 1},
+			{x: c.x, y: c.y + 1, z: c.z + 1, w: c.w - 1},
+			{x: c.x, y: c.y, z: c.z + 1, w: c.w - 1},
+			{x: c.x, y: c.y - 1, z: c.z + 1, w: c.w - 1},
+			{x: c.x - 1, y: c.y + 1, z: c.z + 1, w: c.w - 1},
+			{x: c.x - 1, y: c.y, z: c.z + 1, w: c.w - 1},
+			{x: c.x - 1, y: c.y - 1, z: c.z + 1, w: c.w - 1},
+
+			// same layer
+			{x: c.x + 1, y: c.y + 1, z: c.z, w: c.w - 1},
+			{x: c.x + 1, y: c.y, z: c.z, w: c.w - 1},
+			{x: c.x + 1, y: c.y - 1, z: c.z, w: c.w - 1},
+			{x: c.x, y: c.y + 1, z: c.z, w: c.w - 1},
+			{x: c.x, y: c.y, z: c.z, w: c.w - 1},
+			{x: c.x, y: c.y - 1, z: c.z, w: c.w - 1},
+			{x: c.x - 1, y: c.y + 1, z: c.z, w: c.w - 1},
+			{x: c.x - 1, y: c.y, z: c.z, w: c.w - 1},
+			{x: c.x - 1, y: c.y - 1, z: c.z, w: c.w - 1},
+
+			// below
+			{x: c.x + 1, y: c.y + 1, z: c.z - 1, w: c.w - 1},
+			{x: c.x + 1, y: c.y, z: c.z - 1, w: c.w - 1},
+			{x: c.x + 1, y: c.y - 1, z: c.z - 1, w: c.w - 1},
+			{x: c.x, y: c.y + 1, z: c.z - 1, w: c.w - 1},
+			{x: c.x, y: c.y, z: c.z - 1, w: c.w - 1},
+			{x: c.x, y: c.y - 1, z: c.z - 1, w: c.w - 1},
+			{x: c.x - 1, y: c.y + 1, z: c.z - 1, w: c.w - 1},
+			{x: c.x - 1, y: c.y, z: c.z - 1, w: c.w - 1},
+			{x: c.x - 1, y: c.y - 1, z: c.z - 1, w: c.w - 1},
+		}...)
+	}
+
+	// "normal" space
+	cells = append(cells, []cell{
+		// above
+		{x: c.x + 1, y: c.y + 1, z: c.z + 1, w: c.w},
+		{x: c.x + 1, y: c.y, z: c.z + 1, w: c.w},
+		{x: c.x + 1, y: c.y - 1, z: c.z + 1, w: c.w},
+		{x: c.x, y: c.y + 1, z: c.z + 1, w: c.w},
+		{x: c.x, y: c.y, z: c.z + 1, w: c.w},
+		{x: c.x, y: c.y - 1, z: c.z + 1, w: c.w},
+		{x: c.x - 1, y: c.y + 1, z: c.z + 1, w: c.w},
+		{x: c.x - 1, y: c.y, z: c.z + 1, w: c.w},
+		{x: c.x - 1, y: c.y - 1, z: c.z + 1, w: c.w},
+
+		// same layer
+		{x: c.x + 1, y: c.y + 1, z: c.z, w: c.w},
+		{x: c.x + 1, y: c.y, z: c.z, w: c.w},
+		{x: c.x + 1, y: c.y - 1, z: c.z, w: c.w},
+		{x: c.x, y: c.y + 1, z: c.z, w: c.w},
+		// <-- original cell
+		{x: c.x, y: c.y - 1, z: c.z, w: c.w},
+		{x: c.x - 1, y: c.y + 1, z: c.z, w: c.w},
+		{x: c.x - 1, y: c.y, z: c.z, w: c.w},
+		{x: c.x - 1, y: c.y - 1, z: c.z, w: c.w},
+
+		// below
+		{x: c.x + 1, y: c.y + 1, z: c.z - 1, w: c.w},
+		{x: c.x + 1, y: c.y, z: c.z - 1, w: c.w},
+		{x: c.x + 1, y: c.y - 1, z: c.z - 1, w: c.w},
+		{x: c.x, y: c.y + 1, z: c.z - 1, w: c.w},
+		{x: c.x, y: c.y, z: c.z - 1, w: c.w},
+		{x: c.x, y: c.y - 1, z: c.z - 1, w: c.w},
+		{x: c.x - 1, y: c.y + 1, z: c.z - 1, w: c.w},
+		{x: c.x - 1, y: c.y, z: c.z - 1, w: c.w},
+		{x: c.x - 1, y: c.y - 1, z: c.z - 1, w: c.w},
+	}...)
+
+	return cells
+}
+
+func checkNeighbors(u universe, c cell) bool {
+	sum := 0
+	for _, c := range makeNeighborsList(c, u.hyper) {
+		if u.cells[c] {
+			sum = sum + 1
+		}
+	}
+	if sum == 3 {
+		return true
+	} else if u.cells[c] && sum == 2 {
+		return true
+	}
+	return false
+}
+
+func step(old universe) universe {
+	new := universe{
+		cells: make(map[cell]bool, len(old.cells)),
+		hyper: old.hyper,
+	}
+	cellsToCheck := make([]cell, 0, len(old.cells)*80)
+	for c := range old.cells {
+		cellsToCheck = append(cellsToCheck, makeNeighborsList(c, old.hyper)...)
+	}
+	for _, c := range cellsToCheck {
+		if checkNeighbors(old, c) {
+			new.cells[c] = true
+		}
+	}
+	return new
+}
+
+func newUniverse(input []string, hyper bool) universe {
+	u := universe{
+		cells: make(map[cell]bool, len(input)),
+		hyper: hyper,
+	}
+
+	for i, r := range input {
+		for j, c := range r {
+			if c == '#' {
+				u.cells[cell{z: 0, x: i, y: j, w: 0}] = true
+			}
+		}
+	}
+
+	return u
+}
+
+func countActivesInUniverse(u universe) {
+	fmt.Println(len(u.cells))
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	var input []string
+
+	for scanner.Scan() {
+		input = append(input, scanner.Text())
+	}
+
+	u := newUniverse(input, false)
+	u = step(u)
+	u = step(u)
+	u = step(u)
+	u = step(u)
+	u = step(u)
+	u = step(u)
+	countActivesInUniverse(u)
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	var input []string
+	for scanner.Scan() {
+		input = append(input, scanner.Text())
+	}
+
+	u := newUniverse(input, true)
+	u = step(u)
+	u = step(u)
+	u = step(u)
+	u = step(u)
+	u = step(u)
+	u = step(u)
+	countActivesInUniverse(u)
+}

18/main.go

@@ -0,0 +1,180 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+// an expression is a list of expressions and a list of operators that intersperse those expressions, OR a number.
+// as a sanity check, the length of the body list should always be one more than operators, unless both are 0,
+// in which case number must be set. Luckily, we don't have to worry about 0 (the "empty" state for numbers).
+type expression struct {
+	body      []expression
+	operators []op
+	number    int
+}
+
+// op can be one of two things.
+type op struct {
+	plus     bool
+	multiply bool
+}
+
+// 1 + 2 should parse to
+// expression{
+// 		body:[
+// 			expression{number: 1},
+// 			expression{number: 2},
+// 		],
+// 		operators:[
+// 			op{plus: true},
+// 		],
+// }
+
+// (2 * 2) + 2 should parse to:
+// expression{
+// 		body:[
+// 			expression{
+// 				body: [
+// 					expression{number: 2},
+// 					expression{number: 2},
+// 				],
+// 				operators: [
+// 					op{multiply: true},
+// 				],
+// 			},
+// 			expression{number: 2},
+// 		],
+// 		operators:[
+// 			op{plus:true}
+// 		]
+// }
+
+func parseExpression(e expression, s string) (expression, string) {
+	for i := 0; i < len(s); {
+		switch s[i] {
+		case '(':
+			sub, remainder := parseExpression(expression{}, s[i+1:])
+			e.body = append(e.body, sub)
+			i = len(s) - len(remainder) + 1 // skip everything we just parsed in the subexpression
+		case ')':
+			return e, s[i:]
+		case '*':
+			e.operators = append(e.operators, op{multiply: true})
+			i = i + 1
+		case '+':
+			e.operators = append(e.operators, op{plus: true})
+			i = i + 1
+		case ' ':
+			i = i + 1
+		case '1', '2', '3', '4', '5', '6', '7', '8', '9':
+			// luckily, it's all single-digit numbers and I don't need to track parser state
+			// an ASCII byte number minus the rune value of 0 equals the integer itself: '1' - '0' == 49 - 48;
+			// rune math returns a platform-specific int, so cast to generic int.
+			e.body = append(e.body, expression{number: int(s[i] - '0')})
+			i = i + 1
+		}
+	}
+	return e, ""
+}
+
+func printExpression(e expression) string {
+	result := ""
+	for i := 0; i < len(e.body); i = i + 1 {
+		if e.body[i].number != 0 {
+			result = result + fmt.Sprintf("%d ", e.body[i].number)
+		} else {
+			result = result + "(" + printExpression(e.body[i]) + ") "
+		}
+		if i < len(e.operators) {
+			if e.operators[i].multiply {
+				result = result + "* "
+			} else if e.operators[i].plus {
+				result = result + "+ "
+			}
+		}
+	}
+	return strings.TrimSpace(result)
+}
+
+// evaluation starts at the root then descends into the sub-expressions.
+func evalLeftToRight(e expression) int {
+	var result int
+	if e.number != 0 {
+		result = e.number
+	} else {
+		result = evalLeftToRight(e.body[0])
+	}
+	for i := 0; i < len(e.operators); i = i + 1 {
+		if e.operators[i].multiply {
+			result = result * evalLeftToRight(e.body[i+1])
+		}
+		if e.operators[i].plus {
+			result = result + evalLeftToRight(e.body[i+1])
+		}
+	}
+	return result
+}
+
+// previous approach might not work...
+func evalPlusThenMultiplication(e expression) int {
+	var result int
+	if e.number != 0 {
+		result = e.number
+	} else {
+		result = evalPlusThenMultiplication(e.body[0])
+	}
+	for i := 0; i < len(e.operators); i = i + 1 {
+		if e.operators[i].multiply {
+			result = result * evalPlusThenMultiplication(e.body[i+1])
+		}
+		if e.operators[i].plus {
+			result = result + evalPlusThenMultiplication(e.body[i+1])
+		}
+	}
+	return result
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	total := 0
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		e, _ := parseExpression(expression{}, line)
+		total = total + evalLeftToRight(e)
+	}
+
+	fmt.Println(total)
+}
+
+func partTwo() {
+	f, _ := os.Open("testinput")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	total := 0
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		e, _ := parseExpression(expression{}, line)
+		fmt.Println(printExpression(e), "=", evalPlusThenMultiplication(e))
+	}
+
+	fmt.Println(total)
+}

19/main.go

@@ -0,0 +1,175 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+// options: 20 => [][20]; 20 10 => [][20, 10]; 20 | 10 => [][20][10]; 20 10 | 39 12 => [][20, 10][39, 12];
+// token is only set for the two "bottom" rules of "a" and "b"
+// options and token will never both be set
+// ints all point to rule numbers in overall rule list
+type rule struct {
+	name    int
+	options [][]int
+	token   rune
+}
+
+func parseRule(s string) rule {
+	r := rule{
+		options: make([][]int, 0),
+	}
+	sp := strings.Split(s, ": ")
+	r.name, _ = strconv.Atoi(sp[0])
+
+	// either it's a token rule...
+	if sp[1] == `"a"` || sp[1] == `"b"` {
+		r.token = []rune(strings.Trim(sp[1], `"`))[0]
+		return r
+	}
+
+	// ...or an options rule.
+	opts := strings.Split(sp[1], " | ")
+	for _, o := range opts {
+		opt := []int{}
+		for _, i := range strings.Split(o, " ") {
+			ii, _ := strconv.Atoi(i)
+			opt = append(opt, ii)
+		}
+		r.options = append(r.options, opt)
+	}
+	return r
+}
+
+func validateString(rules []rule, currentRule int, s string) (bool, string) {
+	activeRule := rules[currentRule]
+	// if we're run out of string, the rule obviously cannot match
+	if len(s) == 0 {
+		return false, ""
+	}
+	// we've reached a leaf; check if the current head of the string matches our expected rune
+	if len(activeRule.options) == 0 {
+		return []rune(s)[0] == activeRule.token, s[1:]
+	}
+	// if there's only one option, attempt to apply the rules
+	if len(activeRule.options) == 1 {
+		option := activeRule.options[0]
+		if len(option) == 1 {
+			return validateString(rules, option[0], s)
+		}
+		if len(option) == 2 {
+			ok, remainder := validateString(rules, option[0], s)
+			if ok {
+				return validateString(rules, option[1], remainder)
+			}
+		}
+		if len(option) == 3 {
+			ok, remainder := validateString(rules, option[0], s)
+			if ok {
+				ok, remainder = validateString(rules, option[1], remainder)
+			}
+			if ok {
+				return validateString(rules, option[2], remainder)
+			}
+		}
+	}
+	// otherwise, we need to branch into our options and try and apply their rules
+	if len(activeRule.options) == 2 {
+		ok := false
+		remainder := ""
+		optionOne := activeRule.options[0]
+		if len(optionOne) == 1 {
+			ok, remainder = validateString(rules, optionOne[0], s)
+		}
+		if len(optionOne) == 2 {
+			ok, remainder = validateString(rules, optionOne[0], s)
+			if ok {
+				ok, remainder = validateString(rules, optionOne[1], remainder)
+			}
+		}
+		if len(optionOne) == 3 {
+			ok, remainder = validateString(rules, optionOne[0], s)
+			if ok {
+				ok, remainder = validateString(rules, optionOne[1], remainder)
+			}
+			if ok {
+				ok, remainder = validateString(rules, optionOne[2], remainder)
+			}
+		}
+		if ok {
+			return ok, remainder
+		}
+		optionTwo := activeRule.options[1]
+		if len(optionTwo) == 1 {
+			ok, _ = validateString(rules, optionTwo[0], s)
+		}
+		if len(optionTwo) == 2 {
+			ok, remainder = validateString(rules, optionTwo[0], s)
+			if ok {
+				return validateString(rules, optionTwo[1], remainder)
+			}
+		}
+		if len(optionTwo) == 3 {
+			ok, remainder = validateString(rules, optionTwo[0], s)
+			if ok {
+				ok, remainder = validateString(rules, optionTwo[1], remainder)
+			}
+			if ok {
+				return validateString(rules, optionTwo[2], remainder)
+			}
+		}
+	}
+	return false, s
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	rules := make([]rule, 134)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			break
+		}
+		rule := parseRule(line)
+		rules[rule.name] = rule
+	}
+
+	total := 0
+	for scanner.Scan() {
+		line := scanner.Text()
+		if ok, _ := validateString(rules, 0, line); ok {
+			fmt.Println("VALID:", line)
+			total = total + 1
+		} else {
+			fmt.Println("INVALID:", line)
+		}
+	}
+
+	fmt.Println(total)
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	for scanner.Scan() {
+		// line := scanner.Text()
+	}
+}

20/main.go

@@ -0,0 +1,244 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"math"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+type tile struct {
+	id        int
+	neighbors map[int]int // key matches the edge key, so we can skip existing matches if we want
+	edges     map[int]string
+	image     map[int]string
+}
+
+// edges are always defined clockwise starting from 0:north
+// as long as there's only ever one match for an edge, I don't _think_ I need to keep track of piece orientation...?
+func parseTile(header string, body []string) tile {
+	id, _ := strconv.Atoi(strings.Trim(header, "Tile :"))
+	t := tile{
+		id:        id,
+		neighbors: make(map[int]int, 4),
+		edges:     make(map[int]string, 4),
+		image:     make(map[int]string, 8),
+	}
+	// make edges and extract picture data...
+	left := []byte{}
+	right := []byte{}
+	for i := range body {
+		left = append(left, body[i][0])
+		right = append(right, body[i][9])
+		// image has edges removed; still want 0-indexed for sanity later!
+		if i != 0 && i != 9 {
+			t.image[i-1] = body[i-1][1:9]
+		}
+	}
+	t.edges[0] = body[0]
+	t.edges[1] = string(right)
+	t.edges[2] = body[9]
+	t.edges[3] = string(left)
+	return t
+}
+
+// return is match, inverted
+func testEdge(one, two string) (bool, bool) {
+	if one == two {
+		return true, false
+	}
+	// go doesn't have a built-in "string reversal" method, so we just check it the hard way
+	// 9 is our magic number for a 10-character string, which we know they all are.
+	// This might not be 100% sound for unicode (byte != rune), but our string is ASCII so it's... fine.
+	for i := range one {
+		if one[i] != two[9-i] {
+			return false, false
+		}
+	}
+	return true, true
+}
+
+// this is wasteful in that it happily checks edges that we've already determined neighbors for,
+// but it's performant enough that I don't really care to fix it
+func findPartners(id int, tiles map[int]tile) map[int]tile {
+	currentTile := tiles[id]
+	for tile := range tiles {
+		if tile != id {
+			for edgeID := range tiles[tile].edges {
+				for i := range currentTile.edges {
+					if ok, _ := testEdge(tiles[tile].edges[edgeID], currentTile.edges[i]); ok {
+						tiles[tile].neighbors[edgeID] = tiles[id].id
+						tiles[id].neighbors[i] = tiles[tile].id
+					}
+				}
+			}
+		}
+	}
+	return tiles
+}
+
+// Starting from a corner, we can "walk" through the image by:
+// stepping to the next tile in a known direction
+// checking where _that_ tile thinks we came from
+// then re-orienting ourselves and stepping again.
+// For example, if we take a corner and go to its 0 neighbor,
+// and that neighbor thinks our previous tile was 3,
+// a "straight line" along the edge would be 1. Repeat as necessary until you hit an edge.
+// Once we hit an edge, return to the start of the row,
+// step to its 90° neighbor, then walk parallel to the initial edge.
+// Repeat until we reach the far corner.
+// Additionally, we need to "line up" the edges, so there's some alignment checking that has to happen.
+func mergeTiles(corner int, tiles map[int]tile) []string {
+	// just to make the counting a little easier
+	sideLength := int(math.Sqrt(float64(len(tiles))))
+	result := make([]string, sideLength*8)
+	headOfRow := tiles[corner]
+	currentRowDirection := 0
+	nextRowDirection := 0
+	// what an absurd way to "pick" between two keys in a two-key map
+	for i := range headOfRow.neighbors {
+		if currentRowDirection == 0 {
+			currentRowDirection = i
+		} else if nextRowDirection == 0 {
+			nextRowDirection = i
+		}
+	}
+	currentTile := tiles[corner]
+	image := imageFromEdge(currentTile, 0)
+	for row, s := range image {
+		result[row] = result[row] + s
+	}
+	for i := 0; i < sideLength; i = i + 1 {
+		nextID := currentTile.neighbors[currentRowDirection]
+		for e, id := range tiles[nextID].neighbors {
+			if id == currentTile.id {
+				_, invert := testEdge(currentTile.edges[currentRowDirection], tiles[nextID].edges[e])
+				image := imageFromEdge(tiles[nextID], (e+1)%4)
+				if !invert {
+					for row, s := range image {
+						result[row] = result[row] + s
+					}
+				} else {
+					for row, s := range image {
+						result[7-row] = result[7-row] + s
+					}
+				}
+				currentRowDirection = (e + 2) % 4 // flip 180 around the compass
+			}
+		}
+		currentTile = tiles[nextID]
+	}
+	return result
+}
+
+// returns the image "rotated" so that the provided edge is on "top" (the 0 position)
+func imageFromEdge(t tile, edge int) map[int]string {
+	if edge == 1 {
+		newImage := make(map[int]string, 8)
+		for i := 0; i < 8; i = i + 1 {
+			b := []byte{t.image[0][7-i], t.image[1][7-i], t.image[2][7-i], t.image[3][7-i], t.image[4][7-i], t.image[5][7-i], t.image[6][7-i], t.image[7][7-i]}
+			newImage[i] = string(b)
+		}
+		return newImage
+	}
+	if edge == 2 {
+		newImage := make(map[int]string, 8)
+		for i := 0; i < 8; i = i + 1 {
+			b := []byte{t.image[7-i][7], t.image[7-i][6], t.image[7-i][5], t.image[7-i][4], t.image[7-i][3], t.image[7-i][2], t.image[7-i][1], t.image[7-i][0]}
+			newImage[i] = string(b)
+		}
+		return newImage
+	}
+	if edge == 3 {
+		newImage := make(map[int]string, 8)
+		for i := 0; i < 8; i = i + 1 {
+			b := []byte{t.image[7][i], t.image[6][i], t.image[5][i], t.image[4][i], t.image[3][i], t.image[2][i], t.image[1][i], t.image[0][i]}
+			newImage[i] = string(b)
+		}
+		return newImage
+	}
+	// edge == 0 does nothing
+	return t.image
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	buffer := []string{}
+	tiles := make(map[int]tile)
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			tile := parseTile(buffer[0], buffer[1:])
+			tiles[tile.id] = tile
+			buffer = []string{}
+		} else {
+			buffer = append(buffer, line)
+		}
+	}
+	// catch that last tile
+	// weirdly, I'd think the trailing newline in the input would have done this for us
+	tile := parseTile(buffer[0], buffer[1:])
+	tiles[tile.id] = tile
+
+	corners := 1
+	for id := range tiles {
+		tiles = findPartners(id, tiles)
+		if len(tiles[id].neighbors) == 2 {
+			corners = corners * id
+		}
+	}
+
+	fmt.Println(corners)
+}
+
+func partTwo() {
+	f, _ := os.Open("testinput")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	buffer := []string{}
+	tiles := make(map[int]tile)
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			tile := parseTile(buffer[0], buffer[1:])
+			tiles[tile.id] = tile
+			buffer = []string{}
+		} else {
+			buffer = append(buffer, line)
+		}
+	}
+	// catch that last tile
+	tile := parseTile(buffer[0], buffer[1:])
+	tiles[tile.id] = tile
+
+	corner := 0
+	for id := range tiles {
+		tiles = findPartners(id, tiles)
+		// grab a corner to start the merge from
+		if len(tiles[id].neighbors) == 2 && corner == 0 {
+			corner = id
+		}
+	}
+
+	for _, line := range mergeTiles(corner, tiles) {
+		fmt.Println(line)
+	}
+}

21/main.go

@@ -0,0 +1,140 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+type food struct {
+	allergens   []string
+	ingredients []string
+}
+
+func intersection(a, b []string) []string {
+	// special-case starting out
+	if len(a) == 0 {
+		return b
+	} else if len(b) == 0 {
+		return a
+	}
+	result := []string{}
+	for _, s := range b {
+		for _, t := range a {
+			if s == t {
+				result = append(result, s)
+			}
+		}
+	}
+	return result
+}
+
+func countSafeIngredients(f food, allergens map[string][]string) int {
+	for c, i := range f.ingredients {
+		for _, l := range allergens {
+			for _, a := range l {
+				if i == a {
+					f.ingredients[c] = ""
+				}
+			}
+		}
+	}
+	result := 0
+	for _, i := range f.ingredients {
+		if i != "" {
+			result = result + 1
+		}
+	}
+	return result
+}
+
+func remove(ingredients []string, ingredient string) []string {
+	for i, j := range ingredients {
+		if j == ingredient {
+			ingredients[i] = ingredients[len(ingredients)-1]
+			ingredients[len(ingredients)-1] = ""
+			return ingredients[:len(ingredients)-1]
+		}
+	}
+	return ingredients
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	foods := []food{}
+	allergens := map[string][]string{}
+
+	for scanner.Scan() {
+		line := strings.Split(scanner.Text(), " (contains ")
+		f := food{
+			ingredients: strings.Split(line[0], " "),
+			allergens:   strings.Split(strings.TrimSuffix(line[1], ")"), ", "),
+		}
+		foods = append(foods, f)
+		for _, a := range f.allergens {
+			allergens[a] = intersection(f.ingredients, allergens[a])
+		}
+	}
+
+	sum := 0
+	for _, f := range foods {
+		sum = sum + countSafeIngredients(f, allergens)
+	}
+	fmt.Println(sum)
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	foods := []food{}
+	allergens := map[string][]string{}
+
+	for scanner.Scan() {
+		line := strings.Split(scanner.Text(), " (contains ")
+		f := food{
+			ingredients: strings.Split(line[0], " "),
+			allergens:   strings.Split(strings.TrimSuffix(line[1], ")"), ", "),
+		}
+		foods = append(foods, f)
+		for _, a := range f.allergens {
+			allergens[a] = intersection(f.ingredients, allergens[a])
+		}
+	}
+
+	// hey, this is the same reduction logic as day 16!
+	trueAllergens := map[string]string{}
+	var lastAllergenSolved string
+	for i := 0; i < len(allergens); i = i + 1 {
+		for i, a := range allergens {
+			if len(a) == 1 {
+				trueAllergens[i] = a[0]
+				lastAllergenSolved = a[0]
+			}
+		}
+		for i := range allergens {
+			allergens[i] = remove(allergens[i], lastAllergenSolved)
+		}
+	}
+
+	formatted := "" // in theory we shouldn't hard-code this allergen order...
+	for _, a := range []string{"dairy", "eggs", "fish", "nuts", "peanuts", "sesame", "soy", "wheat"} {
+		formatted = formatted + trueAllergens[a] + ","
+	}
+	fmt.Println(strings.TrimSuffix(formatted, ","))
+}

22/main.go

@@ -0,0 +1,155 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+type deck struct {
+	cards []int
+}
+
+type cacheKey struct {
+	one int
+	two int
+}
+
+func countWinningDeck(deck []int) int {
+	sum := 0
+	for i := range deck {
+		sum = sum + ((len(deck) - i) * deck[i])
+	}
+	return sum
+}
+
+// return values is who won/their score: true for player one, false for player two
+func playCombat(one, two []int) (bool, int) {
+	round := 0
+	for len(one) > 0 && len(two) > 0 {
+		round = round + 1
+		if one[0] > two[0] {
+			one = append(one[1:], one[0], two[0])
+			two = two[1:]
+		} else if one[0] < two[0] {
+			two = append(two[1:], two[0], one[0])
+			one = one[1:]
+		}
+	}
+	if len(one) == 0 {
+		return false, countWinningDeck(two)
+	}
+	return true, countWinningDeck(one)
+}
+
+func playRecursiveCombat(one, two []int) (bool, int) {
+	loopCache := make(map[cacheKey]bool, 0)
+	round := 0
+	for len(one) > 0 && len(two) > 0 {
+		// check for loops
+		if loopCache[cacheKey{one: countWinningDeck(one), two: countWinningDeck(two)}] {
+			return true, countWinningDeck(one)
+		}
+		loopCache[cacheKey{one: countWinningDeck(one), two: countWinningDeck(two)}] = true
+
+		// otherwise, play a round
+		round = round + 1
+		if len(one[1:]) >= one[0] && len(two[1:]) >= two[0] {
+			// fuckin' go slice internals
+			subDeckOne := make([]int, len(one))
+			subDeckTwo := make([]int, len(two))
+			copy(subDeckOne, one)
+			copy(subDeckTwo, two)
+			winner, _ := playRecursiveCombat(subDeckOne[1:one[0]+1], subDeckTwo[1:two[0]+1])
+			if winner { // p1 wins
+				one = append(one[1:], one[0], two[0])
+				two = two[1:]
+			} else { // p2 wins
+				two = append(two[1:], two[0], one[0])
+				one = one[1:]
+			}
+		} else if one[0] > two[0] {
+			one = append(one[1:], one[0], two[0])
+			two = two[1:]
+		} else if one[0] < two[0] {
+			two = append(two[1:], two[0], one[0])
+			one = one[1:]
+		}
+	}
+	if len(one) == 0 {
+		return false, countWinningDeck(two)
+	}
+	return true, countWinningDeck(one)
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	// deal the decks
+	playerone := deck{}
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			break
+		}
+		i, err := strconv.Atoi(line)
+		if err == nil {
+			playerone.cards = append(playerone.cards, i)
+		}
+	}
+	playertwo := deck{}
+	for scanner.Scan() {
+		line := scanner.Text()
+		i, err := strconv.Atoi(line)
+		if err == nil {
+			playertwo.cards = append(playertwo.cards, i)
+		}
+	}
+
+	// play a game and determine a winner
+	_, score := playCombat(playerone.cards, playertwo.cards)
+	fmt.Println(score)
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	// deal the decks
+	playerone := deck{}
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line == "" {
+			break
+		}
+		i, err := strconv.Atoi(line)
+		if err == nil {
+			playerone.cards = append(playerone.cards, i)
+		}
+	}
+	playertwo := deck{}
+	for scanner.Scan() {
+		line := scanner.Text()
+		i, err := strconv.Atoi(line)
+		if err == nil {
+			playertwo.cards = append(playertwo.cards, i)
+		}
+	}
+
+	_, score := playRecursiveCombat(playerone.cards, playertwo.cards)
+	fmt.Println(score)
+}

23/main.go

@@ -0,0 +1,137 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+func cupInList(cup int, cups []int) bool {
+	for _, c := range cups {
+		if c == cup {
+			return true
+		}
+	}
+	return false
+}
+
+func generateFullCupset(cups []int, totalCups int) []int {
+	for i := len(cups); i < totalCups; i = i + 1 {
+		cups = append(cups, i)
+	}
+	return cups
+}
+
+func findTargetCupIndex(cup int, cups []int) int {
+	for i, c := range cups {
+		if c == cup {
+			return i
+		}
+	}
+	return 0 // uh-oh
+}
+
+func step(cups []int) []int {
+	cupCount := len(cups)
+	currentCup := cups[0]
+	removedCups := cups[1:4]
+	newList := make([]int, 0, cupCount)
+	newList = append([]int{cups[0]}, cups[4:]...)
+	var targetCup int
+	if currentCup == 1 {
+		targetCup = cupCount
+	} else {
+		targetCup = currentCup - 1
+	}
+	for cupInList(targetCup, removedCups) {
+		targetCup = targetCup - 1
+		if targetCup == 0 {
+			targetCup = cupCount
+		}
+	}
+	tc := findTargetCupIndex(targetCup, newList)
+	result := make([]int, 0, cupCount)
+	result = append(result, newList[1:tc+1]...)
+	result = append(result, removedCups...)
+	result = append(result, newList[tc+1:]...)
+	result = append(result, newList[0])
+	return result
+}
+
+func runabout(cups []int) string {
+	oneIndex := findTargetCupIndex(1, cups)
+	result := ""
+	for i := oneIndex + 1; i < len(cups); i = i + 1 {
+		result = result + strconv.Itoa(cups[i])
+	}
+	for i := 0; i < len(cups[:oneIndex]); i = i + 1 {
+		result = result + strconv.Itoa(cups[i])
+	}
+	return result
+}
+
+func nextTwo(cups []int) (int, int) {
+	oneIndex := findTargetCupIndex(1, cups)
+	if oneIndex+1 > len(cups) {
+		return cups[0], cups[1]
+	}
+	if oneIndex+2 > len(cups) {
+		return cups[len(cups)], cups[0]
+	}
+	return cups[oneIndex+1], cups[oneIndex+2]
+}
+
+func partOne() {
+	f, _ := os.Open("testinput")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	cups := []int{}
+
+	scanner.Scan()
+	for _, s := range strings.Split(scanner.Text(), "") {
+		i, _ := strconv.Atoi(s)
+		cups = append(cups, i)
+	}
+
+	for i := 0; i < 100; i = i + 1 {
+		cups = step(cups)
+	}
+
+	fmt.Println(runabout(cups))
+
+}
+
+func partTwo() {
+	f, _ := os.Open("testinput")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	cups := []int{}
+
+	scanner.Scan()
+	for _, s := range strings.Split(scanner.Text(), "") {
+		i, _ := strconv.Atoi(s)
+		cups = append(cups, i)
+	}
+
+	allCups := generateFullCupset(cups, 10000000)
+	for i := 0; i < 10000000; i = i + 1 {
+		allCups = step(allCups)
+		fmt.Println(i)
+	}
+
+	fmt.Println(nextTwo(allCups))
+}

24/main.go

@@ -0,0 +1,186 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+// Our grid is a hex, so let's represent that as follows:
+// x runs east-west, y runs north-south. Yes, these coordinates here are backward.
+//       2,-2      2,0     2,2     2,4
+//  1,-2      1,-1     1,1     1,2
+//       0,-2      0,0     0,2     0,4
+// -1,-2     -1,-1    -1,1    -1,2
+//      -2,-2     -2,0    -2,2    -2,4
+// 0,0 ->  e ->  0, 2
+//     ->  w ->  0,-2
+//     -> se ->	-1, 1
+//     -> sw -> -1,-1
+//     -> ne ->  1, 1
+//     -> nw ->  1,-1
+
+// 0 == white, 1 == black
+type tiles map[coord]int
+
+type coord struct {
+	x int
+	y int
+}
+
+func walkGrid(directions []string) (int, int) {
+	var y, x int
+	for _, d := range directions {
+		switch d {
+		case "e":
+			x = x + 2
+		case "se":
+			y = y - 1
+			x = x + 1
+		case "sw":
+			y = y - 1
+			x = x - 1
+		case "w":
+			x = x - 2
+		case "ne":
+			y = y + 1
+			x = x + 1
+		case "nw":
+			y = y + 1
+			x = x - 1
+		}
+	}
+	return y, x
+}
+
+func parseLine(s string) []string {
+	directions := []string{}
+	for i := 0; i < len(s); {
+		switch s[i] {
+		case 'e':
+			directions = append(directions, "e")
+			i = i + 1
+		case 's':
+			if s[i+1] == 'e' {
+				directions = append(directions, "se")
+				i = i + 2
+			} else if s[i+1] == 'w' {
+				directions = append(directions, "sw")
+				i = i + 2
+			}
+		case 'w':
+			directions = append(directions, "w")
+			i = i + 1
+		case 'n':
+			if s[i+1] == 'e' {
+				directions = append(directions, "ne")
+				i = i + 2
+			} else if s[i+1] == 'w' {
+				directions = append(directions, "nw")
+				i = i + 2
+			}
+		}
+	}
+	return directions
+}
+
+func generation(t tiles) tiles {
+	newTiles := tiles{}
+	seenWhiteTiles := tiles{}
+	for coord, tile := range t {
+		// walk black tile list
+		if tile == 1 {
+			neighbors := getNeighbors(coord)
+			bn := 0
+			for _, n := range neighbors {
+				if t[n] == 0 {
+					seenWhiteTiles[n] = seenWhiteTiles[n] + 1
+				} else {
+					bn = bn + 1
+				}
+			}
+			// black tiles only survive if they have one or two black neighbors
+			if bn == 1 || bn == 2 {
+				newTiles[coord] = 1
+			}
+		}
+	}
+	// white tiles turn black if they have two black neighbors
+	for c, n := range seenWhiteTiles {
+		if n == 2 {
+			newTiles[c] = 1
+		}
+	}
+	return newTiles
+}
+
+func getNeighbors(c coord) []coord {
+	return []coord{
+		{x: c.x + 2, y: c.y},     // e
+		{x: c.x - 2, y: c.y},     // w
+		{x: c.x + 1, y: c.y + 1}, // ne
+		{x: c.x + 1, y: c.y - 1}, // nw
+		{x: c.x - 1, y: c.y + 1}, // se
+		{x: c.x - 1, y: c.y - 1}, // sw
+	}
+}
+
+func countBlackTiles(t tiles) int {
+	sum := 0
+	for _, tile := range t {
+		sum = sum + tile
+	}
+	return sum
+}
+
+func partOne() {
+	f, _ := os.Open("testinput")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	t := tiles{}
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		x, y := walkGrid(parseLine(line))
+		if t[coord{x: x, y: y}] == 0 {
+			t[coord{x: x, y: y}] = 1
+		} else {
+			t[coord{x: x, y: y}] = 0
+		}
+	}
+	fmt.Println(countBlackTiles(t))
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	t := tiles{}
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		y, x := walkGrid(parseLine(line))
+		if t[coord{x: x, y: y}] == 0 {
+			t[coord{x: x, y: y}] = 1
+		} else {
+			t[coord{x: x, y: y}] = 0
+		}
+	}
+	for day := 1; day <= 100; day = day + 1 {
+		t = generation(t)
+	}
+	fmt.Println(countBlackTiles(t))
+
+}

25/main.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	fmt.Printf("p1: %s, p2: n/a\n", duration)
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	scanner.Scan()
+	doorPubKey, _ := strconv.Atoi(scanner.Text())
+	scanner.Scan()
+	cardPubKey, _ := strconv.Atoi(scanner.Text())
+
+	subjectNumber := 7
+	value := 1
+	magicPrime := 20201227
+	loopCount := 1
+	doorLoopCount := 0
+	cardLoopCount := 0
+	for {
+		value = (value * subjectNumber) % magicPrime
+		if value == doorPubKey {
+			doorLoopCount = loopCount
+		}
+		if value == cardPubKey {
+			cardLoopCount = loopCount
+		}
+		loopCount = loopCount + 1
+		if doorLoopCount != 0 && cardLoopCount != 0 {
+			break
+		}
+	}
+
+	value = 1
+	subjectNumber = cardPubKey
+	for i := 0; i < doorLoopCount; i = i + 1 {
+		value = (value * subjectNumber) % magicPrime
+	}
+	fmt.Println(value)
+}

main.go.tmpl

@@ -0,0 +1,37 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	for scanner.Scan() {
+		// line := scanner.Text()
+	}
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	for scanner.Scan() {
+		// line := scanner.Text()
+	}
+}

new.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+if [ "${1}" != "" ]; then
+    mkdir -p ${1}
+    touch ${1}/{input,testinput}
+    cp -n main.go.tmpl ${1}/main.go
+fi

run.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 
 if [ "${1}" != "" ]; then
-    cd ${01}
+    cd ${1}
     go run *.go
     cd - > /dev/null
 fi