From 559d91b9a6c6743789988bb38f1373aef4937cea Mon Sep 17 00:00:00 2001
From: David Ashby <delta.mu.alpha@gmail.com>
Date: Sun, 13 Dec 2020 11:46:57 -0500
Subject: [PATCH] part one works, part two... eh.

---
 13/main.go | 89 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)
 create mode 100644 13/main.go

diff --git a/13/main.go b/13/main.go
new file mode 100644
index 0000000..13cfc71
--- /dev/null
+++ b/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),
+			})
+		}
+	}
+
+	// walking the list works fine for the small test inputs but takes far too long on the real schedule
+	t := float64(0)
+	for {
+	loop:
+		t = t + buses[0].route
+		for i := range buses {
+			if math.Remainder(t+buses[i].offset, buses[i].route) != 0 {
+				goto loop
+			}
+		}
+		break
+	}
+
+	fmt.Println(int(t))
+}