day fifteen, with a pretty slow part 2

15/main.go

@@ -0,0 +1,86 @@
+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)
+}
+
+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[string]int)
+
+	// load in numbers
+	for i, v := range numbers[:len(numbers)-1] {
+		seen[v] = i
+	}
+
+	for {
+		i := len(numbers) - 1
+		lastNumber := numbers[i]
+		if len(numbers) == 30000000 {
+			fmt.Println(numbers[30000000-1])
+			return
+		}
+		if v, ok := seen[lastNumber]; !ok {
+			// never seen that before
+			numbers = append(numbers, "0")
+			seen[lastNumber] = i
+			continue
+		} else {
+			// a number matched; insert the difference between the last number and when we last saw it
+			n := fmt.Sprintf("%d", i-v)
+			numbers = append(numbers, n)
+			seen[lastNumber] = i
+			continue
+		}
+	}
+
+}