Day 13

13/input.txt

@@ -0,0 +1,2 @@
+1002561
+17,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,409,x,29,x,x,x,x,x,x,x,x,x,x,13,x,x,x,x,x,x,x,x,x,23,x,x,x,x,x,x,x,373,x,x,x,x,x,x,x,x,x,41,x,x,x,x,x,x,x,x,19

13/main.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"math"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func main() {
+	var earliest int
+	var buslines []int
+
+	file, _ := os.Open("input.txt")
+	defer file.Close()
+	scanner := bufio.NewScanner(file)
+	scanner.Scan()
+	earliest, _ = strconv.Atoi(scanner.Text())
+	scanner.Scan()
+	for _, s := range strings.Split(scanner.Text(), ",") {
+		if s != "x" {
+			line, _ := strconv.Atoi(s)
+			buslines = append(buslines, line)
+		} else {
+			buslines = append(buslines, -1)
+		}
+	}
+
+	favLine := 0
+	waitTime := math.MaxInt64
+	for _, l := range buslines {
+		if l != -1 {
+			if t := l - earliest%l; t < waitTime {
+				favLine = l
+				waitTime = t
+			}
+		}
+	}
+
+	fmt.Println("Part 1:", favLine*waitTime)
+
+	t, step := 0, 1
+	for i, l := range buslines {
+		if l == -1 {
+			continue
+		}
+		for (t+i)%l != 0 {
+			t += step
+		}
+		step *= l
+	}
+
+	fmt.Println("Part 2:", t)
+}