Day 13

$ time ./d13
Hello
2845
487905974205117

real    0m0.008s
user    0m0.000s
sys     0m0.009s

Signed-off-by: Benjamin Hesmans <[email protected]>

d13/d13.go

@@ -0,0 +1,122 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func panicOnError(e error) {
+	if e != nil {
+		panic(e)
+	}
+}
+
+type line struct {
+	start   int
+	pos, id int
+}
+
+type bus struct {
+	edt   int
+	lines []line
+}
+
+func readInput(inputName string) bus {
+	file, err := os.Open(inputName)
+	panicOnError(err)
+	defer file.Close()
+
+	b := bus{}
+
+	scanner := bufio.NewScanner(file)
+
+	scanner.Scan()
+	l := scanner.Text()
+	v, _ := strconv.Atoi(l)
+	b.edt = int(v)
+
+	scanner.Scan()
+	l = scanner.Text()
+	t := strings.Split(l, ",")
+	for i := 0; i < len(t); i++ {
+		if t[i] == "x" {
+			continue
+		}
+		v, _ := strconv.Atoi(t[i])
+		b.lines = append(b.lines, line{pos: i, id: int(v)})
+	}
+
+	err = scanner.Err()
+	panicOnError(err)
+
+	return b
+}
+
+func (b bus) busAt(at int) int {
+	for _, line := range b.lines {
+		if at%line.id == 0 {
+			return line.id
+		}
+	}
+
+	return -1
+}
+
+func part1(b bus) int {
+	at, line := b.edt, -1
+
+	for line < 0 {
+		at++
+		line = b.busAt(at)
+	}
+
+	return (at - b.edt) * line
+}
+
+func mergeLines(l1, l2 line) line {
+	x0, y0, diff := l1.start, l2.start, l2.pos-l1.pos
+	visited := make(map[int]int)
+
+	cycle := false
+	for !cycle {
+		visited[y0-x0] = x0
+		if x0 < y0 {
+			x0 += l1.id
+		} else {
+			y0 += l2.id
+		}
+		_, cycle = visited[y0-x0]
+	}
+
+	firstOcc, _ := visited[y0-x0]
+	cycleLength := x0 - firstOcc
+
+	return line{start: visited[diff], id: cycleLength, pos: l1.pos}
+}
+
+func reduce(b bus) bus {
+	nb := bus{}
+	for i := 0; i < len(b.lines)-1; i++ {
+		nb.lines = append(nb.lines, mergeLines(b.lines[i], b.lines[i+1]))
+	}
+
+	return nb
+}
+
+func part22(b bus) int {
+	for len(b.lines) != 1 {
+		b = reduce(b)
+	}
+	return b.lines[0].start
+}
+
+func main() {
+	fmt.Println("Hello")
+	// b := readInput("small_input.txt")
+	b := readInput("input.txt")
+	fmt.Printf("%#v\n", part1(b))
+	fmt.Printf("%#v\n", part22(b))
+}

d13/input.txt

@@ -0,0 +1,2 @@
+1007125
+13,x,x,41,x,x,x,x,x,x,x,x,x,569,x,29,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,19,x,x,x,23,x,x,x,x,x,x,x,937,x,x,x,x,x,37,x,x,x,x,x,x,x,x,x,x,17

d13/small_input.txt

@@ -0,0 +1,2 @@
+939
+7,13,x,x,59,x,31,19