Solution for 2024, day 20

Author: lucianoq

2024/20/Makefile

@@ -0,0 +1,20 @@
+input:
+	http "https://adventofcode.com/2024/day/20/input" "Cookie:session=${AOC_SESSION};" >input
+
+main1:
+	go build -o main1 main1.go common.go
+
+main2:
+	go build -o main2 main2.go common.go
+
+.PHONY: run1 run2 clean
+
+run1: main1 input
+	./main1 <input
+
+run2: main2 input
+	./main2 <input
+
+clean:
+	rm -f main1 main2 input
+

2024/20/common.go

@@ -0,0 +1,102 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"slices"
+)
+
+type P struct{ x, y int }
+
+var Delta = []P{{0, 1}, {1, 0}, {-1, 0}, {0, -1}}
+
+func main() {
+	walls, start, end := parseInput()
+
+	path := bfs(walls, start, end)
+
+	sum := 0
+	for i := 0; i < len(path)-1; i++ {
+		for j := i + 1; j < len(path); j++ {
+			dist := manhattan(path[i], path[j])
+			if dist > 0 && dist <= CheatTime {
+				saved := j - i - dist
+				if saved >= MinSavedTime {
+					sum++
+				}
+			}
+		}
+	}
+	fmt.Println(sum)
+}
+
+func parseInput() (map[P]struct{}, P, P) {
+	walls := map[P]struct{}{}
+	var start, end P
+	scanner := bufio.NewScanner(os.Stdin)
+	for i := 0; scanner.Scan(); i++ {
+		for j, ch := range scanner.Text() {
+			switch ch {
+			case '#':
+				walls[P{i, j}] = struct{}{}
+			case 'S':
+				start = P{i, j}
+			case 'E':
+				end = P{i, j}
+			}
+		}
+	}
+	return walls, start, end
+}
+
+func bfs(walls map[P]struct{}, start, end P) []P {
+	queue := []P{start}
+	visited := map[P]struct{}{start: {}}
+	parent := map[P]P{}
+
+	for len(queue) > 0 {
+		var curr P
+		curr, queue = queue[0], queue[1:]
+
+		if curr == end {
+			return getPath(parent, start, end)
+		}
+
+		for _, d := range Delta {
+			next := P{curr.x + d.x, curr.y + d.y}
+
+			if _, ok := walls[next]; ok {
+				continue
+			}
+
+			if _, ok := visited[next]; !ok {
+				visited[next] = struct{}{}
+				parent[next] = curr
+				queue = append(queue, next)
+			}
+		}
+	}
+	return nil
+}
+
+func getPath(parent map[P]P, start, end P) []P {
+	l := make([]P, 0)
+	for p := end; p != start; p = parent[p] {
+		l = append(l, p)
+	}
+	l = append(l, start)
+	slices.Reverse(l)
+	return l
+}
+
+func manhattan(p1, p2 P) int {
+	return abs(p2.x-p1.x) + abs(p2.y-p1.y)
+}
+
+func abs(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return x
+}

2024/20/main1.go

@@ -0,0 +1,6 @@
+package main
+
+const (
+	CheatTime    = 2
+	MinSavedTime = 100
+)

2024/20/main2.go

@@ -0,0 +1,6 @@
+package main
+
+const (
+	CheatTime    = 20
+	MinSavedTime = 100
+)