Day 20

4 files changed, 270 insertions, 0 deletions

diff --git a/day20/data.txt b/day20/data.txt
new file mode 100644
index 0000000..f71b7bb
--- /dev/null
+++ b/day20/data.txt
@@ -0,0 +1,141 @@
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+#.#.#.#.###.#.###.###########.#.#####.#.#.#.#.#.#########.###.#.#.#.#.#########.#.#.#.###.###.#.###.#.#.#.#.#.#.#.#.#####.#.###.#.#.#.#.#.#.#

+#...#...###...###.............#.......#...#...#...........###...#...#...........#...#.....###...###...#...#...#...#.......#.....#...#...#...#

+#############################################################################################################################################

diff --git a/day20/example.txt b/day20/example.txt
new file mode 100644
index 0000000..6f4af0d
--- /dev/null
+++ b/day20/example.txt
@@ -0,0 +1,15 @@
+###############

+#...#...#.....#

+#.#.#.#.#.###.#

+#S#...#.#.#...#

+#######.#.#.###

+#######.#.#...#

+#######.#.###.#

+###..E#...#...#

+###.#######.###

+#...###...#...#

+#.#####.#.###.#

+#.#...#.#.#...#

+#.#.#.#.#.#.###

+#...#...#...###

+###############

diff --git a/day20/go.mod b/day20/go.mod
new file mode 100644
index 0000000..fdcaa89
--- /dev/null
+++ b/day20/go.mod
@@ -0,0 +1,3 @@
+module stevenlr.com/aoc2024/day20
+
+go 1.23.3
diff --git a/day20/main.go b/day20/main.go
new file mode 100644
index 0000000..3c5921e
--- /dev/null
+++ b/day20/main.go
@@ -0,0 +1,111 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"math"
+	"os"
+	"slices"
+	"strings"
+)
+
+func main() {
+	fmt.Println(part1(readData("example.txt")))
+	fmt.Println(1358)
+	fmt.Println(part1(readData("data.txt")))
+}
+
+func IntAbs(x int) int {
+	if x >= 0 {
+		return x
+	} else {
+		return -x
+	}
+}
+
+func part1(maze [][]byte) (score1, score2 int) {
+	startX, startY := findInMaze(maze, 'S')
+	endX, endY := findInMaze(maze, 'E')
+	distMap := buildDistMap(maze, startX, startY, endX, endY)
+
+	dirs := [][2]int{}
+	for dy := -20; dy <= 20; dy++ {
+		for dx := -20; dx <= 20; dx++ {
+			d := IntAbs(dx) + IntAbs(dy)
+			if d >= 1 && d <= 20 {
+				dirs = append(dirs, [2]int{dx, dy})
+			}
+		}
+	}
+
+	x, y := startX, startY
+	for x != endX || y != endY {
+		distNow := distMap[y][x]
+		xNext, yNext := x, y
+		for _, dir := range dirs {
+			x2, y2 := x+dir[0], y+dir[1]
+			dist := IntAbs(dir[0]) + IntAbs(dir[1])
+
+			if dist == 1 && maze[y2][x2] != '#' && distMap[y2][x2] > distNow {
+				xNext, yNext = x2, y2
+			}
+
+			if dist > 1 && x2 >= 0 && y2 >= 0 && y2 < len(maze) && x2 < len(maze[y2]) && distMap[y2][x2] > distNow && maze[y2][x2] != '#' {
+				saved := distMap[y2][x2] - distNow - dist
+				if dist == 2 && saved >= 100 {
+					score1++
+				}
+				if saved >= 100 {
+					score2++
+				}
+			}
+		}
+		x, y = xNext, yNext
+	}
+
+	return
+}
+
+func buildDistMap(maze [][]byte, x, y int, endX, endY int) (distMap [][]int) {
+	distMap = make([][]int, len(maze))
+	for y, _ := range distMap {
+		distMap[y] = make([]int, len(maze[0]))
+		for x := range distMap[y] {
+			distMap[y][x] = math.MaxInt
+		}
+	}
+
+	distMap[y][x] = 0
+	var dirs = [4][2]int{{1, 0}, {-1, 0}, {0, 1}, {0, -1}}
+
+	for x != endX || y != endY {
+		for _, dir := range dirs {
+			x2, y2 := x+dir[0], y+dir[1]
+			if maze[y2][x2] != '#' && distMap[y2][x2] == math.MaxInt {
+				distMap[y2][x2] = distMap[y][x] + 1
+				x, y = x2, y2
+				break
+			}
+		}
+	}
+
+	return
+}
+
+func findInMaze(maze [][]byte, needle byte) (x, y int) {
+	for y, line := range maze {
+		if x := slices.Index(line, needle); x >= 0 {
+			return x, y
+		}
+	}
+	panic("not found")
+}
+
+func readData(fileName string) (maze [][]byte) {
+	fp, _ := os.Open(fileName)
+	scanner := bufio.NewScanner(fp)
+	for scanner.Scan() {
+		maze = append(maze, []byte(strings.TrimSpace(scanner.Text())))
+	}
+	return
+}