day20

day20p1/solution.go

@@ -0,0 +1,95 @@
+package day20p1
+
+import (
+	"io"
+
+	"aoc/utils"
+)
+
+func Solve(r io.Reader) any {
+	lines := utils.ReadLines(r)
+
+	track := make([][]rune, len(lines))
+	startPos := utils.Point{}
+	endPos := utils.Point{}
+	for i, line := range lines {
+		for j, c := range line {
+			if c == 'S' {
+				startPos = utils.Point{X: j, Y: i}
+			} else if c == 'E' {
+				endPos = utils.Point{X: j, Y: i}
+			}
+		}
+		track[i] = []rune(line)
+	}
+
+	stepsFwd, fwdVisited := bfs(track, startPos, endPos)
+	stepsBwd, bwdVisited := bfs(track, endPos, startPos)
+	if stepsFwd == -1 || stepsBwd == -1 || stepsFwd != stepsBwd {
+		return -1
+	}
+
+	nCheats := 0
+
+	const MAX_CHEAT_TIME = 2
+	const SAVE_MIN = 100
+
+	for pos, step := range fwdVisited {
+		if step > stepsFwd-SAVE_MIN {
+			continue
+		}
+		for dy := -MAX_CHEAT_TIME; dy <= MAX_CHEAT_TIME; dy++ {
+			timeLeft := MAX_CHEAT_TIME - utils.Abs(dy)
+			for dx := -timeLeft; dx <= timeLeft; dx++ {
+				cheatPos := pos.Add(utils.Point{X: dx, Y: dy})
+				if cheatPos.X < 0 || cheatPos.X >= len(track[0]) || cheatPos.Y < 0 || cheatPos.Y >= len(track) {
+					continue
+				}
+				if track[cheatPos.Y][cheatPos.X] == '#' {
+					continue
+				}
+				time := utils.Abs(dy) + utils.Abs(dx)
+				if bwdStep, ok := bwdVisited[cheatPos]; ok {
+					if step+time+bwdStep <= stepsFwd-SAVE_MIN {
+						nCheats++
+					}
+				}
+			}
+		}
+	}
+
+	return nCheats
+
+}
+
+func bfs(track [][]rune, startPos, endPos utils.Point) (int, map[utils.Point]int) {
+	visited := map[utils.Point]int{startPos: 0}
+	queue := []utils.Point{startPos}
+	var curr []utils.Point
+
+	step := 0
+	for len(queue) > 0 {
+		step++
+		curr, queue = queue, curr[:0]
+		for _, pos := range curr {
+			if pos == endPos {
+				return step - 1, visited
+			}
+			for _, dir := range utils.Directions {
+				nextPos := pos.Add(dir)
+				if nextPos.X < 0 || nextPos.X >= len(track[0]) || nextPos.Y < 0 || nextPos.Y >= len(track) {
+					continue
+				}
+				if track[nextPos.Y][nextPos.X] == '#' {
+					continue
+				}
+				if _, ok := visited[nextPos]; ok {
+					continue
+				}
+				visited[nextPos] = step
+				queue = append(queue, nextPos)
+			}
+		}
+	}
+	return -1, visited
+}

day20p1/solution_test.go

@@ -0,0 +1,47 @@
+package day20p1
+
+import (
+	"strings"
+	"testing"
+
+	"aoc/utils"
+)
+
+var testInput = `###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############`
+
+func TestSolve(t *testing.T) {
+	tests := []struct {
+		input  string
+		answer int
+	}{
+		{testInput, 0}, // i trust, test case would be too short for a result
+	}
+
+	if testing.Verbose() {
+		utils.Verbose = true
+	}
+
+	for _, test := range tests {
+		r := strings.NewReader(test.input)
+
+		result := Solve(r).(int)
+
+		if result != test.answer {
+			t.Errorf("Expected %d, got %d", test.answer, result)
+		}
+	}
+}

day20p2/solution.go

@@ -0,0 +1,95 @@
+package day20p2
+
+import (
+	"io"
+
+	"aoc/utils"
+)
+
+func Solve(r io.Reader) any {
+	lines := utils.ReadLines(r)
+
+	track := make([][]rune, len(lines))
+	startPos := utils.Point{}
+	endPos := utils.Point{}
+	for i, line := range lines {
+		for j, c := range line {
+			if c == 'S' {
+				startPos = utils.Point{X: j, Y: i}
+			} else if c == 'E' {
+				endPos = utils.Point{X: j, Y: i}
+			}
+		}
+		track[i] = []rune(line)
+	}
+
+	stepsFwd, fwdVisited := bfs(track, startPos, endPos)
+	stepsBwd, bwdVisited := bfs(track, endPos, startPos)
+	if stepsFwd == -1 || stepsBwd == -1 || stepsFwd != stepsBwd {
+		return -1
+	}
+
+	nCheats := 0
+
+	const MAX_CHEAT_TIME = 20
+	const SAVE_MIN = 100
+
+	for pos, step := range fwdVisited {
+		if step > stepsFwd-SAVE_MIN {
+			continue
+		}
+		for dy := -MAX_CHEAT_TIME; dy <= MAX_CHEAT_TIME; dy++ {
+			timeLeft := MAX_CHEAT_TIME - utils.Abs(dy)
+			for dx := -timeLeft; dx <= timeLeft; dx++ {
+				cheatPos := pos.Add(utils.Point{X: dx, Y: dy})
+				if cheatPos.X < 0 || cheatPos.X >= len(track[0]) || cheatPos.Y < 0 || cheatPos.Y >= len(track) {
+					continue
+				}
+				if track[cheatPos.Y][cheatPos.X] == '#' {
+					continue
+				}
+				time := utils.Abs(dy) + utils.Abs(dx)
+				if bwdStep, ok := bwdVisited[cheatPos]; ok {
+					if step+time+bwdStep <= stepsFwd-SAVE_MIN {
+						nCheats++
+					}
+				}
+			}
+		}
+	}
+
+	return nCheats
+
+}
+
+func bfs(track [][]rune, startPos, endPos utils.Point) (int, map[utils.Point]int) {
+	visited := map[utils.Point]int{startPos: 0}
+	queue := []utils.Point{startPos}
+	var curr []utils.Point
+
+	step := 0
+	for len(queue) > 0 {
+		step++
+		curr, queue = queue, curr[:0]
+		for _, pos := range curr {
+			if pos == endPos {
+				return step - 1, visited
+			}
+			for _, dir := range utils.Directions {
+				nextPos := pos.Add(dir)
+				if nextPos.X < 0 || nextPos.X >= len(track[0]) || nextPos.Y < 0 || nextPos.Y >= len(track) {
+					continue
+				}
+				if track[nextPos.Y][nextPos.X] == '#' {
+					continue
+				}
+				if _, ok := visited[nextPos]; ok {
+					continue
+				}
+				visited[nextPos] = step
+				queue = append(queue, nextPos)
+			}
+		}
+	}
+	return -1, visited
+}

day20p2/solution_test.go

@@ -0,0 +1,47 @@
+package day20p2
+
+import (
+	"strings"
+	"testing"
+
+	"aoc/utils"
+)
+
+var testInput = `###############
+#...#...#.....#
+#.#.#.#.#.###.#
+#S#...#.#.#...#
+#######.#.#.###
+#######.#.#...#
+#######.#.###.#
+###..E#...#...#
+###.#######.###
+#...###...#...#
+#.#####.#.###.#
+#.#...#.#.#...#
+#.#.#.#.#.#.###
+#...#...#...###
+###############`
+
+func TestSolve(t *testing.T) {
+	tests := []struct {
+		input  string
+		answer int
+	}{
+		{testInput, 0}, // i trust, test case would be too short for a result
+	}
+
+	if testing.Verbose() {
+		utils.Verbose = true
+	}
+
+	for _, test := range tests {
+		r := strings.NewReader(test.input)
+
+		result := Solve(r).(int)
+
+		if result != test.answer {
+			t.Errorf("Expected %d, got %d", test.answer, result)
+		}
+	}
+}