Day18

2024/gareth/day18/day18.go

@@ -0,0 +1,156 @@
+package day18
+
+import (
+	"container/heap"
+	"strconv"
+	"strings"
+)
+
+type Point struct {
+	x, y int
+}
+
+// Item represents a node in the priority queue.
+type Item struct {
+	point    Point
+	distance int
+	index    int
+}
+
+func Part1(input string) int {
+	grid := parseInput(input, 1024)
+	_, distance := dijkstra(grid)
+	return distance
+}
+
+func Part2(input string) int {
+	lines := strings.Split(strings.TrimSpace(input), "\n")
+	numLines := len(lines)
+	for i := 1; i <= numLines; i++ {
+		grid := parseInput(input, i)
+		_, distance := dijkstra(grid)
+		if distance == -1 {
+			println(lines[i-1])
+			return 2
+		}
+	}
+	return -1
+}
+
+func parseInput(input string, upToLine int) [][]rune {
+	grid := make([][]rune, 71)
+	for i := range grid {
+		grid[i] = make([]rune, 71)
+		for j := range grid[i] {
+			grid[i][j] = '.'
+		}
+	}
+
+	lines := strings.Split(strings.TrimSpace(input), "\n")
+
+	for index, line := range lines {
+		if index == upToLine {
+			break
+		}
+		cords := strings.Split(strings.TrimSpace(line), ",")
+		x, _ := strconv.Atoi(cords[0])
+		y, _ := strconv.Atoi(cords[1])
+		grid[x][y] = '#'
+	}
+	return grid
+}
+
+// PriorityQueue implements a priority queue for Dijkstra's algorithm.
+type PriorityQueue []*Item
+
+func (pq PriorityQueue) Len() int { return len(pq) }
+func (pq PriorityQueue) Less(i, j int) bool {
+	return pq[i].distance < pq[j].distance
+}
+func (pq PriorityQueue) Swap(i, j int) {
+	pq[i], pq[j] = pq[j], pq[i]
+	pq[i].index = i
+	pq[j].index = j
+}
+
+func (pq *PriorityQueue) Push(x interface{}) {
+	n := len(*pq)
+	item := x.(*Item)
+	item.index = n
+	*pq = append(*pq, item)
+}
+
+func (pq *PriorityQueue) Pop() interface{} {
+	old := *pq
+	n := len(old)
+	item := old[n-1]
+	item.index = -1
+	*pq = old[0 : n-1]
+	return item
+}
+
+func isValid(grid [][]rune, x, y int) bool {
+	return x >= 0 && x < len(grid) && y >= 0 && y < len(grid[0]) && grid[x][y] == '.'
+}
+
+func dijkstra(grid [][]rune) ([][]rune, int) {
+	directions := []Point{{0, 1}, {1, 0}, {0, -1}, {-1, 0}}
+	rows, cols := len(grid), len(grid[0])
+
+	// Validate start and end points
+	if grid[0][0] != '.' || grid[rows-1][cols-1] != '.' {
+		return grid, -1 // No path if start or end is not valid
+	}
+
+	dist := make([][]int, rows)
+	for i := range dist {
+		dist[i] = make([]int, cols)
+		for j := range dist[i] {
+			dist[i][j] = 1 << 30
+		}
+	}
+
+	pq := &PriorityQueue{}
+	heap.Init(pq)
+	heap.Push(pq, &Item{point: Point{0, 0}, distance: 0})
+	dist[0][0] = 0
+
+	path := make([][]Point, rows)
+	for i := range path {
+		path[i] = make([]Point, cols)
+	}
+
+	for pq.Len() > 0 {
+		item := heap.Pop(pq).(*Item)
+		x, y := item.point.x, item.point.y
+		if x == rows-1 && y == cols-1 {
+			break
+		}
+		for _, dir := range directions {
+			nx, ny := x+dir.x, y+dir.y
+			if isValid(grid, nx, ny) {
+				newDist := dist[x][y] + 1
+				if newDist < dist[nx][ny] {
+					dist[nx][ny] = newDist
+					heap.Push(pq, &Item{point: Point{nx, ny}, distance: newDist})
+					path[nx][ny] = Point{x, y}
+				}
+			}
+		}
+	}
+
+	// Check if no path exists
+	if dist[rows-1][cols-1] == 1<<30 {
+		return grid, -1 // No path found
+	}
+
+	// Reconstruct the path
+	x, y := rows-1, cols-1
+	for x != 0 || y != 0 {
+		grid[x][y] = 'O'
+		x, y = path[x][y].x, path[x][y].y
+	}
+	grid[0][0] = 'O'
+
+	return grid, dist[rows-1][cols-1]
+}

2024/gareth/day18/day18_test.go

@@ -0,0 +1,65 @@
+package day18
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPart1(t *testing.T) {
+	r := Part1(`5,4
+4,2
+4,5
+3,0
+2,1
+6,3
+2,4
+1,5
+0,6
+3,3
+2,6
+5,1
+1,2
+5,5
+2,5
+6,5
+1,4
+0,4
+6,4
+1,1
+6,1
+1,0
+0,5
+1,6
+2,0`)
+	assert.Equal(t, 22, r)
+}
+
+func TestPart2(t *testing.T) {
+	r := Part2(`5,4
+4,2
+4,5
+3,0
+2,1
+6,3
+2,4
+1,5
+0,6
+3,3
+2,6
+5,1
+1,2
+5,5
+2,5
+6,5
+1,4
+0,4
+6,4
+1,1
+6,1
+1,0
+0,5
+1,6
+2,0`)
+	assert.Equal(t, 0, r)
+}

2024/gareth/main.go

@@ -1,7 +1,7 @@
 package main
 
 import (
-	"aoc2024/day13"
+	"aoc2024/day18"
 	"fmt"
 	"os"
 	"time"
@@ -9,9 +9,9 @@ import (
 
 func main() {
 	start := time.Now()
-	data, _ := os.ReadFile("day13/input.txt")
-	fmt.Printf("part 1: %d\n", day13.Part1(string(data)))
-	fmt.Printf("part 2: %d\n", day13.Part2(string(data)))
+	data, _ := os.ReadFile("day18/input.txt")
+	fmt.Printf("part 1: %d\n", day18.Part1(string(data)))
+	fmt.Printf("part 2: %d\n", day18.Part2(string(data)))
 	elapsed := time.Since(start)
 	fmt.Printf("Execution time: %s\n", elapsed)
 }