Day04

2024/gareth/day04/day04.go

@@ -0,0 +1,91 @@
+package day04
+
+import (
+	"strings"
+)
+
+var directions = [8][2]int{
+	{0, 1},
+	{0, -1},
+	{1, 0},
+	{-1, 0},
+	{1, 1},
+	{1, -1},
+	{-1, 1},
+	{-1, -1},
+}
+
+func Part1(input string) int {
+	grid := parseInput(input)
+	return countXMAS(grid)
+}
+
+func Part2(input string) int {
+	grid := parseInput(input)
+	return countXMASPatterns(grid)
+}
+
+func parseInput(input string) [][]rune {
+	lines := strings.Split(strings.TrimSpace(input), "\n")
+	grid := make([][]rune, len(lines))
+	for i, line := range lines {
+		grid[i] = []rune(line)
+	}
+	return grid
+}
+
+func countXMAS(grid [][]rune) int {
+	rows := len(grid)
+	cols := len(grid[0])
+	target := "XMAS"
+	total := 0
+
+	for r := 0; r < rows; r++ {
+		for c := 0; c < cols; c++ {
+			for _, d := range directions {
+				dr, dc := d[0], d[1]
+				found := true
+
+				for k := 0; k < len(target); k++ {
+					nr := r + k*dr
+					nc := c + k*dc
+					if nr < 0 || nr >= rows || nc < 0 || nc >= cols || grid[nr][nc] != rune(target[k]) {
+						found = false
+						break
+					}
+				}
+
+				if found {
+					total++
+				}
+			}
+		}
+	}
+	return total
+}
+
+func countXMASPatterns(grid [][]rune) int {
+	rows := len(grid)
+	cols := len(grid[0])
+	total := 0
+
+	for r := 1; r < rows-1; r++ {
+		for c := 1; c < cols-1; c++ {
+
+			topLeft := grid[r-1][c-1]
+			topRight := grid[r-1][c+1]
+			center := grid[r][c]
+			bottomLeft := grid[r+1][c-1]
+			bottomRight := grid[r+1][c+1]
+
+			if center == 'A' && ((topLeft == 'S' && topRight == 'S' && bottomLeft == 'M' && bottomRight == 'M') ||
+				(topLeft == 'S' && topRight == 'M' && bottomLeft == 'S' && bottomRight == 'M') ||
+				(topLeft == 'M' && topRight == 'M' && bottomLeft == 'S' && bottomRight == 'S') ||
+				(topLeft == 'M' && topRight == 'S' && bottomLeft == 'M' && bottomRight == 'S')) {
+				total++
+			}
+		}
+	}
+
+	return total
+}