day4: solve first part

Author: fxnn

day4/example-focus.txt

@@ -0,0 +1,10 @@
+....XXMAS.
+.SAMXMS...
+...S..A...
+..A.A.MS.X
+XMASAMX.MM
+X.....XA.A
+S.S.S.S.SS
+.A.A.A.A.A
+..M.M.M.MM
+.X.X.XMASX

day4/example.txt

@@ -0,0 +1,10 @@
+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX

day4/main.go

@@ -0,0 +1,105 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+const NEEDLE = "XMAS"
+
+type point struct {
+	y int
+	x int
+}
+
+func charAt(lines []string, p point) (byte, bool) {
+	if p.y < 0 || p.x < 0 {
+		return 0, false
+	}
+	if p.y >= len(lines) {
+		return 0, false
+	}
+	if p.x >= len(lines[p.y]) {
+		return 0, false
+	}
+	return lines[p.y][p.x], true
+}
+
+func generateMatchPoints(at point, dy, dx int) []point {
+	result := []point{}
+	for i := range NEEDLE {
+		p := point{y: at.y + dy*i, x: at.x + dx*i}
+		result = append(result, p)
+	}
+	return result
+}
+
+func matchPointGenerators() []func(point) []point {
+	return []func(point) []point{
+		func(p point) []point { return generateMatchPoints(p, -1, 0) },
+		func(p point) []point { return generateMatchPoints(p, -1, -1) },
+		func(p point) []point { return generateMatchPoints(p, 0, -1) },
+		func(p point) []point { return generateMatchPoints(p, 1, -1) },
+		func(p point) []point { return generateMatchPoints(p, 1, 0) },
+		func(p point) []point { return generateMatchPoints(p, 1, 1) },
+		func(p point) []point { return generateMatchPoints(p, 0, 1) },
+		func(p point) []point { return generateMatchPoints(p, -1, 1) },
+	}
+}
+
+func isAMatch(points []point, lines []string) bool {
+	for i, p := range points {
+		if c, ok := charAt(lines, p); !ok || c != NEEDLE[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func countMatchesFrom(source point, lines []string, matches [][]byte) int {
+	matchCount := 0
+	for _, generator := range matchPointGenerators() {
+		matchPoints := generator(source)
+		if !isAMatch(matchPoints, lines) {
+			continue
+		}
+
+		for i, p := range matchPoints {
+			matches[p.y][p.x] = NEEDLE[i]
+		}
+		matchCount++
+	}
+	return matchCount
+}
+
+func main() {
+	scanner := bufio.NewScanner(os.Stdin)
+	scanner.Split(bufio.ScanLines)
+
+	var lines []string
+	for scanner.Scan() {
+		lines = append(lines, scanner.Text())
+	}
+
+	matches := make([][]byte, len(lines))
+	for y := range lines {
+		matches[y] = make([]byte, len(lines[y]))
+		for x := range lines[y] {
+			matches[y][x] = byte('.')
+		}
+	}
+
+	matchCount := 0
+	for y := range lines {
+		line := lines[y]
+		for x := range line {
+			matchCount += countMatchesFrom(point{y: y, x: x}, lines, matches)
+		}
+	}
+
+	fmt.Fprintf(os.Stderr, "matches: %d\n", matchCount)
+	for _, lineMatches := range matches {
+		fmt.Printf("%s\n", string(lineMatches))
+	}
+}

go.mod

@@ -1,3 +1,3 @@
 module github.com/fxnn/adventofcode2024
 
-go 1.23.2
+go 1.23