2025 day 7

Author: lucianoq

2025/7/Makefile

@@ -0,0 +1,20 @@
+input:
+	http "https://adventofcode.com/2025/day/7/input" "Cookie:session=${AOC_SESSION};" >input
+
+main1:
+	go build -o main1 main1.go common.go
+
+main2:
+	go build -o main2 main2.go common.go
+
+.PHONY: run1 run2 clean
+
+run1: main1 input
+	./main1 <input
+
+run2: main2 input
+	./main2 <input
+
+clean:
+	rm -f main1 main2 input
+

2025/7/common.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"bufio"
+	"os"
+)
+
+type P struct{ x, y int }
+
+func parseInput() (map[P]struct{}, P, int) {
+	var splitters = map[P]struct{}{}
+	var start P
+
+	scanner := bufio.NewScanner(os.Stdin)
+	var i int
+	for i = 0; scanner.Scan(); i++ {
+		line := scanner.Text()
+
+		for j, c := range line {
+			switch c {
+			case 'S':
+				start = P{i, j}
+			case '^':
+				splitters[P{i, j}] = struct{}{}
+			}
+		}
+	}
+	return splitters, start, i
+}

2025/7/main1.go

@@ -0,0 +1,23 @@
+package main
+
+import "fmt"
+
+func main() {
+	splitters, start, last := parseInput()
+	beams := map[P]struct{}{P{start.x + 1, start.y}: {}}
+	splits := 0
+	for i := 2; i < last; i++ {
+		nextBeams := map[P]struct{}{}
+		for b := range beams {
+			if _, ok := splitters[P{i, b.y}]; ok {
+				splits++
+				nextBeams[P{i, b.y - 1}] = struct{}{}
+				nextBeams[P{i, b.y + 1}] = struct{}{}
+			} else {
+				nextBeams[P{i, b.y}] = struct{}{}
+			}
+		}
+		beams = nextBeams
+	}
+	fmt.Println(splits)
+}

2025/7/main2.go

@@ -0,0 +1,27 @@
+package main
+
+import "fmt"
+
+func main() {
+	splitters, start, last := parseInput()
+	beams := map[P]int{P{start.x + 1, start.y}: 1}
+	for i := 2; i < last; i++ {
+		nextBeams := map[P]int{}
+		for b, journeys := range beams {
+			if _, ok := splitters[P{i, b.y}]; ok {
+				nextBeams[P{i, b.y - 1}] += journeys
+				nextBeams[P{i, b.y + 1}] += journeys
+			} else {
+				nextBeams[P{i, b.y}] += journeys
+			}
+		}
+		beams = nextBeams
+	}
+
+	paths := 0
+	for _, journeys := range beams {
+		paths += journeys
+	}
+
+	fmt.Println(paths)
+}