Solution for 2024, day 12

Author: lucianoq

2024/12/Makefile

@@ -0,0 +1,19 @@
+input:
+	http "https://adventofcode.com/2024/day/12/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

2024/12/common.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+type P struct{ x, y int }
+
+func parseInput() (map[P]uint8, int) {
+	scanner := bufio.NewScanner(os.Stdin)
+	m := map[P]uint8{}
+	var i int
+	for i = 0; scanner.Scan(); i++ {
+		line := scanner.Text()
+		for j, c := range line {
+			m[P{i, j}] = uint8(c)
+		}
+	}
+	return m, i
+}
+
+func main() {
+	m, size := parseInput()
+	price := 0
+	visited := map[P]struct{}{}
+	for i := 0; i < size; i++ {
+		for j := 0; j < size; j++ {
+			if _, ok := visited[P{i, j}]; ok {
+				continue
+			}
+			area, fence := explore(m, P{i, j}, visited)
+			price += area * fence
+		}
+	}
+	fmt.Println(price)
+}

2024/12/main1.go

@@ -0,0 +1,33 @@
+package main
+
+// BFS
+func explore(m map[P]uint8, p P, visited map[P]struct{}) (int, int) {
+
+	visited[p] = struct{}{}
+
+	perimeter, area := 0, 0
+
+	var c P
+	toDo := []P{p}
+	for len(toDo) > 0 {
+		c, toDo = toDo[0], toDo[1:]
+
+		area++
+
+		for _, d := range []P{{-1, 0}, {1, 0}, {0, -1}, {0, 1}} {
+			next := P{c.x + d.x, c.y + d.y}
+			val, ok := m[next]
+			if ok && val == m[p] {
+				if _, ok := visited[next]; !ok {
+					toDo = append(toDo, next)
+					visited[next] = struct{}{}
+				}
+			} else {
+				perimeter++
+			}
+		}
+
+	}
+
+	return area, perimeter
+}

2024/12/main2.go

@@ -0,0 +1,46 @@
+package main
+
+// BFS
+func explore(m map[P]uint8, p P, visited map[P]struct{}) (int, int) {
+	visited[p] = struct{}{}
+
+	id := m[p]
+
+	area, corners := 0, 0
+
+	var c P
+	toDo := []P{p}
+	for len(toDo) > 0 {
+		c, toDo = toDo[0], toDo[1:]
+
+		area++
+
+		for _, d := range []P{{-1, 0}, {1, 0}, {0, -1}, {0, 1}} {
+			next := P{c.x + d.x, c.y + d.y}
+			val, ok := m[next]
+			if ok && val == id {
+				if _, ok := visited[next]; !ok {
+					toDo = append(toDo, next)
+					visited[next] = struct{}{}
+				}
+			}
+		}
+
+		for _, d := range []P{{-1, -1}, {1, 1}, {1, -1}, {-1, 1}} {
+			// convex corner
+			if m[P{c.x + d.x, c.y}] != id &&
+				m[P{c.x, c.y + d.y}] != id {
+				corners++
+			}
+			// concave corner
+			if m[P{c.x + d.x, c.y}] == id &&
+				m[P{c.x, c.y + d.y}] == id &&
+				m[P{c.x + d.x, c.y + d.y}] != id {
+				corners++
+			}
+		}
+
+	}
+
+	return area, corners
+}