day 4 part 1

day4/main.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/kelindar/bitmap"
+)
+
+func readLines(filename string) []string {
+	raw, err := os.ReadFile(filename)
+	if err != nil {
+		panic(err)
+	}
+	return strings.Split(string(raw), "\n")
+}
+
+type warehouse struct {
+	boxes  bitmap.Bitmap
+	width  uint32
+	height uint32
+}
+
+func buildWarehouse(lines []string) warehouse {
+	wh := warehouse{}
+	wh.height = uint32(len(lines))
+	if wh.height == 0 {
+		return wh
+	}
+	wh.width = uint32(len(lines[0]))
+	if wh.width == 0 {
+		return wh
+	}
+	for y, line := range lines {
+		for x, char := range line {
+			if char == '@' {
+				set(&wh.boxes, wh.width, wh.height, uint32(x), uint32(y))
+			}
+		}
+	}
+	return wh
+}
+
+func set(area *bitmap.Bitmap, width uint32, height uint32, x uint32, y uint32) {
+	if x >= width || y >= height {
+		return
+	}
+	area.Set(y*width + x)
+}
+
+func debug(area *bitmap.Bitmap, width uint32) {
+	area.Range(func(offset uint32) {
+		fmt.Printf("(%d, %d)\n", offset%width, offset/width)
+	})
+}
+
+func buildNeighbors(width uint32, height uint32, x uint32, y uint32) *bitmap.Bitmap {
+	var neighbors bitmap.Bitmap
+	set(&neighbors, width, height, x+1, y)
+	set(&neighbors, width, height, x-1, y)
+	set(&neighbors, width, height, x+1, y+1)
+	set(&neighbors, width, height, x-1, y+1)
+	set(&neighbors, width, height, x+1, y-1)
+	set(&neighbors, width, height, x-1, y-1)
+	set(&neighbors, width, height, x, y+1)
+	set(&neighbors, width, height, x, y-1)
+	return &neighbors
+}
+
+func (wh warehouse) countAccessibleBoxes() uint32 {
+	boxes := wh.boxes.Clone(nil)
+	boxes.Filter(func(offset uint32) bool {
+		y := offset / wh.width
+		x := offset % wh.width
+		neighbors := buildNeighbors(wh.width, wh.height, x, y)
+		neighbors.And(wh.boxes)
+		accessible := neighbors.Count() < 4
+		return accessible
+	})
+	return uint32(boxes.Count())
+}
+
+func main() {
+	filename := "input0.txt"
+	if len(os.Args) > 1 {
+		filename = os.Args[1]
+	}
+	lines := readLines(filename)
+	wh := buildWarehouse(lines)
+	fmt.Printf("%v\n", wh.countAccessibleBoxes())
+}