refactor: extract region logic into separate package

cmd/12/region/region.go

@@ -0,0 +1,158 @@
+// Package region provides a model and methods for manipulating a region
+package region
+
+import (
+	"sort"
+	"strconv"
+	"strings"
+
+	"github.com/StevanFreeborn/advent-of-code-2025/cmd/12/shape"
+)
+
+type Region interface {
+	CanFit(shapes map[int][]shape.Shape) bool
+}
+
+type region struct {
+	width          int
+	height         int
+	requiredShapes []int
+}
+
+func From(line string) Region {
+	parts := strings.Split(line, ":")
+	dims := strings.Split(parts[0], "x")
+	width, _ := strconv.Atoi(dims[0])
+	height, _ := strconv.Atoi(dims[1])
+
+	countsStr := strings.Fields(parts[1])
+	requiredShapes := []int{}
+
+	for id, s := range countsStr {
+		count, _ := strconv.Atoi(s)
+
+		for range count {
+			requiredShapes = append(requiredShapes, id)
+		}
+	}
+
+	return region{
+		width:          width,
+		height:         height,
+		requiredShapes: requiredShapes,
+	}
+}
+
+func (r region) CanFit(shapes map[int][]shape.Shape) bool {
+	totalArea := 0
+
+	itemsToPlace := r.createItems(shapes)
+
+	for _, item := range itemsToPlace {
+		totalArea += item.area
+	}
+
+	if totalArea > r.area() {
+		return false
+	}
+
+	return r.checkFit(0, itemsToPlace, r.createGrid())
+}
+
+type item struct {
+	id       int
+	area     int
+	variants []shape.Shape
+}
+
+func (r region) checkFit(index int, itemsToPlace []item, grid [][]bool) bool {
+	if index == len(itemsToPlace) {
+		return true
+	}
+
+	currentItem := itemsToPlace[index]
+
+	for _, variant := range currentItem.variants {
+		maxRow := variant.MaxRow()
+		maxColumn := variant.MaxColumn()
+
+		lastRow := r.height - maxRow
+		lastColumn := r.width - maxColumn
+
+		for rowNumber := range lastRow {
+			for columnNumber := range lastColumn {
+				if canPlace(grid, rowNumber, columnNumber, variant) {
+					place(grid, rowNumber, columnNumber, variant)
+
+					if r.checkFit(index+1, itemsToPlace, grid) {
+						return true
+					}
+
+					unplace(grid, rowNumber, columnNumber, variant)
+				}
+			}
+		}
+	}
+
+	return false
+}
+
+func (r region) area() int {
+	return r.width * r.height
+}
+
+func (r region) createGrid() [][]bool {
+	grid := make([][]bool, r.height)
+
+	for i := range grid {
+		grid[i] = make([]bool, r.width)
+	}
+
+	return grid
+}
+
+func (r region) createItems(shapes map[int][]shape.Shape) []item {
+	itemsToPlace := make([]item, 0, len(r.requiredShapes))
+
+	for _, id := range r.requiredShapes {
+		shapeVariants := shapes[id]
+		area := shapeVariants[0].Area()
+		itemsToPlace = append(itemsToPlace, item{id: id, area: area, variants: shapeVariants})
+	}
+
+	sort.Slice(itemsToPlace, func(i, j int) bool {
+		return itemsToPlace[i].area > itemsToPlace[j].area
+	})
+
+	return itemsToPlace
+}
+
+func canPlace(grid [][]bool, r, c int, variant shape.Shape) bool {
+	for _, position := range variant.Positions() {
+		absoluteRow := r + position.Row()
+		absoluteColumn := c + position.Column()
+		isOccupied := grid[absoluteRow][absoluteColumn]
+
+		if isOccupied {
+			return false
+		}
+	}
+
+	return true
+}
+
+func place(grid [][]bool, r, c int, variant shape.Shape) {
+	for _, position := range variant.Positions() {
+		absoluteRow := r + position.Row()
+		absoluteColumn := c + position.Column()
+		grid[absoluteRow][absoluteColumn] = true
+	}
+}
+
+func unplace(grid [][]bool, r, c int, variant shape.Shape) {
+	for _, position := range variant.Positions() {
+		absoluteRow := r + position.Row()
+		absoluteColumn := c + position.Column()
+		grid[absoluteRow][absoluteColumn] = false
+	}
+}