day 4

go.mod

@@ -1,3 +1,7 @@
 module git.bizdoc.ro/gabi-public/Advent-of-Code.git
 
 go 1.25.1
+
+require git.bizdoc.ro/private/devkit.git v0.0.0-20250902190935-21cc248cb135
+
+require golang.org/x/exp v0.0.0-20241108190413-2d47ceb2692f // indirect

go.sum

@@ -0,0 +1,4 @@
+git.bizdoc.ro/private/devkit.git v0.0.0-20250902190935-21cc248cb135 h1:mUorxNlLlyvmPuZMKP80RFJVRcI1j85u1g8TDPht7AM=
+git.bizdoc.ro/private/devkit.git v0.0.0-20250902190935-21cc248cb135/go.mod h1:r/tE3YYNlrYNQc9DF4ph2Ur5WYKCtaqOM+8jqvzSoAQ=
+golang.org/x/exp v0.0.0-20241108190413-2d47ceb2692f h1:XdNn9LlyWAhLVp6P/i8QYBW+hlyhrhei9uErw2B5GJo=
+golang.org/x/exp v0.0.0-20241108190413-2d47ceb2692f/go.mod h1:D5SMRVC3C2/4+F/DB1wZsLRnSNimn2Sp/NPsCrsv8ak=

year25/day4.go

@@ -0,0 +1,78 @@
+package year25
+
+import (
+	"fmt"
+	"io"
+
+	"git.bizdoc.ro/private/devkit.git/collections/geometry/v2"
+)
+
+const Day4PaperRoll = '@'
+
+func Day4Part1(r io.Reader, l Logger) (any, error) {
+	// 4 rolls in 8 adjacent positions
+
+	var accessibleRolls int
+	g, err := geometry.ReaderToByteGrid(r)
+	if err != nil {
+		return nil, fmt.Errorf("day4: failed to read grid: %w", err)
+	}
+
+	for c := range g.Points() {
+		if g.At(c) != Day4PaperRoll {
+			continue
+		}
+
+		var count int
+		for p := range g.SubgridPointsIter(c.MoveUpLeft(), c.MoveDownRight()) {
+			if g.At(p) == Day4PaperRoll {
+				count += 1
+			}
+		}
+
+		if count <= 4 {
+			accessibleRolls += 1
+		}
+	}
+
+	return accessibleRolls, nil
+}
+
+func Day4Part2(r io.Reader, l Logger) (any, error) {
+	// 4 rolls in 8 adjacent positions
+
+	var totalRemovedRolls int
+	g, err := geometry.ReaderToByteGrid(r)
+	if err != nil {
+		return nil, fmt.Errorf("day4: failed to read grid: %w", err)
+	}
+
+	for {
+		removedRolls := 0
+		for c := range g.Points() {
+			if g.At(c) != Day4PaperRoll {
+				continue
+			}
+
+			var count int
+			for p := range g.SubgridPointsIter(c.MoveUpLeft(), c.MoveDownRight()) {
+				if g.At(p) == Day4PaperRoll {
+					count += 1
+				}
+			}
+
+			if count <= 4 {
+				removedRolls += 1
+				g.Set(c, '.') // remove grid
+			}
+		}
+
+		if removedRolls == 0 {
+			break
+		}
+
+		totalRemovedRolls += removedRolls
+	}
+
+	return totalRemovedRolls, nil
+}