day 13 tricky edge cases with reflections

2023/day13/main.go

@@ -0,0 +1,165 @@
+package main
+
+import (
+	_ "embed"
+	"flag"
+	"fmt"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+//go:embed input.txt
+var input string
+
+func init() {
+	// do this in init (not main) so test file has same input
+	input = strings.TrimRight(input, "\n")
+	if len(input) == 0 {
+		panic("empty input.txt file")
+	}
+}
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	if part == 1 {
+		ans := part1(input)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	} else {
+		ans := part2(input)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	}
+}
+
+func part1(input string) int {
+	patterns := parseInput(input)
+
+	ans := 0
+	for _, pattern := range patterns {
+		maybeMirrorRow := findMirrorRow(pattern, -1)
+		if maybeMirrorRow != -1 {
+			ans += 100 * maybeMirrorRow
+		} else {
+			maybeMirrorCol := findMirrorCol(pattern, -1)
+			if maybeMirrorCol == -1 {
+				panic("did not find mirror row or col")
+			}
+			ans += maybeMirrorCol
+		}
+	}
+
+	return ans
+}
+
+// returns the zero-index of the line, if the line is between index 3 and 4, it returns 4
+// ignoreRow is for part2 where we want to ignore the original mirrored row because it may still be
+// valid in the un-smudged pattern
+func findMirrorRow(pattern [][]string, ignoreRow int) int {
+	// combine the string slices into a string so they're easier to compare
+	combinedRows := []string{}
+	for _, row := range pattern {
+		combinedRows = append(combinedRows, strings.Join(row, ""))
+	}
+
+	for i := 1; i < len(combinedRows); i++ {
+		mismatchFound := false
+		for offset := 1; i-offset >= 0 && i+offset-1 < len(combinedRows); offset++ {
+			// fmt.Println("combined row indexes", i-offset, i+offset-1)
+			// fmt.Println(combinedRows[i-offset], "\n", combinedRows[i+offset-1])
+			if combinedRows[i-offset] != combinedRows[i+offset-1] {
+				mismatchFound = true
+				// fmt.Println("mismatch found")
+				break
+			}
+		}
+
+		if !mismatchFound {
+			if i != ignoreRow {
+				return i
+			}
+		}
+	}
+	// none found
+	return -1
+}
+
+func findMirrorCol(pattern [][]string, ignoreCol int) int {
+	// rotate the grid, maintaining the indices for easier maths later
+	// then just pass it into the findMirrorRow func
+	rotatedGrid := [][]string{}
+	for c := 0; c < len(pattern[0]); c++ {
+		newRow := []string{}
+		for r := 0; r < len(pattern); r++ {
+			newRow = append(newRow, pattern[r][c])
+		}
+		rotatedGrid = append(rotatedGrid, newRow)
+	}
+
+	return findMirrorRow(rotatedGrid, ignoreCol)
+}
+
+func part2(input string) int {
+	patterns := parseInput(input)
+
+	ans := 0
+
+	for _, pattern := range patterns {
+
+		// store the original row and col so they can be ignored in the find mirror row func
+		originalMirrorRow := findMirrorRow(pattern, -1)
+		originalMirrorCol := findMirrorCol(pattern, -1)
+
+	traverse:
+		// labels suck but without the breaks this all has to go into a separate function which is
+		// arguably less readable. and the break is necessary to not double count reflections
+		for r, row := range pattern {
+			for c, val := range row {
+				if val == "." {
+					pattern[r][c] = "#"
+					if maybeMirrorRow := findMirrorRow(pattern, originalMirrorRow); maybeMirrorRow != -1 {
+						ans += 100 * maybeMirrorRow
+						break traverse
+					}
+					if maybeMirrorCol := findMirrorCol(pattern, originalMirrorCol); maybeMirrorCol != -1 {
+						ans += maybeMirrorCol
+						break traverse
+					}
+					pattern[r][c] = "."
+				} else if val == "#" {
+					pattern[r][c] = "."
+					if maybeMirrorRow := findMirrorRow(pattern, originalMirrorRow); maybeMirrorRow != -1 {
+						ans += 100 * maybeMirrorRow
+						break traverse
+					}
+					if maybeMirrorCol := findMirrorCol(pattern, originalMirrorCol); maybeMirrorCol != -1 {
+						ans += maybeMirrorCol
+						break traverse
+					}
+					pattern[r][c] = "#"
+
+				} else {
+					panic("expected input: " + val)
+				}
+			}
+		}
+	}
+
+	return ans
+}
+
+func parseInput(input string) (ans [][][]string) {
+	for _, section := range strings.Split(input, "\n\n") {
+		grid := [][]string{}
+		for _, line := range strings.Split(section, "\n") {
+			grid = append(grid, strings.Split(line, ""))
+		}
+		ans = append(ans, grid)
+	}
+	return ans
+}

2023/day13/main_test.go

@@ -0,0 +1,73 @@
+package main
+
+import (
+	"testing"
+)
+
+var example = `#.##..##.
+..#.##.#.
+##......#
+##......#
+..#.##.#.
+..##..##.
+#.#.##.#.
+
+#...##..#
+#....#..#
+..##..###
+#####.##.
+#####.##.
+..##..###
+#....#..#`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  405,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  30575,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part1(tt.input); got != tt.want {
+				t.Errorf("part1() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_part2(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  400,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  37478,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part2(tt.input); got != tt.want {
+				t.Errorf("part2() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}