Day 13

README.md

@@ -15,7 +15,7 @@ A small practice project created to get familiar with Go syntax.
 - [x] day 10 
 - [x] day 11
 - [x] day 12
-- [ ] day 13
+- [x] day 13
 - [ ] day 14
 - [ ] day 15
 - [ ] day 16

day13/day13.go

@@ -5,8 +5,8 @@ import (
 	"fmt"
 	"log"
 	"os"
-	"slices"
-	"strings"
+	// "slices"
+	// "strings"
 )
 
 func Solution1(filepath string) int {
@@ -40,7 +40,109 @@ func Solution1(filepath string) int {
 }
 
 func Solution2(filepath string) int {
-	return -1
+	file, err := os.Open(filepath)
+	if err != nil {
+		log.Fatalf("Failed to open the input file with: %v\n", err.Error())
+	}
+
+	result := 0
+	scanner := bufio.NewScanner(file)
+	var currentMatrix []string
+	for scanner.Scan() {
+		currentLine := scanner.Text()
+
+		if len(currentLine) == 0 {
+			result += calcReflection2(currentMatrix)
+			currentMatrix = make([]string, 0)
+			continue
+		}
+
+		currentMatrix = append(currentMatrix, currentLine)
+	}
+	if len(currentMatrix) != 0 {
+		result += calcReflection2(currentMatrix)
+	}
+	if err = scanner.Err(); err != nil {
+		log.Fatalf("Error during input file read: %v\n", err.Error())
+	}
+
+	return result
+}
+
+func calcReflection2(m []string) int {
+	// vertical reflection line
+	for i := len(m) - 1; i >= 1; i-- {
+		step := 0
+		diff := 0
+		for {
+			if diff > 1 {
+				break
+			}
+			// imaginary reflection line is between upIdx=i-1 and downIdx=i, then we start with offset 0 and just keep increasing it by 1 on each iteration
+			// we keep walking until:
+			// a) success - we've reached one of the ends => return
+			// b) failure - we've encountered m[upIdx] != m[downIdx] => break and move onto the next i
+			upIdx := i - 1 - step
+			downIdx := i + step
+
+			if upIdx < 0 || downIdx >= len(m) {
+				// new reflections only
+				if diff != 1 {
+					break
+				}
+				// i == number of rows above the current i
+				// separator := strings.Repeat("-", len(m[0]))
+				// m = slices.Insert(m, i, separator)
+				// printMatrix(m)
+				return 100 * i
+			}
+
+			for j := 0; j < len(m[0]); j++ {
+				if m[upIdx][j] != m[downIdx][j] {
+					diff++
+				}
+			}
+
+			step++
+		}
+	}
+
+	// horizontal reflection line
+	for i := len(m[0]) - 1; i >= 1; i-- {
+		step := 0
+		diff := 0
+		for {
+			if diff > 1 {
+				break
+			}
+			leftIdx := i - 1 - step
+			rightIdx := i + step
+
+			if leftIdx < 0 || rightIdx >= len(m[0]) {
+				// new reflections only
+				if diff != 1 {
+					break
+				}
+				// separator := "|"
+				// for j := 0; j < len(m); j++ {
+				// 	m[j] = m[j][:i] + separator + m[j][i:]
+				// }
+				// printMatrix(m)
+				// i == number of cols left of i
+				return i
+			}
+
+			for j := 0; j < len(m); j++ {
+				currentRow := m[j]
+				if currentRow[leftIdx] != currentRow[rightIdx] {
+					diff++
+				}
+			}
+			step++
+		}
+	}
+
+	return 0
 }
 
 func calcReflection(m []string) int {
@@ -57,14 +159,13 @@ func calcReflection(m []string) int {
 
 			if upIdx < 0 || downIdx >= len(m) {
 				// i == number of rows above the current i
-				separator := strings.Repeat("-", len(m[0]))
-				m = slices.Insert(m, i, separator)
-				printMatrix(m)
+				// separator := strings.Repeat("-", len(m[0]))
+				// m = slices.Insert(m, i, separator)
+				// printMatrix(m)
 				return 100 * i
 			}
 
 			if m[upIdx] != m[downIdx] {
-				fmt.Printf("i=%d, step=%d: %s != %s\n", i, step, m[upIdx], m[downIdx])
 				break
 			}
 			step++
@@ -79,11 +180,11 @@ func calcReflection(m []string) int {
 			rightIdx := i + step
 
 			if leftIdx < 0 || rightIdx >= len(m[0]) {
-				separator := "|"
-				for j := 0; j < len(m); j++ {
-					m[j] = m[j][:i] + separator + m[j][i:]
-				}
-				printMatrix(m)
+				// separator := "|"
+				// for j := 0; j < len(m); j++ {
+				// 	m[j] = m[j][:i] + separator + m[j][i:]
+				// }
+				// printMatrix(m)
 				// i == number of cols left of i
 				return i
 			}

day13/day13_test.go

@@ -21,3 +21,19 @@ func TestSolution1(t *testing.T) {
 		}
 	}
 }
+
+func TestSolution2(t *testing.T) {
+	cases := map[string]int{
+		"test_input11": 400,
+	}
+
+	for input, expectedResult := range cases {
+		result := Solution2(input)
+
+		if result == expectedResult {
+			fmt.Printf("Solution2()=%d, OK\n", result)
+		} else {
+			t.Fatalf("Solution2()=%d, expecting %d, FAIL\n", result, expectedResult)
+		}
+	}
+}