feat: solve part two

internal/2016/DayFour/code.go

@@ -0,0 +1,119 @@
+package dayfour
+
+import (
+	"advent-of-code/internal/registry"
+	"os"
+	"regexp"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+func init() {
+	registry.Register("2016D4", ParseInput, PartOne, PartTwo)
+}
+
+func ParseInput(filepath string) []string {
+	content, _ := os.ReadFile(filepath)
+	return strings.Split(string(content), "\n")
+}
+
+func isValidRoom(encryptedName string, expectedChecksum string) bool {
+	letterFrequency := [26]int{}
+
+	for _, character := range encryptedName {
+		if character >= 'a' && character <= 'z' {
+			letterFrequency[character-'a']++
+		}
+	}
+
+	type letterFrequencyPair struct {
+		letter rune
+		count  int
+	}
+
+	letterFrequencyPairs := make([]letterFrequencyPair, 0, 26)
+	for index, frequency := range letterFrequency {
+		if frequency > 0 {
+			letterFrequencyPairs = append(letterFrequencyPairs, letterFrequencyPair{
+				letter: rune('a' + index),
+				count:  frequency,
+			})
+		}
+	}
+
+	sort.Slice(letterFrequencyPairs, func(i, j int) bool {
+		if letterFrequencyPairs[i].count != letterFrequencyPairs[j].count {
+			return letterFrequencyPairs[i].count > letterFrequencyPairs[j].count
+		}
+		return letterFrequencyPairs[i].letter < letterFrequencyPairs[j].letter
+	})
+
+	if len(letterFrequencyPairs) < 5 {
+		return false
+	}
+
+	for index := range 5 {
+		if letterFrequencyPairs[index].letter != rune(expectedChecksum[index]) {
+			return false
+		}
+	}
+
+	return true
+}
+
+func decryptRoomName(encryptedName string, sectorID int) string {
+	result := strings.Builder{}
+	result.Grow(len(encryptedName))
+	shift := sectorID % 26
+	for _, char := range encryptedName {
+		if char == '-' {
+			result.WriteByte(' ')
+		} else if char >= 'a' && char <= 'z' {
+			shifted := ((int(char-'a') + shift) % 26) + 'a'
+			result.WriteByte(byte(shifted))
+		}
+	}
+	return result.String()
+}
+
+var roomPattern = regexp.MustCompile(`^(.+)-(\d+)(?:\[([a-z]{5})\])?$`)
+
+func PartOne(data []string) int {
+	sum := 0
+	for _, line := range data {
+		if line == "" {
+			continue
+		}
+		matches := roomPattern.FindStringSubmatch(line)
+		if len(matches) < 4 || matches[3] == "" {
+			continue
+		}
+		encryptedName := matches[1]
+		sectorIdentifier, _ := strconv.Atoi(matches[2])
+		actualChecksum := matches[3]
+		if isValidRoom(encryptedName, actualChecksum) {
+			sum += sectorIdentifier
+		}
+	}
+	return sum
+}
+
+func PartTwo(data []string) int {
+	for _, line := range data {
+		matches := roomPattern.FindStringSubmatch(line)
+		encryptedName := matches[1]
+		sectorIdentifier, _ := strconv.Atoi(matches[2])
+		checksum := matches[3]
+
+		if !isValidRoom(encryptedName, checksum) {
+			continue
+		}
+
+		decrypted := decryptRoomName(encryptedName, sectorIdentifier)
+		if strings.Contains(decrypted, "northpole") {
+			return sectorIdentifier
+		}
+	}
+	return 0
+}

internal/2016/DayFour/code_test.go

@@ -1,18 +1,29 @@
 package dayfour
 
-import "testing"
+import (
+	"testing"
+)
 
 var testInput = []string{
 	"aaaaa-bbb-z-y-x-123[abxyz]",
 	"a-b-c-d-e-f-g-h-987[abcde]",
 	"not-a-real-room-404[oarel]",
 	"totally-real-room-200[decoy]",
+	"ijmockjgz-storage-5[gjoac]",
 }
 
 func TestPartOne(t *testing.T) {
-	expected := 1514
+	expected := 1519
 	got := PartOne(testInput)
 	if got != expected {
 		t.Errorf("PartOne() = %d, want %d", got, expected)
 	}
 }
+
+func TestPartTwo(t *testing.T) {
+	expected := 5
+	got := PartTwo(testInput)
+	if got != expected {
+		t.Errorf("PartTwo() = %d, want %d", got, expected)
+	}
+}