|
| 1 | +package main |
| 2 | + |
| 3 | +import ( |
| 4 | + "fmt" |
| 5 | + "os" |
| 6 | + "strings" |
| 7 | +) |
| 8 | + |
| 9 | +func main() { |
| 10 | + f, _ := os.ReadFile("input.txt") |
| 11 | + lines := strings.Split(strings.TrimSpace(string(f)), "\n") |
| 12 | + initialState := strings.TrimPrefix(strings.TrimSpace(lines[0]), "initial state: ") |
| 13 | + rules := createRules(lines[2:]) |
| 14 | + |
| 15 | + tunnel := make([]string, 500) |
| 16 | + applyInitialState(&tunnel, initialState) |
| 17 | + for range 20 { |
| 18 | + applyRules(&tunnel, rules) |
| 19 | + } |
| 20 | + fmt.Println("Part 1 | Sum: ", calculateSum(&tunnel)) |
| 21 | + |
| 22 | + partTwo(initialState, rules) |
| 23 | +} |
| 24 | + |
| 25 | +func partTwo(initialState string, rules map[string]bool) { |
| 26 | + // The tunnel has to be up sized for the ammount of generations we're computing. |
| 27 | + tunnel := make([]string, 1500) |
| 28 | + applyInitialState(&tunnel, initialState) |
| 29 | + lastValue := 0 |
| 30 | + for i := range 500 { |
| 31 | + temp := make([]string, len(tunnel)) |
| 32 | + copy(temp, tunnel) |
| 33 | + for range i { |
| 34 | + applyRules(&temp, rules) |
| 35 | + } |
| 36 | + sum := calculateSum(&temp) |
| 37 | + //@NOTE: Debug purposes, uncomment following print statements. |
| 38 | + // fmt.Printf("Part 2 | For %d Generations, Sum is: %d\n", i, sum) |
| 39 | + // You'll see this value converges to 72 in my case. |
| 40 | + // fmt.Printf("Gen %d | Difference to the last generation: %d\n", i, sum-lastValue) |
| 41 | + lastValue = sum |
| 42 | + } |
| 43 | + // The fifty billion iterations, minus the ones we did above (499) times the constant. |
| 44 | + lastValue += ((50000000000 - 499) * (72)) |
| 45 | + fmt.Printf("Part 2 | Answer: %d\n", lastValue) |
| 46 | +} |
| 47 | + |
| 48 | +func calculateSum(tunnel *[]string) int { |
| 49 | + center := len(*tunnel) / 2 |
| 50 | + sum := 0 |
| 51 | + for i, value := range *tunnel { |
| 52 | + if value == "#" { |
| 53 | + sum += (i - center) |
| 54 | + } |
| 55 | + } |
| 56 | + return sum |
| 57 | +} |
| 58 | + |
| 59 | +func applyRules(tunnel *[]string, rules map[string]bool) { |
| 60 | + str := strings.Builder{} |
| 61 | + // We cant write straight to the array, because we override the state of the adjacent positions while we iterate. |
| 62 | + // Instead we keep a note of the changes to apply after. Its better than to copy the initial state i think. |
| 63 | + changes := map[int]string{} |
| 64 | + |
| 65 | + for i := 2; i < len(*tunnel)-2; i++ { |
| 66 | + // Write to the temporary string the 5 adjacent values of this position |
| 67 | + str.WriteString((*tunnel)[i-2]) |
| 68 | + str.WriteString((*tunnel)[i-1]) |
| 69 | + str.WriteString((*tunnel)[i]) |
| 70 | + str.WriteString((*tunnel)[i+1]) |
| 71 | + str.WriteString((*tunnel)[i+2]) |
| 72 | + |
| 73 | + // If this string matches any rule: |
| 74 | + if value, ok := rules[str.String()]; ok { |
| 75 | + if value { |
| 76 | + changes[i] = "#" |
| 77 | + } else { |
| 78 | + changes[i] = "." |
| 79 | + } |
| 80 | + } else { |
| 81 | + changes[i] = "." |
| 82 | + } |
| 83 | + // Reset the temp string |
| 84 | + str.Reset() |
| 85 | + } |
| 86 | + // Apply changes |
| 87 | + for key, value := range changes { |
| 88 | + (*tunnel)[key] = value |
| 89 | + } |
| 90 | +} |
| 91 | + |
| 92 | +func applyInitialState(tunnel *[]string, initialState string) { |
| 93 | + center := len(*tunnel) / 2 |
| 94 | + for i := range len(*tunnel) { |
| 95 | + (*tunnel)[i] = "." |
| 96 | + } |
| 97 | + for i, char := range initialState { |
| 98 | + if char == '#' { |
| 99 | + (*tunnel)[center+i] = "#" |
| 100 | + } else { |
| 101 | + (*tunnel)[center+i] = "." |
| 102 | + } |
| 103 | + } |
| 104 | + |
| 105 | +} |
| 106 | + |
| 107 | +func createRules(lines []string) map[string]bool { |
| 108 | + rules := map[string]bool{} |
| 109 | + for _, line := range lines { |
| 110 | + fields := strings.Fields(strings.TrimSpace(line)) |
| 111 | + if fields[2] == "#" { |
| 112 | + rules[fields[0]] = true |
| 113 | + } else { |
| 114 | + rules[fields[0]] = false |
| 115 | + } |
| 116 | + } |
| 117 | + return rules |
| 118 | +} |
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