add part 2 of day 11

day11/stones.go

@@ -69,7 +69,7 @@ func main() {
         numbers = append(numbers, num)
     }
 
-    for range 75 {
+    for range 25 {
         numbers = apply_rules(numbers)
     }
     fmt.Println(len(numbers))

day11/stones_cached.go

@@ -0,0 +1,93 @@
+package main
+
+import (
+    "os"
+    "fmt"
+    "strconv"
+    "strings"
+)
+
+
+// ---------------------------------------
+type Stone int64
+
+func (s Stone) blink() [2]Stone {
+    stones  := [...]Stone{-1, -1}
+    num_str := strconv.Itoa(int(s))
+
+    switch {
+        // rule 1
+        case s == 0:
+            stones[0] = 1
+        // rule 2
+        case len(num_str) % 2 == 0:
+            num1, err := strconv.Atoi(num_str[:len(num_str) / 2])
+            check(err)
+            num2, err := strconv.Atoi(num_str[len(num_str) / 2:])
+            check(err)
+    
+            stones[0] = Stone(num1)
+            stones[1] = Stone(num2)
+        // rule 3
+        default:
+            stones[0] = s * 2024
+    }
+
+    return stones
+}
+
+
+// ---------------------------------------
+type Pair struct {
+    s Stone
+    n_steps uint32
+}
+var population_number_cache map[Pair]uint64
+
+func population_number(s Stone, n_steps uint32) uint64 {
+    cache := population_number_cache[Pair{s, n_steps}]
+    if cache != 0 { return cache }
+
+    new_stones := s.blink()
+
+    if n_steps == 0 { return 1 }
+
+    p := population_number(new_stones[0], n_steps - 1)
+    if new_stones[1] > -1 {
+        p += population_number(new_stones[1], n_steps - 1)
+    }
+
+    population_number_cache[Pair{s, n_steps}] = p
+    return p
+}
+
+func check(e error) { if e != nil { panic(e) } }
+
+
+func main() {
+
+    // read in file
+    dat, err := os.ReadFile("data.txt")
+    check(err)
+    dat_str := strings.TrimSpace(string(dat))
+    line_arr := strings.Split(dat_str, " ")
+
+    // convert to list of numbers
+    stones := make([]Stone, 0, len(line_arr))
+
+    for _, e := range line_arr {
+        num, err := strconv.Atoi(e)
+        check(err)
+        stones = append(stones, Stone(num))
+    }
+
+    // init cache
+    population_number_cache = make(map[Pair]uint64)
+
+    // main calc
+    len_acc := uint64(0)
+    for _, stone := range stones {
+        len_acc += population_number(stone, 75)
+    }
+    fmt.Println(len_acc)
+}