Add solution day 22 part 2

Author: terminalnode

README.md

@@ -54,7 +54,7 @@ automatically rebuilt and redeployed every time the `common` module or their own
 | 06  | ⭐ ⭐      | 19  | ⭐ ⭐      |
 | 07  | ⭐ ⭐      | 20  | ⭐ ⭐      |
 | 08  | ⭐ ⭐      | 21  |          |
-| 09  | ⭐ ⭐      | 22  | ⭐        |
+| 09  | ⭐ ⭐      | 22  | ⭐ ⭐      |
 | 10  | ⭐ ⭐      | 23  |          |
 | 11  | ⭐ ⭐      | 24  |          |
 | 12  | ⭐ ⭐      | 25  |          |

solutions/day22/main.go

@@ -8,7 +8,7 @@ import (
 )
 
 func main() {
-	common.Setup(22, part1, nil)
+	common.Setup(22, part1, part2)
 }
 
 func part1(
@@ -30,6 +30,73 @@ func part1(
 	return fmt.Sprintf("Sum of all secret numbers after 2k rounds: %d", sum)
 }
 
+type priceDiff struct {
+	diff  int
+	price int
+}
+
+func part2(
+	input string,
+) string {
+	secrets, err := parse(input)
+	if err != nil {
+		return fmt.Sprintf("Failed to parse input: %v", err)
+	}
+
+	allKeys := make(map[string]bool)
+	allPriceMaps := make([]map[string]int, len(secrets))
+
+	for i, secret := range secrets {
+		ds := make([]priceDiff, 2000)
+		price := priceOf(secret)
+		visited := make(map[string]bool)
+		priceMap := make(map[string]int)
+
+		for n := 0; n < 2000; n++ {
+			secret = evolveSecretNumber(secret)
+			newPrice := priceOf(secret)
+			ds[n] = priceDiff{diff: newPrice - price, price: newPrice}
+			price = newPrice
+		}
+
+		for n := 3; n <= 1999; n++ {
+			key := fmt.Sprintf(
+				"%d,%d,%d,%d",
+				ds[n-3].diff, ds[n-2].diff, ds[n-1].diff, ds[n].diff)
+			if visited[key] {
+				continue
+			}
+
+			allKeys[key] = true
+			visited[key] = true
+			priceMap[key] = ds[n].price
+		}
+		allPriceMaps[i] = priceMap
+	}
+
+	bestKey := ""
+	bestSum := 0
+	for key, _ := range allKeys {
+		sum := 0
+		for _, pm := range allPriceMaps {
+			sum += pm[key]
+		}
+
+		if sum > bestSum {
+			bestSum = sum
+			bestKey = key
+		}
+	}
+
+	return fmt.Sprintf("Max number of bananas %d (%s)", bestSum, bestKey)
+}
+
+func priceOf(n int) int {
+	s := strconv.Itoa(n)
+	ch := s[len(s)-1]
+	return int(ch - '0')
+}
+
 func parse(
 	input string,
 ) ([]int, error) {