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2020/day10/main.swift

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+import Foundation
+
+let input = loadData(day: 10)
+let scanner = Scanner(string: input)
+
+var adapters = scanner.integers().sorted { $0 > $1 }
+adapters.append(0)
+
+
+let device = adapters[0] + 3
+
+var current = device
+var ones = 0
+var threes = 0
+
+for adapter in adapters {
+    let step = current - adapter
+    if step == 3 {
+        threes += 1
+    } else if step == 1 {
+        ones += 1
+    } else if step != 2 {
+        print("fail")
+    }
+    current = adapter
+}
+
+print("part 1", ones * threes)
+
+struct Key: Hashable {
+    var start: Int
+    var max: Int
+}
+var memo: [Key: Int] = [:]
+
+func findPaths(start: Int, max: Int) -> Int {
+    if let value = memo[Key(start: start, max: max)] {
+        return value
+    }
+
+    let diff = max - adapters[start]
+    if diff > 3 || diff < 1 {
+        return 0
+    }
+
+    if start == adapters.count - 1 {
+        return 1
+    }
+
+    var result = 0
+    for n in (start + 1)..<(adapters.count) {
+        let next = findPaths(start: n, max: adapters[start])
+        if next == 0 {
+            break
+        }
+        result += next
+    }
+
+    memo[Key(start: start, max: max)] = result
+
+    return result
+}
+
+print(findPaths(start: 0, max: device))