Add 2015

2015/Day10.playground/Contents.swift

@@ -0,0 +1,23 @@
+func step(_ input: String) -> String {
+    var position = input.startIndex
+    var output = ""
+    while position < input.endIndex {
+        let next = input[position...].firstIndex(where: { $0 != input[position] }) ?? input.endIndex
+
+        let distance = input.distance(from: position, to: next)
+
+        output += "\(distance)\(input[position])"
+
+        position = next
+    }
+
+    return output
+}
+
+var current = "1321131112"
+for _ in 0..<50 {
+    current = step(current)
+}
+current
+current.count
+