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2020/day13/main.swift

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+import Foundation
+
+let input = loadData(day: 13)
+
+
+let scanner = Scanner(string: input)
+
+let startTime = scanner.scanInt()!
+
+var busLines: [(Int, position: Int)] = []
+
+var position = 0
+repeat {
+    if let line = scanner.scanInt() {
+        busLines.append((line, position))
+    } else if scanner.string("x") {
+        // Ignore
+    }
+    position += 1
+} while scanner.string(",")
+
+let min = busLines
+    .map { $0.0 }
+    .map { ((startTime / $0) * $0 + $0, $0) }
+    .print()
+    .min(by: { $0.0 < $1.0 })!
+
+
+print((min.0 - startTime) * min.1)
+
+let max = busLines.max(by: { $0.0 < $1.0 })!
+print(max)
+
+/*
+ brute force - takes too long
+
+var s = sequence(first: 0, next: { $0 + max.0 })
+    .lazy
+    .filter { t in busLines.allSatisfy { (t - max.position + $0.position).isMultiple(of: $0.0 )} }
+    .makeIterator()
+
+print(s.next())
+
+*/
+
+var step = 0
+var time = 0
+
+outer: while true {
+    time += step
+
+    step = 1
+    for (bus, position) in busLines {
+        guard (time + position).isMultiple(of: bus) else { continue outer }
+        step *= bus
+    }
+
+    break
+}
+
+print("part2", time)