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2020/day5/main.swift

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+let input = loadData(day: 5).lines()
+
+func decode(_ s: Substring, lower: Character, upper: Character, range: Range<Int>) -> (Int, Substring)
+{
+    var range = range
+    var start = s.startIndex
+    while start != s.endIndex && (s[start] == lower || s[start] == upper) {
+        let mid = (range.lowerBound + range.upperBound) / 2
+        if s[start] == lower {
+            range = range.lowerBound..<mid
+        } else if s[start] == upper {
+            range = mid..<range.upperBound
+        }
+        start = s.index(after: start)
+    }
+
+    return (range.lowerBound, s[start...])
+}
+
+func decodeSeat(_ s: String) -> Int {
+    let (row, rest) = decode(s[...], lower: "F", upper: "B", range: 0..<128)
+    let (column, _) = decode(rest, lower: "L", upper: "R", range: 0..<8)
+    let id = row * 8 + column
+    print(s, row, column, id)
+    return id
+}
+
+let foundSeats = Set(input.map { decodeSeat($0) })
+print("max seat id", foundSeats.max() ?? -1)
+
+let allSeats = Set(0...(7+127*8))
+let freeSeats = allSeats.subtracting(foundSeats)
+
+let yours = freeSeats.first(where: { foundSeats.contains($0 + 1) && foundSeats.contains($0 - 1)})
+print("yours", yours ?? -1)