Solution_2018_02.swift

import Foundation

struct Solution_2018_02: Solution {
    var input: Input
    func run() throws {
        let ids = try input.get()
            .components(separatedBy: .whitespacesAndNewlines)

        // ------- Part 1 -------
        func countLetters(_ s:String) -> NSCountedSet {
            return s.reduce(into: NSCountedSet()) { acc, c in
                acc.add(c)
            }
        }

        func solve(_ s:String) -> (Bool, Bool) {
            let counted = countLetters(s)

            let twice = counted.contains(where: { counted.count(for: $0) == 2 })
            let thrice = counted.contains(where: { counted.count(for: $0) == 3 })
            return (twice, thrice)
        }

        let b2i: (Bool) -> Int = { $0 ? 1 : 0 }

        let (twice, thrice) = ids.reduce(into: (0,0), { (res, id) in
            let (twice, thrice) = solve(id)
            res.0 += b2i(twice)
            res.1 += b2i(thrice)
        })

        let part1 = twice * thrice
        print(part1)

        // ------- Part 2 -------

        func distance(_ s1: String, _ s2: String) -> Int {
            //strings are the same length
            return s1.indices.reduce(into: 0, { (res, i) in
                res += b2i(s1[i] != s2[i])
            })
        }

        func commonPart(_ s1: String, _ s2: String) -> String {
            var resultString = s1
            if let index = s1.indices.first(where: { s1[$0] != s2[$0] }) {
                resultString.remove(at: index)
            }
            return resultString
        }

        //let lazyIds = ids.lazy
        //let pairs = lazyIds.flatMap({ id1 in lazyIds.map ({ id2 in (id1, id2) }) })
        //guard let pairIndex = pairs.firstIndex(where: { distance($0, $1) == 1 }) else {
        //    fatalError("Should have found a pair...")
        //}
        //
        //let pair = pairs[pairIndex]
        //let part2 = commonPart(pair.0, pair.1)

        func solve2(_ ids: [String]) -> String {
            for i in ids.indices {
                for j in (i + 1)..<ids.endIndex {
                    if distance(ids[i], ids[j]) == 1 {
                        return commonPart(ids[i], ids[j])
                    }
                }
            }
            fatalError("Shouldn't happen")
        }

        let part2 = solve2(ids)
        print(part2)

        // ------- Test -------

        assert(part1 == 8715, "WA")
        assert(part2 == "fvstwblgqkhpuixdrnevmaycd", "WA")
    }
}