aoc2021/Sources/14/14.swift

//
//  File.swift
//
//
//  Created by Max Nuding on 14.12.21.
//

import Foundation
import Runner

struct Replacement {
    let from: String
    let to: String
    
    init(line: String) {
        let r = line.components(separatedBy: " -> ")
        from = r.first!
        to = r.last!
    }
    
    func replace(pair: String) -> String.Element? { pair == from ? to.first : nil }
}

class Day14: Runnable {
    let inputPath: String
    
    required init(inputPath: String) {
        self.inputPath = inputPath
    }
    
    public func run() {
        let input = try! String(contentsOfFile: inputPath)
        let parts = input
            .trimmingCharacters(in: .newlines)
            .components(separatedBy: "\n\n")
        let startingPolymer = parts.first!
        let replacements = parts.last!
            .components(separatedBy: .newlines)
            .map {$0.components(separatedBy: " -> ")}
            .reduce(into: [String:String](), { p, c in
                p[c.first!] = c.last!
            })
        
        var pairCounts = [String:Int]()
        for i in startingPolymer.indices {
            let next = startingPolymer.index(i, offsetBy: 1)
            guard next != startingPolymer.endIndex else {
                continue
            }
            let pair = "\(startingPolymer[i])\(startingPolymer[next])"
            pairCounts[pair, default: 0] += 1
        }
        var charCount = startingPolymer.reduce(into: [String.Element:Int](), {
            $0[$1, default: 0] += 1
        })
        
        for step in 1...40 {
            let tmpPairCounts = pairCounts
            for pairCount in tmpPairCounts {
                let pair = pairCount.key
                let replacement = replacements[pair]!
                pairCounts[pair]! -= pairCount.value
                let firstCharacter = pair.first!
                let secondCharacter = pair.last!
                pairCounts["\(firstCharacter)\(replacement)", default: 0] += pairCount.value
                pairCounts["\(replacement)\(secondCharacter)", default: 0] += pairCount.value
                charCount[replacement.first!, default: 0] += pairCount.value
            }
            if step == 10 {
                printResult(charCount: charCount)
            }
        }
        printResult(charCount: charCount)
    }
    
    func printResult(charCount: [String.Element:Int]) {
        let maxC = charCount.max(by: {$0.value < $1.value })!
        let minC = charCount.min(by: {$0.value < $1.value })!
        print(maxC.value - minC.value)
    }
}