aoc2021/Sources/14/14.swift

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

import Foundation
import Runner

struct Pair: Hashable {
    let first: String.Element
    let second: String.Element
    
    init(_ str: String) {
        first = str.first!
        second = str.last!
    }
    
    init(first: String.Element, second: String.Element) {
        self.first = first
        self.second = second
    }
}

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: [Pair:String.Element](), {
                $0[Pair($1.first!)] = $1.last!.first!
            })
        
        var pairCounts = [Pair:Int]()
        for i in startingPolymer.indices {
            let next = startingPolymer.index(i, offsetBy: 1)
            guard next != startingPolymer.endIndex else {
                continue
            }
            pairCounts[Pair(first: startingPolymer[i], second: startingPolymer[next]), 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
                pairCounts[Pair(first: pair.first, second: replacement), default: 0] += pairCount.value
                pairCounts[Pair(first: replacement, second: pair.second), default: 0] += pairCount.value
                charCount[replacement, default: 0] += pairCount.value
            }
            if step == 10 {
                printResult(charCount: charCount)
            }
        }
        printResult(charCount: charCount)
    }
    
    func printResult(charCount: [String.Element:Int]) {
        // Sort instead of min/max so we need to only go through the dict once
        let sorted = charCount.sorted(by: {$0.value > $1.value })
        let maxC = sorted.first!
        let minC = sorted.last!
        print(maxC.value - minC.value)
    }
}