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")
        var startingPolymer = parts.first!
        let replacements = parts.last!
            .components(separatedBy: .newlines)
            .map(Replacement.init)
        
        for step in 1...10 {
            var inserts = [(String.Index, Character)]()
            for i in startingPolymer.indices {
                let next = startingPolymer.index(i, offsetBy: 1)
                guard next != startingPolymer.endIndex else {
                    continue
                }
                var pair = startingPolymer[i]/* + startingPolymer[next]*/
                let p = "\(startingPolymer[i])\(startingPolymer[next])"
                for replacement in replacements {
                    if let c = replacement.replace(pair: p) {
                        inserts.append((next, c))
                        break
                    }
                }
            }
            
            var tmp = startingPolymer
            var count = 0
            for r in inserts {
                let idx = tmp.index(r.0, offsetBy: count)
                tmp.insert(r.1, at: idx)
                count += 1
            }
            startingPolymer = tmp
        }
        let charCounts = startingPolymer.reduce(into: [String.Element: Int](), {
            $0[$1, default: 0] += 1
        })
        let maxC = charCounts.max(by: {$0.value < $1.value })!
        let minC = charCounts.min(by: {$0.value < $1.value })!
        print(maxC.value - minC.value)
    }
}