Advent of Code 2019 Day 2.swift

func calculate(_ inputs: [Int], _ idx: Int = 0) -> [Int] {
    let opcode = inputs[idx]
    if opcode == 99 { return inputs }
    let x = inputs[inputs[idx+1]]
    let y = inputs[inputs[idx+2]]
    let out = inputs[idx+3]
    
    var result : [Int] = inputs
    switch opcode {
        case 1: result[out] = x+y
        case 2: result[out] = x*y
        default: return result // throw?
    }
    
    return calculate(result, idx+4)
}

Computer.swift

class Computer {
    private let initialState : [Int]
    private var memory : [Int]
    private var index : Int = 0
    
    init(state: [Int]) {
        self.initialState = state
        self.memory = state
    }
    
    enum OSError: Error {
        case InvalidOpcode
    }
    
    private func step() throws {
        let opcode = memory[index]
        let p1 = memory[memory[index+1]]
        let p2 = memory[memory[index+2]]
        switch opcode {
        case 1: memory[memory[index+3]] = p1 + p2
        case 2: memory[memory[index+3]] = p1 * p2
        default: throw OSError.InvalidOpcode
        }
        self.index = index+4
    }
    
    private func reset(noun: Int, verb: Int) {
        self.reset()
        memory[1] = noun
        memory[2] = verb
    }
    
    private func run() throws {
        while memory[index] != 99 {
            try step()
        }
    }
    
    private func reset() {
        self.memory = initialState
        self.index = 0
    }
    
    func run(noun: Int, verb: Int) throws -> Int {
        self.reset(noun: noun, verb: verb)
        try self.run()
        return memory[0]
    }
}