#!/usr/bin/env julia

using Test

struct Instruction
  cycles :: UInt
  add    :: Int
end

function parse_instruction_line(line)
  parts = split(line, limit=2)
  if parts[1] == "noop"
    return Instruction(1, 0)
  elseif parts[1] == "addx"
    return Instruction(2, parse(Int, parts[2]))
  else
    throw(DomainError("Unknown instruction: " * line))
  end
end

function execute_program(io; end_cycle=220)
  X :: Int = 1
  cycle :: Int = 1
  signal_strength_sum :: Int = 0
  next_test_cycle = 20
  for line in eachline(io)
    if mod(cycle - 20, 40) == 0 && cycle <= end_cycle
      println(cycle, " ", X)
      signal_strength_sum += cycle * X
      next_test_cycle += 40
    end
    inst = parse_instruction_line(line)
    new_cycle = cycle + inst.cycles
    if new_cycle > next_test_cycle
      signal_strength_sum += next_test_cycle * X
      next_test_cycle += 40
    end
    cycle = new_cycle
    X += inst.add
  end
  println("last inst cycle ", cycle)
  signal_strength_sum += X * sum(filter(x -> x >= cycle, 20:40:end_cycle))
  return signal_strength_sum
end

function test()
  @testset "signal strength sum" verbose=true begin
    @test execute_program("example.txt") == 13140
    @test execute_program("input.txt") == 13220
  end
end

function main()
  if size(ARGS, 1) == 0
    test()
  else
    infile = ARGS[1]
    println("infile = ", infile)
    println("signal strength sum = ", execute_program(infile))
  end
end

main()