#!/usr/bin/env julia

using Test

mutable struct Monkey
  index :: UInt
  items :: Vector{Int}
  op :: Function
  op_value :: Int
  test_divisor :: Int
  test_result :: NTuple{2, UInt}
  inspected :: Int
end

function parse_monkey(text)
  lines = split(text, '\n')
  # Monkey 0:
  match_index = match(r"Monkey (\d+):", lines[1])
  index = parse(UInt, match_index.captures[1])
  #   Starting items: 79, 98
  match_items = match(r" *Starting items: (.*)", lines[2])
  items_list = split(match_items.captures[1], ", ")
  items = parse.(Int, items_list)
  #   Operation: new = old * 19
  match_op = match(r" *Operation: new = old (.) (old|\d+)", lines[3])
  if match_op.captures[1] == "+"
    op = +
  elseif match_op.captures[1] == "*"
    op = *
  else
    throw(DomainError("Unknown op: " * match_op.captures[1]))
  end
  if match_op.captures[2] == "old"
    op = ^
    op_value = 2
  else
    op_value = parse(Int, match_op.captures[2])
  end
  #   Test: divisible by 13
  match_test = match(r" *Test: divisible by (\d+)", lines[4])
  test_divisor = parse(Int, match_test.captures[1])
  #   If true: throw to monkey 1
  #   If false: throw to monkey 3
  match_true = match(r" *If true: throw to monkey (\d+)", lines[5])
  match_false = match(r" *If false: throw to monkey (\d+)", lines[6])
  test_result = (parse(UInt, match_true.captures[1]) + 1,
                 parse(UInt, match_false.captures[1]) + 1)
  return Monkey(index, items, op, op_value, test_divisor, test_result, 0)
end

function read_monkeys(infile)
  open(infile) do io
    monkey_strings = split(read(io, String), "\n\n")
    return parse_monkey.(monkey_strings)
  end
end

function simian_shenanigans(monkeys :: Vector{Monkey})
  for m in monkeys
    for item in m.items
      new_value = m.op(item, m.op_value)
      new_value = Int(floor(new_value / 3))
      new_monkey = (mod(new_value, m.test_divisor) == 0
                    ? m.test_result[1] : m.test_result[2])
      push!(monkeys[new_monkey].items, new_value)
      m.inspected += 1
    end
    empty!(m.items)
  end
end

function monkey_business(monkeys; rounds=20)
  for _ in 1:rounds
    simian_shenanigans(monkeys)
  end
  return prod(sort([m.inspected for m in monkeys])[end-1:end])
end

function test()
  @testset "monkey business" verbose=true begin
    @test monkey_business(read_monkeys("example.txt")) == 10605
    @test monkey_business(read_monkeys("input.txt")) == 64032
  end
end

function main()
  if size(ARGS, 1) == 0
    test()
  else
    infile = ARGS[1]
    println("infile = ", infile)
    monkeys = read_monkeys(infile)
    display(monkeys)
    println()
    println("monkey business ", monkey_business(monkeys))
  end
end

main()