aoc-2019/01/main.pony

use "files"
use "format"
use "itertools"

primitive Fuel
  // calculate the amount of fuel needed
  fun calc(x: U64) : U64 => (x / 3) - 2

  // calculate the amount of fuel needed, naively recursively taking
  //in account the weight of the fuel
  fun calc_recursive(x: U64) : U64 =>
    let weight = calc(x)
    if weight > 6 then
      weight + calc_recursive(weight)
    else
      weight
    end

actor Main
  new create(env: Env) =>
    // use a command-line flag to figure out which version of the fuel
    //calculation to use
    let part_one = env.args.contains("--part-one", {(l, r) => l == r})
    let part_two = env.args.contains("--part-two", {(l, r) => l == r})

    if part_one or part_two then
      calculate_fuel(env, part_one)
    else
      env.err.print("Usage: 01 [--part-one|--part-two]")
      env.exitcode(99)
    end

  fun calculate_fuel(env: Env, part_one: Bool) =>
    // since we're storing the input list externally, we'll need the
    //ability to read and also stat files, so build that capability
    let caps = recover val FileCaps.>set(FileRead).>set(FileStat) end

    try
      // try opening the file, whose name we've hard-coded
      with file = OpenFile(FilePath(env.root as AmbientAuth, "input.txt", caps)?) as File
    do
        // iterate through the lines of the file
        let sum = Iter[String](file.lines())
                    // try to parse all the lines as a number
                    .filter_map[U64]({(str) => try str.u64()? else None end})
                    // calculate the amount of fuel required for each one
                    .map[U64]({(x) => if part_one then Fuel.calc(x) else Fuel.calc_recursive(x) end })
                    // and sum them up
                    .fold[U64](0, {(sum, x) => sum + x})
        // print out the resulting number
        env.out.print(Format.int[U64](sum))
      end
    else
      // if something failed, then print an error message of some kind and exit
      env.err.print("Couldn't read expected file `input.txt'")
      env.exitcode(99)
    end