06.rhm

#lang rhombus

// Rhombus port of Brian Adkins solution
// <https://github.com/lojic/LearningRacket/blob/master/advent-of-code-2021/solutions/day06/day06.rkt>

import:
  #{advent-of-code} as aoc:
    rename:
      #{open-aoc-input} as puzzle_input
      #{find-session} as find_session
  racket/base:
    rename:
      #{zero?}          as is_zero
      #{make-vector}    as make_vector
      #{string->number} as string_to_number
      #{vector-length}  as vector_length
  racket/port:
    rename:
      #{port->string} as to_string
  racket/string:
    rename:
      #{string-split} as split
      #{string-trim}  as trim
  rhombus/compat/extra open

fun vector_update(vec, i, f):
  vec[i] := f(vec[i])


fun iterate(f, seed, count):
  if base.is_zero(count)
  | seed
  | iterate(f, f(seed), count - 1)


fun vector_sum(vec):
  fun step(i, sum):
    if base.is_zero(i)
    | sum + vec[i]
    | step(i - 1, sum + vec[i])
  step(base.vector_length(vec) - 1, 0)      


// `|>` operator is (currently) not as fancy as the threading macro, which is
// why there is some explicit grouping and literal funs.
val fish:
  aoc.puzzle_input(aoc.find_session(), 2021, 6, ~cache: #true)
    |> port.to_string
    |> string.trim  
    |> (fun(s): string.split(s, ","))
    |> (fun(ss): base.map(base.string_to_number, ss))
    |> (fun(ns):
          base.foldl(fun(n, vec): vector_update(vec, n, base.add1); vec,
                     base.make_vector(9, 0),
                     ns))


fun solve(fish, n):
  fun spawn(fish):
    Array(fish[1], fish[2], fish[3], fish[4], fish[5],
          fish[6], fish[0] + fish[7], fish[8], fish[0])
  vector_sum(iterate(spawn, fish, n))


solve(fish, 256)