09.rhm

#lang rhombus/static

import:
  "util/advent_of_code.rhm" as aoc
  "util/misc.rhm" as util
  "util/parsec.rhm" as p

@util.example_input(test_input){
0 3 6 9 12 15
1 3 6 10 15 21
10 13 16 21 30 45@("\n")
}

def spaces_p:
  p.many1(p.try(p.char(#{#\space})))

def line_p:
  p.parse_sequence:
    n = p.integer
    ns = p.many1(p.try(p.parse_sequence:
                         spaces_p
                         p.integer))
    p.char(#{#\newline})
    p.pure(List.cons(n, ns))

def input_p:
  p.many1(line_p)

check:
  p.parse_string(line_p, "0 3 6 9\n") ~is values([0, 3, 6, 9], [])
  p.parse_string(input_p, "0 1 2\n0 2 4\n") ~is:
    values([[0, 1, 2], [0, 2, 4]], [])

fun all_zero(nums :: List.of(Int)) :: Boolean:
  match nums
  | []: error("empty list")
  | [0]: #true
  | [0, &rest]: all_zero(rest)
  | _: #false

fun delta(nums :: List.of(Int)) :~ values(List.of(Int), Int):
  match nums
  | [a]: values([], a)
  | Pair(a, Pair(b, _) && rest):
      let values(rest, last) = delta(rest)
      values([b - a, &rest], last)

fun next_number(nums :: List.of(Int)):
  let values(nums, last) = delta(nums)
  if all_zero(nums)
  | last
  | next_number(nums) + last

check:
  next_number([1, 1]) ~is 1
  next_number([0, 3, 6, 9, 12, 15]) ~is 18
  next_number([1, 3, 6, 10, 15, 21]) ~is 28
  next_number([10, 13, 16, 21, 30, 45]) ~is 68
  next_number([-1, 0, 2, -6, 27, 94, 10]) ~is -708

fun run1(input):
  def values(lines :: List.of(List.of(Int)), []):
    p.parse_string(input_p, input)
  for values(sum = 0):
    each line: lines
    sum + next_number(line)

check:
  run1(test_input) ~is 114

module part1:
  run1(aoc.fetch_input(aoc.find_session(), 2023, 9))

fun prev_number(nums :: List.of(Int)):
  def first = nums[0]
  let values(nums, _last) = delta(nums)
  if all_zero(nums)
  | first
  | first - prev_number(nums)

check:
  prev_number([1, 1]) ~is 1
  prev_number([0, 3, 6, 9, 12, 15]) ~is -3
  prev_number([1, 3, 6, 10, 15, 21]) ~is 0
  prev_number([10, 13, 16, 21, 30, 45]) ~is 5


fun run2(input):
  def values(lines :: List.of(List.of(Int)), []):
    p.parse_string(input_p, input)
  for values(sum = 0):
    each line: lines
    sum + prev_number(line)

check:
  run2(test_input) ~is 2

module part2:
  run2(aoc.fetch_input(aoc.find_session(), 2023, 9))