2020/thirteen.nim


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# Day Thirteen: Shuttle Search
import os, strutils, sequtils

let input: string = paramStr(1)
let arrival: int = parseInt(splitLines(readFile(input))[0])
let buses: seq[int] = map(split(splitLines(readFile(input))[1], ','),
  func (bus: string): int =
    if bus == "x":
      0
    else:
      parseInt(bus))

var earliest: tuple[id, wait: int] = (999, 999)
for bus in buses:
  if bus > 0:
    if bus - (arrival mod bus) < earliest.wait:
      earliest = (bus, bus - (arrival mod bus))
echo earliest.id * earliest.wait

# blatantly stolen code - what on earth is a chinese remainder theorem?
# (c: lizthegrey)
var timestamp: int = 0
var step: int = 1
for i, bus in buses:
  if bus != 0:
    while (timestamp+i) mod bus != 0:
      timestamp += step
    step *= bus
echo timestamp