bf.nim

import os
import streams
import system
import tables

var data: array[30_000, byte]
var dataPointer: int = 0

var instructions: seq[char]
var instructionPointer: int = 0

proc panic(message: string) =
  stderr.writeLine("ERROR: " & message)
  system.quit(-1)

proc incrementByte() =
  inc(data[dataPointer])

proc decrementByte() =
  dec(data[dataPointer])

proc outputByte() =
  stdout.write(data[dataPointer])

proc incrementDataPointer() =
  inc(dataPointer)

proc decrementDataPointer() =
  dec(dataPointer)

proc conditionalJumpForward() =
  if data[dataPointer] == 0:
    var nestedPairs = 1
    while instructions[instructionPointer] != ']' and nestedPairs != 0:
      inc(instructionPointer)
      if instructions[instructionPointer] == '[':
        inc(nestedPairs)
      elif instructions[instructionPointer] == ']':
        dec(nestedPairs)

proc conditionalJumpBackwards() =
  if data[dataPointer] != 0:
    var nestedPairs = 1
    while instructions[instructionPointer] != '[' and nestedPairs != 0:
      dec(instructionPointer)
      if instructions[instructionPointer] == ']':
        inc(nestedPairs)
      elif instructions[instructionPointer] == '[':
        dec(nestedPairs)

proc readByte() =
  var buf: array[1, byte]
  let numBytesRead = stdin.readBytes(buf, 0, 1)
  if numBytesRead != 1:
    panic("Failed to read byte from stdin.")
  data[dataPointer] = buf[0]

var instructionToProc = {
  '+': incrementByte,
  '-': decrementByte,
  '>': incrementDataPointer,
  '<': decrementDataPointer,
  '[': conditionalJumpForward,
  ']': conditionalJumpBackwards,
  ',': readByte,
  '.': outputByte
  }.newTable

proc loadCode(filename: string): seq[char] =
  var strm = newFileStream(filename, fmRead)
  var sourceCode = strm.readAll()
  strm.close()

  for ch in sourceCode:
    if instructionToProc.hasKey(ch):
      result.add(ch)

when isMainModule:
  instructions = loadCode(paramStr(1))

  while true:
    instructionToProc[instructions[instructionPointer]]()
    inc(instructionPointer)
    if instructionPointer >= len(instructions):
      system.quit(0)