Encoder.js

// This file includes code which was modified from https://github.com/openai/gpt-2
const fs = require('fs')
const path = require('path');

const encoder = JSON.parse(fs.readFileSync(path.join(__dirname, './encoder.json')));
const bpe_file = fs.readFileSync(path.join(__dirname, './vocab.bpe'), 'utf-8');

const range = (x, y) => {
  const res = Array.from(Array(y).keys()).slice(x)
  return res
}

const ord = x => {
  return x.charCodeAt(0)
}

const chr = x => {
  return String.fromCharCode(x)
}

const textEncoder = new TextEncoder("utf-8")
const encodeStr = str => {
  return Array.from(textEncoder.encode(str)).map(x => x.toString())
}

const textDecoder = new TextDecoder("utf-8")
const decodeStr = arr => {
  return textDecoder.decode(new Uint8Array(arr));
}

const dictZip = (x, y) => {
  const result = {}
  x.map((_, i) => { result[x[i]] = y[i] })
  return result
}

function bytes_to_unicode() {
  const bs = range(ord('!'), ord('~') + 1).concat(range(ord('¡'), ord('¬') + 1), range(ord('®'), ord('ÿ') + 1))

  let cs = bs.slice()
  let n = 0
  for (let b = 0; b < 2 ** 8; b++) {
    if (!bs.includes(b)) {
      bs.push(b)
      cs.push(2 ** 8 + n)
      n = n + 1
    }
  }

  cs = cs.map(x => chr(x))

  const result = {}
  bs.map((_, i) => { result[bs[i]] = cs[i] })
  return result
}

function get_pairs(word) {
  const pairs = new Set()
  let prev_char = word[0]
  for (let i = 1; i < word.length; i++) {
    const char = word[i]
    pairs.add([prev_char, char])
    prev_char = char
  }
  return pairs
}

const pat = /'s|'t|'re|'ve|'m|'ll|'d| ?\p{L}+| ?\p{N}+| ?[^\s\p{L}\p{N}]+|\s+(?!\S)|\s+/gu

const decoder = {}
Object.keys(encoder).map(x => { decoder[encoder[x]] = x })

const lines = bpe_file.split('\n')

// bpe_merges = [tuple(merge_str.split()) for merge_str in bpe_data.split("\n")[1:-1]]
const bpe_merges = lines.slice(1, lines.length - 1).map(x => {
  return x.split(/(\s+)/).filter(function(e) { return e.trim().length > 0 })
})

const byte_encoder = bytes_to_unicode()
const byte_decoder = {}
Object.keys(byte_encoder).map(x => { byte_decoder[byte_encoder[x]] = x })

const bpe_ranks = dictZip(bpe_merges, range(0, bpe_merges.length))
const cache = new Map;

function bpe(token) {
  if (cache.has(token)) {
    return cache.get(token)
  }``

  let word = token.split('')

  let pairs = get_pairs(word)

  if (!pairs) {
    return token
  }

  while (true) {
    const minPairs = {}
    Array.from(pairs).map(pair => {
      const rank = bpe_ranks[pair]
      minPairs[(isNaN(rank) ? 10e10 : rank)] = pair
    })



    const bigram = minPairs[Math.min(...Object.keys(minPairs).map(x => {
      return parseInt(x)
    }
    ))]

    if (!(bigram in bpe_ranks)) {
      break
    }

    const first = bigram[0]
    const second = bigram[1]
    let new_word = []
    let i = 0

    while (i < word.length) {
      const j = word.indexOf(first, i)
      if (j === -1) {
        new_word = new_word.concat(word.slice(i))
        break
      }
      new_word = new_word.concat(word.slice(i, j))
      i = j

      if (word[i] === first && i < word.length - 1 && word[i + 1] === second) {
        new_word.push(first + second)
        i = i + 2
      } else {
        new_word.push(word[i])
        i = i + 1
      }
    }

    word = new_word
    if (word.length === 1) {
      break
    } else {
      pairs = get_pairs(word)
    }
  }

  word = word.join(' ')
  cache.set(token, word)

  return word
}

function encode(text) {
  let bpe_tokens = []
  const matches = Array.from(text.matchAll(pat)).map(x => x[0])
  for (let token of matches) {
    token = encodeStr(token).map(x => {
      return byte_encoder[x]
    }).join('')
    
    const new_tokens = bpe(token).split(' ').map(x => encoder[x])
    bpe_tokens = bpe_tokens.concat(new_tokens)
  }
  return bpe_tokens
}

function decode(tokens) {
  let text = tokens.map(x => decoder[x]).join('')
  text = decodeStr(text.split('').map(x => byte_decoder[x]))
  return text
}

module.exports = {
  encode,
  decode
};