PHP's pack in TypeScript

How to use

Install via yarn add locutus and import: import { pack } from 'locutus/php/misc/pack'.

Or with CommonJS: const { pack } = require('locutus/php/misc/pack')

Use a bundler that supports tree-shaking so you only ship the functions you actually use. Vite, webpack, Rollup, and Parcel all handle this. For server-side use this is less of a concern.

Examples

These examples are extracted from test cases that automatically verify our functions against their native counterparts.

#	code	expected result
1	pack('nvc*', 0x1234, 0x5678, 65, 66)	'\u00124xVAB'
2	pack('H4', '2345')	'#E'
3	pack('H*', 'D5')	'Õ'
4	pack('d', -100.876)	"\u0000\u0000\u0000\u0000\u00008YÀ"

Notes

• Float encoding by: Jonas Raoni Soares Silva Home: https://www.kingsquare.nl/blog/12-12-2009/13507444 Feedback: phpjs-pack@kingsquare.nl “machine dependent byte order and size” aren’t applicable for JavaScript; pack works as on a 32bit, little endian machine.

Here's what our current TypeScript equivalent to PHP's pack looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

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type PackArgument = string | number | boolean | null | undefined

export function pack(format: string, ...inputArgs: PackArgument[]): string {
  //  discuss at: https://locutus.io/php/pack/
  // original by: Tim de Koning (https://www.kingsquare.nl)
  //    parts by: Jonas Raoni Soares Silva (https://www.jsfromhell.com)
  // bugfixed by: Tim de Koning (https://www.kingsquare.nl)
  //      note 1: Float encoding by: Jonas Raoni Soares Silva
  //      note 1: Home: https://www.kingsquare.nl/blog/12-12-2009/13507444
  //      note 1: Feedback: phpjs-pack@kingsquare.nl
  //      note 1: "machine dependent byte order and size" aren't
  //      note 1: applicable for JavaScript; pack works as on a 32bit,
  //      note 1: little endian machine.
  //   example 1: pack('nvc*', 0x1234, 0x5678, 65, 66)
  //   returns 1: '\u00124xVAB'
  //   example 2: pack('H4', '2345')
  //   returns 2: '#E'
  //   example 3: pack('H*', 'D5')
  //   returns 3: 'Õ'
  //   example 4: pack('d', -100.876)
  //   returns 4: "\u0000\u0000\u0000\u0000\u00008YÀ"

  let formatPointer = 0
  let argumentPointer = 0
  let result = ''
  let argument: string | number = ''
  let i = 0
  let r: string[] = []
  let instruction = ''
  let quantifier: number | '*' = 1
  let word = ''
  let precisionBits = 0
  let exponentBits = 0
  let extraNullCount = 0

  // vars used by float encoding
  let bias = 0
  let minExp = 0
  let maxExp = 0
  let minUnnormExp = 0
  let status = 0
  let exp = 0
  let len = 0
  let bin: number[] = []
  let signal = 0
  let n = 0
  let intPart = 0
  let floatPart = 0
  let lastBit = 0
  let rounded = 0
  let j = 0
  let k = 0
  let tmpResult = ''

  const getRemainingArgumentCount = (): number => inputArgs.length - argumentPointer
  const getCurrentArgument = (): PackArgument => inputArgs[argumentPointer]
  const getCurrentNumericArgument = (): number => Number(getCurrentArgument() ?? 0)
  const getBin = (index: number): number => bin[index] ?? 0

  while (formatPointer < format.length) {
    instruction = format.charAt(formatPointer)
    let quantifierText = ''
    formatPointer++
    while (formatPointer < format.length && /[\d*]/.test(format.charAt(formatPointer))) {
      quantifierText += format.charAt(formatPointer)
      formatPointer++
    }
    if (quantifierText === '') {
      quantifier = 1
    } else if (quantifierText === '*') {
      quantifier = '*'
    } else {
      quantifier = Number.parseInt(quantifierText, 10)
    }

    // Now pack variables: 'quantifier' times 'instruction'
    switch (instruction) {
      case 'a':
      case 'A': {
        // NUL-padded string
        // SPACE-padded string
        if (typeof getCurrentArgument() === 'undefined') {
          throw new Error('Warning:  pack() Type ' + instruction + ': not enough arguments')
        }
        argument = String(getCurrentArgument())
        const argString = String(argument)
        const count = quantifier === '*' ? argString.length : quantifier
        for (i = 0; i < count; i++) {
          if (typeof argString[i] === 'undefined') {
            if (instruction === 'a') {
              result += String.fromCharCode(0)
            } else {
              result += ' '
            }
          } else {
            result += argString[i]
          }
        }
        argumentPointer++
        break
      }
      case 'h':
      case 'H': {
        // Hex string, low nibble first
        // Hex string, high nibble first
        if (typeof getCurrentArgument() === 'undefined') {
          throw new Error('Warning: pack() Type ' + instruction + ': not enough arguments')
        }
        argument = String(getCurrentArgument())
        const argString = String(argument)
        const count = quantifier === '*' ? argString.length : quantifier
        if (count > argString.length) {
          throw new Error('Warning: pack() Type ' + instruction + ': not enough characters in string')
        }

        for (i = 0; i < count; i += 2) {
          // Always get per 2 bytes...
          const first = argString[i] ?? ''
          const second = i + 1 >= count || typeof argString[i + 1] === 'undefined' ? '0' : (argString[i + 1] ?? '0')
          word = first + second
          // The fastest way to reverse?
          if (instruction === 'h') {
            word = (word[1] ?? '0') + (word[0] ?? '0')
          }
          result += String.fromCharCode(Number.parseInt(word, 16))
        }
        argumentPointer++
        break
      }

      case 'c':
      case 'C': {
        // signed char
        // unsigned char
        // c and C is the same in pack
        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning:  pack() Type ' + instruction + ': too few arguments')
        }

        for (i = 0; i < count; i++) {
          result += String.fromCharCode(getCurrentNumericArgument())
          argumentPointer++
        }
        break
      }

      case 's':
      case 'S':
      case 'v': {
        // signed short (always 16 bit, machine byte order)
        // unsigned short (always 16 bit, machine byte order)
        // s and S is the same in pack
        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning:  pack() Type ' + instruction + ': too few arguments')
        }

        for (i = 0; i < count; i++) {
          const value = getCurrentNumericArgument()
          result += String.fromCharCode(value & 0xff)
          result += String.fromCharCode((value >> 8) & 0xff)
          argumentPointer++
        }
        break
      }

      case 'n': {
        // unsigned short (always 16 bit, big endian byte order)
        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning: pack() Type ' + instruction + ': too few arguments')
        }

        for (i = 0; i < count; i++) {
          const value = getCurrentNumericArgument()
          result += String.fromCharCode((value >> 8) & 0xff)
          result += String.fromCharCode(value & 0xff)
          argumentPointer++
        }
        break
      }

      case 'i':
      case 'I':
      case 'l':
      case 'L':
      case 'V': {
        // signed integer (machine dependent size and byte order)
        // unsigned integer (machine dependent size and byte order)
        // signed long (always 32 bit, machine byte order)
        // unsigned long (always 32 bit, machine byte order)
        // unsigned long (always 32 bit, little endian byte order)
        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning:  pack() Type ' + instruction + ': too few arguments')
        }

        for (i = 0; i < count; i++) {
          const value = getCurrentNumericArgument()
          result += String.fromCharCode(value & 0xff)
          result += String.fromCharCode((value >> 8) & 0xff)
          result += String.fromCharCode((value >> 16) & 0xff)
          result += String.fromCharCode((value >> 24) & 0xff)
          argumentPointer++
        }

        break
      }
      case 'N': {
        // unsigned long (always 32 bit, big endian byte order)
        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning: pack() Type ' + instruction + ': too few arguments')
        }

        for (i = 0; i < count; i++) {
          const value = getCurrentNumericArgument()
          result += String.fromCharCode((value >> 24) & 0xff)
          result += String.fromCharCode((value >> 16) & 0xff)
          result += String.fromCharCode((value >> 8) & 0xff)
          result += String.fromCharCode(value & 0xff)
          argumentPointer++
        }
        break
      }

      case 'f':
      case 'd': {
        // float (machine dependent size and representation)
        // double (machine dependent size and representation)
        // version based on IEEE754
        precisionBits = 23
        exponentBits = 8
        if (instruction === 'd') {
          precisionBits = 52
          exponentBits = 11
        }

        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning:  pack() Type ' + instruction + ': too few arguments')
        }
        for (i = 0; i < count; i++) {
          argument = String(getCurrentArgument() ?? '')
          bias = Math.pow(2, exponentBits - 1) - 1
          minExp = -bias + 1
          maxExp = bias
          minUnnormExp = minExp - precisionBits
          n = Number.parseFloat(String(argument))
          status = Number.isNaN(n) || n === -Infinity || n === Infinity ? n : 0
          exp = 0
          len = 2 * bias + 1 + precisionBits + 3
          bin = new Array<number>(len)
          signal = (n = status !== 0 ? 0 : n) < 0 ? 1 : 0
          n = Math.abs(n)
          intPart = Math.floor(n)
          floatPart = n - intPart

          for (k = len; k; ) {
            bin[--k] = 0
          }
          for (k = bias + 2; intPart && k; ) {
            bin[--k] = intPart % 2
            intPart = Math.floor(intPart / 2)
          }
          for (k = bias + 1; floatPart > 0 && k; --floatPart) {
            bin[++k] = Number((floatPart *= 2) >= 1)
          }
          for (k = -1; ++k < len && !getBin(k); ) {
            /* skip leading zeros */
          }

          // @todo: Make this more readable:
          const key =
            (lastBit =
              precisionBits -
              1 +
              (k = (exp = bias + 1 - k) >= minExp && exp <= maxExp ? k + 1 : bias + 1 - (exp = minExp - 1))) + 1

          if (getBin(key)) {
            if (!(rounded = getBin(lastBit))) {
              for (j = lastBit + 2; !rounded && j < len; rounded = getBin(j++)) {
                /* find if any trailing bit is set */
              }
            }
            for (j = lastBit + 1; rounded && --j >= 0; (bin[j] = Number(!getBin(j))) && (rounded = 0)) {
              /* propagate rounding carry */
            }
          }

          for (k = k - 2 < 0 ? -1 : k - 3; ++k < len && !getBin(k); ) {
            /* skip leading zeros after rounding */
          }

          if ((exp = bias + 1 - k) >= minExp && exp <= maxExp) {
            ++k
          } else if (exp < minExp) {
            if (exp !== bias + 1 - len && exp < minUnnormExp) {
              // "encodeFloat::float underflow"
            }
            k = bias + 1 - (exp = minExp - 1)
          }

          if (intPart || status !== 0) {
            exp = maxExp + 1
            k = bias + 2
            if (status === -Infinity) {
              signal = 1
            } else if (Number.isNaN(status)) {
              bin[k] = 1
            }
          }

          n = Math.abs(exp + bias)
          tmpResult = ''

          for (j = exponentBits + 1; --j; ) {
            tmpResult = (n % 2) + tmpResult
            n >>= 1
          }

          n = 0
          j = 0
          k = (tmpResult = (signal ? '1' : '0') + tmpResult + bin.slice(k, k + precisionBits).join('')).length
          r = []

          for (; k; ) {
            n += (1 << j) * Number(tmpResult.charAt(--k))
            if (j === 7) {
              r.push(String.fromCharCode(n))
              n = 0
            }
            j = (j + 1) % 8
          }

          r.push(n ? String.fromCharCode(n) : '')
          result += r.join('')
          argumentPointer++
        }
        break
      }

      case 'x': {
        // NUL byte
        if (quantifier === '*') {
          throw new Error("Warning: pack(): Type x: '*' ignored")
        }
        for (i = 0; i < quantifier; i++) {
          result += String.fromCharCode(0)
        }
        break
      }

      case 'X': {
        // Back up one byte
        if (quantifier === '*') {
          throw new Error("Warning: pack(): Type X: '*' ignored")
        }
        for (i = 0; i < quantifier; i++) {
          if (result.length === 0) {
            throw new Error('Warning: pack(): Type X:' + ' outside of string')
          }
          result = result.slice(0, -1)
        }
        break
      }

      case '@': {
        // NUL-fill to absolute position
        if (quantifier === '*') {
          throw new Error("Warning: pack(): Type X: '*' ignored")
        }
        if (quantifier > result.length) {
          extraNullCount = quantifier - result.length
          for (i = 0; i < extraNullCount; i++) {
            result += String.fromCharCode(0)
          }
        }
        if (quantifier < result.length) {
          result = result.slice(0, quantifier)
        }
        break
      }

      default:
        throw new Error('Warning: pack() Type ' + instruction + ': invalid format code')
    }
  }
  if (argumentPointer < inputArgs.length) {
    throw new Error('Warning: pack(): ' + (inputArgs.length - argumentPointer) + ' arguments unused')
  }

  return result
}

export function pack(format, ...inputArgs) {
  //  discuss at: https://locutus.io/php/pack/
  // original by: Tim de Koning (https://www.kingsquare.nl)
  //    parts by: Jonas Raoni Soares Silva (https://www.jsfromhell.com)
  // bugfixed by: Tim de Koning (https://www.kingsquare.nl)
  //      note 1: Float encoding by: Jonas Raoni Soares Silva
  //      note 1: Home: https://www.kingsquare.nl/blog/12-12-2009/13507444
  //      note 1: Feedback: phpjs-pack@kingsquare.nl
  //      note 1: "machine dependent byte order and size" aren't
  //      note 1: applicable for JavaScript; pack works as on a 32bit,
  //      note 1: little endian machine.
  //   example 1: pack('nvc*', 0x1234, 0x5678, 65, 66)
  //   returns 1: '\u00124xVAB'
  //   example 2: pack('H4', '2345')
  //   returns 2: '#E'
  //   example 3: pack('H*', 'D5')
  //   returns 3: 'Õ'
  //   example 4: pack('d', -100.876)
  //   returns 4: "\u0000\u0000\u0000\u0000\u00008YÀ"

  let formatPointer = 0
  let argumentPointer = 0
  let result = ''
  let argument = ''
  let i = 0
  let r = []
  let instruction = ''
  let quantifier = 1
  let word = ''
  let precisionBits = 0
  let exponentBits = 0
  let extraNullCount = 0

  // vars used by float encoding
  let bias = 0
  let minExp = 0
  let maxExp = 0
  let minUnnormExp = 0
  let status = 0
  let exp = 0
  let len = 0
  let bin = []
  let signal = 0
  let n = 0
  let intPart = 0
  let floatPart = 0
  let lastBit = 0
  let rounded = 0
  let j = 0
  let k = 0
  let tmpResult = ''

  const getRemainingArgumentCount = () => inputArgs.length - argumentPointer
  const getCurrentArgument = () => inputArgs[argumentPointer]
  const getCurrentNumericArgument = () => Number(getCurrentArgument() ?? 0)
  const getBin = (index) => bin[index] ?? 0

  while (formatPointer < format.length) {
    instruction = format.charAt(formatPointer)
    let quantifierText = ''
    formatPointer++
    while (formatPointer < format.length && /[\d*]/.test(format.charAt(formatPointer))) {
      quantifierText += format.charAt(formatPointer)
      formatPointer++
    }
    if (quantifierText === '') {
      quantifier = 1
    } else if (quantifierText === '*') {
      quantifier = '*'
    } else {
      quantifier = Number.parseInt(quantifierText, 10)
    }

    // Now pack variables: 'quantifier' times 'instruction'
    switch (instruction) {
      case 'a':
      case 'A': {
        // NUL-padded string
        // SPACE-padded string
        if (typeof getCurrentArgument() === 'undefined') {
          throw new Error('Warning:  pack() Type ' + instruction + ': not enough arguments')
        }
        argument = String(getCurrentArgument())
        const argString = String(argument)
        const count = quantifier === '*' ? argString.length : quantifier
        for (i = 0; i < count; i++) {
          if (typeof argString[i] === 'undefined') {
            if (instruction === 'a') {
              result += String.fromCharCode(0)
            } else {
              result += ' '
            }
          } else {
            result += argString[i]
          }
        }
        argumentPointer++
        break
      }
      case 'h':
      case 'H': {
        // Hex string, low nibble first
        // Hex string, high nibble first
        if (typeof getCurrentArgument() === 'undefined') {
          throw new Error('Warning: pack() Type ' + instruction + ': not enough arguments')
        }
        argument = String(getCurrentArgument())
        const argString = String(argument)
        const count = quantifier === '*' ? argString.length : quantifier
        if (count > argString.length) {
          throw new Error('Warning: pack() Type ' + instruction + ': not enough characters in string')
        }

        for (i = 0; i < count; i += 2) {
          // Always get per 2 bytes...
          const first = argString[i] ?? ''
          const second = i + 1 >= count || typeof argString[i + 1] === 'undefined' ? '0' : (argString[i + 1] ?? '0')
          word = first + second
          // The fastest way to reverse?
          if (instruction === 'h') {
            word = (word[1] ?? '0') + (word[0] ?? '0')
          }
          result += String.fromCharCode(Number.parseInt(word, 16))
        }
        argumentPointer++
        break
      }

      case 'c':
      case 'C': {
        // signed char
        // unsigned char
        // c and C is the same in pack
        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning:  pack() Type ' + instruction + ': too few arguments')
        }

        for (i = 0; i < count; i++) {
          result += String.fromCharCode(getCurrentNumericArgument())
          argumentPointer++
        }
        break
      }

      case 's':
      case 'S':
      case 'v': {
        // signed short (always 16 bit, machine byte order)
        // unsigned short (always 16 bit, machine byte order)
        // s and S is the same in pack
        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning:  pack() Type ' + instruction + ': too few arguments')
        }

        for (i = 0; i < count; i++) {
          const value = getCurrentNumericArgument()
          result += String.fromCharCode(value & 0xff)
          result += String.fromCharCode((value >> 8) & 0xff)
          argumentPointer++
        }
        break
      }

      case 'n': {
        // unsigned short (always 16 bit, big endian byte order)
        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning: pack() Type ' + instruction + ': too few arguments')
        }

        for (i = 0; i < count; i++) {
          const value = getCurrentNumericArgument()
          result += String.fromCharCode((value >> 8) & 0xff)
          result += String.fromCharCode(value & 0xff)
          argumentPointer++
        }
        break
      }

      case 'i':
      case 'I':
      case 'l':
      case 'L':
      case 'V': {
        // signed integer (machine dependent size and byte order)
        // unsigned integer (machine dependent size and byte order)
        // signed long (always 32 bit, machine byte order)
        // unsigned long (always 32 bit, machine byte order)
        // unsigned long (always 32 bit, little endian byte order)
        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning:  pack() Type ' + instruction + ': too few arguments')
        }

        for (i = 0; i < count; i++) {
          const value = getCurrentNumericArgument()
          result += String.fromCharCode(value & 0xff)
          result += String.fromCharCode((value >> 8) & 0xff)
          result += String.fromCharCode((value >> 16) & 0xff)
          result += String.fromCharCode((value >> 24) & 0xff)
          argumentPointer++
        }

        break
      }
      case 'N': {
        // unsigned long (always 32 bit, big endian byte order)
        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning: pack() Type ' + instruction + ': too few arguments')
        }

        for (i = 0; i < count; i++) {
          const value = getCurrentNumericArgument()
          result += String.fromCharCode((value >> 24) & 0xff)
          result += String.fromCharCode((value >> 16) & 0xff)
          result += String.fromCharCode((value >> 8) & 0xff)
          result += String.fromCharCode(value & 0xff)
          argumentPointer++
        }
        break
      }

      case 'f':
      case 'd': {
        // float (machine dependent size and representation)
        // double (machine dependent size and representation)
        // version based on IEEE754
        precisionBits = 23
        exponentBits = 8
        if (instruction === 'd') {
          precisionBits = 52
          exponentBits = 11
        }

        const count = quantifier === '*' ? getRemainingArgumentCount() : quantifier
        if (count > getRemainingArgumentCount()) {
          throw new Error('Warning:  pack() Type ' + instruction + ': too few arguments')
        }
        for (i = 0; i < count; i++) {
          argument = String(getCurrentArgument() ?? '')
          bias = Math.pow(2, exponentBits - 1) - 1
          minExp = -bias + 1
          maxExp = bias
          minUnnormExp = minExp - precisionBits
          n = Number.parseFloat(String(argument))
          status = Number.isNaN(n) || n === -Infinity || n === Infinity ? n : 0
          exp = 0
          len = 2 * bias + 1 + precisionBits + 3
          bin = new Array(len)
          signal = (n = status !== 0 ? 0 : n) < 0 ? 1 : 0
          n = Math.abs(n)
          intPart = Math.floor(n)
          floatPart = n - intPart

          for (k = len; k; ) {
            bin[--k] = 0
          }
          for (k = bias + 2; intPart && k; ) {
            bin[--k] = intPart % 2
            intPart = Math.floor(intPart / 2)
          }
          for (k = bias + 1; floatPart > 0 && k; --floatPart) {
            bin[++k] = Number((floatPart *= 2) >= 1)
          }
          for (k = -1; ++k < len && !getBin(k); ) {
            /* skip leading zeros */
          }

          // @todo: Make this more readable:
          const key =
            (lastBit =
              precisionBits -
              1 +
              (k = (exp = bias + 1 - k) >= minExp && exp <= maxExp ? k + 1 : bias + 1 - (exp = minExp - 1))) + 1

          if (getBin(key)) {
            if (!(rounded = getBin(lastBit))) {
              for (j = lastBit + 2; !rounded && j < len; rounded = getBin(j++)) {
                /* find if any trailing bit is set */
              }
            }
            for (j = lastBit + 1; rounded && --j >= 0; (bin[j] = Number(!getBin(j))) && (rounded = 0)) {
              /* propagate rounding carry */
            }
          }

          for (k = k - 2 < 0 ? -1 : k - 3; ++k < len && !getBin(k); ) {
            /* skip leading zeros after rounding */
          }

          if ((exp = bias + 1 - k) >= minExp && exp <= maxExp) {
            ++k
          } else if (exp < minExp) {
            if (exp !== bias + 1 - len && exp < minUnnormExp) {
              // "encodeFloat::float underflow"
            }
            k = bias + 1 - (exp = minExp - 1)
          }

          if (intPart || status !== 0) {
            exp = maxExp + 1
            k = bias + 2
            if (status === -Infinity) {
              signal = 1
            } else if (Number.isNaN(status)) {
              bin[k] = 1
            }
          }

          n = Math.abs(exp + bias)
          tmpResult = ''

          for (j = exponentBits + 1; --j; ) {
            tmpResult = (n % 2) + tmpResult
            n >>= 1
          }

          n = 0
          j = 0
          k = (tmpResult = (signal ? '1' : '0') + tmpResult + bin.slice(k, k + precisionBits).join('')).length
          r = []

          for (; k; ) {
            n += (1 << j) * Number(tmpResult.charAt(--k))
            if (j === 7) {
              r.push(String.fromCharCode(n))
              n = 0
            }
            j = (j + 1) % 8
          }

          r.push(n ? String.fromCharCode(n) : '')
          result += r.join('')
          argumentPointer++
        }
        break
      }

      case 'x': {
        // NUL byte
        if (quantifier === '*') {
          throw new Error("Warning: pack(): Type x: '*' ignored")
        }
        for (i = 0; i < quantifier; i++) {
          result += String.fromCharCode(0)
        }
        break
      }

      case 'X': {
        // Back up one byte
        if (quantifier === '*') {
          throw new Error("Warning: pack(): Type X: '*' ignored")
        }
        for (i = 0; i < quantifier; i++) {
          if (result.length === 0) {
            throw new Error('Warning: pack(): Type X:' + ' outside of string')
          }
          result = result.slice(0, -1)
        }
        break
      }

      case '@': {
        // NUL-fill to absolute position
        if (quantifier === '*') {
          throw new Error("Warning: pack(): Type X: '*' ignored")
        }
        if (quantifier > result.length) {
          extraNullCount = quantifier - result.length
          for (i = 0; i < extraNullCount; i++) {
            result += String.fromCharCode(0)
          }
        }
        if (quantifier < result.length) {
          result = result.slice(0, quantifier)
        }
        break
      }

      default:
        throw new Error('Warning: pack() Type ' + instruction + ': invalid format code')
    }
  }
  if (argumentPointer < inputArgs.length) {
    throw new Error('Warning: pack(): ' + (inputArgs.length - argumentPointer) + ' arguments unused')
  }

  return result
}

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