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---
layout: page
title: "JavaScript pack function"
comments: true
sharing: true
footer: true
alias:
- /functions/view/pack:880
- /functions/view/pack
- /functions/view/880
- /functions/pack:880
- /functions/880
---
<!-- Generated by Rakefile:build -->
A JavaScript equivalent of PHP's pack
{% codeblock misc/pack.js lang:js https://raw.github.com/kvz/phpjs/master/functions/misc/pack.js raw on github %}
function pack (format) {
// From: http://phpjs.org/functions
// + original by: Tim de Koning (http://www.kingsquare.nl)
// + parts by: Jonas Raoni Soares Silva (http://www.jsfromhell.com)
// + bugfixed by: Tim de Koning (http://www.kingsquare.nl)
// % note 1: Float encoding by: Jonas Raoni Soares Silva
// % note 2: Home: http://www.kingsquare.nl/blog/12-12-2009/13507444
// % note 3: Feedback: phpjs-pack@kingsquare.nl
// % note 4: 'machine dependent byte order and size' aren't
// % note 4: applicable for JavaScript; pack works as on a 32bit,
// % note 4: little endian machine
// * example 1: pack('nvc*', 0x1234, 0x5678, 65, 66);
// * returns 1: '4xVAB'
var formatPointer = 0,
argumentPointer = 1,
result = '',
argument = '',
i = 0,
r = [],
instruction, quantifier, word, precisionBits, exponentBits, extraNullCount;
// vars used by float encoding
var bias, minExp, maxExp, minUnnormExp, status, exp, len, bin, signal, n, intPart, floatPart, lastBit, rounded, j, k, tmpResult;
while (formatPointer < format.length) {
instruction = format.charAt(formatPointer);
quantifier = '';
formatPointer++;
while ((formatPointer < format.length) && (format.charAt(formatPointer).match(/[\d\*]/) !== null)) {
quantifier += format.charAt(formatPointer);
formatPointer++;
}
if (quantifier === '') {
quantifier = '1';
}
// Now pack variables: 'quantifier' times 'instruction'
switch (instruction) {
case 'a':
// NUL-padded string
case 'A':
// SPACE-padded string
if (typeof arguments[argumentPointer] === 'undefined') {
throw new Error('Warning: pack() Type ' + instruction + ': not enough arguments');
} else {
argument = String(arguments[argumentPointer]);
}
if (quantifier === '*') {
quantifier = argument.length;
}
for (i = 0; i < quantifier; i++) {
if (typeof argument[i] === 'undefined') {
if (instruction === 'a') {
result += String.fromCharCode(0);
} else {
result += ' ';
}
} else {
result += argument[i];
}
}
argumentPointer++;
break;
case 'h':
// Hex string, low nibble first
case 'H':
// Hex string, high nibble first
if (typeof arguments[argumentPointer] === 'undefined') {
throw new Error('Warning: pack() Type ' + instruction + ': not enough arguments');
} else {
argument = arguments[argumentPointer];
}
if (quantifier === '*') {
quantifier = argument.length;
}
if (quantifier > argument.length) {
throw new Error('Warning: pack() Type ' + instruction + ': not enough characters in string');
}
for (i = 0; i < quantifier; i += 2) {
// Always get per 2 bytes...
word = argument[i];
if (((i + 1) >= quantifier) || typeof argument[i + 1] === 'undefined') {
word += '0';
} else {
word += argument[i + 1];
}
// The fastest way to reverse?
if (instruction === 'h') {
word = word[1] + word[0];
}
result += String.fromCharCode(parseInt(word, 16));
}
argumentPointer++;
break;
case 'c':
// signed char
case 'C':
// unsigned char
// c and C is the same in pack
if (quantifier === '*') {
quantifier = arguments.length - argumentPointer;
}
if (quantifier > (arguments.length - argumentPointer)) {
throw new Error('Warning: pack() Type ' + instruction + ': too few arguments');
}
for (i = 0; i < quantifier; i++) {
result += String.fromCharCode(arguments[argumentPointer]);
argumentPointer++;
}
break;
case 's':
// signed short (always 16 bit, machine byte order)
case 'S':
// unsigned short (always 16 bit, machine byte order)
case 'v':
// s and S is the same in pack
if (quantifier === '*') {
quantifier = arguments.length - argumentPointer;
}
if (quantifier > (arguments.length - argumentPointer)) {
throw new Error('Warning: pack() Type ' + instruction + ': too few arguments');
}
for (i = 0; i < quantifier; i++) {
result += String.fromCharCode(arguments[argumentPointer] & 0xFF);
result += String.fromCharCode(arguments[argumentPointer] >> 8 & 0xFF);
argumentPointer++;
}
break;
case 'n':
// unsigned short (always 16 bit, big endian byte order)
if (quantifier === '*') {
quantifier = arguments.length - argumentPointer;
}
if (quantifier > (arguments.length - argumentPointer)) {
throw new Error('Warning: pack() Type ' + instruction + ': too few arguments');
}
for (i = 0; i < quantifier; i++) {
result += String.fromCharCode(arguments[argumentPointer] >> 8 & 0xFF);
result += String.fromCharCode(arguments[argumentPointer] & 0xFF);
argumentPointer++;
}
break;
case 'i':
// signed integer (machine dependent size and byte order)
case 'I':
// unsigned integer (machine dependent size and byte order)
case 'l':
// signed long (always 32 bit, machine byte order)
case 'L':
// unsigned long (always 32 bit, machine byte order)
case 'V':
// unsigned long (always 32 bit, little endian byte order)
if (quantifier === '*') {
quantifier = arguments.length - argumentPointer;
}
if (quantifier > (arguments.length - argumentPointer)) {
throw new Error('Warning: pack() Type ' + instruction + ': too few arguments');
}
for (i = 0; i < quantifier; i++) {
result += String.fromCharCode(arguments[argumentPointer] & 0xFF);
result += String.fromCharCode(arguments[argumentPointer] >> 8 & 0xFF);
result += String.fromCharCode(arguments[argumentPointer] >> 16 & 0xFF);
result += String.fromCharCode(arguments[argumentPointer] >> 24 & 0xFF);
argumentPointer++;
}
break;
case 'N':
// unsigned long (always 32 bit, big endian byte order)
if (quantifier === '*') {
quantifier = arguments.length - argumentPointer;
}
if (quantifier > (arguments.length - argumentPointer)) {
throw new Error('Warning: pack() Type ' + instruction + ': too few arguments');
}
for (i = 0; i < quantifier; i++) {
result += String.fromCharCode(arguments[argumentPointer] >> 24 & 0xFF);
result += String.fromCharCode(arguments[argumentPointer] >> 16 & 0xFF);
result += String.fromCharCode(arguments[argumentPointer] >> 8 & 0xFF);
result += String.fromCharCode(arguments[argumentPointer] & 0xFF);
argumentPointer++;
}
break;
case 'f':
// float (machine dependent size and representation)
case 'd':
// double (machine dependent size and representation)
// version based on IEEE754
precisionBits = 23;
exponentBits = 8;
if (instruction === 'd') {
precisionBits = 52;
exponentBits = 11;
}
if (quantifier === '*') {
quantifier = arguments.length - argumentPointer;
}
if (quantifier > (arguments.length - argumentPointer)) {
throw new Error('Warning: pack() Type ' + instruction + ': too few arguments');
}
for (i = 0; i < quantifier; i++) {
argument = arguments[argumentPointer];
bias = Math.pow(2, exponentBits - 1) - 1;
minExp = -bias + 1;
maxExp = bias;
minUnnormExp = minExp - precisionBits;
status = isNaN(n = parseFloat(argument)) || 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;
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] = ((floatPart *= 2) >= 1) - 0);
}
for (k = -1; ++k < len && !bin[k];) {}
if (bin[(lastBit = precisionBits - 1 + (k = (exp = bias + 1 - k) >= minExp && exp <= maxExp ? k + 1 : bias + 1 - (exp = minExp - 1))) + 1]) {
if (!(rounded = bin[lastBit])) {
for (j = lastBit + 2; !rounded && j < len; rounded = bin[j++]) {}
}
for (j = lastBit + 1; rounded && --j >= 0;
(bin[j] = !bin[j] - 0) && (rounded = 0)) {}
}
for (k = k - 2 < 0 ? -1 : k - 3; ++k < len && !bin[k];) {}
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 (isNaN(status)) {
bin[k] = 1;
}
}
n = Math.abs(exp + bias);
tmpResult = '';
for (j = exponentBits + 1; --j;) {
tmpResult = (n % 2) + tmpResult;
n = 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) * tmpResult.charAt(--k);
if (j === 7) {
r[r.length] = String.fromCharCode(n);
n = 0;
}
j = (j + 1) % 8;
}
r[r.length] = 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');
} else {
result = result.substring(0, result.length - 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.substring(0, quantifier);
}
break;
default:
throw new Error('Warning: pack() Type ' + instruction + ': unknown format code');
}
}
if (argumentPointer < arguments.length) {
throw new Error('Warning: pack(): ' + (arguments.length - argumentPointer) + ' arguments unused');
}
return result;
}
{% endcodeblock %}
- [Raw function on GitHub](https://github.com/kvz/phpjs/blob/master/functions/misc/pack.js)
Please note that php.js uses JavaScript objects as substitutes for PHP arrays, they are
the closest match to this hashtable-like data structure.
Please also note that php.js offers community built functions and goes by the
[McDonald's Theory](https://medium.com/what-i-learned-building/9216e1c9da7d). We'll put online
functions that are far from perfect, in the hopes to spark better contributions.
Do you have one? Then please just:
- [Edit on GitHub](https://github.com/kvz/phpjs/edit/master/functions/misc/pack.js)
### Example 1
This code
{% codeblock lang:js example %}
pack('nvc*', 0x1234, 0x5678, 65, 66);
{% endcodeblock %}
Should return
{% codeblock lang:js returns %}
'4xVAB'
{% endcodeblock %}
### Other PHP functions in the misc extension
{% render_partial _includes/custom/misc.html %}