ArrayOptimiser Tool

What is this tool for?

Let's take an array that looks like this:

var arr1 = [1,2,3,4,5,6,7, 1000000]

In memory each element will be stored on 32 bits (PACKED_ELEMENTS) If we decompose this array into two sub-arrays we get:

var arr2 = [[1,2,3,4,5,6,7], // PACKED_SMI_ELEMENTS (int8_t) + Array overhead (16 bytes)
            [1000000]] // PACKED_ELEMENTS (int32_t) + Array overhead (16 bytes)

In terms of size we get (knowing that we need to add the size of the header of the tables themselves):

• arr1 = (8 * 4bytes = 32 bytes) + (1 x 16bytes = 16 bytes)
• arr2 = (7 * 1byte + 1 * 4bytes = 11 bytes) + (3 x 16bytes = 48 bytes)

TOTAL:

• arr1 = 48 bytes
• arr2 = 59 bytes ⭕❌ (very bad)
  
  

The tool can also use a String representation like:

var arr2 = ["\x01\x02\x03\x04\x05\x06\x07", // (one-byte String) + String overhead (12 bytes) + Math.ceil(length/4)
            [1000000]] // PACKED_ELEMENTS (int32_t) + Array overhead (16 bytes)

TOTAL:

• arr1 = 48 bytes
• arr2 = 55 bytes ⭕ (bad)
  
  

Container sizes

n = elements

• String(1byte) = 12 + 4 * Math.ceil(n/4)
• String(2byte) = 12 + 4 * Math.ceil(n/2)
• Array(1byte) = 16 + 4 * Math.ceil(n/4)
• Array(4bytes) = 16 + 4 * n
  
  

Conclusion and tips

• Worth for large arrays with large variance
• In javascript the String object is the smallest container for data's !
  
  

Usage

Here are the functions to access the 2d table, they are provided in 2 versions:

• 1D and 2D

function arrayAt(a,i) { // 1d and 2d array's (slow)
  var w = a[0].length;
  if (w) { a = a[(i/w)|0]; i %= w; }
  return a?(a.charCodeAt?a.charCodeAt(i):a[i]):undefined;
}
function arrayLength(a) { // 1d and 2d array's (slow)
  var w = a[0].length, h = a.length-1;
  return w?((w*h)+a[h].length):(h+1);
}

• 2D only

function array2dAt(a,i) {
  var w = a[0].length, t = (i/w)|0;
  i -= w*t; a = a[t];
  return a.charCodeAt? a.charCodeAt(i) : a[i];
}
function array2dLength(a) {
  var w = a[0].length, h = a.length-1;
  return w*h + a[h].length;
}

• Call

for (var i = 0, len = array2dLength(arr2); i < len; i++) {
  console.log(array2dAt(arr2,i), " vs ", arr1[i]);
}