phaser/src/utils/array/StableSort.js

/**
 * @author       Richard Davey <rich@photonstorm.com>
 * @copyright    2018 Photon Storm Ltd.
 * @license      {@link https://github.com/photonstorm/phaser/blob/master/license.txt|MIT License}
 */

//! stable.js 0.1.6, https://github.com/Two-Screen/stable
//! © 2017 Angry Bytes and contributors. MIT licensed.

(function() {

// A stable array sort, because `Array#sort()` is not guaranteed stable.
// This is an implementation of merge sort, without recursion.

var stable = function(arr, comp) {
    return exec(arr.slice(), comp);
};

stable.inplace = function(arr, comp) {
    var result = exec(arr, comp);

    // This simply copies back if the result isn't in the original array,
    // which happens on an odd number of passes.
    if (result !== arr) {
        pass(result, null, arr.length, arr);
    }

    return arr;
};

// Execute the sort using the input array and a second buffer as work space.
// Returns one of those two, containing the final result.
function exec(arr, comp) {
    if (typeof(comp) !== 'function') {
        comp = function(a, b) {
            return String(a).localeCompare(b);
        };
    }

    // Short-circuit when there's nothing to sort.
    var len = arr.length;
    if (len <= 1) {
        return arr;
    }

    // Rather than dividing input, simply iterate chunks of 1, 2, 4, 8, etc.
    // Chunks are the size of the left or right hand in merge sort.
    // Stop when the left-hand covers all of the array.
    var buffer = new Array(len);
    for (var chk = 1; chk < len; chk *= 2) {
        pass(arr, comp, chk, buffer);

        var tmp = arr;
        arr = buffer;
        buffer = tmp;
    }

    return arr;
}

// Run a single pass with the given chunk size.
var pass = function(arr, comp, chk, result) {
    var len = arr.length;
    var i = 0;
    // Step size / double chunk size.
    var dbl = chk * 2;
    // Bounds of the left and right chunks.
    var l, r, e;
    // Iterators over the left and right chunk.
    var li, ri;

    // Iterate over pairs of chunks.
    for (l = 0; l < len; l += dbl) {
        r = l + chk;
        e = r + chk;
        if (r > len) r = len;
        if (e > len) e = len;

        // Iterate both chunks in parallel.
        li = l;
        ri = r;
        while (true) {
            // Compare the chunks.
            if (li < r && ri < e) {
                // This works for a regular `sort()` compatible comparator,
                // but also for a simple comparator like: `a > b`
                if (comp(arr[li], arr[ri]) <= 0) {
                    result[i++] = arr[li++];
                }
                else {
                    result[i++] = arr[ri++];
                }
            }
            // Nothing to compare, just flush what's left.
            else if (li < r) {
                result[i++] = arr[li++];
            }
            else if (ri < e) {
                result[i++] = arr[ri++];
            }
            // Both iterators are at the chunk ends.
            else {
                break;
            }
        }
    }
};

// Export using CommonJS or to the window.
if (typeof(module) !== 'undefined') {
    module.exports = stable;
}
else {
    window.stable = stable;
}

})();