This is a vanilla shift_and implementation in JS. Lacking the context it's hard to see what's going on. The interesting part of the algorithm is the bit parallelism techniques it uses to match prefixes. The implementation is based on this book: http://www.amazon.com/Flexible-Pattern-Matching-Strings-Algorithms/dp/0521039932/ Blog post explaining…

shift_and.js

/**
  * string p: pattern to search (needle);
  * string text: haystack
**/
function shift_and(p, text) {
    var b = {};
    var l = p.length;
    var tl = text.length;

    // initialize bitmask table
    for (var i = 0; i < l; i++) {
        var ch = p.charAt(i);
        b[ch] = 0;
    }

    //build bitmask table;
    for (var i = 0; i < l; i++) {
        b[p.charAt(i)] = b[p.charAt(i)] | (1 << i);
    }

    var d = 0;
    var matchMask = << l-1;
    for (var i = 0; i < tl; i++) {
        d = ((d << 1) | 1) & (b[text.charAt(i)] | 0);
        var matched = (d & matchMask);
        if (matched != 0) {
            return i - l + 1;
        }
    }
    return -1;
}