Simple implementation of a runlength-encoding algorithm

runlength-encoding.js

function encodeFrame(frame, idx) {
    // RLE: MSB indicates a run-length (2-127) for the subsequent character
    return frame.replace(/(.)\1{2,127}/g, function(s) {
        return String.fromCharCode(128 + s.length) + s[0];
    }); 
}

var decodeCache = {};
function decodeFrame(frame) {
    return frame.replace(/[\u0080-\u00FF]./g, function replaceCallback(s) {
        var c=s[1],
            buf=decodeCache[c] || c,
            n=s.charCodeAt(0)-128;

        while(buf.length <= n) { buf=buf+buf; }
        decodeCache[c]=buf;

        return buf.slice(0,n);
    }); 
}