// http://blog.dkbza.org/2007/05/scanning-data-for-entropy-anomalies.html
// https://github.com/dxa4481/truffleHog/blob/master/truffleHog.py

var Math_log2 = Math.log2 || (x) => {
    return Math.log(x) * Math.LOG2E;
};

exports.SHANNON_ENTROPY_BASE64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
exports.SHANNON_ENTROPY_HEX    = "1234567890abcdefABCDEF";

exports.shannonEntropy = (data, charRange) => {
    if (!data) {
        return 0;
    }
    var lenOfData = data.length;
    var charCodeCountMap = {};
    for (var i = 0; i < data.length; i++) {
        var charCode = data.charCodeAt(i);
        charCodeCountMap[charCode] = charCodeCountMap[charCode] || 0;
        charCodeCountMap[charCode] += 1;
    }
    var entropy = 0;
    for (var i = 0; i < charRange.length; i++) {
        var charCode = charRange.charCodeAt(i);
        var charCodeCount = charCodeCountMap[charCode];
        if (charCodeCount != null) {
            var pCharCode = charCodeCount / lenOfData;
            entropy += - pCharCode * Math_log2(pCharCode);
        }
    }
    return entropy;
};