var Match = require ('../match');

/**
 * Charset recognizer for UTF-8
 */
module.exports = function() {
    this.name = function() {
        return "UTF-8";
    };
    this.match = function(det) {

        var hasBOM = false,
            numValid = 0,
            numInvalid = 0,
            input = det.fRawInput,
            i,
            trailBytes = 0,
            confidence;

        if (det.fRawLength >= 3 &&
            (input[0] & 0xFF) == 0xef && (input[1] & 0xFF) == 0xbb && (input[2] & 0xFF) == 0xbf) {
            hasBOM = true;
        }

        // Scan for multi-byte sequences
        for (i=0; i<det.fRawLength; i++) {
            var b = input[i];
            if ((b & 0x80) == 0) {
                continue;   // ASCII
            }

            // Hi bit on char found.  Figure out how long the sequence should be
            if ((b & 0x0e0) == 0x0c0) {
                trailBytes = 1;
            } else if ((b & 0x0f0) == 0x0e0) {
                trailBytes = 2;
            } else if ((b & 0x0f8) == 0xf0) {
                trailBytes = 3;
            } else {
                numInvalid++;
                if (numInvalid > 5) {
                    break;
                }
                trailBytes = 0;
            }

            // Verify that we've got the right number of trail bytes in the sequence
            for (;;) {
                i++;
                if (i>=det.fRawLength) {
                    break;
                }
                b = input[i];
                if ((b & 0xc0) != 0x080) {
                    numInvalid++;
                    break;
                }
                if (--trailBytes == 0) {
                    numValid++;
                    break;
                }
            }
        }

        // Cook up some sort of confidence score, based on presense of a BOM
        //    and the existence of valid and/or invalid multi-byte sequences.
        confidence = 0;
        if (hasBOM && numInvalid==0) {
            confidence = 100;
        } else if (hasBOM && numValid > numInvalid*10) {
            confidence = 80;
        } else if (numValid > 3 && numInvalid == 0) {
            confidence = 100;
        } else if (numValid > 0 && numInvalid == 0) {
            confidence = 80;
        } else if (numValid == 0 && numInvalid == 0) {
            // Plain ASCII.
            confidence = 10;
        } else if (numValid > numInvalid*10) {
            // Probably corruput utf-8 data.  Valid sequences aren't likely by chance.
            confidence = 25;
        }
        return confidence == 0 ? null : new Match(det, this, confidence);
    };
};