root/as3/Marilena/trunk/src/deng/utils/ChecksumUtil.as

package deng.utils
{
        import flash.utils.ByteArray;
        
        public class ChecksumUtil
        {
                /**
                 * @private
                 */             
                private static var crcTable:Array = makeCRCTable();
                
                /**
                 * @private
                 */             
                private static function makeCRCTable():Array {
                        var table:Array = [];
                        var i:uint;
                        var j:uint;
                        var c:uint;
                        for (i = 0; i < 256; i++) {
                                c = i;
                                for (j = 0; j < 8; j++) {
                                        if (c & 1) {
                                                c = 0xEDB88320 ^ (c >>> 1);
                                        } else {
                                                c >>>= 1;
                                        }
                                }
                                table.push(c);
                        }
                        return table;
                }
                
                /**
                 * Calculates a CRC-32 checksum over a ByteArray
                 * 
                 * @see http://www.w3.org/TR/PNG/#D-CRCAppendix
                 * 
                 * @param data 
                 * @param len
                 * @param start
                 * @return CRC-32 checksum
                 */             
                public static function CRC32(data:ByteArray, start:uint = 0, len:uint = 0):uint {
                        if (start >= data.length) { start = data.length; }
                        if (len == 0) { len = data.length - start; }
                        if (len + start > data.length) { len = data.length - start; }
                        var i:uint;
                        var c:uint = 0xffffffff;
                        for (i = start; i < len; i++) {
                                c = uint(crcTable[(c ^ data[i]) & 0xff]) ^ (c >>> 8);
                        }
                        return (c ^ 0xffffffff);
                }
                
                /**
                 * Calculates an Adler-32 checksum over a ByteArray
                 * 
                 * @see http://en.wikipedia.org/wiki/Adler-32#Example_implementation
                 * 
                 * @param data 
                 * @param len
                 * @param start
                 * @return Adler-32 checksum
                 */             
                public static function Adler32(data:ByteArray, start:uint = 0, len:uint = 0):uint {
                        if (start >= data.length) { start = data.length; }
                        if (len == 0) { len = data.length - start; }
                        if (len + start > data.length) { len = data.length - start; }
                        var i:uint = start;
                        var a:uint = 1;
                        var b:uint = 0;
                        while (len) {
                                var tlen:uint = (len > 5550) ? 5550 : len;
                                len -= tlen;
                                do {
                                        a += data[i++];
                                        b += a;
                                } while (--tlen);
                                a = (a & 0xffff) + (a >> 16) * 15;
                                b = (b & 0xffff) + (b >> 16) * 15;
                        }
                        if (a >= 65521) { a -= 65521; }
                        b = (b & 0xffff) + (b >> 16) * 15;
                        if (b >= 65521) { b -= 65521; }
                        return (b << 16) | a;
                }
        }
}