Widget:CRCprefix: Difference between revisions

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+<html>
+<head lang="en">
+<meta http-equiv="content-type" content="text/html; charset=UTF-8">
+<title>CRC Prefix Calculator by L0scher</title>
+<script type="text/javascript">
+//<![CDATA[
+function calculateInit () {
+    str = document.crcform.hexstring.value.toString();
+    var prefix = crc.calculateInitFunc(str);
+    document.getElementById("initcrc").textContent =
+                    prefix.toString(16).toUpperCase().padStart(4, "0");
+}
+var crc = (function() {
+    applyCRC = function(crc, value) {
+        crc ^= value << 8;
+        crc &= 0xffff;
+        for (bit = 0; bit < 8; bit++) {
+            var olddata = crc & 0x8000;
+            crc <<= 1;
+            if (olddata) {
+                crc ^= 0x1021;
+            }
+            crc &= 0xffff;
+        }
+        return crc;
+    }
+    calcCRC = function(pkt, crc) {
+        return pkt.reduce(applyCRC, crc);
+    }
+    stringToBytes = function(s){
+        return s.split(/([a-fA-F0-9][a-fA-F0-9])/g).filter(p => !!p).map(c => parseInt(c, 16));
+    }
+    bytesToString = function(b) {
+        return b.map(c => c.toString(16).padStart(2,"0")).join("");
+    }
+    return {
+        calculateInitFunc : function (str) {
+            str = str.toUpperCase().replace(/\s/g,"");
+            var bytes = stringToBytes(str);
+            for (testvalue = 0; testvalue < 65536; testvalue+=1) {
+                if (calcCRC(bytes,testvalue) === 0) {
+                    return testvalue;
+                }
+            }
+        }
+    };
+})();
+//]]>
+</script>
+</head>
+<body>
+<h1>CRC Prefix Calculator</h1>
+<p>Enter a string of hex bytes in the box and hit the <b>Find CRC init</b> button searches for the initial CRC value which would make the calculated CRC equal to zero.  It uses the 0x1021 polynomial value.  If it cannot find such a value, it will leave the value unchanged.
+</p>
+<form name="crcform">
+<textarea name="hexstring" rows="5" cols="80">30FFFFFFFFFFFF50CF5DD9E2C0B80065F5D1A483F0FBBC6F01001E6C043B517E90B286
+</textarea>
+<br>
+<label>Initial CRC value (hex):</label>
+<label id="initcrc">5FD6</label>
+<br>
+<input name="submit" type="button" value="Find CRC init" onclick="calculateInit()">
+</form>
+</body></html>