1 files changed, 143 insertions, 0 deletions
diff --git a/src/zcrc.js b/src/zcrc.js
new file mode 100644
index 0000000..831f835
--- /dev/null
+++ b/src/zcrc.js
@@ -0,0 +1,143 @@
+"use strict";
+
+const CRC32_MOD = require('crc-32');
+
+var Zmodem = module.exports;
+
+Object.assign(
+    Zmodem,
+    require("./zerror"),
+    require("./encode")
+);
+
+//----------------------------------------------------------------------
+// BEGIN adapted from crc-js by Johannes Rudolph
+
+var _crctab;
+
+const
+    crc_width = 16,
+    crc_polynomial = 0x1021,
+    crc_castmask = 0xffff,
+    crc_msbmask = 1 << (crc_width - 1)
+;
+
+function _compute_crctab() {
+    _crctab = new Array(256);
+
+    var divident_shift = crc_width - 8;
+
+    for (var divident = 0; divident < 256; divident++) {
+        var currByte = (divident << divident_shift) & crc_castmask;
+
+        for (var bit = 0; bit < 8; bit++) {
+
+            if ((currByte & crc_msbmask) !== 0) {
+                currByte <<= 1;
+                currByte ^= crc_polynomial;
+            }
+            else {
+                currByte <<= 1;
+            }
+        }
+
+        _crctab[divident] = (currByte & crc_castmask);
+    }
+}
+
+// END adapted from crc-js by Johannes Rudolph
+//----------------------------------------------------------------------
+
+function _updcrc(cp, crc) {
+    if (!_crctab) _compute_crctab();
+
+    return(
+        _crctab[((crc >> 8) & 255)]
+        ^ ((255 & crc) << 8)
+        ^ cp
+    );
+}
+
+function __verify(expect, got) {
+    var err;
+
+    if ( expect.join() !== got.join() ) {
+        throw new Zmodem.Error("crc", got, expect);
+    }
+}
+
+//TODO: use external implementation(s)
+Zmodem.CRC = {
+
+    //https://www.lammertbies.nl/comm/info/crc-calculation.html
+    //CRC-CCITT (XModem)
+
+    /**
+     * Deduce a given set of octet values’ CRC16, as per the CRC16
+     * variant that ZMODEM uses (CRC-CCITT/XModem).
+     *
+     * @param {Array} octets - The array of octet values.
+     *      Each array member should be an 8-bit unsigned integer (0-255).
+     *
+     * @returns {Array} crc - The CRC, expressed as an array of octet values.
+     */
+    crc16: function crc16(octet_nums) {
+        var crc = octet_nums[0];
+        for (var b=1; b<octet_nums.length; b++) {
+            crc = _updcrc( octet_nums[b], crc );
+        }
+
+        crc = _updcrc( 0, _updcrc(0, crc) );
+
+        //a big-endian 2-byte sequence
+        return Zmodem.ENCODELIB.pack_u16_be(crc);
+    },
+
+    /**
+     * Deduce a given set of octet values’ CRC32.
+     *
+     * @param {Array} octets - The array of octet values.
+     *      Each array member should be an 8-bit unsigned integer (0-255).
+     *
+     * @returns {Array} crc - The CRC, expressed as an array of octet values.
+     */
+    crc32: function crc32(octet_nums) {
+        return Zmodem.ENCODELIB.pack_u32_le(
+            CRC32_MOD.buf(octet_nums) >>> 0     //bit-shift to get unsigned
+        );
+    },
+
+    /**
+     * Verify a given set of octet values’ CRC16.
+     * An exception is thrown on failure.
+     *
+     * @param {Array} bytes_arr - The array of octet values.
+     *      Each array member should be an 8-bit unsigned integer (0-255).
+     *
+     * @param {Array} crc - The CRC to check against, expressed as
+     *      an array of octet values.
+     */
+    verify16: function verify16(bytes_arr, got) {
+        return __verify( this.crc16(bytes_arr), got );
+    },
+
+    /**
+     * Verify a given set of octet values’ CRC32.
+     * An exception is thrown on failure.
+     *
+     * @param {Array} bytes_arr - The array of octet values.
+     *      Each array member should be an 8-bit unsigned integer (0-255).
+     *
+     * @param {Array} crc - The CRC to check against, expressed as
+     *      an array of octet values.
+     */
+    verify32: function verify32(bytes_arr, crc) {
+        try {
+            __verify( this.crc32(bytes_arr), crc );
+        }
+        catch(err) {
+            err.input = bytes_arr.slice(0);
+            throw err;
+        }
+    },
+};