100 lines
3.8 KiB
Java
100 lines
3.8 KiB
Java
package com.bytezone.diskbrowser.nib;
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// -----------------------------------------------------------------------------------//
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public class DiskReaderGCR extends DiskReader
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// -----------------------------------------------------------------------------------//
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{
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static final int TAG_SIZE = 12;
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private final ByteTranslator byteTranslator = new ByteTranslator6and2 ();
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// ---------------------------------------------------------------------------------//
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DiskReaderGCR ()
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// ---------------------------------------------------------------------------------//
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{
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super (0);
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}
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// ---------------------------------------------------------------------------------//
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@Override
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byte[] decodeSector (byte[] inBuffer, int inPtr) throws DiskNibbleException
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// ---------------------------------------------------------------------------------//
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{
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byte[] outBuffer = new byte[BLOCK_SIZE + TAG_SIZE]; // 524 bytes
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int outPtr = 0;
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int[] checksums = new int[3];
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// decode four disk bytes into three data bytes (174 * 3 + 2 = 524)
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while (true)
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{
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// ROL first checksum
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checksums[0] = (checksums[0] & 0xFF) << 1; // shift left
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if ((checksums[0] > 0xFF)) // check for overflow
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++checksums[0]; // set bit 0
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// 6&2 translation
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byte d3 = byteTranslator.decode (inBuffer[inPtr++]); // composite byte
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byte d0 = byteTranslator.decode (inBuffer[inPtr++]);
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byte d1 = byteTranslator.decode (inBuffer[inPtr++]);
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// reassemble bytes
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byte b0 = (byte) (d0 | ((d3 & 0x30) << 2));
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byte b1 = (byte) (d1 | ((d3 & 0x0C) << 4));
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// calculate running checksums
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outBuffer[outPtr++] = checksum (b0, checksums, 0, 2);
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outBuffer[outPtr++] = checksum (b1, checksums, 2, 1);
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if (outPtr == outBuffer.length)
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break;
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byte d2 = byteTranslator.decode (inBuffer[inPtr++]); // translate
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byte b2 = (byte) (d2 | (d3 << 6)); // reassemble
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outBuffer[outPtr++] = checksum (b2, checksums, 1, 0); // checksum
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}
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// decode four disk bytes into three checksum bytes
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byte d3 = byteTranslator.decode (inBuffer[inPtr++]); // composite byte
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byte d0 = byteTranslator.decode (inBuffer[inPtr++]);
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byte d1 = byteTranslator.decode (inBuffer[inPtr++]);
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byte d2 = byteTranslator.decode (inBuffer[inPtr++]);
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// reassemble bytes
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byte b0 = (byte) ((d0 & 0x3F) | ((d3 & 0x30) << 2));
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byte b1 = (byte) ((d1 & 0x3F) | ((d3 & 0x0C) << 4));
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byte b2 = (byte) ((d2 & 0x3F) | ((d3 & 0x03) << 6));
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// compare disk checksums with calculated checksums
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if ((checksums[0] & 0xFF) != (b2 & 0xFF) //
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|| (checksums[1] & 0xFF) != (b1 & 0xFF) //
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|| (checksums[2] & 0xFF) != (b0 & 0xFF))
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throw new DiskNibbleException ("Checksum failed");
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return outBuffer;
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}
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// ---------------------------------------------------------------------------------//
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private byte checksum (byte diskByte, int[] checksums, int current, int next)
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// ---------------------------------------------------------------------------------//
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{
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int val = (diskByte ^ checksums[current]) & 0xFF;
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checksums[next] += val; // prepare next checksum
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if (checksums[current] > 0xFF) // is there a carry?
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{
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++checksums[next]; // pass it on
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checksums[current] &= 0xFF; // back to 8 bits
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}
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return (byte) val; // converted data byte
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}
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// ---------------------------------------------------------------------------------//
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@Override
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byte[] encodeSector (byte[] buffer)
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// ---------------------------------------------------------------------------------//
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{
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System.out.println ("encodeSector() not written");
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return null;
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}
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}
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