ReedSolomonDecoder::decode() - Code Metrics - Inspection of ":shower: some docblocks and typos" - chillerlan/php-qrcode - Measure and Improve Code Quality continuously with Scrutinizer

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by smiley

created 2023-01-31 22:55 UTC

ReedSolomonDecoder::decode() A

↳ Parent: ReedSolomonDecoder

Complexity

Conditions	3
Paths	3

Size

Total Lines	15
Code Lines	8

Duplication

Lines	0
Ratio	0 %

Importance

Changes	2
Bugs	0	Features	0

Metric	Value
cc	3
eloc	8
c	2
b	0
f	0
nc	3
nop	1
dl	0
loc	15
rs	10

<?php
/**
 * Class ReedSolomonDecoder
 *
 * @created      24.01.2021
 * @author       ZXing Authors
 * @author       Smiley <[email protected]>
 * @copyright    2021 Smiley
 * @license      Apache-2.0
 */

namespace chillerlan\QRCode\Common;

use chillerlan\QRCode\QRCodeException;
use function array_fill, array_reverse, count;

/**
 * Implements Reed-Solomon decoding, as the name implies.
 *
 * The algorithm will not be explained here, but the following references were helpful
 * in creating this implementation:
 *
 * - Bruce Maggs "Decoding Reed-Solomon Codes" (see discussion of Forney's Formula)
 *   http://www.cs.cmu.edu/afs/cs.cmu.edu/project/pscico-guyb/realworld/www/rs_decode.ps
 * - J.I. Hall. "Chapter 5. Generalized Reed-Solomon Codes" (see discussion of Euclidean algorithm)
 *   https://users.math.msu.edu/users/halljo/classes/codenotes/GRS.pdf
 *
 * Much credit is due to William Rucklidge since portions of this code are an indirect
 * port of his C++ Reed-Solomon implementation.
 *
 * @author Sean Owen
 * @author William Rucklidge
 * @author sanfordsquires
 */
final class ReedSolomonDecoder{

	private Version  $version;
	private EccLevel $eccLevel;

	/**
	 * ReedSolomonDecoder constructor
	 */
	public function __construct(Version $version, EccLevel $eccLevel){
		$this->version  = $version;
		$this->eccLevel = $eccLevel;
	}

	/**
	 * Error-correct and copy data blocks together into a stream of bytes
	 */
	public function decode(array $rawCodewords):BitBuffer{
		$dataBlocks  = $this->deinterleaveRawBytes($rawCodewords);
		$dataBytes   = [];

		foreach($dataBlocks as $dataBlock){
			[$numDataCodewords, $codewordBytes] = $dataBlock;

			$corrected = $this->correctErrors($codewordBytes, $numDataCodewords);

			for($i = 0; $i < $numDataCodewords; $i++){
				$dataBytes[] = $corrected[$i];
			}
		}

		return new BitBuffer($dataBytes);
	}

	/**
	 * When QR Codes use multiple data blocks, they are actually interleaved.
	 * That is, the first byte of data block 1 to n is written, then the second bytes, and so on. This
	 * method will separate the data into original blocks.
	 *
	 * @throws \chillerlan\QRCode\Decoder\QRCodeDecoderException
	 */
	private function deinterleaveRawBytes(array $rawCodewords):array{
		// Figure out the number and size of data blocks used by this version and
		// error correction level
		[$numEccCodewords, $eccBlocks] = $this->version->getRSBlocks($this->eccLevel);

		// Now establish DataBlocks of the appropriate size and number of data codewords
		$result          = [];//new DataBlock[$totalBlocks];
		$numResultBlocks = 0;

		foreach($eccBlocks as $blockData){
			[$numEccBlocks, $eccPerBlock] = $blockData;

			for($i = 0; $i < $numEccBlocks; $i++, $numResultBlocks++){
				$result[$numResultBlocks] = [$eccPerBlock, array_fill(0, $numEccCodewords + $eccPerBlock, 0)];
			}
		}

		// All blocks have the same amount of data, except that the last n
		// (where n may be 0) have 1 more byte. Figure out where these start.
		/** @phan-suppress-next-line PhanTypePossiblyInvalidDimOffset */
		$shorterBlocksTotalCodewords = count($result[0][1]);
		$longerBlocksStartAt         = count($result) - 1;

		while($longerBlocksStartAt >= 0){
			$numCodewords = count($result[$longerBlocksStartAt][1]);

			if($numCodewords == $shorterBlocksTotalCodewords){
				break;
			}

			$longerBlocksStartAt--;
		}

		$longerBlocksStartAt++;

		$shorterBlocksNumDataCodewords = $shorterBlocksTotalCodewords - $numEccCodewords;
		// The last elements of result may be 1 element longer;
		// first fill out as many elements as all of them have
		$rawCodewordsOffset = 0;

		for($i = 0; $i < $shorterBlocksNumDataCodewords; $i++){
			for($j = 0; $j < $numResultBlocks; $j++){
				$result[$j][1][$i] = $rawCodewords[$rawCodewordsOffset++];
			}
		}

		// Fill out the last data block in the longer ones
		for($j = $longerBlocksStartAt; $j < $numResultBlocks; $j++){
			$result[$j][1][$shorterBlocksNumDataCodewords] = $rawCodewords[$rawCodewordsOffset++];
		}

		// Now add in error correction blocks
		/** @phan-suppress-next-line PhanTypePossiblyInvalidDimOffset */
		$max = count($result[0][1]);

		for($i = $shorterBlocksNumDataCodewords; $i < $max; $i++){
			for($j = 0; $j < $numResultBlocks; $j++){
				$iOffset                 = $j < $longerBlocksStartAt ? $i : $i + 1;
				$result[$j][1][$iOffset] = $rawCodewords[$rawCodewordsOffset++];
			}
		}

		// DataBlocks containing original bytes, "de-interleaved" from representation in the QR Code
		return $result;
	}

	/**
	 * Given data and error-correction codewords received, possibly corrupted by errors, attempts to
	 * correct the errors in-place using Reed-Solomon error correction.
	 */
	private function correctErrors(array $codewordBytes, int $numDataCodewords):array{
		// First read into an array of ints
		$codewordsInts = [];

		foreach($codewordBytes as $codewordByte){
			$codewordsInts[] = $codewordByte & 0xFF;
		}

		$decoded = $this->decodeWords($codewordsInts, (count($codewordBytes) - $numDataCodewords));

		// Copy back into array of bytes -- only need to worry about the bytes that were data
		// We don't care about errors in the error-correction codewords
		for($i = 0; $i < $numDataCodewords; $i++){
			$codewordBytes[$i] = $decoded[$i];
		}

		return $codewordBytes;
	}

	/**
	 * Decodes given set of received codewords, which include both data and error-correction
	 * codewords. Really, this means it uses Reed-Solomon to detect and correct errors, in-place,
	 * in the input.
	 *
	 * @param array $received        data and error-correction codewords
	 * @param int   $numEccCodewords number of error-correction codewords available
	 *
	 * @return int[]
	 * @throws \chillerlan\QRCode\QRCodeException if decoding fails for any reason
	 */
	private function decodeWords(array $received, int $numEccCodewords):array{
		$poly                 = new GenericGFPoly($received);
		$syndromeCoefficients = [];
		$error                = false;

		for($i = 0; $i < $numEccCodewords; $i++){
			$syndromeCoefficients[$i] = $poly->evaluateAt(GF256::exp($i));

			if($syndromeCoefficients[$i] !== 0){
				$error = true;
			}
		}

		if(!$error){

			return $received;
		}

		[$sigma, $omega] = $this->runEuclideanAlgorithm(
			GF256::buildMonomial($numEccCodewords, 1),
			new GenericGFPoly(array_reverse($syndromeCoefficients)),
			$numEccCodewords
		);

		$errorLocations      = $this->findErrorLocations($sigma);
		$errorMagnitudes     = $this->findErrorMagnitudes($omega, $errorLocations);
		$errorLocationsCount = count($errorLocations);
		$receivedCount       = count($received);

		for($i = 0; $i < $errorLocationsCount; $i++){
			$position = $receivedCount - 1 - GF256::log($errorLocations[$i]);

			if($position < 0){
				throw new QRCodeException('Bad error location');
			}

			$received[$position] ^= $errorMagnitudes[$i];
		}

		return $received;
	}

	/**
	 * @return \chillerlan\QRCode\Common\GenericGFPoly[] [sigma, omega]
	 * @throws \chillerlan\QRCode\QRCodeException
	 */
	private function runEuclideanAlgorithm(GenericGFPoly $a, GenericGFPoly $b, int $R):array{
		// Assume a's degree is >= b's
		if($a->getDegree() < $b->getDegree()){
			$temp = $a;
			$a    = $b;
			$b    = $temp;
		}

		$rLast = $a;
		$r     = $b;
		$tLast = new GenericGFPoly([0]);
		$t     = new GenericGFPoly([1]);

		// Run Euclidean algorithm until r's degree is less than R/2
		while(2 * $r->getDegree() >= $R){
			$rLastLast = $rLast;
			$tLastLast = $tLast;
			$rLast     = $r;
			$tLast     = $t;

			// Divide rLastLast by rLast, with quotient in q and remainder in r
			[$q, $r] = $rLastLast->divide($rLast);

			$t = $q->multiply($tLast)->addOrSubtract($tLastLast);

			if($r->getDegree() >= $rLast->getDegree()){
				throw new QRCodeException('Division algorithm failed to reduce polynomial?');
			}
		}

		$sigmaTildeAtZero = $t->getCoefficient(0);

		if($sigmaTildeAtZero === 0){
			throw new QRCodeException('sigmaTilde(0) was zero');
		}

		$inverse = GF256::inverse($sigmaTildeAtZero);

		return [$t->multiplyInt($inverse), $r->multiplyInt($inverse)];
	}

	/**
	 * @throws \chillerlan\QRCode\QRCodeException
	 */
	private function findErrorLocations(GenericGFPoly $errorLocator):array{
		// This is a direct application of Chien's search
		$numErrors = $errorLocator->getDegree();

		if($numErrors === 1){ // shortcut
			return [$errorLocator->getCoefficient(1)];
		}

		$result = array_fill(0, $numErrors, 0);
		$e      = 0;

		for($i = 1; $i < 256 && $e < $numErrors; $i++){
			if($errorLocator->evaluateAt($i) === 0){
				$result[$e] = GF256::inverse($i);
				$e++;
			}
		}

		if($e !== $numErrors){
			throw new QRCodeException('Error locator degree does not match number of roots');
		}

		return $result;
	}

	/**
	 *
	 */
	private function findErrorMagnitudes(GenericGFPoly $errorEvaluator, array $errorLocations):array{
		// This is directly applying Forney's Formula
		$s      = count($errorLocations);
		$result = [];

		for($i = 0; $i < $s; $i++){
			$xiInverse   = GF256::inverse($errorLocations[$i]);
			$denominator = 1;

			for($j = 0; $j < $s; $j++){
				if($i !== $j){
#					$denominator = GF256::multiply($denominator, GF256::addOrSubtract(1, GF256::multiply($errorLocations[$j], $xiInverse)));
					// Above should work but fails on some Apple and Linux JDKs due to a Hotspot bug.
					// Below is a funny-looking workaround from Steven Parkes
					$term        = GF256::multiply($errorLocations[$j], $xiInverse);
					$denominator = GF256::multiply($denominator, (($term & 0x1) === 0 ? $term | 1 : $term & ~1));
				}
			}

			$result[$i] = GF256::multiply($errorEvaluator->evaluateAt($xiInverse), GF256::inverse($denominator));
		}

		return $result;
	}

}


1			<?php
2			/**
3			* Class ReedSolomonDecoder
4			*
5			* @created 24.01.2021
6			* @author ZXing Authors
7			* @author Smiley <[email protected]>
8			* @copyright 2021 Smiley
9			* @license Apache-2.0
10			*/
11
12			namespace chillerlan\QRCode\Common;
13
14			use chillerlan\QRCode\QRCodeException;
15			use function array_fill, array_reverse, count;
16
17			/**
18			* Implements Reed-Solomon decoding, as the name implies.
19			*
20			* The algorithm will not be explained here, but the following references were helpful
21			* in creating this implementation:
22			*
23			* - Bruce Maggs "Decoding Reed-Solomon Codes" (see discussion of Forney's Formula)
24			* http://www.cs.cmu.edu/afs/cs.cmu.edu/project/pscico-guyb/realworld/www/rs_decode.ps
25			* - J.I. Hall. "Chapter 5. Generalized Reed-Solomon Codes" (see discussion of Euclidean algorithm)
26			* https://users.math.msu.edu/users/halljo/classes/codenotes/GRS.pdf
27			*
28			* Much credit is due to William Rucklidge since portions of this code are an indirect
29			* port of his C++ Reed-Solomon implementation.
30			*
31			* @author Sean Owen
32			* @author William Rucklidge
33			* @author sanfordsquires
34			*/
35			final class ReedSolomonDecoder{
36
37			private Version $version;
38			private EccLevel $eccLevel;
39
40			/**
41			* ReedSolomonDecoder constructor
42			*/
43			public function __construct(Version $version, EccLevel $eccLevel){
44			$this->version = $version;
45			$this->eccLevel = $eccLevel;
46			}
47
48			/**
49			* Error-correct and copy data blocks together into a stream of bytes
50			*/
51			public function decode(array $rawCodewords):BitBuffer{
52			$dataBlocks = $this->deinterleaveRawBytes($rawCodewords);
53			$dataBytes = [];
54
55			foreach($dataBlocks as $dataBlock){
56			[$numDataCodewords, $codewordBytes] = $dataBlock;
57
58			$corrected = $this->correctErrors($codewordBytes, $numDataCodewords);
59
60			for($i = 0; $i < $numDataCodewords; $i++){
61			$dataBytes[] = $corrected[$i];
62			}
63			}
64
65			return new BitBuffer($dataBytes);
66			}
67
68			/**
69			* When QR Codes use multiple data blocks, they are actually interleaved.
70			* That is, the first byte of data block 1 to n is written, then the second bytes, and so on. This
71			* method will separate the data into original blocks.
72			*
73			* @throws \chillerlan\QRCode\Decoder\QRCodeDecoderException
74			*/
75			private function deinterleaveRawBytes(array $rawCodewords):array{
76			// Figure out the number and size of data blocks used by this version and
77			// error correction level
78			[$numEccCodewords, $eccBlocks] = $this->version->getRSBlocks($this->eccLevel);
79
80			// Now establish DataBlocks of the appropriate size and number of data codewords
81			$result = [];//new DataBlock[$totalBlocks];
82			$numResultBlocks = 0;
83
84			foreach($eccBlocks as $blockData){
85			[$numEccBlocks, $eccPerBlock] = $blockData;
86
87			for($i = 0; $i < $numEccBlocks; $i++, $numResultBlocks++){
88			$result[$numResultBlocks] = [$eccPerBlock, array_fill(0, $numEccCodewords + $eccPerBlock, 0)];
89			}
90			}
91
92			// All blocks have the same amount of data, except that the last n
93			// (where n may be 0) have 1 more byte. Figure out where these start.
94			/** @phan-suppress-next-line PhanTypePossiblyInvalidDimOffset */
95			$shorterBlocksTotalCodewords = count($result[0][1]);
96			$longerBlocksStartAt = count($result) - 1;
97
98			while($longerBlocksStartAt >= 0){
99			$numCodewords = count($result[$longerBlocksStartAt][1]);
100
101			if($numCodewords == $shorterBlocksTotalCodewords){
102			break;
103			}
104
105			$longerBlocksStartAt--;
106			}
107
108			$longerBlocksStartAt++;
109
110			$shorterBlocksNumDataCodewords = $shorterBlocksTotalCodewords - $numEccCodewords;
111			// The last elements of result may be 1 element longer;
112			// first fill out as many elements as all of them have
113			$rawCodewordsOffset = 0;
114
115			for($i = 0; $i < $shorterBlocksNumDataCodewords; $i++){
116			for($j = 0; $j < $numResultBlocks; $j++){
117			$result[$j][1][$i] = $rawCodewords[$rawCodewordsOffset++];
118			}
119			}
120
121			// Fill out the last data block in the longer ones
122			for($j = $longerBlocksStartAt; $j < $numResultBlocks; $j++){
123			$result[$j][1][$shorterBlocksNumDataCodewords] = $rawCodewords[$rawCodewordsOffset++];
124			}
125
126			// Now add in error correction blocks
127			/** @phan-suppress-next-line PhanTypePossiblyInvalidDimOffset */
128			$max = count($result[0][1]);
129
130			for($i = $shorterBlocksNumDataCodewords; $i < $max; $i++){
131			for($j = 0; $j < $numResultBlocks; $j++){
132			$iOffset = $j < $longerBlocksStartAt ? $i : $i + 1;
133			$result[$j][1][$iOffset] = $rawCodewords[$rawCodewordsOffset++];
134			}
135			}
136
137			// DataBlocks containing original bytes, "de-interleaved" from representation in the QR Code
138			return $result;
139			}
140
141			/**
142			* Given data and error-correction codewords received, possibly corrupted by errors, attempts to
143			* correct the errors in-place using Reed-Solomon error correction.
144			*/
145			private function correctErrors(array $codewordBytes, int $numDataCodewords):array{
146			// First read into an array of ints
147			$codewordsInts = [];
148
149			foreach($codewordBytes as $codewordByte){
150			$codewordsInts[] = $codewordByte & 0xFF;
151			}
152
153			$decoded = $this->decodeWords($codewordsInts, (count($codewordBytes) - $numDataCodewords));
154
155			// Copy back into array of bytes -- only need to worry about the bytes that were data
156			// We don't care about errors in the error-correction codewords
157			for($i = 0; $i < $numDataCodewords; $i++){
158			$codewordBytes[$i] = $decoded[$i];
159			}
160
161			return $codewordBytes;
162			}
163
164			/**
165			* Decodes given set of received codewords, which include both data and error-correction
166			* codewords. Really, this means it uses Reed-Solomon to detect and correct errors, in-place,
167			* in the input.
168			*
169			* @param array $received data and error-correction codewords
170			* @param int $numEccCodewords number of error-correction codewords available
171			*
172			* @return int[]
173			* @throws \chillerlan\QRCode\QRCodeException if decoding fails for any reason
174			*/
175			private function decodeWords(array $received, int $numEccCodewords):array{
176			$poly = new GenericGFPoly($received);
177			$syndromeCoefficients = [];
178			$error = false;
179
180			for($i = 0; $i < $numEccCodewords; $i++){
181			$syndromeCoefficients[$i] = $poly->evaluateAt(GF256::exp($i));
182
183			if($syndromeCoefficients[$i] !== 0){
184			$error = true;
185			}
186			}
187
188			if(!$error){
			0 ignored issues – show introduced 2022-05-10 12:59 UTC by Report Bug Copy Issue Report The condition `$error` is always `false`. Loading history...
189			return $received;
190			}
191
192			[$sigma, $omega] = $this->runEuclideanAlgorithm(
193			GF256::buildMonomial($numEccCodewords, 1),
194			new GenericGFPoly(array_reverse($syndromeCoefficients)),
195			$numEccCodewords
196			);
197
198			$errorLocations = $this->findErrorLocations($sigma);
199			$errorMagnitudes = $this->findErrorMagnitudes($omega, $errorLocations);
200			$errorLocationsCount = count($errorLocations);
201			$receivedCount = count($received);
202
203			for($i = 0; $i < $errorLocationsCount; $i++){
204			$position = $receivedCount - 1 - GF256::log($errorLocations[$i]);
205
206			if($position < 0){
207			throw new QRCodeException('Bad error location');
208			}
209
210			$received[$position] ^= $errorMagnitudes[$i];
211			}
212
213			return $received;
214			}
215
216			/**
217			* @return \chillerlan\QRCode\Common\GenericGFPoly[] [sigma, omega]
218			* @throws \chillerlan\QRCode\QRCodeException
219			*/
220			private function runEuclideanAlgorithm(GenericGFPoly $a, GenericGFPoly $b, int $R):array{
221			// Assume a's degree is >= b's
222			if($a->getDegree() < $b->getDegree()){
223			$temp = $a;
224			$a = $b;
225			$b = $temp;
226			}
227
228			$rLast = $a;
229			$r = $b;
230			$tLast = new GenericGFPoly([0]);
231			$t = new GenericGFPoly([1]);
232
233			// Run Euclidean algorithm until r's degree is less than R/2
234			while(2 * $r->getDegree() >= $R){
235			$rLastLast = $rLast;
236			$tLastLast = $tLast;
237			$rLast = $r;
238			$tLast = $t;
239
240			// Divide rLastLast by rLast, with quotient in q and remainder in r
241			[$q, $r] = $rLastLast->divide($rLast);
242
243			$t = $q->multiply($tLast)->addOrSubtract($tLastLast);
244
245			if($r->getDegree() >= $rLast->getDegree()){
246			throw new QRCodeException('Division algorithm failed to reduce polynomial?');
247			}
248			}
249
250			$sigmaTildeAtZero = $t->getCoefficient(0);
251
252			if($sigmaTildeAtZero === 0){
253			throw new QRCodeException('sigmaTilde(0) was zero');
254			}
255
256			$inverse = GF256::inverse($sigmaTildeAtZero);
257
258			return [$t->multiplyInt($inverse), $r->multiplyInt($inverse)];
259			}
260
261			/**
262			* @throws \chillerlan\QRCode\QRCodeException
263			*/
264			private function findErrorLocations(GenericGFPoly $errorLocator):array{
265			// This is a direct application of Chien's search
266			$numErrors = $errorLocator->getDegree();
267
268			if($numErrors === 1){ // shortcut
269			return [$errorLocator->getCoefficient(1)];
270			}
271
272			$result = array_fill(0, $numErrors, 0);
273			$e = 0;
274
275			for($i = 1; $i < 256 && $e < $numErrors; $i++){
276			if($errorLocator->evaluateAt($i) === 0){
277			$result[$e] = GF256::inverse($i);
278			$e++;
279			}
280			}
281
282			if($e !== $numErrors){
283			throw new QRCodeException('Error locator degree does not match number of roots');
284			}
285
286			return $result;
287			}
288
289			/**
290			*
291			*/
292			private function findErrorMagnitudes(GenericGFPoly $errorEvaluator, array $errorLocations):array{
293			// This is directly applying Forney's Formula
294			$s = count($errorLocations);
295			$result = [];
296
297			for($i = 0; $i < $s; $i++){
298			$xiInverse = GF256::inverse($errorLocations[$i]);
299			$denominator = 1;
300
301			for($j = 0; $j < $s; $j++){
302			if($i !== $j){
303			# $denominator = GF256::multiply($denominator, GF256::addOrSubtract(1, GF256::multiply($errorLocations[$j], $xiInverse)));
304			// Above should work but fails on some Apple and Linux JDKs due to a Hotspot bug.
305			// Below is a funny-looking workaround from Steven Parkes
306			$term = GF256::multiply($errorLocations[$j], $xiInverse);
307			$denominator = GF256::multiply($denominator, (($term & 0x1) === 0 ? $term \| 1 : $term & ~1));
308			}
309			}
310
311			$result[$i] = GF256::multiply($errorEvaluator->evaluateAt($xiInverse), GF256::inverse($denominator));
312			}
313
314			return $result;
315			}
316
317			}
318

chillerlan / php-qrcode

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ReedSolomonDecoder::decode() A

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Duplication

Importance

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