This class covers the QR Code Model 2 specification, supporting all versions (sizes) * from 1 to 40, all 4 error correction levels, and 4 character encoding modes.
*/ public final class QrCode { /*---- Static factory functions (high level) ----*/ /** * Returns a QR Code symbol representing the specified Unicode text string at the specified error correction level. * As a conservative upper bound, this function is guaranteed to succeed for strings that have 738 or fewer * Unicode code points (not UTF-16 code units) if the low error correction level is used. The smallest possible * QR Code version is automatically chosen for the output. The ECC level of the result may be higher than the * ecl argument if it can be done without increasing the version. * @param text the text to be encoded, which can be any Unicode string * @param ecl the error correction level to use (will be boosted) * @return a QR Code representing the text * @throws NullPointerException if the text or error correction level is {@code null} * @throws IllegalArgumentException if the text fails to fit in the * largest version QR Code at the ECL, which means it is too long */ public static QrCode encodeText(String text, Ecc ecl) { Objects.requireNonNull(text); Objects.requireNonNull(ecl); ListThis function allows the user to create a custom sequence of segments that switches * between modes (such as alphanumeric and binary) to encode text more efficiently. * This function is considered to be lower level than simply encoding text or binary data.
* @param segs the segments to encode * @param ecl the error correction level to use (will be boosted) * @return a QR Code representing the segments * @throws NullPointerException if the list of segments, any segment, or the error correction level is {@code null} * @throws IllegalArgumentException if the segments fail to fit in the * largest version QR Code at the ECL, which means they are too long */ public static QrCode encodeSegments(ListThis function allows the user to create a custom sequence of segments that switches * between modes (such as alphanumeric and binary) to encode text more efficiently. * This function is considered to be lower level than simply encoding text or binary data.
* @param segs the segments to encode * @param ecl the error correction level to use (may be boosted) * @param minVersion the minimum allowed version of the QR symbol (at least 1) * @param maxVersion the maximum allowed version of the QR symbol (at most 40) * @param mask the mask pattern to use, which is either -1 for automatic choice or from 0 to 7 for fixed choice * @param boostEcl increases the error correction level if it can be done without increasing the version number * @return a QR Code representing the segments * @throws NullPointerException if the list of segments, any segment, or the error correction level is {@code null} * @throws IllegalArgumentException if 1 ≤ minVersion ≤ maxVersion ≤ 40 * is violated, or if mask < −1 or mask > 7, or if the segments fail * to fit in the maxVersion QR Code at the ECL, which means they are too long */ public static QrCode encodeSegments(ListThis is a cumbersome low-level constructor that should not be invoked directly by the user. * To go one level up, see the {@link #encodeSegments(List,Ecc,int,int,int,boolean)} function.
* @param ver the version number to use, which must be in the range 1 to 40 (inclusive) * @param ecl the error correction level to use * @param dataCodewords the bytes representing segments to encode (without ECC) * @param mask the mask pattern to use, which is either −1 for automatic choice or from 0 to 7 for fixed choice * @throws NullPointerException if the byte array or error correction level is {@code null} * @throws IllegalArgumentException if the version or mask value is out of range, * or if the data is the wrong length for the specified version and error correction level */ public QrCode(int ver, Ecc ecl, byte[] dataCodewords, int mask) { // Check arguments and initialize fields if (ver < MIN_VERSION || ver > MAX_VERSION) throw new IllegalArgumentException("Version value out of range"); if (mask < -1 || mask > 7) throw new IllegalArgumentException("Mask value out of range"); version = ver; size = ver * 4 + 17; errorCorrectionLevel = Objects.requireNonNull(ecl); Objects.requireNonNull(dataCodewords); modules = new boolean[size][size]; // Initially all white isFunction = new boolean[size][size]; // Compute ECC, draw modules, do masking drawFunctionPatterns(); byte[] allCodewords = addEccAndInterleave(dataCodewords); drawCodewords(allCodewords); this.mask = handleConstructorMasking(mask); isFunction = null; } /*---- Public instance methods ----*/ /** * Returns the color of the module (pixel) at the specified coordinates, which is either * false for white or true for black. The top left corner has the coordinates (x=0, y=0). * If the specified coordinates are out of bounds, then false (white) is returned. * @param x the x coordinate, where 0 is the left edge and size−1 is the right edge * @param y the y coordinate, where 0 is the top edge and size−1 is the bottom edge * @return the module's color, which is either false (white) or true (black) */ public boolean getModule(int x, int y) { return 0 <= x && x < size && 0 <= y && y < size && modules[y][x]; } /** * Returns a new image object representing this QR Code, with the specified module scale and number * of border modules. For example, the arguments scale=10, border=4 means to pad the QR Code symbol * with 4 white border modules on all four edges, then use 10*10 pixels to represent each module. * The resulting image only contains the hex colors 000000 and FFFFFF. * @param scale the module scale factor, which must be positive * @param border the number of border modules to add, which must be non-negative * @return an image representing this QR Code, with padding and scaling * @throws IllegalArgumentException if the scale or border is out of range, or if * {scale, border, size} cause the image dimensions to exceed Integer.MAX_VALUE */ public BufferedImage toImage(int scale, int border) { if (scale <= 0 || border < 0) throw new IllegalArgumentException("Value out of range"); if (border > Integer.MAX_VALUE / 2 || size + border * 2L > Integer.MAX_VALUE / scale) throw new IllegalArgumentException("Scale or border too large"); BufferedImage result = new BufferedImage((size + border * 2) * scale, (size + border * 2) * scale, BufferedImage.TYPE_INT_RGB); for (int y = 0; y < result.getHeight(); y++) { for (int x = 0; x < result.getWidth(); x++) { boolean color = getModule(x / scale - border, y / scale - border); result.setRGB(x, y, color ? 0x000000 : 0xFFFFFF); } } return result; } /** * Returns a string of SVG XML code representing an image of this QR Code symbol with the specified * number of border modules. Note that Unix newlines (\n) are always used, regardless of the platform. * @param border the number of border modules to add, which must be non-negative * @return a string representing this QR Code as an SVG document * @throws IllegalArgumentException if the border is negative */ public String toSvgString(int border) { if (border < 0) throw new IllegalArgumentException("Border must be non-negative"); long brd = border; StringBuilder sb = new StringBuilder() .append("\n") .append("\n") .append(String.format("\n") .toString(); } /*---- Private helper methods for constructor: Drawing function modules ----*/ // Reads this object's version field, and draws and marks all function modules. private void drawFunctionPatterns() { // Draw horizontal and vertical timing patterns for (int i = 0; i < size; i++) { setFunctionModule(6, i, i % 2 == 0); setFunctionModule(i, 6, i % 2 == 0); } // Draw 3 finder patterns (all corners except bottom right; overwrites some timing modules) drawFinderPattern(3, 3); drawFinderPattern(size - 4, 3); drawFinderPattern(3, size - 4); // Draw numerous alignment patterns int[] alignPatPos = getAlignmentPatternPositions(); int numAlign = alignPatPos.length; for (int i = 0; i < numAlign; i++) { for (int j = 0; j < numAlign; j++) { // Don't draw on the three finder corners if (!(i == 0 && j == 0 || i == 0 && j == numAlign - 1 || i == numAlign - 1 && j == 0)) drawAlignmentPattern(alignPatPos[i], alignPatPos[j]); } } // Draw configuration data drawFormatBits(0); // Dummy mask value; overwritten later in the constructor drawVersion(); } // Draws two copies of the format bits (with its own error correction code) // based on the given mask and this object's error correction level field. private void drawFormatBits(int mask) { // Calculate error correction code and pack bits int data = errorCorrectionLevel.formatBits << 3 | mask; // errCorrLvl is uint2, mask is uint3 int rem = data; for (int i = 0; i < 10; i++) rem = (rem << 1) ^ ((rem >>> 9) * 0x537); int bits = (data << 10 | rem) ^ 0x5412; // uint15 assert bits >>> 15 == 0; // Draw first copy for (int i = 0; i <= 5; i++) setFunctionModule(8, i, getBit(bits, i)); setFunctionModule(8, 7, getBit(bits, 6)); setFunctionModule(8, 8, getBit(bits, 7)); setFunctionModule(7, 8, getBit(bits, 8)); for (int i = 9; i < 15; i++) setFunctionModule(14 - i, 8, getBit(bits, i)); // Draw second copy for (int i = 0; i <= 7; i++) setFunctionModule(size - 1 - i, 8, getBit(bits, i)); for (int i = 8; i < 15; i++) setFunctionModule(8, size - 15 + i, getBit(bits, i)); setFunctionModule(8, size - 8, true); // Always black } // Draws two copies of the version bits (with its own error correction code), // based on this object's version field, iff 7 <= version <= 40. private void drawVersion() { if (version < 7) return; // Calculate error correction code and pack bits int rem = version; // version is uint6, in the range [7, 40] for (int i = 0; i < 12; i++) rem = (rem << 1) ^ ((rem >>> 11) * 0x1F25); int bits = version << 12 | rem; // uint18 assert bits >>> 18 == 0; // Draw two copies for (int i = 0; i < 18; i++) { boolean bit = getBit(bits, i); int a = size - 11 + i % 3; int b = i / 3; setFunctionModule(a, b, bit); setFunctionModule(b, a, bit); } } // Draws a 9*9 finder pattern including the border separator, // with the center module at (x, y). Modules can be out of bounds. private void drawFinderPattern(int x, int y) { for (int dy = -4; dy <= 4; dy++) { for (int dx = -4; dx <= 4; dx++) { int dist = Math.max(Math.abs(dx), Math.abs(dy)); // Chebyshev/infinity norm int xx = x + dx, yy = y + dy; if (0 <= xx && xx < size && 0 <= yy && yy < size) setFunctionModule(xx, yy, dist != 2 && dist != 4); } } } // Draws a 5*5 alignment pattern, with the center module // at (x, y). All modules must be in bounds. private void drawAlignmentPattern(int x, int y) { for (int dy = -2; dy <= 2; dy++) { for (int dx = -2; dx <= 2; dx++) setFunctionModule(x + dx, y + dy, Math.max(Math.abs(dx), Math.abs(dy)) != 1); } } // Sets the color of a module and marks it as a function module. // Only used by the constructor. Coordinates must be in bounds. private void setFunctionModule(int x, int y, boolean isBlack) { modules[y][x] = isBlack; isFunction[y][x] = true; } /*---- Private helper methods for constructor: Codewords and masking ----*/ // Returns a new byte string representing the given data with the appropriate error correction // codewords appended to it, based on this object's version and error correction level. private byte[] addEccAndInterleave(byte[] data) { Objects.requireNonNull(data); if (data.length != getNumDataCodewords(version, errorCorrectionLevel)) throw new IllegalArgumentException(); // Calculate parameter numbers int numBlocks = NUM_ERROR_CORRECTION_BLOCKS[errorCorrectionLevel.ordinal()][version]; int blockEccLen = ECC_CODEWORDS_PER_BLOCK [errorCorrectionLevel.ordinal()][version]; int rawCodewords = getNumRawDataModules(version) / 8; int numShortBlocks = numBlocks - rawCodewords % numBlocks; int shortBlockLen = rawCodewords / numBlocks; // Split data into blocks and append ECC to each block byte[][] blocks = new byte[numBlocks][]; ReedSolomonGenerator rs = new ReedSolomonGenerator(blockEccLen); for (int i = 0, k = 0; i < numBlocks; i++) { byte[] dat = Arrays.copyOfRange(data, k, k + shortBlockLen - blockEccLen + (i < numShortBlocks ? 0 : 1)); k += dat.length; byte[] block = Arrays.copyOf(dat, shortBlockLen + 1); byte[] ecc = rs.getRemainder(dat); System.arraycopy(ecc, 0, block, block.length - blockEccLen, ecc.length); blocks[i] = block; } // Interleave (not concatenate) the bytes from every block into a single sequence byte[] result = new byte[rawCodewords]; for (int i = 0, k = 0; i < blocks[0].length; i++) { for (int j = 0; j < blocks.length; j++) { // Skip the padding byte in short blocks if (i != shortBlockLen - blockEccLen || j >= numShortBlocks) { result[k] = blocks[j][i]; k++; } } } return result; } // Draws the given sequence of 8-bit codewords (data and error correction) onto the entire // data area of this QR Code symbol. Function modules need to be marked off before this is called. private void drawCodewords(byte[] data) { Objects.requireNonNull(data); if (data.length != getNumRawDataModules(version) / 8) throw new IllegalArgumentException(); int i = 0; // Bit index into the data // Do the funny zigzag scan for (int right = size - 1; right >= 1; right -= 2) { // Index of right column in each column pair if (right == 6) right = 5; for (int vert = 0; vert < size; vert++) { // Vertical counter for (int j = 0; j < 2; j++) { int x = right - j; // Actual x coordinate boolean upward = ((right + 1) & 2) == 0; int y = upward ? size - 1 - vert : vert; // Actual y coordinate if (!isFunction[y][x] && i < data.length * 8) { modules[y][x] = getBit(data[i >>> 3], 7 - (i & 7)); i++; } // If this QR Code has any remainder bits (0 to 7), they were assigned as // 0/false/white by the constructor and are left unchanged by this method } } } assert i == data.length * 8; } // XORs the codeword modules in this QR Code with the given mask pattern. // The function modules must be marked and the codeword bits must be drawn // before masking. Due to the arithmetic of XOR, calling applyMask() with // the same mask value a second time will undo the mask. A final well-formed // QR Code symbol needs exactly one (not zero, two, etc.) mask applied. private void applyMask(int mask) { if (mask < 0 || mask > 7) throw new IllegalArgumentException("Mask value out of range"); for (int y = 0; y < size; y++) { for (int x = 0; x < size; x++) { boolean invert; switch (mask) { case 0: invert = (x + y) % 2 == 0; break; case 1: invert = y % 2 == 0; break; case 2: invert = x % 3 == 0; break; case 3: invert = (x + y) % 3 == 0; break; case 4: invert = (x / 3 + y / 2) % 2 == 0; break; case 5: invert = x * y % 2 + x * y % 3 == 0; break; case 6: invert = (x * y % 2 + x * y % 3) % 2 == 0; break; case 7: invert = ((x + y) % 2 + x * y % 3) % 2 == 0; break; default: throw new AssertionError(); } modules[y][x] ^= invert & !isFunction[y][x]; } } } // A messy helper function for the constructor. This QR Code must be in an unmasked state when this // method is called. The given argument is the requested mask, which is -1 for auto or 0 to 7 for fixed. // This method applies and returns the actual mask chosen, from 0 to 7. private int handleConstructorMasking(int mask) { if (mask == -1) { // Automatically choose best mask int minPenalty = Integer.MAX_VALUE; for (int i = 0; i < 8; i++) { drawFormatBits(i); applyMask(i); int penalty = getPenaltyScore(); if (penalty < minPenalty) { mask = i; minPenalty = penalty; } applyMask(i); // Undoes the mask due to XOR } } assert 0 <= mask && mask <= 7; drawFormatBits(mask); // Overwrite old format bits applyMask(mask); // Apply the final choice of mask return mask; // The caller shall assign this value to the final-declared field } // Calculates and returns the penalty score based on state of this QR Code's current modules. // This is used by the automatic mask choice algorithm to find the mask pattern that yields the lowest score. private int getPenaltyScore() { int result = 0; // Adjacent modules in row having same color for (int y = 0; y < size; y++) { boolean colorX = false; for (int x = 0, runX = 0; x < size; x++) { if (x == 0 || modules[y][x] != colorX) { colorX = modules[y][x]; runX = 1; } else { runX++; if (runX == 5) result += PENALTY_N1; else if (runX > 5) result++; } } } // Adjacent modules in column having same color for (int x = 0; x < size; x++) { boolean colorY = false; for (int y = 0, runY = 0; y < size; y++) { if (y == 0 || modules[y][x] != colorY) { colorY = modules[y][x]; runY = 1; } else { runY++; if (runY == 5) result += PENALTY_N1; else if (runY > 5) result++; } } } // 2*2 blocks of modules having same color for (int y = 0; y < size - 1; y++) { for (int x = 0; x < size - 1; x++) { boolean color = modules[y][x]; if ( color == modules[y][x + 1] && color == modules[y + 1][x] && color == modules[y + 1][x + 1]) result += PENALTY_N2; } } // Finder-like pattern in rows for (int y = 0; y < size; y++) { for (int x = 0, bits = 0; x < size; x++) { bits = ((bits << 1) & 0b11111111111) | (modules[y][x] ? 1 : 0); if (x >= 10 && (bits == 0b00001011101 || bits == 0b10111010000)) // Needs 11 bits accumulated result += PENALTY_N3; } } // Finder-like pattern in columns for (int x = 0; x < size; x++) { for (int y = 0, bits = 0; y < size; y++) { bits = ((bits << 1) & 0b11111111111) | (modules[y][x] ? 1 : 0); if (y >= 10 && (bits == 0b00001011101 || bits == 0b10111010000)) // Needs 11 bits accumulated result += PENALTY_N3; } } // Balance of black and white modules int black = 0; for (boolean[] row : modules) { for (boolean color : row) { if (color) black++; } } int total = size * size; // Note that size is odd, so black/total != 1/2 // Compute the smallest integer k >= 0 such that (45-5k)% <= black/total <= (55+5k)% int k = (Math.abs(black * 20 - total * 10) + total - 1) / total - 1; result += k * PENALTY_N4; return result; } /*---- Private helper functions ----*/ // Returns an ascending list of positions of alignment patterns for this version number. // Each position is in the range [0,177), and are used on both the x and y axes. // This could be implemented as lookup table of 40 variable-length lists of unsigned bytes. private int[] getAlignmentPatternPositions() { if (version == 1) return new int[]{}; else { int numAlign = version / 7 + 2; int step; if (version == 32) // Special snowflake step = 26; else // step = ceil[(size - 13) / (numAlign*2 - 2)] * 2 step = (version*4 + numAlign*2 + 1) / (numAlign*2 - 2) * 2; int[] result = new int[numAlign]; result[0] = 6; for (int i = result.length - 1, pos = size - 7; i >= 1; i--, pos -= step) result[i] = pos; return result; } } // Returns the number of data bits that can be stored in a QR Code of the given version number, after // all function modules are excluded. This includes remainder bits, so it might not be a multiple of 8. // The result is in the range [208, 29648]. This could be implemented as a 40-entry lookup table. private static int getNumRawDataModules(int ver) { if (ver < MIN_VERSION || ver > MAX_VERSION) throw new IllegalArgumentException("Version number out of range"); int size = ver * 4 + 17; int result = size * size; // Number of modules in the whole QR symbol square result -= 8 * 8 * 3; // Subtract the three finders with separators result -= 15 * 2 + 1; // Subtract the format information and black module result -= (size - 16) * 2; // Subtract the timing patterns (excluding finders) // The five lines above are equivalent to: int result = (16 * ver + 128) * ver + 64; if (ver >= 2) { int numAlign = ver / 7 + 2; result -= (numAlign - 1) * (numAlign - 1) * 25; // Subtract alignment patterns not overlapping with timing patterns result -= (numAlign - 2) * 2 * 20; // Subtract alignment patterns that overlap with timing patterns // The two lines above are equivalent to: result -= (25 * numAlign - 10) * numAlign - 55; if (ver >= 7) result -= 6 * 3 * 2; // Subtract version information } return result; } // Returns the number of 8-bit data (i.e. not error correction) codewords contained in any // QR Code of the given version number and error correction level, with remainder bits discarded. // This stateless pure function could be implemented as a (40*4)-cell lookup table. static int getNumDataCodewords(int ver, Ecc ecl) { return getNumRawDataModules(ver) / 8 - ECC_CODEWORDS_PER_BLOCK [ecl.ordinal()][ver] * NUM_ERROR_CORRECTION_BLOCKS[ecl.ordinal()][ver]; } // Returns true iff the i'th bit of x is set to 1. static boolean getBit(int x, int i) { return ((x >>> i) & 1) != 0; } /*---- Constants and tables ----*/ public static final int MIN_VERSION = 1; public static final int MAX_VERSION = 40; // For use in getPenaltyScore(), when evaluating which mask is best. private static final int PENALTY_N1 = 3; private static final int PENALTY_N2 = 3; private static final int PENALTY_N3 = 40; private static final int PENALTY_N4 = 10; private static final byte[][] ECC_CODEWORDS_PER_BLOCK = { // Version: (note that index 0 is for padding, and is set to an illegal value) //0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40 Error correction level {-1, 7, 10, 15, 20, 26, 18, 20, 24, 30, 18, 20, 24, 26, 30, 22, 24, 28, 30, 28, 28, 28, 28, 30, 30, 26, 28, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30}, // Low {-1, 10, 16, 26, 18, 24, 16, 18, 22, 22, 26, 30, 22, 22, 24, 24, 28, 28, 26, 26, 26, 26, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28}, // Medium {-1, 13, 22, 18, 26, 18, 24, 18, 22, 20, 24, 28, 26, 24, 20, 30, 24, 28, 28, 26, 30, 28, 30, 30, 30, 30, 28, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30}, // Quartile {-1, 17, 28, 22, 16, 22, 28, 26, 26, 24, 28, 24, 28, 22, 24, 24, 30, 28, 28, 26, 28, 30, 24, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30}, // High }; private static final byte[][] NUM_ERROR_CORRECTION_BLOCKS = { // Version: (note that index 0 is for padding, and is set to an illegal value) //0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40 Error correction level {-1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 4, 4, 4, 4, 4, 6, 6, 6, 6, 7, 8, 8, 9, 9, 10, 12, 12, 12, 13, 14, 15, 16, 17, 18, 19, 19, 20, 21, 22, 24, 25}, // Low {-1, 1, 1, 1, 2, 2, 4, 4, 4, 5, 5, 5, 8, 9, 9, 10, 10, 11, 13, 14, 16, 17, 17, 18, 20, 21, 23, 25, 26, 28, 29, 31, 33, 35, 37, 38, 40, 43, 45, 47, 49}, // Medium {-1, 1, 1, 2, 2, 4, 4, 6, 6, 8, 8, 8, 10, 12, 16, 12, 17, 16, 18, 21, 20, 23, 23, 25, 27, 29, 34, 34, 35, 38, 40, 43, 45, 48, 51, 53, 56, 59, 62, 65, 68}, // Quartile {-1, 1, 1, 2, 4, 4, 4, 5, 6, 8, 8, 11, 11, 16, 16, 18, 16, 19, 21, 25, 25, 25, 34, 30, 32, 35, 37, 40, 42, 45, 48, 51, 54, 57, 60, 63, 66, 70, 74, 77, 81}, // High }; /*---- Public helper enumeration ----*/ /** * Represents the error correction level used in a QR Code symbol. */ public enum Ecc { // These enum constants must be declared in ascending order of error protection, // for the sake of the implicit ordinal() method and values() function. LOW(1), MEDIUM(0), QUARTILE(3), HIGH(2); // In the range 0 to 3 (unsigned 2-bit integer). final int formatBits; // Constructor. private Ecc(int fb) { formatBits = fb; } } /*---- Private helper class ----*/ /** * Computes the Reed-Solomon error correction codewords for a sequence of data codewords * at a given degree. Objects are immutable, and the state only depends on the degree. * This class exists because each data block in a QR Code shares the same the divisor polynomial. */ private static final class ReedSolomonGenerator { /*-- Field --*/ // Coefficients of the divisor polynomial, stored from highest to lowest power, excluding the leading term which // is always 1. For example the polynomial x^3 + 255x^2 + 8x + 93 is stored as the uint8 array {255, 8, 93}. private final byte[] coefficients; /*-- Constructor --*/ /** * Constructs a Reed-Solomon ECC generator for the specified degree. This could be implemented * as a lookup table over all possible parameter values, instead of as an algorithm. * @param degree the divisor polynomial degree, which must be between 1 and 255 (inclusive) * @throws IllegalArgumentException if degree < 1 or degree > 255 */ public ReedSolomonGenerator(int degree) { if (degree < 1 || degree > 255) throw new IllegalArgumentException("Degree out of range"); // Start with the monomial x^0 coefficients = new byte[degree]; coefficients[degree - 1] = 1; // Compute the product polynomial (x - r^0) * (x - r^1) * (x - r^2) * ... * (x - r^{degree-1}), // drop the highest term, and store the rest of the coefficients in order of descending powers. // Note that r = 0x02, which is a generator element of this field GF(2^8/0x11D). int root = 1; for (int i = 0; i < degree; i++) { // Multiply the current product by (x - r^i) for (int j = 0; j < coefficients.length; j++) { coefficients[j] = (byte)multiply(coefficients[j] & 0xFF, root); if (j + 1 < coefficients.length) coefficients[j] ^= coefficients[j + 1]; } root = multiply(root, 0x02); } } /*-- Method --*/ /** * Computes and returns the Reed-Solomon error correction codewords for the specified * sequence of data codewords. The returned object is always a new byte array. * This method does not alter this object's state (because it is immutable). * @param data the sequence of data codewords * @return the Reed-Solomon error correction codewords * @throws NullPointerException if the data is {@code null} */ public byte[] getRemainder(byte[] data) { Objects.requireNonNull(data); // Compute the remainder by performing polynomial division byte[] result = new byte[coefficients.length]; for (byte b : data) { int factor = (b ^ result[0]) & 0xFF; System.arraycopy(result, 1, result, 0, result.length - 1); result[result.length - 1] = 0; for (int i = 0; i < result.length; i++) result[i] ^= multiply(coefficients[i] & 0xFF, factor); } return result; } /*-- Static function --*/ // Returns the product of the two given field elements modulo GF(2^8/0x11D). The arguments and result // are unsigned 8-bit integers. This could be implemented as a lookup table of 256*256 entries of uint8. private static int multiply(int x, int y) { assert x >>> 8 == 0 && y >>> 8 == 0; // Russian peasant multiplication int z = 0; for (int i = 7; i >= 0; i--) { z = (z << 1) ^ ((z >>> 7) * 0x11D); z ^= ((y >>> i) & 1) * x; } assert z >>> 8 == 0; return z; } } }