Small speedup, per issue 422

[zxing.git] / core / src / com / google / zxing / common / reedsolomon / GF256.java

diff --git a/core/src/com/google/zxing/common/reedsolomon/GF256.java b/core/src/com/google/zxing/common/reedsolomon/GF256.java
index fec0312..9c58275 100644 (file)
--- a/core/src/com/google/zxing/common/reedsolomon/GF256.java
+++ b/core/src/com/google/zxing/common/reedsolomon/GF256.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2007 Google Inc.
+ * Copyright 2007 ZXing authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,41 +18,76 @@ package com.google.zxing.common.reedsolomon;
 
 /**
  * <p>This class contains utility methods for performing mathematical operations over
- * the Galois Field GF(256). Operations use the primitive polynomial
- * x^8 + x^4 + x^3 + x^2 + 1 in calculations.</p>
+ * the Galois Field GF(256). Operations use a given primitive polynomial in calculations.</p>
  *
  * <p>Throughout this package, elements of GF(256) are represented as an <code>int</code>
  * for convenience and speed (but at the cost of memory).
  * Only the bottom 8 bits are really used.</p>
  *
- * @author srowen@google.com (Sean Owen)
+ * @author Sean Owen
  */
-final class GF256 {
+public final class GF256 {
 
-  private static final int PRIMITIVE = 0x011D;
-  private static final int[] exp = new int[256];
-  private static final int[] log = new int[256];
-  static {
+  public static final GF256 QR_CODE_FIELD = new GF256(0x011D); // x^8 + x^4 + x^3 + x^2 + 1
+  public static final GF256 DATA_MATRIX_FIELD = new GF256(0x012D); // x^8 + x^5 + x^3 + x^2 + 1
+
+  private final int[] expTable;
+  private final int[] logTable;
+  private final GF256Poly zero;
+  private final GF256Poly one;
+
+  /**
+   * Create a representation of GF(256) using the given primitive polynomial.
+   *
+   * @param primitive irreducible polynomial whose coefficients are represented by
+   *  the bits of an int, where the least-significant bit represents the constant
+   *  coefficient
+   */
+  private GF256(int primitive) {
+    expTable = new int[256];
+    logTable = new int[256];
     int x = 1;
     for (int i = 0; i < 256; i++) {
-      exp[i] = x;
+      expTable[i] = x;
       x <<= 1; // x = x * 2; we're assuming the generator alpha is 2
       if (x >= 0x100) {
-        x ^= PRIMITIVE;
+        x ^= primitive;
       }
     }
     for (int i = 0; i < 255; i++) {
-      log[exp[i]] = i;
+      logTable[expTable[i]] = i;
     }
-    // log[0] == 0 but this should never be used
+    // logTable[0] == 0 but this should never be used
+    zero = new GF256Poly(this, new int[]{0});
+    one = new GF256Poly(this, new int[]{1});
+  }
+
+  GF256Poly getZero() {
+    return zero;
+  }
+
+  GF256Poly getOne() {
+    return one;
   }
 
-  private GF256() {
+  /**
+   * @return the monomial representing coefficient * x^degree
+   */
+  GF256Poly buildMonomial(int degree, int coefficient) {
+    if (degree < 0) {
+      throw new IllegalArgumentException();
+    }
+    if (coefficient == 0) {
+      return zero;
+    }
+    int[] coefficients = new int[degree + 1];
+    coefficients[0] = coefficient;
+    return new GF256Poly(this, coefficients);
   }
 
   /**
    * Implements both addition and subtraction -- they are the same in GF(256).
-   * 
+   *
    * @return sum/difference of a and b
    */
   static int addOrSubtract(int a, int b) {
@@ -62,47 +97,43 @@ final class GF256 {
   /**
    * @return 2 to the power of a in GF(256)
    */
-  static int exp(int a) {
-    return exp[a];
+  int exp(int a) {
+    return expTable[a];
   }
 
   /**
    * @return base 2 log of a in GF(256)
    */
-  static int log(int a) {
+  int log(int a) {
     if (a == 0) {
       throw new IllegalArgumentException();
     }
-    return log[a];
+    return logTable[a];
   }
 
   /**
    * @return multiplicative inverse of a
    */
-  static int inverse(int a) {
+  int inverse(int a) {
     if (a == 0) {
       throw new ArithmeticException();
     }
-    return exp[255 - log[a]];
+    return expTable[255 - logTable[a]];
   }
 
   /**
-   *
    * @param a
    * @param b
    * @return product of a and b in GF(256)
    */
-  static int multiply(int a, int b) {
+  int multiply(int a, int b) {
     if (a == 0 || b == 0) {
       return 0;
     }
-    if (a == 1) {
-      return b;
-    }
-    if (b == 1) {
-      return a;
-    }
-    return exp[(log[a] + log[b]) % 255];
+    int logSum = logTable[a] + logTable[b];
+    // index is a sped-up alternative to logSum % 255 since sum
+    // is in [0,510]. Thanks to jmsachs for the idea
+    return expTable[(logSum & 0xFF) + (logSum >>> 8)];
   }
 
-}
\ No newline at end of file
+}