/*
- * 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.
package com.google.zxing.qrcode.decoder;
/**
- * @author srowen@google.com (Sean Owen)
+ * <p>Encapsulates a QR Code's format information, including the data mask used and
+ * error correction level.</p>
+ *
+ * @author Sean Owen
+ * @see DataMask
+ * @see ErrorCorrectionLevel
*/
final class FormatInformation {
/**
* See ISO 18004:2006, Annex C, Table C.1
*/
- private static final int[][] FORMAT_INFO_DECODE_LOOKUP = new int[][]{
+ private static final int[][] FORMAT_INFO_DECODE_LOOKUP = {
{0x5412, 0x00},
{0x5125, 0x01},
{0x5E7C, 0x02},
{0x2BED, 0x1F},
};
+ /**
+ * Offset i holds the number of 1 bits in the binary representation of i
+ */
private static final int[] BITS_SET_IN_HALF_BYTE =
- new int[]{0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4};
+ {0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4};
private final ErrorCorrectionLevel errorCorrectionLevel;
private final byte dataMask;
private FormatInformation(int formatInfo) {
// Bits 3,4
- errorCorrectionLevel =
- ErrorCorrectionLevel.forBits((formatInfo >> 3) & 0x03);
+ errorCorrectionLevel = ErrorCorrectionLevel.forBits((formatInfo >> 3) & 0x03);
// Bottom 3 bits
dataMask = (byte) (formatInfo & 0x07);
}
static int numBitsDiffering(int a, int b) {
- a ^= b;
- return
- BITS_SET_IN_HALF_BYTE[a & 0x0F] +
- BITS_SET_IN_HALF_BYTE[(a >>> 4 & 0x0F)] +
- BITS_SET_IN_HALF_BYTE[(a >>> 8 & 0x0F)] +
- BITS_SET_IN_HALF_BYTE[(a >>> 12 & 0x0F)] +
- BITS_SET_IN_HALF_BYTE[(a >>> 16 & 0x0F)] +
- BITS_SET_IN_HALF_BYTE[(a >>> 20 & 0x0F)] +
- BITS_SET_IN_HALF_BYTE[(a >>> 24 & 0x0F)] +
- BITS_SET_IN_HALF_BYTE[(a >>> 28 & 0x0F)];
+ a ^= b; // a now has a 1 bit exactly where its bit differs with b's
+ // Count bits set quickly with a series of lookups:
+ return BITS_SET_IN_HALF_BYTE[a & 0x0F] +
+ BITS_SET_IN_HALF_BYTE[(a >>> 4 & 0x0F)] +
+ BITS_SET_IN_HALF_BYTE[(a >>> 8 & 0x0F)] +
+ BITS_SET_IN_HALF_BYTE[(a >>> 12 & 0x0F)] +
+ BITS_SET_IN_HALF_BYTE[(a >>> 16 & 0x0F)] +
+ BITS_SET_IN_HALF_BYTE[(a >>> 20 & 0x0F)] +
+ BITS_SET_IN_HALF_BYTE[(a >>> 24 & 0x0F)] +
+ BITS_SET_IN_HALF_BYTE[(a >>> 28 & 0x0F)];
}
- static FormatInformation decodeFormatInformation(int rawFormatInfo) {
- FormatInformation formatInfo = doDecodeFormatInformation(rawFormatInfo);
+ /**
+ * @param maskedFormatInfo1 format info indicator, with mask still applied
+ * @param maskedFormatInfo2 second copy of same info; both are checked at the same time
+ * to establish best match
+ * @return information about the format it specifies, or <code>null</code>
+ * if doesn't seem to match any known pattern
+ */
+ static FormatInformation decodeFormatInformation(int maskedFormatInfo1, int maskedFormatInfo2) {
+ FormatInformation formatInfo = doDecodeFormatInformation(maskedFormatInfo1, maskedFormatInfo2);
if (formatInfo != null) {
return formatInfo;
}
// Should return null, but, some QR codes apparently
- // do not mask this info. Try again, first masking the raw bits so
- // the function will unmask
- return doDecodeFormatInformation(rawFormatInfo ^ FORMAT_INFO_MASK_QR);
+ // do not mask this info. Try again by actually masking the pattern
+ // first
+ return doDecodeFormatInformation(maskedFormatInfo1 ^ FORMAT_INFO_MASK_QR,
+ maskedFormatInfo2 ^ FORMAT_INFO_MASK_QR);
}
- private static FormatInformation doDecodeFormatInformation(
- int rawFormatInfo) {
- // Unmask:
- int unmaskedFormatInfo = rawFormatInfo ^ FORMAT_INFO_MASK_QR;
+ private static FormatInformation doDecodeFormatInformation(int maskedFormatInfo1, int maskedFormatInfo2) {
+ // Find the int in FORMAT_INFO_DECODE_LOOKUP with fewest bits differing
int bestDifference = Integer.MAX_VALUE;
int bestFormatInfo = 0;
for (int i = 0; i < FORMAT_INFO_DECODE_LOOKUP.length; i++) {
int[] decodeInfo = FORMAT_INFO_DECODE_LOOKUP[i];
int targetInfo = decodeInfo[0];
- if (targetInfo == unmaskedFormatInfo) {
+ if (targetInfo == maskedFormatInfo1 || targetInfo == maskedFormatInfo2) {
+ // Found an exact match
return new FormatInformation(decodeInfo[1]);
}
- int bitsDifference = numBitsDiffering(unmaskedFormatInfo, targetInfo);
+ int bitsDifference = numBitsDiffering(maskedFormatInfo1, targetInfo);
if (bitsDifference < bestDifference) {
bestFormatInfo = decodeInfo[1];
bestDifference = bitsDifference;
}
+ if (maskedFormatInfo1 != maskedFormatInfo2) {
+ // also try the other option
+ bitsDifference = numBitsDiffering(maskedFormatInfo2, targetInfo);
+ if (bitsDifference < bestDifference) {
+ bestFormatInfo = decodeInfo[1];
+ bestDifference = bitsDifference;
+ }
+ }
}
+ // Hamming distance of the 32 masked codes is 7, by construction, so <= 3 bits
+ // differing means we found a match
if (bestDifference <= 3) {
return new FormatInformation(bestFormatInfo);
}