zxing/csharp/qrcode/decoder/FormatInformation.cs

/*
 * 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.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace com.google.zxing.qrcode.decoder
{

	/// <summary>
	/// <p>Encapsulates a QR Code's format information, including the data mask used and
	/// error correction level.</p>
	/// 
	/// @author Sean Owen </summary>
	/// <seealso cref= DataMask </seealso>
	/// <seealso cref= ErrorCorrectionLevel </seealso>
	internal sealed class FormatInformation
	{

	  private const int FORMAT_INFO_MASK_QR = 0x5412;

	  /// <summary>
	  /// See ISO 18004:2006, Annex C, Table C.1
	  /// </summary>
	  private static readonly int[][] FORMAT_INFO_DECODE_LOOKUP = {new int[] {0x5412, 0x00}, new int[] {0x5125, 0x01}, new int[] {0x5E7C, 0x02}, new int[] {0x5B4B, 0x03}, new int[] {0x45F9, 0x04}, new int[] {0x40CE, 0x05}, new int[] {0x4F97, 0x06}, new int[] {0x4AA0, 0x07}, new int[] {0x77C4, 0x08}, new int[] {0x72F3, 0x09}, new int[] {0x7DAA, 0x0A}, new int[] {0x789D, 0x0B}, new int[] {0x662F, 0x0C}, new int[] {0x6318, 0x0D}, new int[] {0x6C41, 0x0E}, new int[] {0x6976, 0x0F}, new int[] {0x1689, 0x10}, new int[] {0x13BE, 0x11}, new int[] {0x1CE7, 0x12}, new int[] {0x19D0, 0x13}, new int[] {0x0762, 0x14}, new int[] {0x0255, 0x15}, new int[] {0x0D0C, 0x16}, new int[] {0x083B, 0x17}, new int[] {0x355F, 0x18}, new int[] {0x3068, 0x19}, new int[] {0x3F31, 0x1A}, new int[] {0x3A06, 0x1B}, new int[] {0x24B4, 0x1C}, new int[] {0x2183, 0x1D}, new int[] {0x2EDA, 0x1E}, new int[] {0x2BED, 0x1F}};

	  /// <summary>
	  /// Offset i holds the number of 1 bits in the binary representation of i
	  /// </summary>
	  private static readonly int[] BITS_SET_IN_HALF_BYTE = {0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4};

	  private readonly ErrorCorrectionLevel errorCorrectionLevel;
	  private readonly sbyte dataMask;

	  private FormatInformation(int formatInfo)
	  {
		// Bits 3,4
		errorCorrectionLevel = ErrorCorrectionLevel.forBits((formatInfo >> 3) & 0x03);
		// Bottom 3 bits
		dataMask = (sbyte)(formatInfo & 0x07);
	  }

	  internal static int numBitsDiffering(int a, int b)
	  {
		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[((int)((uint)a >> 4 & 0x0F))] + BITS_SET_IN_HALF_BYTE[((int)((uint)a >> 8 & 0x0F))] + BITS_SET_IN_HALF_BYTE[((int)((uint)a >> 12 & 0x0F))] + BITS_SET_IN_HALF_BYTE[((int)((uint)a >> 16 & 0x0F))] + BITS_SET_IN_HALF_BYTE[((int)((uint)a >> 20 & 0x0F))] + BITS_SET_IN_HALF_BYTE[((int)((uint)a >> 24 & 0x0F))] + BITS_SET_IN_HALF_BYTE[((int)((uint)a >> 28 & 0x0F))];
	  }

	  /// <param name="maskedFormatInfo1"> format info indicator, with mask still applied </param>
	  /// <param name="maskedFormatInfo2"> second copy of same info; both are checked at the same time
	  ///  to establish best match </param>
	  /// <returns> information about the format it specifies, or {@code null}
	  ///  if doesn't seem to match any known pattern </returns>
	  internal 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 by actually masking the pattern
		// first
		return doDecodeFormatInformation(maskedFormatInfo1 ^ FORMAT_INFO_MASK_QR, maskedFormatInfo2 ^ 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 = int.MaxValue;
		int bestFormatInfo = 0;
		foreach (int[] decodeInfo in FORMAT_INFO_DECODE_LOOKUP)
		{
		  int targetInfo = decodeInfo[0];
		  if (targetInfo == maskedFormatInfo1 || targetInfo == maskedFormatInfo2)
		  {
			// Found an exact match
			return new FormatInformation(decodeInfo[1]);
		  }
		  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);
		}
		return null;
	  }

	  internal ErrorCorrectionLevel ErrorCorrectionLevel
	  {
		  get
		  {
			return errorCorrectionLevel;
		  }
	  }

	  internal sbyte DataMask
	  {
		  get
		  {
			return dataMask;
		  }
	  }

	  public override int GetHashCode()
	  {
		return (errorCorrectionLevel.ordinal() << 3) | (int) dataMask;
	  }

	  public override bool Equals(object o)
	  {
		if (!(o is FormatInformation))
		{
		  return false;
		}
		FormatInformation other = (FormatInformation) o;
		return this.errorCorrectionLevel == other.errorCorrectionLevel && this.dataMask == other.dataMask;
	  }

	}

}