Mercurial > jhg

annotate src/com/tmate/hgkit/ll/Nodeid.java @ 46:4022c34a4804

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Better hashCode impl
author Artem Tikhomirov <tikhomirov.artem@gmail.com>
date Fri, 14 Jan 2011 04:56:53 +0100
parents 858d1b2458cb
children
rev   line source
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1 /*
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2 * Copyright (c) 2010, 2011 Artem Tikhomirov
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3 */
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4 package com.tmate.hgkit.ll;
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6 import static com.tmate.hgkit.ll.DigestHelper.toHexString;
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7
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8 import java.util.Arrays;
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11
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12 /**
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13 * Whether to store fixed size array (20 bytes) - ease of manipulation (e.g. hashcode/equals), or
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14 * memory effective - reuse supplied array, keep significant bits only?
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15 * Fixed size array looks most appealing to me now - I doubt one can save any significant amount of memory.
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16 * There'd always 20 non-zero bytes, the difference is only for any extra bytes one may pass to constructor
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17 * @author artem
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18 *
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19 */
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20 public final class Nodeid {
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22 public static final Nodeid NULL = new Nodeid(new byte[20], false);
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23 private final byte[] binaryData;
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25 /**
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26 * @param binaryRepresentation - byte[20], kept by reference
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27 * @param shallClone - true if array is subject to future modification and shall be copied, not referenced
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28 */
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29 public Nodeid(byte[] binaryRepresentation, boolean shallClone) {
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30 // 5 int fields => 32 bytes
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31 // byte[20] => 48 bytes
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32 if (binaryRepresentation == null || binaryRepresentation.length != 20) {
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33 throw new IllegalArgumentException();
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34 }
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35 this.binaryData = shallClone ? binaryRepresentation.clone() : binaryRepresentation;
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36 }
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37
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38 @Override
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39 public int hashCode() {
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40 // digest (part thereof) seems to be nice candidate for the hashCode
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41 byte[] b = binaryData;
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42 return b[0] << 24 | (b[1] & 0xFF) << 16 | (b[2] & 0xFF) << 8 | (b[3] & 0xFF);
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43 }
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44
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45 @Override
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46 public boolean equals(Object o) {
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47 if (o instanceof Nodeid) {
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48 return Arrays.equals(this.binaryData, ((Nodeid) o).binaryData);
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49 }
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50 return false;
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51 }
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52
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53 public boolean equalsTo(byte[] buf) {
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54 return Arrays.equals(this.binaryData, buf);
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55 }
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56
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57 @Override
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58 public String toString() {
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59 // XXX may want to output just single 0 for the NULL id?
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60 return toHexString(binaryData, 0, binaryData.length);
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61 }
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62
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63 public String shortNotation() {
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64 return toHexString(binaryData, 0, 6);
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65 }
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66
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67 public boolean isNull() {
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68 if (this == NULL) {
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69 return true;
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70 }
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71 for (int i = 0; i < 20; i++) {
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72 if (this.binaryData[i] != 0) {
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73 return false;
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74 }
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75 }
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76 return true;
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77 }
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78
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79 // primary purpose is to give DigestHelper access to internal structure. Despite it's friends-only (package visibility), it's still makes sense to
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80 // return a copy, to avoid any accidental modification (same reason field is not made visible, nor any callback, e.g. Output.write(byte[]) was introduced)
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81 /*package-local*/byte[] cloneData() {
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82 return binaryData.clone();
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83 }
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84
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85 // primary difference with cons is handling of NULL id (this method returns constant)
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86 // always makes a copy of an array passed
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87 public static Nodeid fromBinary(byte[] binaryRepresentation, int offset) {
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88 if (binaryRepresentation == null || binaryRepresentation.length - offset < 20) {
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89 throw new IllegalArgumentException();
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90 }
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91 int i = 0;
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92 while (i < 20 && binaryRepresentation[offset+i] == 0) i++;
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93 if (i == 20) {
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94 return NULL;
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95 }
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96 if (offset == 0 && binaryRepresentation.length == 20) {
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97 return new Nodeid(binaryRepresentation, true);
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98 }
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99 byte[] b = new byte[20]; // create new instance if no other reasonable guesses possible
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100 System.arraycopy(binaryRepresentation, offset, b, 0, 20);
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101 return new Nodeid(b, false);
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102 }
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103
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104 // binascii.unhexlify()
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105 public static Nodeid fromAscii(byte[] asciiRepresentation, int offset, int length) {
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106 if (length != 40) {
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107 throw new IllegalArgumentException();
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108 }
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109 byte[] data = new byte[20];
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110 boolean zeroBytes = true;
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111 for (int i = 0, j = offset; i < data.length; i++) {
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112 int hiNibble = Character.digit(asciiRepresentation[j++], 16);
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113 int lowNibble = Character.digit(asciiRepresentation[j++], 16);
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114 byte b = (byte) (((hiNibble << 4) | lowNibble) & 0xFF);
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115 data[i] = b;
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116 zeroBytes = zeroBytes && b == 0;
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117 }
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118 if (zeroBytes) {
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119 return NULL;
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120 }
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121 return new Nodeid(data, false);
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122 }
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123 }