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628522ec JH |
1 | #include "cache.h" |

2 | #include "sha1-lookup.h" | |

3 | ||

4 | /* | |

5 | * Conventional binary search loop looks like this: | |

6 | * | |

7 | * unsigned lo, hi; | |

8 | * do { | |

9 | * unsigned mi = (lo + hi) / 2; | |

10 | * int cmp = "entry pointed at by mi" minus "target"; | |

11 | * if (!cmp) | |

12 | * return (mi is the wanted one) | |

13 | * if (cmp > 0) | |

14 | * hi = mi; "mi is larger than target" | |

15 | * else | |

16 | * lo = mi+1; "mi is smaller than target" | |

17 | * } while (lo < hi); | |

18 | * | |

19 | * The invariants are: | |

20 | * | |

21 | * - When entering the loop, lo points at a slot that is never | |

22 | * above the target (it could be at the target), hi points at a | |

23 | * slot that is guaranteed to be above the target (it can never | |

24 | * be at the target). | |

25 | * | |

26 | * - We find a point 'mi' between lo and hi (mi could be the same | |

27 | * as lo, but never can be as same as hi), and check if it hits | |

28 | * the target. There are three cases: | |

29 | * | |

30 | * - if it is a hit, we are happy. | |

31 | * | |

32 | * - if it is strictly higher than the target, we set it to hi, | |

33 | * and repeat the search. | |

34 | * | |

35 | * - if it is strictly lower than the target, we update lo to | |

36 | * one slot after it, because we allow lo to be at the target. | |

37 | * | |

38 | * If the loop exits, there is no matching entry. | |

39 | * | |

40 | * When choosing 'mi', we do not have to take the "middle" but | |

41 | * anywhere in between lo and hi, as long as lo <= mi < hi is | |

42 | * satisfied. When we somehow know that the distance between the | |

43 | * target and lo is much shorter than the target and hi, we could | |

44 | * pick mi that is much closer to lo than the midway. | |

45 | * | |

46 | * Now, we can take advantage of the fact that SHA-1 is a good hash | |

47 | * function, and as long as there are enough entries in the table, we | |

48 | * can expect uniform distribution. An entry that begins with for | |

49 | * example "deadbeef..." is much likely to appear much later than in | |

50 | * the midway of the table. It can reasonably be expected to be near | |

51 | * 87% (222/256) from the top of the table. | |

52 | * | |

12ecb011 JH |
53 | * However, we do not want to pick "mi" too precisely. If the entry at |

54 | * the 87% in the above example turns out to be higher than the target | |

55 | * we are looking for, we would end up narrowing the search space down | |

56 | * only by 13%, instead of 50% we would get if we did a simple binary | |

57 | * search. So we would want to hedge our bets by being less aggressive. | |

58 | * | |

628522ec JH |
59 | * The table at "table" holds at least "nr" entries of "elem_size" |

60 | * bytes each. Each entry has the SHA-1 key at "key_offset". The | |

61 | * table is sorted by the SHA-1 key of the entries. The caller wants | |

62 | * to find the entry with "key", and knows that the entry at "lo" is | |

63 | * not higher than the entry it is looking for, and that the entry at | |

64 | * "hi" is higher than the entry it is looking for. | |

65 | */ | |

66 | int sha1_entry_pos(const void *table, | |

67 | size_t elem_size, | |

68 | size_t key_offset, | |

69 | unsigned lo, unsigned hi, unsigned nr, | |

70 | const unsigned char *key) | |

71 | { | |

72 | const unsigned char *base = table; | |

73 | const unsigned char *hi_key, *lo_key; | |

74 | unsigned ofs_0; | |

75 | static int debug_lookup = -1; | |

76 | ||

77 | if (debug_lookup < 0) | |

78 | debug_lookup = !!getenv("GIT_DEBUG_LOOKUP"); | |

79 | ||

80 | if (!nr || lo >= hi) | |

81 | return -1; | |

82 | ||

83 | if (nr == hi) | |

84 | hi_key = NULL; | |

85 | else | |

86 | hi_key = base + elem_size * hi + key_offset; | |

87 | lo_key = base + elem_size * lo + key_offset; | |

88 | ||

89 | ofs_0 = 0; | |

90 | do { | |

91 | int cmp; | |

92 | unsigned ofs, mi, range; | |

93 | unsigned lov, hiv, kyv; | |

94 | const unsigned char *mi_key; | |

95 | ||

96 | range = hi - lo; | |

97 | if (hi_key) { | |

98 | for (ofs = ofs_0; ofs < 20; ofs++) | |

99 | if (lo_key[ofs] != hi_key[ofs]) | |

100 | break; | |

101 | ofs_0 = ofs; | |

102 | /* | |

103 | * byte 0 thru (ofs-1) are the same between | |

104 | * lo and hi; ofs is the first byte that is | |

105 | * different. | |

106 | */ | |

107 | hiv = hi_key[ofs_0]; | |

108 | if (ofs_0 < 19) | |

109 | hiv = (hiv << 8) | hi_key[ofs_0+1]; | |

110 | } else { | |

111 | hiv = 256; | |

112 | if (ofs_0 < 19) | |

113 | hiv <<= 8; | |

114 | } | |

115 | lov = lo_key[ofs_0]; | |

116 | kyv = key[ofs_0]; | |

117 | if (ofs_0 < 19) { | |

118 | lov = (lov << 8) | lo_key[ofs_0+1]; | |

119 | kyv = (kyv << 8) | key[ofs_0+1]; | |

120 | } | |

121 | assert(lov < hiv); | |

122 | ||

123 | if (kyv < lov) | |

124 | return -1 - lo; | |

125 | if (hiv < kyv) | |

126 | return -1 - hi; | |

127 | ||

12ecb011 JH |
128 | /* |

129 | * Even if we know the target is much closer to 'hi' | |

130 | * than 'lo', if we pick too precisely and overshoot | |

131 | * (e.g. when we know 'mi' is closer to 'hi' than to | |

132 | * 'lo', pick 'mi' that is higher than the target), we | |

133 | * end up narrowing the search space by a smaller | |

134 | * amount (i.e. the distance between 'mi' and 'hi') | |

135 | * than what we would have (i.e. about half of 'lo' | |

136 | * and 'hi'). Hedge our bets to pick 'mi' less | |

137 | * aggressively, i.e. make 'mi' a bit closer to the | |

138 | * middle than we would otherwise pick. | |

139 | */ | |

140 | kyv = (kyv * 6 + lov + hiv) / 8; | |

141 | if (lov < hiv - 1) { | |

142 | if (kyv == lov) | |

143 | kyv++; | |

144 | else if (kyv == hiv) | |

145 | kyv--; | |

146 | } | |

628522ec JH |
147 | mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo; |

148 | ||

149 | if (debug_lookup) { | |

150 | printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi); | |

151 | printf("ofs %u lov %x, hiv %x, kyv %x\n", | |

152 | ofs_0, lov, hiv, kyv); | |

153 | } | |

154 | if (!(lo <= mi && mi < hi)) | |

155 | die("assertion failure lo %u mi %u hi %u %s", | |

156 | lo, mi, hi, sha1_to_hex(key)); | |

157 | ||

158 | mi_key = base + elem_size * mi + key_offset; | |

159 | cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0); | |

160 | if (!cmp) | |

161 | return mi; | |

162 | if (cmp > 0) { | |

163 | hi = mi; | |

164 | hi_key = mi_key; | |

12ecb011 | 165 | } else { |

628522ec JH |
166 | lo = mi + 1; |

167 | lo_key = mi_key + elem_size; | |

168 | } | |

169 | } while (lo < hi); | |

170 | return -lo-1; | |

171 | } |