1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
|
From 9b987f8c98763ee569bde90b5268b43474ca106c Mon Sep 17 00:00:00 2001
From: Christopher Swenson <chris@caswenson.com>
Date: Fri, 27 Feb 2015 14:55:49 +0800
Subject: [PATCH] Fix timsort invariant loop re: Envisage article
See http://envisage-project.eu/proving-android-java-and-python-sorting-algorithm-is-broken-and-how-to-fix-it/
We use a "runLen" array of size 128, so it should be nearly impossible
to have our implementation overflow.
But in any case, the fix is relatively simple -- checking two extra
conditions in the invariant calculation.
I also took this opportunity to remove some redundancy in the
left/right merge logic in the invariant loop.
---
timsort.h | 74 +++++++++++++++++++++++++++++++++------------------------------
1 file changed, 39 insertions(+), 35 deletions(-)
diff --git a/timsort.h b/timsort.h
index efa3aab..795f272 100644
--- a/timsort.h
+++ b/timsort.h
@@ -392,62 +392,66 @@ static void TIM_SORT_MERGE(SORT_TYPE *dst, const TIM_SORT_RUN_T *stack, const in
static int TIM_SORT_COLLAPSE(SORT_TYPE *dst, TIM_SORT_RUN_T *stack, int stack_curr, TEMP_STORAGE_T *store, const size_t size)
{
- while (1)
- {
- int64_t A, B, C;
+ while (1) {
+ int64_t A, B, C, D;
+ int ABC, BCD, BD, CD;
+
/* if the stack only has one thing on it, we are done with the collapse */
- if (stack_curr <= 1) break;
+ if (stack_curr <= 1) {
+ break;
+ }
+
/* if this is the last merge, just do it */
- if ((stack_curr == 2) &&
- (stack[0].length + stack[1].length == (int64_t) size))
- {
+ if ((stack_curr == 2) && (stack[0].length + stack[1].length == size)) {
TIM_SORT_MERGE(dst, stack, stack_curr, store);
stack[0].length += stack[1].length;
stack_curr--;
break;
}
/* check if the invariant is off for a stack of 2 elements */
- else if ((stack_curr == 2) && (stack[0].length <= stack[1].length))
- {
+ else if ((stack_curr == 2) && (stack[0].length <= stack[1].length)) {
TIM_SORT_MERGE(dst, stack, stack_curr, store);
stack[0].length += stack[1].length;
stack_curr--;
break;
- }
- else if (stack_curr == 2)
+ } else if (stack_curr == 2) {
break;
+ }
- A = stack[stack_curr - 3].length;
- B = stack[stack_curr - 2].length;
- C = stack[stack_curr - 1].length;
+ B = stack[stack_curr - 3].length;
+ C = stack[stack_curr - 2].length;
+ D = stack[stack_curr - 1].length;
- /* check first invariant */
- if (A <= B + C)
- {
- if (A < C)
- {
- TIM_SORT_MERGE(dst, stack, stack_curr - 1, store);
- stack[stack_curr - 3].length += stack[stack_curr - 2].length;
- stack[stack_curr - 2] = stack[stack_curr - 1];
- stack_curr--;
- }
- else
- {
- TIM_SORT_MERGE(dst, stack, stack_curr, store);
- stack[stack_curr - 2].length += stack[stack_curr - 1].length;
- stack_curr--;
- }
+ if (stack_curr >= 4) {
+ A = stack[stack_curr - 4].length;
+ ABC = (A <= B + C);
+ } else {
+ ABC = 0;
}
- /* check second invariant */
- else if (B <= C)
- {
+
+ BCD = (B <= C + D) || ABC;
+ CD = (C <= D);
+ BD = (B < D);
+
+ /* Both invariants are good */
+ if (!BCD && !CD) {
+ break;
+ }
+
+ /* left merge */
+ if (BCD && !CD) {
+ TIM_SORT_MERGE(dst, stack, stack_curr - 1, store);
+ stack[stack_curr - 3].length += stack[stack_curr - 2].length;
+ stack[stack_curr - 2] = stack[stack_curr - 1];
+ stack_curr--;
+ } else {
+ /* right merge */
TIM_SORT_MERGE(dst, stack, stack_curr, store);
stack[stack_curr - 2].length += stack[stack_curr - 1].length;
stack_curr--;
}
- else
- break;
}
+
return stack_curr;
}
--
2.3.5
|
