summaryrefslogtreecommitdiff
path: root/lib/skiplist.c
blob: 2cea6d8dfbeecc7665081fa46bf94cef3d69d89b (plain)
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
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
/* Copyright (C) 2016 Hewlett Packard Enterprise Development LP
 * All Rights Reserved.
 *
 * 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.
 */

/* Skiplist implementation based on:
 * "Skip List: A Probabilistic Alternative to Balanced Trees",
 * by William Pugh. */

#include <config.h>
#include <stdio.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>

#include "skiplist.h"
#include "random.h"
#include "util.h"

/*
 * A maximum height level of 32 should be more than sufficient for
 * anticipated use cases, delivering good expected performance with
 * up to 2**32 list nodes. Changes to this limit will also require
 * changes in skiplist_determine_level().
 */
#define SKIPLIST_MAX_LEVELS 32

/* Skiplist node container */
struct skiplist_node {
    const void *data;                 /* Pointer to saved data. */
    int height;                       /* Height of this node. */
    struct skiplist_node *forward[];  /* Links to the next nodes. */
};

/* Skiplist container */
struct skiplist {
    struct skiplist_node *header; /* Pointer to head node (not first
                                   * data node). */
    skiplist_comparator *cmp;     /* Pointer to the skiplist's comparison
                                   * function. */
    void *cfg;                    /* Pointer to optional comparison
                                   * configuration, used by the comparator. */
    int level;                    /* Maximum level currently in use. */
    uint32_t size;                /* Current number of nodes in skiplist. */
};

/* Create a new skiplist_node with given level and data. */
static struct skiplist_node *
skiplist_create_node(int level, const void *object)
{
    struct skiplist_node *new_node;
    size_t alloc_size = sizeof *new_node +
                        (level + 1) * sizeof new_node->forward[0];

    new_node = xmalloc(alloc_size);
    new_node->data = object;
    new_node->height = level;
    memset(new_node->forward, 0,
           (level + 1) * sizeof new_node->forward[0]);

    return new_node;
}

/*
 * Create a new skiplist, configured with given data comparison function
 * and configuration.
 */
struct skiplist *
skiplist_create(skiplist_comparator object_comparator, void *configuration)
{
    random_init();
    struct skiplist *sl;

    sl = xmalloc(sizeof (struct skiplist));
    sl->cfg = configuration;
    sl->size = 0;
    sl->level = 0;
    sl->cmp = object_comparator;
    sl->header = skiplist_create_node(SKIPLIST_MAX_LEVELS, NULL);

    return sl;
}

/*
 * Move the cursor forward to the first node with associated data greater than
 * or equal to "value".
 */
static struct skiplist_node *
skiplist_forward_to_(struct skiplist *sl, const void *value,
                     struct skiplist_node **update)
{
    struct skiplist_node *x = sl->header;
    int i;

    /* Loop invariant: x < value */
    for (i = sl->level; i >= 0; i--) {
        while (x->forward[i] &&
               sl->cmp(x->forward[i]->data, value, sl->cfg) < 0) {
            x = x->forward[i];
        }
        /* x < value <= x->forward[1] */
        if (update) {
            update[i] = x;
        }
    }
    /* x < value <= x->forward[1] */
    x = x->forward[0];
    return x;
}

struct skiplist_node *
skiplist_forward_to(struct skiplist *sl, const void *value)
{
    return skiplist_forward_to_(sl, value, NULL);
}

/* Find the first exact match of value in the skiplist. */
struct skiplist_node *
skiplist_find(struct skiplist *sl, const void *value)
{
    struct skiplist_node *x = skiplist_forward_to(sl, value);

    return x && sl->cmp(x->data, value, sl->cfg) == 0 ? x : NULL;
}

/*
 * Determine the level for a skiplist node by choosing a level N with
 * probability P(N) = 1/(2**(N+1)) in the range 0..32, with  the returned
 * level clamped at the current skiplist height plus 1.
 */
static int
skiplist_determine_level(struct skiplist *sl)
{
    int lvl;

    lvl = clz32(random_uint32());

    return MIN(lvl, sl->level + 1);
}

/* Insert data into a skiplist. */
void
skiplist_insert(struct skiplist *list, const void *value)
{
    struct skiplist_node *update[SKIPLIST_MAX_LEVELS + 1];
    struct skiplist_node *x = skiplist_forward_to_(list, value, update);
    int i, lvl;

    if (x && list->cmp(x->data, value, list->cfg) == 0) {
        x->data = value;
    } else {
        lvl = skiplist_determine_level(list);
        if (lvl > list->level) {
            for (i = list->level + 1; i <= lvl; i++) {
                update[i] = list->header;
            }
            list->level = lvl;
        }
        x = skiplist_create_node(lvl, value);
        for (i = 0; i <= lvl; i++) {
            x->forward[i] = update[i]->forward[i];
            update[i]->forward[i] = x;
        }
        list->size++;
    }
}

/* Remove first node with associated data equal to "value" from skiplist. */
void *
skiplist_delete(struct skiplist *list, const void *value)
{
    struct skiplist_node *update[SKIPLIST_MAX_LEVELS + 1];
    struct skiplist_node *x;
    void *data = NULL;
    int i;

    x = skiplist_forward_to_(list, value, update);

    if (x && list->cmp(x->data, value, list->cfg) == 0) {
        for (i = 0; i <= list->level; i++) {
            if (!update[i]->forward[i] ||
                list->cmp(update[i]->forward[i]->data, value,
                          list->cfg) != 0) {
                break;
            }
            update[i]->forward[i] = x->forward[i];
        }
        data = CONST_CAST(void *, x->data);

        free(x);

        while (list->level > 0 && !list->header->forward[list->level]) {
            list->level--;
        }
        list->size--;
    }
    return data;
}

/* Get the associated data value stored in a skiplist node. */
void *
skiplist_get_data(struct skiplist_node *node)
{
    return node ? CONST_CAST(void *, node->data) : NULL;
}

/* Get the number of items in a skiplist. */
uint32_t
skiplist_get_size(struct skiplist *sl)
{
    return sl->size;
}

/* Get the first node in a skiplist.  */
struct skiplist_node *
skiplist_first(struct skiplist *sl)
{
    return sl->header->forward[0];
}

/* Get a node's successor in a skiplist. */
struct skiplist_node *
skiplist_next(struct skiplist_node *node)
{
    return node ? node->forward[0] : NULL;
}

/*
 * Destroy a skiplist and free all nodes in the list.  If the "data_destroy"
 * function pointer is non-NULL, it will be called for each node as it is
 * removed to allow any needed cleanups to be performed on the associated
 * data.
 */
void
skiplist_destroy(struct skiplist *sl, void (*data_destroy)(void *))
{
    struct skiplist_node *node, *next;

    next = node = sl->header;
    while (next != NULL) {
        next = node->forward[0];
        if (data_destroy) {
            data_destroy(CONST_CAST(void *, node->data));
        }
        free(node);
        node = next;
    }
    free(sl);
}