commit 57e0a96f18be94a4af2e382420200d8ec67829f7
Author: Darko <darko@icosahedron.dummy.porta.siemens.net>
Date:   Fri Mar 20 20:32:11 2015 +0100

    Initial commit.

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..b755a9b
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,4 @@
+*.o
+*.so
+sl
+
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..8b7718c
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,28 @@
+Copyright (c) 2015, Darko Poljak
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+* Neither the name of mergesort nor the names of its
+  contributors may be used to endorse or promote products derived from
+  this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
diff --git a/README.rst b/README.rst
new file mode 100644
index 0000000..87a4206
--- /dev/null
+++ b/README.rst
@@ -0,0 +1,4 @@
+skiplist
+=========
+
+Academic example of skip list implementation in C.
diff --git a/main.c b/main.c
new file mode 100644
index 0000000..15cfde7
--- /dev/null
+++ b/main.c
@@ -0,0 +1,134 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include "skiplist.h"
+#include "x.h"
+
+#define MAXBUF 80
+#define KEYCNT 1000
+
+int dodebug = 1;
+
+int main(int argc, char **argv)
+{
+    slist sl;
+    int key;
+    sl_node *x;
+    int res;
+    int i;
+    int keys[KEYCNT];
+    int failed = 0;
+    int cntnew;
+    int cntsame;
+/*
+    char foo[MAXBUF];
+
+    sl_init(&sl);
+    sl_print(&sl);
+
+    printf("addition\n");
+    while (fgets(foo, MAXBUF - 1, stdin) != NULL) {
+        if (foo[0] == '\n') {
+            break;
+        }
+        key = atoi(foo);
+        sl_add(&sl, key, NULL);
+        sl_print(&sl);
+    }
+
+    printf("search\n");
+    while (fgets(foo, MAXBUF - 1, stdin) != NULL) {
+        if (foo[0] == '\n') {
+            break;
+        }
+        key = atoi(foo);
+        x = sl_find(&sl, key);
+        if (x) {
+            printf("found: %d\n", x->key);
+        } else {
+            printf("not found\n");
+        }
+    }
+
+    printf("remove\n");
+    while (fgets(foo, MAXBUF - 1, stdin) != NULL) {
+        if (foo[0] == '\n') {
+            break;
+        }
+        key = atoi(foo);
+        res = sl_remove(&sl, key);
+        sl_print(&sl);
+        if (res) {
+            printf("removed\n");
+        } else {
+            printf("not found\n");
+        }
+    }
+
+    sl_free(&sl);
+*/
+
+    sl_init(&sl);
+    dodebug = 0;
+    failed = 0;
+    cntnew = cntsame = 0;
+    for (i = 0; i < KEYCNT; ++i) {
+        key = random() % KEYCNT;
+        keys[i] = key;
+        res = sl_add(&sl, key, NULL);
+        switch(res) {
+            case SL_ADD_NEW:
+                ++cntnew;
+                break;
+            case SL_ADD_SAME:
+                ++cntsame;
+                break;
+            default:
+                failed = 1;
+                break;
+        }
+    }
+    printf("added %d new keys, %d same keys\n", cntnew, cntsame);
+    dodebug = 1;
+    sl_print(&sl);
+    dodebug = 0;
+    cntnew = cntsame = 0;
+    for (i = 0; i < KEYCNT; ++i) {
+        key = keys[i];
+        x = sl_find(&sl, key);
+        if (!x) {
+            failed = 1;
+            break;
+        }
+    }
+    if (failed) {
+        fatal("find failed, report bug\n");
+    }
+    cntnew = cntsame = 0;
+    for (i = 0; i < KEYCNT; ++i) {
+        key = keys[i];
+        res = sl_remove(&sl, key);
+        switch (res) {
+            case 0:
+                ++cntsame;
+                break;
+            case 1:
+                ++cntnew;
+                break;
+            default:
+                break;
+        }
+        if (sl_find(&sl, key)) {
+            failed = 1;
+            break;
+        }
+    }
+    if (failed) {
+        fatal("remove failed, report bug\n");
+    }
+    printf("removed %d new keys, %d same keys\n", cntnew, cntsame);
+    sl_free(&sl);
+
+    return 0;
+}
+
diff --git a/skiplist.c b/skiplist.c
new file mode 100644
index 0000000..8296e7d
--- /dev/null
+++ b/skiplist.c
@@ -0,0 +1,292 @@
+#include <strings.h>
+#include "skiplist.h"
+#include "x.h"
+
+sl_node *_sl_create_node(int key, void *data)
+{
+    sl_node *x = xmalloc(sizeof(sl_node));
+
+    x->key = key;
+    x->data = data;
+    x->next = x->prev = x->below = x->above = NULL;
+
+    return x;
+}
+
+void sl_init(slist *sl)
+{
+    sl_node *x = _sl_create_node(KEY_INF_MIN, NULL);
+    sl_node *y = _sl_create_node(KEY_INF_MAX, NULL);
+
+    x->next = y;
+    y->prev = x;
+
+    sl->topleft = x;
+    sl->bottomleft = x;
+    sl->topright = y;
+    sl->bottomright = y;
+
+    sl->maxlevel = 0;
+}
+
+void _sl_add_level(slist *sl)
+{
+    sl_node *x = _sl_create_node(KEY_INF_MIN, NULL);
+    sl_node *y = _sl_create_node(KEY_INF_MAX, NULL);
+
+    x->next = y;
+    y->prev = x;
+
+    x->below = sl->topleft;
+    y->below = sl->topright;
+    sl->topleft->above = x;
+    sl->topright->above = y;
+    sl->topleft = x;
+    sl->topright = y;
+
+    ++sl->maxlevel;
+}
+
+void sl_free(slist *sl)
+{
+    sl_node *foo;
+    sl_node *x;
+    sl_node *y;
+
+    for (foo = sl->bottomleft; foo != NULL;) {
+        x = foo;
+        foo = foo->next;
+        while (x != NULL) {
+            y = x;
+            x = x->above;
+            free(y->data);
+            free(y);
+        }
+    }
+}
+
+void _print_key(int key)
+{
+    if (key == KEY_INF_MIN) {
+        debug("-INF -> ");
+    } else if (key == KEY_INF_MAX) {
+        debug("+INF -> ");
+    } else {
+        debug("%d -> ", key);
+    }
+}
+
+void _sl_println()
+{
+    int i;
+
+    for (i = 0; i < 79; ++i) {
+        debug("-");
+    }
+    debug("\n");
+}
+
+void sl_print(const slist *sl)
+{
+    sl_node *foo;
+    sl_node *x;
+    int level = sl->maxlevel;
+
+    _sl_println();
+    for (foo = sl->topleft; foo != NULL; foo = foo->below, --level) {
+        debug("level %d: ", level);
+        for (x = foo; x != NULL; x = x->next) {
+            _print_key(x->key);
+        }
+        debug("\n");
+    }
+    _sl_println();
+    debug("from below to above:\n");
+    for (foo = sl->bottomleft; foo != NULL; foo = foo->next) {
+        for (x = foo; x != NULL; x = x->above) {
+            _print_key(x->key);
+        }
+        debug("\n");
+    }
+    _sl_println();
+}
+
+#define HEAD 1
+#define TAIL 0
+
+int _toss_coin(int maxlevel) {
+    int i;
+    int x;
+    
+    i = 0;
+    while(1) {
+        x = random() & 0x1;
+        if (x == TAIL) {
+            break;
+        }
+        ++i;
+        /* inc level only by one at one iter */
+        if (i == maxlevel + 1) {
+            break;
+        }
+    };
+
+    return i;
+}
+
+/*
+ * Clean top levels with only INFINITY nodes until only one such
+ * level exists.
+ */
+void _sl_clean_top(slist *sl)
+{
+    sl_node *foo;
+
+    for (foo = sl->topleft; foo->below != NULL; foo = foo->below) {
+        if (foo->next->key == KEY_INF_MAX
+                && foo->below->next->key == KEY_INF_MAX) {
+            sl->topleft = foo->below;
+            sl->topright = foo->below->next;
+            sl->topleft->above = NULL;
+            sl->topright->above = NULL;
+            --sl->maxlevel;
+            free(foo->data);
+            free(foo->next);
+            free(foo);
+            debug("cleaned one level\n");
+        }
+    }
+}
+
+int sl_add(slist *sl, int key, void *data)
+{
+    sl_node *foo;
+    sl_node *x;
+    int i;
+    int k;
+    int level;
+    sl_node **update;
+    int res;
+    
+    if (key == KEY_INF_MIN || key == KEY_INF_MAX) {
+        return SL_ADD_INV;
+    }
+    i = _toss_coin(sl->maxlevel);
+    debug("toss coin result: %d\n", i);
+    if (i >= sl->maxlevel) {
+        /* add one more level */
+        _sl_add_level(sl);
+        debug("added level\n");
+    }
+    update = (sl_node **) xmalloc(sizeof(sl_node *) * (sl->maxlevel + 1));
+    bzero(update, sizeof(sl_node *) * (sl->maxlevel + 1));
+    level = sl->maxlevel;
+
+    foo = sl->topleft;
+    while (1) {
+        while (key >= foo->next->key) {
+            foo = foo->next;
+        }
+        if (foo->below == NULL) {
+            break;
+        }
+        update[level--] = foo;
+        foo = foo->below;
+    }
+    update[level] = foo;
+
+    debug("search finished at key: %d\n", foo->key);
+    debug("update list keys:\n");
+    for (k = 0; k <= sl->maxlevel; ++k) {
+        if (update[k] != 0) {
+            debug("%d\n", update[k]->key);
+        }
+    }
+
+    debug("adding\n");
+    if (foo->key == key) {
+        res = SL_ADD_SAME;
+        debug("key exists\n");
+        do {
+            foo->data = data;
+            foo = foo->above;
+        } while (foo != NULL);
+    } else {
+        res = SL_ADD_NEW;
+        debug("new key\n");
+        while (level <= i) {
+            x = _sl_create_node(key, data);
+            x->next = update[level]->next;
+            update[level]->next->prev = x;
+            x->prev = update[level];
+            update[level]->next = x;
+            if (level > 0) {
+                x->below = update[level - 1]->next;
+                update[level - 1]->next->above = x;
+            }
+            ++level;
+        }
+    }
+    _sl_clean_top(sl);
+    free(update);
+
+    return res;
+}
+
+sl_node *sl_find(slist *sl, int key)
+{
+    sl_node *foo;
+    
+    foo = sl->topleft;
+    while (1) {
+        while (key >= foo->next->key) {
+            foo = foo->next;
+        }
+        if (foo->below == NULL) {
+            break;
+        }
+        foo = foo->below;
+    }
+
+    if (foo->key != key) {
+        foo = NULL;
+    }
+    return foo;
+}
+
+int sl_remove(slist *sl, int key)
+{
+    sl_node *foo;
+    sl_node *x;
+    int result;
+    
+    foo = sl->topleft;
+    while (1) {
+        while (key >= foo->next->key) {
+            foo = foo->next;
+        }
+        if (foo->below == NULL) {
+            break;
+        }
+        foo = foo->below;
+    }
+
+    if (foo->key == key) {
+        result = 1;
+
+        while (foo != NULL) {
+            foo->prev->next = foo->next;
+            foo->next->prev = foo->prev;
+            free(foo->data);
+            x = foo;
+            foo = foo->above;
+            free(x);
+        }
+
+        _sl_clean_top(sl);
+    } else {
+        result = 0;
+    }
+
+    return result;
+}
diff --git a/skiplist.h b/skiplist.h
new file mode 100644
index 0000000..301c8ff
--- /dev/null
+++ b/skiplist.h
@@ -0,0 +1,63 @@
+#ifndef __SKIPLIST
+#define __SKIPLIST
+
+#include <inttypes.h>
+#include <limits.h>
+#include <stdio.h>
+
+#define KEY_INF_MIN INT_MIN  /* -INFINITY */
+#define KEY_INF_MAX INT_MAX  /* +INFINITY */
+
+#define SL_ADD_NEW 1  /* added new key */
+#define SL_ADD_SAME 0  /* added existing key */
+#define SL_ADD_INV -1  /* added invalid key (+-INFINITY) */
+
+/* skip list node */
+typedef struct skiplist_node {
+    int key;
+    void *data;
+    struct skiplist_node *next;
+    struct skiplist_node *prev;
+    struct skiplist_node *below;
+    struct skiplist_node *above;
+} sl_node;
+
+/* skiplist object */
+typedef struct skiplist {
+    sl_node *topleft;
+    sl_node *bottomleft;
+    sl_node *topright;
+    sl_node *bottomright;
+    int maxlevel;
+} slist;
+
+/*
+ * Initialize empty skip list.
+ */
+void sl_init(slist *sl);
+/*
+ * Free skip list memory. After free list is not initialized
+ * for subsequent use.
+ */
+void sl_free(slist *sl);
+/*
+ * Print skip list level by level and in down to up form.
+ */
+void sl_print(const slist *sl);
+/*
+ * Add new key with data into skip list.
+ */
+int sl_add(slist *sl, int key, void *data);
+/*
+ * Find node in skip list with given key.
+ * If key is not found returns NULL.
+ */
+sl_node *sl_find(slist *sl, int key);
+/*
+ * Remove node from skip list with given key.
+ * If key is found and removed return 1, otherwise 0.
+ */
+int sl_remove(slist *sl, int key);
+
+#endif
+
diff --git a/x.c b/x.c
new file mode 100644
index 0000000..c45282b
--- /dev/null
+++ b/x.c
@@ -0,0 +1,45 @@
+#include "x.h"
+
+extern int dodebug;
+
+static void _log_stderr(const char *fmt, va_list ap)
+{
+    char buf[MAXLINE];
+
+    vsnprintf(buf, MAXLINE-1, fmt, ap);
+    fflush(stdout);
+    fputs(buf, stderr);
+    fflush(stderr);
+}
+
+void fatal(const char *fmt, ...)
+{
+    va_list ap;
+
+    va_start(ap, fmt);
+    _log_stderr(fmt, ap);
+    va_end(ap);
+    exit(1);
+}
+
+void debug(const char *fmt, ...)
+{
+    va_list ap;
+
+    if (dodebug) {
+        va_start(ap, fmt);
+        _log_stderr(fmt, ap);
+        va_end(ap);
+    }
+}
+
+void *xmalloc(size_t size)
+{
+    void *foo = malloc(size);
+
+    if (foo == NULL) {
+        fatal("Not enough memory\n");
+    }
+    return foo;
+}
+
diff --git a/x.h b/x.h
new file mode 100644
index 0000000..e7c3024
--- /dev/null
+++ b/x.h
@@ -0,0 +1,18 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdarg.h>
+
+#define MAXLINE 1024
+
+/*
+ * Print message and exit.
+ */
+void fatal(const char *fmt, ...);
+/*
+ * malloc, if failed then fatal exit.
+ */
+void *xmalloc(size_t size);
+/*
+ * Print debug message if debug is enabled.
+ */
+void debug(const char *fmt, ...);