Add Skip List implementation and unit test

skip_list.c

@@ -0,0 +1,77 @@
+#include "skip_list.h"
+#include <stdlib.h>
+
+static int xRandomLevel(void) {
+    int level = 1;
+    while ((rand() & 0x1) && level < SKIP_LIST_MAX_LEVEL)
+        level++;
+    return level;
+}
+
+void vSkipListInit(SkipList_t *list) {
+    list->level = 1;
+    list->header = (SkipListNode_t *)calloc(1, sizeof(SkipListNode_t));
+    for (int i = 0; i < SKIP_LIST_MAX_LEVEL; i++)
+        list->header->forward[i] = NULL;
+}
+
+void vSkipListInsert(SkipList_t *list, TickType_t key, void *value) {
+    SkipListNode_t *update[SKIP_LIST_MAX_LEVEL];
+    SkipListNode_t *x = list->header;
+    for (int i = list->level - 1; i >= 0; i--) {
+        while (x->forward[i] && x->forward[i]->key < key)
+            x = x->forward[i];
+        update[i] = x;
+    }
+    x = x->forward[0];
+    if (x && x->key == key) {
+        x->value = value;
+        return;
+    }
+    int lvl = xRandomLevel();
+    if (lvl > list->level) {
+        for (int i = list->level; i < lvl; i++)
+            update[i] = list->header;
+        list->level = lvl;
+    }
+    x = (SkipListNode_t *)calloc(1, sizeof(SkipListNode_t));
+    x->key = key;
+    x->value = value;
+    for (int i = 0; i < lvl; i++) {
+        x->forward[i] = update[i]->forward[i];
+        update[i]->forward[i] = x;
+    }
+}
+
+void vSkipListRemove(SkipList_t *list, TickType_t key) {
+    SkipListNode_t *update[SKIP_LIST_MAX_LEVEL];
+    SkipListNode_t *x = list->header;
+    for (int i = list->level - 1; i >= 0; i--) {
+        while (x->forward[i] && x->forward[i]->key < key)
+            x = x->forward[i];
+        update[i] = x;
+    }
+    x = x->forward[0];
+    if (x && x->key == key) {
+        for (int i = 0; i < list->level; i++) {
+            if (update[i]->forward[i] != x)
+                break;
+            update[i]->forward[i] = x->forward[i];
+        }
+        free(x);
+        while (list->level > 1 && list->header->forward[list->level - 1] == NULL)
+            list->level--;
+    }
+}
+
+void *pvSkipListSearch(SkipList_t *list, TickType_t key) {
+    SkipListNode_t *x = list->header;
+    for (int i = list->level - 1; i >= 0; i--) {
+        while (x->forward[i] && x->forward[i]->key < key)
+            x = x->forward[i];
+    }
+    x = x->forward[0];
+    if (x && x->key == key)
+        return x->value;
+    return NULL;
+}

skip_list.h

@@ -0,0 +1,32 @@
+#ifndef FREERTOS_SKIP_LIST_H
+#define FREERTOS_SKIP_LIST_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+#ifndef TickType_t
+typedef unsigned int TickType_t;
+#endif
+
+#ifndef SKIP_LIST_MAX_LEVEL
+#define SKIP_LIST_MAX_LEVEL 4 // Tunable for memory/complexity tradeoff
+#endif
+
+// Forward declaration
+typedef struct SkipListNode {
+    TickType_t key;
+    void *value;
+    struct SkipListNode *forward[SKIP_LIST_MAX_LEVEL];
+} SkipListNode_t;
+
+typedef struct SkipList {
+    SkipListNode_t *header;
+    int level;
+} SkipList_t;
+
+void vSkipListInit(SkipList_t *list);
+void vSkipListInsert(SkipList_t *list, TickType_t key, void *value);
+void vSkipListRemove(SkipList_t *list, TickType_t key);
+void *pvSkipListSearch(SkipList_t *list, TickType_t key);
+
+#endif // FREERTOS_SKIP_LIST_H

test_skip_list.c

@@ -0,0 +1,62 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include "skip_list.h"
+
+void print_skip_list(SkipList_t *list) {
+    printf("Skip List: ");
+    SkipListNode_t *node = list->header->forward[0];
+    while (node) {
+        printf("(%u, %p) ", node->key, node->value);
+        node = node->forward[0];
+    }
+    printf("\n");
+}
+
+int main(void) {
+    SkipList_t list;
+    vSkipListInit(&list);
+
+    // Insert test
+    printf("Inserting values...\n");
+    for (TickType_t i = 10; i > 0; --i) {
+        int *val = (int *)malloc(sizeof(int));
+        *val = i * 100;
+        vSkipListInsert(&list, i, val);
+    }
+    print_skip_list(&list);
+
+    // Search test
+    printf("Searching for key 5...\n");
+    int *found = (int *)pvSkipListSearch(&list, 5);
+    if (found) {
+        printf("Found key 5 with value %d\n", *found);
+    } else {
+        printf("Key 5 not found!\n");
+    }
+
+    printf("Searching for key 42...\n");
+    found = (int *)pvSkipListSearch(&list, 42);
+    if (found) {
+        printf("Found key 42 with value %d\n", *found);
+    } else {
+        printf("Key 42 not found!\n");
+    }
+
+    // Remove test
+    printf("Removing key 5...\n");
+    vSkipListRemove(&list, 5);
+    print_skip_list(&list);
+    found = (int *)pvSkipListSearch(&list, 5);
+    if (!found) {
+        printf("Key 5 successfully removed.\n");
+    }
+
+    // Cleanup
+    for (TickType_t i = 1; i <= 10; ++i) {
+        int *val = (int *)pvSkipListSearch(&list, i);
+        if (val) free(val);
+    }
+    // Free header node
+    free(list.header);
+    return 0;
+}