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+/*
+ * Copyright (c) 2011-Present, Redis Ltd.
+ * All rights reserved.
+ *
+ * Licensed under your choice of (a) the Redis Source Available License 2.0
+ * (RSALv2); or (b) the Server Side Public License v1 (SSPLv1); or (c) the
+ * GNU Affero General Public License v3 (AGPLv3).
+ * 
+ *
+ * FENWICK TREE (Binary Indexed Tree)
+ * ----------------------------------
+ * A Fenwick tree is a data structure that efficiently supports:
+ * - Point updates: Add/subtract values at specific indices in O(log n) time
+ * - Prefix sum queries: Calculate cumulative sums from index 0 to any index in O(log n) time
+ * - Range queries: Calculate sums over any range [i,j] in O(log n) time
+ * - Space complexity: O(n)
+ *
+ * USAGE IN REDIS
+ * --------------
+ * This implementation is used by KVSTORE and ESTORE to efficiently track:
+ * - Cumulative key counts across dictionary slots (KVSTORE)
+ * - Cumulative item counts across expiration buckets (ESTORE)
+ *
+ * This enables efficient operations like:
+ * - Finding which dictionary/bucket contains the Nth key/item
+ * - Iterating through non-empty dictionaries/buckets
+ * - Load balancing and random key selection
+ *
+ * IMPLEMENTATION NOTES
+ * -------------------
+ * - The tree uses 1-based indexing internally for mathematical convenience
+ * - The public API uses 0-based indexing for consistency with Redis codebase
+ * - Tree size must be a power of 2 (specified as sizeBits where size = 2^sizeBits)
+ * - All operations have O(log n) time complexity where n is the tree size
+ *
+ * REFERENCES
+ * ----------
+ * For more details on Fenwick trees: https://en.wikipedia.org/wiki/Fenwick_tree
+ */
+
+#ifndef __FWTREE_H
+#define __FWTREE_H
+
+#include <stdint.h>
+
+/* Forward declaration of the fenwick tree structure */
+typedef struct _fenwickTree fenwickTree;
+
+/* Fenwick Tree API */
+
+fenwickTree *fwTreeCreate(int sizeBits);
+
+void fwTreeDestroy(fenwickTree *ft);
+
+unsigned long long fwTreePrefixSum(fenwickTree *ft, int idx);
+
+void fwTreeUpdate(fenwickTree *ft, int idx, long long delta);
+
+int fwTreeFindIndex(fenwickTree *ft, unsigned long long target);
+
+int fwTreeFindFirstNonEmpty(fenwickTree *ft);
+
+int fwTreeFindNextNonEmpty(fenwickTree *ft, int idx);
+
+void fwTreeClear(fenwickTree *ft);
+
+#ifdef REDIS_TEST
+int fwtreeTest(int argc, char *argv[], int flags);
+#endif
+
+#endif /* __FWTREE_H */