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+/*
+ *
+ * Copyright 2015, Google Inc.
+ * 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 Google Inc. 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
+ * OWNER 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.
+ *
+ */
+
+#ifndef GRPC_CORE_EXT_CENSUS_HASH_TABLE_H
+#define GRPC_CORE_EXT_CENSUS_HASH_TABLE_H
+
+#include <stddef.h>
+
+#include <grpc/support/port_platform.h>
+
+/* A chain based hash table with fixed number of buckets.
+   Your probably shouldn't use this code directly. It is implemented for the
+   use case in census trace store and stats store, where number of entries in
+   the table is in the scale of upto several thousands, entries are added and
+   removed from the table very frequently (~100k/s), the frequency of find()
+   operations is roughly several times of the frequency of insert() and erase()
+   Comparing to find(), the insert(), erase() and get_all_entries() operations
+   are much less freqent (<1/s).
+
+   Per bucket memory overhead is about (8 + sizeof(intptr_t) bytes.
+   Per entry memory overhead is about (8 + 2 * sizeof(intptr_t) bytes.
+
+   All functions are not thread-safe. Synchronization will be provided in the
+   upper layer (in trace store and stats store).
+*/
+
+/* Opaque hash table struct */
+typedef struct unresizable_hash_table census_ht;
+
+/* Currently, the hash_table can take two types of keys. (uint64 for trace
+   store and const char* for stats store). */
+typedef union {
+  uint64_t val;
+  void *ptr;
+} census_ht_key;
+
+typedef enum census_ht_key_type {
+  CENSUS_HT_UINT64 = 0,
+  CENSUS_HT_POINTER = 1
+} census_ht_key_type;
+
+typedef struct census_ht_option {
+  /* Type of hash key */
+  census_ht_key_type key_type;
+  /* Desired number of buckets, preferably a prime number */
+  int32_t num_buckets;
+  /* Fucntion to calculate uint64 hash value of the key. Only takes effect if
+     key_type is POINTER. */
+  uint64_t (*hash)(const void *);
+  /* Function to compare two keys, returns 0 iff equal. Only takes effect if
+     key_type is POINTER */
+  int (*compare_keys)(const void *k1, const void *k2);
+  /* Value deleter. NULL if no specialized delete function is needed. */
+  void (*delete_data)(void *);
+  /* Key deleter. NULL if table does not own the key. (e.g. key is part of the
+     value or key is not owned by the table.) */
+  void (*delete_key)(void *);
+} census_ht_option;
+
+/* Creates a hashtable with fixed number of buckets according to the settings
+   specified in 'options' arg. Function pointers "hash" and "compare_keys" must
+   be provided if key_type is POINTER. Asserts if fail to create. */
+census_ht *census_ht_create(const census_ht_option *options);
+
+/* Deletes hash table instance. Frees all dynamic memory owned by ht.*/
+void census_ht_destroy(census_ht *ht);
+
+/* Inserts the input key-val pair into hash_table. If an entry with the same key
+   exists in the table, the corresponding value will be overwritten by the input
+   val. */
+void census_ht_insert(census_ht *ht, census_ht_key key, void *val);
+
+/* Returns pointer to data, returns NULL if not found. */
+void *census_ht_find(const census_ht *ht, census_ht_key key);
+
+/* Erase hash table entry with input key. Noop if key is not found. */
+void census_ht_erase(census_ht *ht, census_ht_key key);
+
+typedef struct census_ht_kv {
+  census_ht_key k;
+  void *v;
+} census_ht_kv;
+
+/* Returns an array of pointers to all values in the hash table. Order of the
+   elements can be arbitrary. Sets 'num' to the size of returned array. Caller
+   owns returned array. */
+census_ht_kv *census_ht_get_all_elements(const census_ht *ht, size_t *num);
+
+/* Returns number of elements kept. */
+size_t census_ht_get_size(const census_ht *ht);
+
+/* Functor applied on each key-value pair while iterating through entries in the
+   table. The functor should not mutate data. */
+typedef void (*census_ht_itr_cb)(census_ht_key key, const void *val_ptr,
+                                 void *state);
+
+/* Iterates through all key-value pairs in the hash_table. The callback function
+   should not invalidate data entries. */
+uint64_t census_ht_for_all(const census_ht *ht, census_ht_itr_cb);
+
+#endif /* GRPC_CORE_EXT_CENSUS_HASH_TABLE_H */