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#ifndef BDL_HASHTABLE_H
#define BDL_HASHTABLE_H
#include "darray.h"
// Minimum table capacity.
#define HT_MIN_CAP 4
#define HT_MIN_SHIFT 2
// Adjust the load factor threshold at which the table will grow on insertion.
#define HT_LOAD_THRESHOLD 0.8
typedef struct HashTablePair {
void *key;
void *value;
} HashTablePair;
struct HashTable;
typedef uint64_t (HashFunction)(const struct HashTable *table, void *bytes);
typedef bool (EqFunc)(void *a, void *b);
typedef struct HashTable {
// All available key-value pairs as a dynamic array.
HashTablePair *pairs;
// Hash function.
HashFunction *hash_func;
// Equality function.
EqFunc *eq_func;
// This table expects the number of buckets to grow in powers of two. To
// speedup the default hashing, we memoize the number of bits equivalent to
// that power of 2:
//
// cap := 1024 = 2 ^ 10, shift_amount := 10
//
uint8_t shift_amount;
} HashTable;
// Hash a byte stream using a circular shift + XOR hash function.
static inline uint64_t
_xor_shift_hash(const char *key, size_t n) {
uint64_t hash = 0x65d9d65f6a19574f;
char *last = (char *)key + n;
while (key != last) {
hash ^= (uint64_t)*key++;
hash = (hash << 8) | (hash >> (64 - 8));
}
return hash;
}
// Use Fibonacci hashing to map a hash to a value in range of the table.
static inline uint64_t
_fibonacci_hash(uint64_t hash, size_t shift_amount) {
return (hash * UINT64_C(11400714819323198485)) >> (64 - shift_amount);
}
static inline float
ht_load_factor(const HashTable *table) {
return (float)array_size(table->pairs) / (float)array_cap(table->pairs);
}
HashTable *
ht_init(HashFunction *hash_func, EqFunc *eq_func) {
HashTable *table = malloc(sizeof(HashTable));
*table = (HashTable){0};
array_init(table->pairs, HT_MIN_CAP);
for (size_t i = 0; i < array_cap(table->pairs); i++) {
table->pairs[i] = (HashTablePair){NULL, NULL};
}
table->shift_amount = HT_MIN_SHIFT;
table->hash_func = hash_func;
table->eq_func = eq_func;
return table;
}
void
_ht_insert(HashTable *table, void *key, void *value) {
size_t position = table->hash_func(table, key);
size_t probe_position = position;
// Verify the key in that position is free. If not, use linear probing to
// find the next free slot.
HashTablePair *pairs = table->pairs;
bool update = false;
while (true) {
if (pairs[probe_position].key == NULL) {
array_head(pairs)->size++;
break;
}
if (table->eq_func(pairs[probe_position].key, key)) {
update = true;
break;
}
if (probe_position == array_cap(pairs) - 1) {
probe_position = 0;
} else {
probe_position++;
}
}
if (!update) {
pairs[probe_position].key = key;
pairs[probe_position].value = value;
} else {
pairs[probe_position].value = value;
}
}
void
_ht_maybe_grow(HashTable *table) {
if (ht_load_factor(table) < HT_LOAD_THRESHOLD) {
return;
}
// Create a new array with 2x capacity.
HashTablePair *old_pairs = table->pairs;
table->pairs = NULL;
array_init(table->pairs, array_cap(old_pairs) * 2);
for (size_t i = 0; i < array_cap(table->pairs); i++) {
table->pairs[i] = (HashTablePair){NULL, NULL};
}
table->shift_amount++;
// Hash everything in the table for the new array capacity.
for (size_t i = 0; i < array_cap(old_pairs); i++) {
if (old_pairs[i].key != NULL) {
_ht_insert(table, old_pairs[i].key, old_pairs[i].value);
}
}
// Free old arrays.
array_free(old_pairs);
}
void
ht_insert(HashTable *table, void *key, void *value) {
_ht_maybe_grow(table);
_ht_insert(table, key, value);
return;
}
void *
ht_lookup(const HashTable *table, void *key) {
size_t position = table->hash_func(table, key);
size_t probe_position = position;
// Verify the key in that position is the same. If not perform linear
// probing to find it.
HashTablePair *pairs = table->pairs;
while (true) {
if (pairs[probe_position].key == NULL) {
return NULL;
}
if (table->eq_func(pairs[probe_position].key, key)) {
break;
}
if (probe_position == array_cap(pairs) - 1) {
probe_position = 0;
} else {
probe_position++;
}
if (probe_position == position) {
return NULL;
}
}
return pairs[probe_position].value;
}
void
ht_free(HashTable *table) {
if (table == NULL) {
return;
}
array_free(table->pairs);
free(table);
}
#endif // BDL_HASHTABLE_H
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