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#ifndef BDL_HASHTABLE_H
#define BDL_HASHTABLE_H
#include "darray.h"
#include "objects.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 {
Object *key;
Object *value;
} HashTablePair;
typedef struct HashTable {
// All available key-value pairs as a dynamic array.
HashTablePair *pairs;
// 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);
}
uint64_t
ht_hash(const HashTable *table, const Object *key) {
uint64_t hash = 0;
switch (key->type) {
case OBJ_TYPE_FIXNUM: {
hash = key->fixnum;
} break;
case OBJ_TYPE_STRING: {
hash = _xor_shift_hash(key->string, array_size(key->string));
} break;
case OBJ_TYPE_SYMBOL: {
hash = _xor_shift_hash(key->symbol, array_size(key->symbol));
} break;
case OBJ_TYPE_BOOL:
case OBJ_TYPE_NIL:
case OBJ_TYPE_PAIR:
case OBJ_TYPE_LAMBDA:
case OBJ_TYPE_PROCEDURE:
case OBJ_TYPE_ERR: {
hash = (uintptr_t)key;
} break;
}
hash = _fibonacci_hash(hash, table->shift_amount);
return hash;
}
static inline float
ht_load_factor(const HashTable *table) {
return (float)array_size(table->pairs) / (float)array_cap(table->pairs);
}
HashTable *
ht_init(void) {
HashTable *table = malloc(sizeof(HashTable));
table->pairs = NULL;
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;
return table;
}
void
_ht_insert(HashTable *table, const Object *key, const Object *value) {
size_t position = ht_hash(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;
while (true) {
if (pairs[probe_position].key == NULL) {
array_head(pairs)->size++;
break;
}
if (obj_eq(pairs[probe_position].key, key)) {
break;
}
if (probe_position == array_cap(pairs) - 1) {
probe_position = 0;
} else {
probe_position++;
}
}
pairs[probe_position].key = (Object *)key;
pairs[probe_position].value = (Object *)value;
}
void
_ht_maybe_grow(HashTable *table) {
HashTablePair *pairs = table->pairs;
if (ht_load_factor(table) < HT_LOAD_THRESHOLD) {
return;
}
// Create a new array with 2x capacity.
table->pairs = NULL;
array_init(table->pairs, array_cap(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(pairs); i++) {
if (pairs[i].key != NULL) {
_ht_insert(table, pairs[i].key, pairs[i].value);
}
}
// Free the old array.
array_free(pairs);
}
void
ht_insert(HashTable *table, const Object *key, const Object *value) {
_ht_maybe_grow(table);
_ht_insert(table, key, value);
return;
}
Object *
ht_lookup(const HashTable *table, const Object *key) {
size_t position = ht_hash(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 (obj_eq(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;
}
if (table->pairs == NULL) {
return;
}
array_free(table->pairs);
free(table);
}
#endif // BDL_HASHTABLE_H
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