#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