v/vlib/builtin/map.v

// Copyright (c) 2019-2021 Alexander Medvednikov. All rights reserved.
// Use of this source code is governed by an MIT license
// that can be found in the LICENSE file.
module builtin

// import hash.wyhash as hash
import hash

/*
This is a highly optimized hashmap implementation. It has several traits that
in combination makes it very fast and memory efficient. Here is a short expl-
anation of each trait. After reading this you should have a basic understand-
ing of how it functions:

1. Hash-function: Wyhash. Wyhash is the fastest hash-function for short keys
passing SMHasher, so it was an obvious choice.

2. Open addressing: Robin Hood Hashing. With this method, a hash-collision is
resolved by probing. As opposed to linear probing, Robin Hood hashing has a
simple but clever twist: As new keys are inserted, old keys are shifted arou-
nd in a way such that all keys stay reasonably close to the slot they origin-
ally hash to. A new key may displace a key already inserted if its probe cou-
nt is larger than that of the key at the current position.

3. Memory layout: key-value pairs are stored in a `DenseArray`. This is a dy-
namic array with a very low volume of unused memory, at the cost of more rea-
llocations when inserting elements. It also preserves the order of the key-v-
alues. This array is named `key_values`. Instead of probing a new key-value,
this map probes two 32-bit numbers collectively. The first number has its 8
most significant bits reserved for the probe-count and the remaining 24 bits
are cached bits from the hash which are utilized for faster re-hashing. This
number is often referred to as `meta`. The other 32-bit number is the index
at which the key-value was pushed to in `key_values`. Both of these numbers
are stored in a sparse array `metas`. The `meta`s and `kv_index`s are stored
at even and odd indices, respectively:

metas = [meta, kv_index, 0, 0, meta, kv_index, 0, 0, meta, kv_index, ...]
key_values = [kv, kv, kv, ...]

4. The size of metas is a power of two. This enables the use of bitwise AND
to convert the 64-bit hash to a bucket/index that doesn't overflow metas. If
the size is power of two you can use "hash & (SIZE - 1)" instead of "hash %
SIZE". Modulo is extremely expensive so using '&' is a big performance impro-
vement. The general concern with this approach is that you only make use of
the lower bits of the hash which can cause more collisions. This is solved by
using a well-dispersed hash-function.

5. The hashmap keeps track of the highest probe_count. The trick is to alloc-
ate `extra_metas` > max(probe_count), so you never have to do any bounds-che-
cking since the extra meta memory ensures that a meta will never go beyond
the last index.

6. Cached rehashing. When the `load_factor` of the map exceeds the `max_load_
factor` the size of metas is doubled and all the key-values are "rehashed" to
find the index for their meta's in the new array. Instead of rehashing compl-
etely, it simply uses the cached-hashbits stored in the meta, resulting in
much faster rehashing.
*/
const (
	// Number of bits from the hash stored for each entry
	hashbits            = 24
	// Number of bits from the hash stored for rehashing
	max_cached_hashbits = 16
	// Initial log-number of buckets in the hashtable
	init_log_capicity   = 5
	// Initial number of buckets in the hashtable
	init_capicity       = 1 << init_log_capicity
	// Maximum load-factor (len / capacity)
	max_load_factor     = 0.8
	// Initial highest even index in metas
	init_even_index     = init_capicity - 2
	// Used for incrementing `extra_metas` when max
	// probe count is too high, to avoid overflow
	extra_metas_inc     = 4
	// Bitmask to select all the hashbits
	hash_mask           = u32(0x00FFFFFF)
	// Used for incrementing the probe-count
	probe_inc           = u32(0x01000000)
)

// fast_string_eq is intended to be fast when
// the strings are very likely to be equal
// TODO: add branch prediction hints
[inline]
fn fast_string_eq(a string, b string) bool {
	if a.len != b.len {
		return false
	}
	unsafe {
		return C.memcmp(a.str, b.str, b.len) == 0
	}
}

// DenseArray represents a dynamic array with very low growth factor
struct DenseArray {
	key_bytes   int
	key_bytes_  int
	value_bytes int
	slot_bytes  int // sum of 2 fields above
mut:
	cap     int
	len     int
	deletes u32 // count
	// array allocated (with `cap` bytes) on first deletion
	// has non-zero element when key deleted
	all_deleted &byte
	data        byteptr // array of interleaved key data and value data
}

[inline]
fn new_dense_array(key_bytes int, value_bytes int) DenseArray {
	mut key_bytes_ := key_bytes
	mut value_bytes_ := value_bytes
	$if gcboehm ? {
		align, mask := $if x64 { 7, int(0xfffffff8) } $else { 3, int(0xfffffffc) }
		key_bytes_ = (key_bytes + align) & mask
		value_bytes_ = (value_bytes + align) & mask
	}
	slot_bytes := key_bytes_ + value_bytes_
	cap := 8
	return DenseArray{
		key_bytes: key_bytes
		key_bytes_: key_bytes_
		value_bytes: value_bytes
		slot_bytes: slot_bytes
		cap: cap
		len: 0
		deletes: 0
		all_deleted: 0
		data: unsafe { malloc(cap * slot_bytes) }
	}
}

[inline]
fn (d &DenseArray) key(i int) voidptr {
	return unsafe { d.data + i * d.slot_bytes }
}

// for cgen
[inline]
fn (d &DenseArray) value(i int) voidptr {
	return unsafe { d.data + i * d.slot_bytes + d.key_bytes_ }
}

[inline]
fn (d &DenseArray) has_index(i int) bool {
	return d.deletes == 0 || unsafe { d.all_deleted[i] } == 0
}

// Make space to append an element and return index
// The growth-factor is roughly 1.125 `(x + (x >> 3))`
[inline]
fn (mut d DenseArray) expand() int {
	old_cap := d.cap
	old_size := d.slot_bytes * old_cap
	if d.cap == d.len {
		d.cap += d.cap >> 3
		unsafe {
			d.data = realloc_data(d.data, old_size, d.slot_bytes * d.cap)
			if d.deletes != 0 {
				d.all_deleted = realloc_data(d.all_deleted, old_cap, d.cap)
				C.memset(d.all_deleted + d.len, 0, d.cap - d.len)
			}
		}
	}
	push_index := d.len
	unsafe {
		if d.deletes != 0 {
			d.all_deleted[push_index] = 0
		}
	}
	d.len++
	return push_index
}

// Move all zeros to the end of the array and resize array
fn (mut d DenseArray) zeros_to_end() {
	// TODO alloca?
	mut tmp_buf := unsafe { malloc(d.slot_bytes) }
	mut count := 0
	for i in 0 .. d.len {
		if d.has_index(i) {
			// swap (TODO: optimize)
			unsafe {
				C.memcpy(tmp_buf, d.key(count), d.slot_bytes)
				C.memcpy(d.key(count), d.key(i), d.slot_bytes)
				C.memcpy(d.key(i), tmp_buf, d.slot_bytes)
			}
			count++
		}
	}
	unsafe {
		free(tmp_buf)
		d.deletes = 0
		// TODO: reallocate instead as more deletes are likely
		free(d.all_deleted)
	}
	d.len = count
	old_cap := d.cap
	d.cap = if count < 8 { 8 } else { count }
	unsafe {
		d.data = realloc_data(d.data, d.slot_bytes * old_cap, d.slot_bytes * d.cap)
	}
}

type MapHashFn = fn (voidptr) u64

type MapEqFn = fn (voidptr, voidptr) bool

type MapCloneFn = fn (voidptr, voidptr)

type MapFreeFn = fn (voidptr)

// map is the internal representation of a V `map` type.
pub struct map {
	// Number of bytes of a key
	key_bytes  int
	key_bytes_ int
	// Number of bytes of a value
	value_bytes int
mut:
	// Highest even index in the hashtable
	even_index u32
	// Number of cached hashbits left for rehashing
	cached_hashbits byte
	// Used for right-shifting out used hashbits
	shift byte
	// Array storing key-values (ordered)
	key_values DenseArray
	// Pointer to meta-data:
	// - Odd indices store kv_index.
	// - Even indices store probe_count and hashbits.
	metas &u32
	// Extra metas that allows for no ranging when incrementing
	// index in the hashmap
	extra_metas     u32
	has_string_keys bool
	hash_fn         MapHashFn
	key_eq_fn       MapEqFn
	clone_fn        MapCloneFn
	free_fn         MapFreeFn
pub mut:
	// Number of key-values currently in the hashmap
	len int
}

fn map_hash_string(pkey voidptr) u64 {
	key := *unsafe { &string(pkey) }
	return hash.wyhash_c(key.str, u64(key.len), 0)
}

fn map_hash_int_1(pkey voidptr) u64 {
	return hash.wyhash64_c(*unsafe { &byte(pkey) }, 0)
}

fn map_hash_int_2(pkey voidptr) u64 {
	return hash.wyhash64_c(*unsafe { &u16(pkey) }, 0)
}

fn map_hash_int_4(pkey voidptr) u64 {
	return hash.wyhash64_c(*unsafe { &u32(pkey) }, 0)
}

fn map_hash_int_8(pkey voidptr) u64 {
	return hash.wyhash64_c(*unsafe { &u64(pkey) }, 0)
}

fn map_eq_string(a voidptr, b voidptr) bool {
	return fast_string_eq(*unsafe { &string(a) }, *unsafe { &string(b) })
}

fn map_eq_int_1(a voidptr, b voidptr) bool {
	return unsafe { C.memcmp(a, b, 1) == 0 }
}

fn map_eq_int_2(a voidptr, b voidptr) bool {
	return unsafe { C.memcmp(a, b, 2) == 0 }
}

fn map_eq_int_4(a voidptr, b voidptr) bool {
	return unsafe { C.memcmp(a, b, 4) == 0 }
}

fn map_eq_int_8(a voidptr, b voidptr) bool {
	return unsafe { C.memcmp(a, b, 8) == 0 }
}

fn map_clone_string(dest voidptr, pkey voidptr) {
	unsafe {
		s := *&string(pkey)
		(*&string(dest)) = s.clone()
	}
}

fn map_clone_int_1(dest voidptr, pkey voidptr) {
	unsafe {
		C.memcpy(dest, pkey, 1)
	}
}

fn map_clone_int_2(dest voidptr, pkey voidptr) {
	unsafe {
		C.memcpy(dest, pkey, 2)
	}
}

fn map_clone_int_4(dest voidptr, pkey voidptr) {
	unsafe {
		C.memcpy(dest, pkey, 4)
	}
}

fn map_clone_int_8(dest voidptr, pkey voidptr) {
	unsafe {
		C.memcpy(dest, pkey, 8)
	}
}

fn map_free_string(pkey voidptr) {
	unsafe {
		(*&string(pkey)).free()
	}
}

fn map_free_nop(_ voidptr) {
}

fn new_map_2(key_bytes int, value_bytes int, hash_fn MapHashFn, key_eq_fn MapEqFn, clone_fn MapCloneFn, free_fn MapFreeFn) map {
	metasize := int(sizeof(u32) * (init_capicity + extra_metas_inc))
	// for now assume anything bigger than a pointer is a string
	has_string_keys := key_bytes > sizeof(voidptr)
	mut key_bytes_ := key_bytes
	$if gcboehm ? {
		align, mask := $if x64 { 7, int(0xfffffff8) } $else { 3, int(0xfffffffc) }
		key_bytes_ = (key_bytes + align) & mask
	}
	return map{
		key_bytes: key_bytes
		key_bytes_: key_bytes_
		value_bytes: value_bytes
		even_index: init_even_index
		cached_hashbits: max_cached_hashbits
		shift: init_log_capicity
		key_values: new_dense_array(key_bytes, value_bytes)
		metas: &u32(vcalloc(metasize))
		extra_metas: extra_metas_inc
		len: 0
		has_string_keys: has_string_keys
		hash_fn: hash_fn
		key_eq_fn: key_eq_fn
		clone_fn: clone_fn
		free_fn: free_fn
	}
}

fn new_map_init_2(hash_fn MapHashFn, key_eq_fn MapEqFn, clone_fn MapCloneFn, free_fn MapFreeFn, n int, key_bytes int, value_bytes int, keys voidptr, values voidptr) map {
	mut out := new_map_2(key_bytes, value_bytes, hash_fn, key_eq_fn, clone_fn, free_fn)
	// TODO pre-allocate n slots
	mut pkey := byteptr(keys)
	mut pval := byteptr(values)
	for _ in 0 .. n {
		unsafe {
			out.set_1(pkey, pval)
			pkey += key_bytes
			pval += value_bytes
		}
	}
	return out
}

pub fn (mut m map) move() map {
	r := *m
	unsafe {
		C.memset(m, 0, sizeof(map))
	}
	return r
}

[inline]
fn (m &map) key_to_index(pkey voidptr) (u32, u32) {
	hash := m.hash_fn(pkey)
	index := hash & m.even_index
	meta := ((hash >> m.shift) & hash_mask) | probe_inc
	return u32(index), u32(meta)
}

[inline]
fn (m &map) meta_less(_index u32, _metas u32) (u32, u32) {
	mut index := _index
	mut meta := _metas
	for meta < unsafe { m.metas[index] } {
		index += 2
		meta += probe_inc
	}
	return index, meta
}

[inline]
fn (mut m map) meta_greater(_index u32, _metas u32, kvi u32) {
	mut meta := _metas
	mut index := _index
	mut kv_index := kvi
	for unsafe { m.metas[index] } != 0 {
		if meta > unsafe { m.metas[index] } {
			unsafe {
				tmp_meta := m.metas[index]
				m.metas[index] = meta
				meta = tmp_meta
				tmp_index := m.metas[index + 1]
				m.metas[index + 1] = kv_index
				kv_index = tmp_index
			}
		}
		index += 2
		meta += probe_inc
	}
	unsafe {
		m.metas[index] = meta
		m.metas[index + 1] = kv_index
	}
	probe_count := (meta >> hashbits) - 1
	m.ensure_extra_metas(probe_count)
}

[inline]
fn (mut m map) ensure_extra_metas(probe_count u32) {
	if (probe_count << 1) == m.extra_metas {
		size_of_u32 := sizeof(u32)
		old_mem_size := (m.even_index + 2 + m.extra_metas)
		m.extra_metas += extra_metas_inc
		mem_size := (m.even_index + 2 + m.extra_metas)
		unsafe {
			x := realloc_data(byteptr(m.metas), int(size_of_u32 * old_mem_size), int(size_of_u32 * mem_size))
			m.metas = &u32(x)
			C.memset(m.metas + mem_size - extra_metas_inc, 0, int(sizeof(u32) * extra_metas_inc))
		}
		// Should almost never happen
		if probe_count == 252 {
			panic('Probe overflow')
		}
	}
}

// Insert new element to the map. The element is inserted if its key is
// not equivalent to the key of any other element already in the container.
// If the key already exists, its value is changed to the value of the new element.
fn (mut m map) set_1(key voidptr, value voidptr) {
	load_factor := f32(m.len << 1) / f32(m.even_index)
	if load_factor > max_load_factor {
		m.expand()
	}
	mut index, mut meta := m.key_to_index(key)
	index, meta = m.meta_less(index, meta)
	// While we might have a match
	for meta == unsafe { m.metas[index] } {
		kv_index := int(unsafe { m.metas[index + 1] })
		pkey := unsafe { m.key_values.key(kv_index) }
		if m.key_eq_fn(key, pkey) {
			unsafe {
				pval := byteptr(pkey) + m.key_bytes_
				C.memcpy(pval, value, m.value_bytes)
			}
			return
		}
		index += 2
		meta += probe_inc
	}
	kv_index := m.key_values.expand()
	unsafe {
		pkey := m.key_values.key(kv_index)
		m.clone_fn(pkey, key)
		C.memcpy(byteptr(pkey) + m.key_bytes_, value, m.value_bytes)
	}
	m.meta_greater(index, meta, u32(kv_index))
	m.len++
}

// Doubles the size of the hashmap
fn (mut m map) expand() {
	old_cap := m.even_index
	m.even_index = ((m.even_index + 2) << 1) - 2
	// Check if any hashbits are left
	if m.cached_hashbits == 0 {
		m.shift += max_cached_hashbits
		m.cached_hashbits = max_cached_hashbits
		m.rehash()
	} else {
		m.cached_rehash(old_cap)
		m.cached_hashbits--
	}
}

// A rehash is the reconstruction of the hash table:
// All the elements in the container are rearranged according
// to their hash value into the newly sized key-value container.
// Rehashes are performed when the load_factor is going to surpass
// the max_load_factor in an operation.
fn (mut m map) rehash() {
	meta_bytes := sizeof(u32) * (m.even_index + 2 + m.extra_metas)
	unsafe {
		// TODO: use realloc_data here too
		x := v_realloc(byteptr(m.metas), int(meta_bytes))
		m.metas = &u32(x)
		C.memset(m.metas, 0, meta_bytes)
	}
	for i := 0; i < m.key_values.len; i++ {
		if !m.key_values.has_index(i) {
			continue
		}
		pkey := unsafe { m.key_values.key(i) }
		mut index, mut meta := m.key_to_index(pkey)
		index, meta = m.meta_less(index, meta)
		m.meta_greater(index, meta, u32(i))
	}
}

// This method works like rehash. However, instead of rehashing the
// key completely, it uses the bits cached in `metas`.
fn (mut m map) cached_rehash(old_cap u32) {
	old_metas := m.metas
	metasize := int(sizeof(u32) * (m.even_index + 2 + m.extra_metas))
	m.metas = &u32(vcalloc(metasize))
	old_extra_metas := m.extra_metas
	for i := u32(0); i <= old_cap + old_extra_metas; i += 2 {
		if unsafe { old_metas[i] } == 0 {
			continue
		}
		old_meta := unsafe { old_metas[i] }
		old_probe_count := ((old_meta >> hashbits) - 1) << 1
		old_index := (i - old_probe_count) & (m.even_index >> 1)
		mut index := (old_index | (old_meta << m.shift)) & m.even_index
		mut meta := (old_meta & hash_mask) | probe_inc
		index, meta = m.meta_less(index, meta)
		kv_index := unsafe { old_metas[i + 1] }
		m.meta_greater(index, meta, kv_index)
	}
	unsafe { free(old_metas) }
}

// This method is used for assignment operators. If the argument-key
// does not exist in the map, it's added to the map along with the zero/default value.
// If the key exists, its respective value is returned.
fn (mut m map) get_and_set_1(key voidptr, zero voidptr) voidptr {
	for {
		mut index, mut meta := m.key_to_index(key)
		for {
			if meta == unsafe { m.metas[index] } {
				kv_index := int(unsafe { m.metas[index + 1] })
				pkey := unsafe { m.key_values.key(kv_index) }
				if m.key_eq_fn(key, pkey) {
					return unsafe { byteptr(pkey) + m.key_values.key_bytes_ }
				}
			}
			index += 2
			meta += probe_inc
			if meta > unsafe { m.metas[index] } {
				break
			}
		}
		// Key not found, insert key with zero-value
		m.set_1(key, zero)
	}
	assert false
	return voidptr(0)
}

// If `key` matches the key of an element in the container,
// the method returns a reference to its mapped value.
// If not, a zero/default value is returned.
fn (m &map) get_1(key voidptr, zero voidptr) voidptr {
	mut index, mut meta := m.key_to_index(key)
	for {
		if meta == unsafe { m.metas[index] } {
			kv_index := int(unsafe { m.metas[index + 1] })
			pkey := unsafe { m.key_values.key(kv_index) }
			if m.key_eq_fn(key, pkey) {
				return unsafe { byteptr(pkey) + m.key_values.key_bytes_ }
			}
		}
		index += 2
		meta += probe_inc
		if meta > unsafe { m.metas[index] } {
			break
		}
	}
	return zero
}

// If `key` matches the key of an element in the container,
// the method returns a reference to its mapped value.
// If not, a zero pointer is returned.
// This is used in `x := m['key'] or { ... }`
fn (m &map) get_1_check(key voidptr) voidptr {
	mut index, mut meta := m.key_to_index(key)
	for {
		if meta == unsafe { m.metas[index] } {
			kv_index := int(unsafe { m.metas[index + 1] })
			pkey := unsafe { m.key_values.key(kv_index) }
			if m.key_eq_fn(key, pkey) {
				return unsafe { byteptr(pkey) + m.key_values.key_bytes_ }
			}
		}
		index += 2
		meta += probe_inc
		if meta > unsafe { m.metas[index] } {
			break
		}
	}
	return 0
}

// Checks whether a particular key exists in the map.
fn (m &map) exists_1(key voidptr) bool {
	mut index, mut meta := m.key_to_index(key)
	for {
		if meta == unsafe { m.metas[index] } {
			kv_index := int(unsafe { m.metas[index + 1] })
			pkey := unsafe { m.key_values.key(kv_index) }
			if m.key_eq_fn(key, pkey) {
				return true
			}
		}
		index += 2
		meta += probe_inc
		if meta > unsafe { m.metas[index] } {
			break
		}
	}
	return false
}

[inline]
fn (mut d DenseArray) delete(i int) {
	if d.deletes == 0 {
		d.all_deleted = vcalloc(d.cap) // sets to 0
	}
	d.deletes++
	unsafe {
		d.all_deleted[i] = 1
	}
}

// delete this
pub fn (mut m map) delete(key string) {
	unsafe {
		m.delete_1(&key)
	}
}

// Removes the mapping of a particular key from the map.
[unsafe]
pub fn (mut m map) delete_1(key voidptr) {
	mut index, mut meta := m.key_to_index(key)
	index, meta = m.meta_less(index, meta)
	// Perform backwards shifting
	for meta == unsafe { m.metas[index] } {
		kv_index := int(unsafe { m.metas[index + 1] })
		pkey := unsafe { m.key_values.key(kv_index) }
		if m.key_eq_fn(key, pkey) {
			for (unsafe { m.metas[index + 2] } >> hashbits) > 1 {
				unsafe {
					m.metas[index] = m.metas[index + 2] - probe_inc
					m.metas[index + 1] = m.metas[index + 3]
				}
				index += 2
			}
			m.len--
			m.key_values.delete(kv_index)
			unsafe {
				m.metas[index] = 0
				m.free_fn(pkey)
				// Mark key as deleted
				C.memset(pkey, 0, m.key_bytes)
			}
			if m.key_values.len <= 32 {
				return
			}
			// Clean up key_values if too many have been deleted
			if m.key_values.deletes >= (m.key_values.len >> 1) {
				m.key_values.zeros_to_end()
				m.rehash()
			}
			return
		}
		index += 2
		meta += probe_inc
	}
}

// bootstrap
// delete this
pub fn (m &map) keys() []string {
	mut keys := []string{len: m.len}
	mut item := unsafe { byteptr(keys.data) }
	for i := 0; i < m.key_values.len; i++ {
		if !m.key_values.has_index(i) {
			continue
		}
		unsafe {
			pkey := m.key_values.key(i)
			m.clone_fn(item, pkey)
			item += m.key_bytes
		}
	}
	return keys
}

// Returns all keys in the map.
fn (m &map) keys_1() array {
	mut keys := __new_array(m.len, 0, m.key_bytes)
	mut item := unsafe { byteptr(keys.data) }
	if m.key_values.deletes == 0 {
		for i := 0; i < m.key_values.len; i++ {
			unsafe {
				pkey := m.key_values.key(i)
				m.clone_fn(item, pkey)
				item += m.key_bytes
			}
		}
		return keys
	}
	for i := 0; i < m.key_values.len; i++ {
		if !m.key_values.has_index(i) {
			continue
		}
		unsafe {
			pkey := m.key_values.key(i)
			m.clone_fn(item, pkey)
			item += m.key_bytes
		}
	}
	return keys
}

// warning: only copies keys, does not clone
[unsafe]
fn (d &DenseArray) clone() DenseArray {
	res := DenseArray{
		key_bytes: d.key_bytes
		key_bytes_: d.key_bytes_
		value_bytes: d.value_bytes
		slot_bytes: d.slot_bytes
		cap: d.cap
		len: d.len
		deletes: d.deletes
		all_deleted: 0
		data: 0
	}
	unsafe {
		if d.deletes != 0 {
			res.all_deleted = memdup(d.all_deleted, d.cap)
		}
		res.data = memdup(d.data, d.cap * d.slot_bytes)
	}
	return res
}

// clone returns a clone of the `map`.
[unsafe]
pub fn (m &map) clone() map {
	metasize := int(sizeof(u32) * (m.even_index + 2 + m.extra_metas))
	res := map{
		key_bytes: m.key_bytes
		key_bytes_: m.key_bytes_
		value_bytes: m.value_bytes
		even_index: m.even_index
		cached_hashbits: m.cached_hashbits
		shift: m.shift
		key_values: unsafe { m.key_values.clone() }
		metas: unsafe { &u32(malloc(metasize)) }
		extra_metas: m.extra_metas
		len: m.len
		has_string_keys: m.has_string_keys
		hash_fn: m.hash_fn
		key_eq_fn: m.key_eq_fn
		clone_fn: m.clone_fn
		free_fn: m.free_fn
	}
	unsafe { C.memcpy(res.metas, m.metas, metasize) }
	if !m.has_string_keys {
		return res
	}
	// clone keys
	for i in 0 .. m.key_values.len {
		if !m.key_values.has_index(i) {
			continue
		}
		m.clone_fn(res.key_values.key(i), m.key_values.key(i))
	}
	return res
}

// free releases all memory resources occupied by the `map`.
[unsafe]
pub fn (m &map) free() {
	unsafe { free(m.metas) }
	if m.key_values.deletes == 0 {
		for i := 0; i < m.key_values.len; i++ {
			unsafe {
				pkey := m.key_values.key(i)
				m.free_fn(pkey)
			}
		}
	} else {
		for i := 0; i < m.key_values.len; i++ {
			if !m.key_values.has_index(i) {
				continue
			}
			unsafe {
				pkey := m.key_values.key(i)
				m.free_fn(pkey)
			}
		}
		unsafe { free(m.key_values.all_deleted) }
	}
	unsafe { free(m.key_values.data) }
}