// Copyright (c) 2019-2020 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 strings
//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_cap = 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)
)
// This function 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, b string) bool {
if a.len != b.len {
return false
}
unsafe {
return C.memcmp(a.str, b.str, b.len) == 0
}
}
// Dynamic array with very low growth factor
struct DenseArray {
value_bytes int
mut:
cap u32
len u32
deletes u32
keys &string
values byteptr
}
[inline]
[unsafe_fn]
fn new_dense_array(value_bytes int) DenseArray {
return DenseArray{
value_bytes: value_bytes
cap: 8
len: 0
deletes: 0
keys: &string(malloc(int(8 * sizeof(string))))
values: malloc(8 * value_bytes)
}
}
// Push element to array and return index
// The growth-factor is roughly 1.125 `(x + (x >> 3))`
[inline]
fn (mut d DenseArray) push(key string, value voidptr) u32 {
if d.cap == d.len {
d.cap += d.cap >> 3
unsafe {
d.keys = &string(v_realloc(d.keys, sizeof(string) * d.cap))
d.values = v_realloc(d.values, u32(d.value_bytes) * d.cap)
}
}
push_index := d.len
unsafe {
d.keys[push_index] = key
C.memcpy(d.values + push_index * u32(d.value_bytes), value, d.value_bytes)
}
d.len++
return push_index
}
fn (d DenseArray) get(i int) voidptr {
$if !no_bounds_checking? {
if i < 0 || i >= int(d.len) {
panic('DenseArray.get: index out of range (i == $i, d.len == $d.len)')
}
}
unsafe {
return byteptr(d.keys) + i * int(sizeof(string))
}
}
// Move all zeros to the end of the array and resize array
fn (mut d DenseArray) zeros_to_end() {
mut tmp_value := malloc(d.value_bytes)
mut count := u32(0)
for i in 0 .. d.len {
if unsafe {d.keys[i]}.str != 0 {
// swap keys
unsafe {
tmp_key := d.keys[count]
d.keys[count] = d.keys[i]
d.keys[i] = tmp_key
}
// swap values (TODO: optimize)
unsafe {
C.memcpy(tmp_value, d.values + count * u32(d.value_bytes), d.value_bytes)
C.memcpy(d.values + count * u32(d.value_bytes), d.values + i * d.value_bytes, d.value_bytes)
C.memcpy(d.values + i * d.value_bytes, tmp_value, d.value_bytes)
}
count++
}
}
free(tmp_value)
d.deletes = 0
d.len = count
d.cap = if count < 8 { u32(8) } else { count }
unsafe {
d.keys = &string(v_realloc(d.keys, sizeof(string) * d.cap))
d.values = v_realloc(d.values, u32(d.value_bytes) * d.cap)
}
}
pub struct map {
// Number of bytes of a value
value_bytes int
mut:
// Highest even index in the hashtable
cap u32
// Number of cached hashbits left for rehasing
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
pub mut:
// Number of key-values currently in the hashmap
len int
}
fn new_map_1(value_bytes int) map {
return map{
value_bytes: value_bytes
cap: init_cap
cached_hashbits: max_cached_hashbits
shift: init_log_capicity
key_values: new_dense_array(value_bytes)
metas: &u32(vcalloc(int(sizeof(u32) * (init_capicity + extra_metas_inc))))
extra_metas: extra_metas_inc
len: 0
}
}
fn new_map_init(n, value_bytes int, keys &string, values voidptr) map {
mut out := new_map_1(value_bytes)
for i in 0 .. n {
unsafe {
out.set(keys[i], byteptr(values) + i * value_bytes)
}
}
return out
}
[inline]
fn (m &map) key_to_index(key string) (u32,u32) {
hash := hash.wyhash_c(key.str, u64(key.len), 0)
index := hash & m.cap
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 := unsafe {m.metas[index + 1]}
unsafe {
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 {
m.extra_metas += extra_metas_inc
mem_size := (m.cap + 2 + m.extra_metas)
unsafe {
m.metas = &u32(v_realloc(m.metas, sizeof(u32) * mem_size))
C.memset(m.metas + mem_size - extra_metas_inc, 0, 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(k string, value voidptr) {
key := k.clone()
load_factor := f32(m.len << 1) / f32(m.cap)
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 := unsafe {m.metas[index + 1]}
if fast_string_eq(key, unsafe {m.key_values.keys[kv_index]}) {
unsafe {
C.memcpy(m.key_values.values + kv_index * u32(m.value_bytes), value, m.value_bytes)
}
return
}
index += 2
meta += probe_inc
}
kv_index := m.key_values.push(key, value)
m.meta_greater(index, meta, kv_index)
m.len++
}
// Doubles the size of the hashmap
fn (mut m map) expand() {
old_cap := m.cap
m.cap = ((m.cap + 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.cap + 2 + m.extra_metas)
unsafe {
m.metas = &u32(v_realloc(m.metas, meta_bytes))
C.memset(m.metas, 0, meta_bytes)
}
for i := u32(0); i < m.key_values.len; i++ {
if unsafe {m.key_values.keys[i]}.str == 0 {
continue
}
mut index,mut meta := m.key_to_index(unsafe {m.key_values.keys[i]})
index,meta = m.meta_less(index, meta)
m.meta_greater(index, meta, 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
m.metas = &u32(vcalloc(int(sizeof(u32) * (m.cap + 2 + m.extra_metas))))
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.cap >> 1)
mut index := (old_index | (old_meta << m.shift)) & m.cap
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/dafault value.
// If the key exists, its respective value is returned.
fn (mut m map) get_and_set(key string, zero voidptr) voidptr {
for {
mut index,mut meta := m.key_to_index(key)
for {
if meta == unsafe {m.metas[index]} {
kv_index := unsafe {m.metas[index + 1]}
if fast_string_eq(key, unsafe {m.key_values.keys[kv_index]}) {
unsafe {
return voidptr(m.key_values.values + kv_index * u32(m.value_bytes))
}
}
}
index += 2
meta += probe_inc
if meta > unsafe {m.metas[index]} { break }
}
// Key not found, insert key with zero-value
m.set(key, 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/default value is returned.
fn (m map) get(key string, zero voidptr) voidptr {
mut index,mut meta := m.key_to_index(key)
for {
if meta == unsafe {m.metas[index]} {
kv_index := unsafe {m.metas[index + 1]}
if fast_string_eq(key, unsafe {m.key_values.keys[kv_index]}) {
unsafe {
return voidptr(m.key_values.values + kv_index * u32(m.value_bytes))
}
}
}
index += 2
meta += probe_inc
if meta > unsafe {m.metas[index]} { break }
}
return zero
}
// Checks whether a particular key exists in the map.
fn (m map) exists(key string) bool {
mut index,mut meta := m.key_to_index(key)
for {
if meta == unsafe {m.metas[index]} {
kv_index := unsafe {m.metas[index + 1]}
if fast_string_eq(key, unsafe {m.key_values.keys[kv_index]}) {
return true
}
}
index += 2
meta += probe_inc
if meta > unsafe {m.metas[index]} { break }
}
return false
}
// Removes the mapping of a particular key from the map.
pub fn (mut m map) delete(key string) {
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 := unsafe {m.metas[index + 1]}
if fast_string_eq(key, unsafe {m.key_values.keys[kv_index]}) {
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--
unsafe {
m.metas[index] = 0
}
m.key_values.deletes++
// Mark key as deleted
unsafe {
m.key_values.keys[kv_index].free()
C.memset(&m.key_values.keys[kv_index], 0, sizeof(string))
}
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()
m.key_values.deletes = 0
}
return
}
index += 2
meta += probe_inc
}
}
// Returns all keys in the map.
// TODO: add optimization in case of no deletes
pub fn (m &map) keys() []string {
mut keys := []string{ len:m.len }
mut j := 0
for i := u32(0); i < m.key_values.len; i++ {
if unsafe {m.key_values.keys[i]}.str == 0 {
continue
}
keys[j] = unsafe {m.key_values.keys[i]}.clone()
j++
}
return keys
}
[unsafe_fn]
pub fn (d DenseArray) clone() DenseArray {
res := DenseArray {
value_bytes: d.value_bytes
cap: d.cap
len: d.len
deletes: d.deletes
keys: unsafe {&string(malloc(int(d.cap * sizeof(string))))}
values: unsafe {byteptr(malloc(int(d.cap * u32(d.value_bytes))))}
}
unsafe {
C.memcpy(res.keys, d.keys, d.cap * sizeof(string))
C.memcpy(res.values, d.values, d.cap * u32(d.value_bytes))
}
return res
}
[unsafe_fn]
pub fn (m map) clone() map {
metas_size := sizeof(u32) * (m.cap + 2 + m.extra_metas)
res := map{
value_bytes: m.value_bytes
cap: m.cap
cached_hashbits: m.cached_hashbits
shift: m.shift
key_values: unsafe {m.key_values.clone()}
metas: &u32(malloc(int(metas_size)))
extra_metas: m.extra_metas
len: m.len
}
unsafe {
C.memcpy(res.metas, m.metas, metas_size)
}
return res
}
[unsafe_fn]
pub fn (m &map) free() {
unsafe {
free(m.metas)
}
for i := u32(0); i < m.key_values.len; i++ {
if unsafe {m.key_values.keys[i]}.str == 0 {
continue
}
unsafe {
m.key_values.keys[i].free()
}
}
unsafe {
free(m.key_values.keys)
free(m.key_values.values)
}
}
pub fn (m map_string) str() string {
if m.len == 0 {
return '{}'
}
mut sb := strings.new_builder(50)
sb.writeln('{')
for key, val in m {
sb.writeln(' "$key" => "$val"')
}
sb.writeln('}')
return sb.str()
}