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v/vlib/builtin/map.v
2020-04-04 14:22:39 +02:00

420 lines
11 KiB
V

// 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
hash.wyhash
)
/*
This is a very fast hashmap implementation. It has several properties that in
combination makes it very fast. Here is a short explanation of each property.
After reading this you should have a basic understanding of how it works:
1. |Hash-function (Wyhash)|. Wyhash is the fastest hash-function passing SMHash-
er, so it was an easy 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 sim-
ple but clever twist: As new keys are inserted, old keys are shifted around in a
way such that all keys stay reasonably close to the slot they originally hash to.
3. |Memory layout|. Key-value pairs are stored in a `DenseArray`, with an avera-
ge of roughly 6.25% unused memory, as opposed to most other dynamic array imple-
mentations with a growth factor of 1.5 or 2. The key-values keep their index in
the array - they are not probed. Instead, this implementation uses another array
"metas" storing "meta"s (meta-data). Each Key-value has a corresponding meta. A
meta stores a reference to its key-value, and its index in "metas" is determined
by the hash of the key and probing. A meta also stores bits from the hash (for
faster rehashing etc.) and how far away it is from the index it was originally
hashed to (probe_count). probe_count is 0 if empty, 1 if not probed, 2 if probed
by 1, etc..
meta (64 bit) = kv_index (32 bit) | probe_count (8 bits) | hashbits (24 bits)
metas = [meta, 0, meta, 0, meta, meta, meta, 0, ...]
key_values = [kv, kv, kv, kv, kv, ...]
4. |Power of two size array|. The size of metas is a power of two. This makes it
possible to find a bucket from a hash code by using "hash & (SIZE -1)" instead
of "abs(hash) % SIZE". Modulo is extremely expensive so using '&' is a big perf-
ormance improvement. The general concern with this is that you only use the low-
er bits of the hash and that can cause more collisions. This is solved by using
good hash-function.
5. |Extra metas|. The hashmap keeps track of the highest probe_count. The trick
is to allocate extra_metas > max(probe_count), so you never have to do any boun-
ds-checking because the extra metas ensures that an element will never go beyond
the last index.
6. |Cached rehashing|. When the load_factor of the map exceeds the max_load_fac-
tor the size of metas is doubled and all the elements need to be "rehashed" to
find the index in the new array. Instead of rehashing completely, it simply uses
the hashbits stored in the meta.
*/
const (
// Number of bits from the hash stored for each entry
hashbits = 24
// Number of bits from the hash stored for rehashing
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 (size / 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)
)
struct KeyValue {
key string
mut:
value voidptr
}
// Dynamic array with very low growth factor
struct DenseArray {
mut:
cap u32
size u32
deletes u32
data &KeyValue
}
[inline]
fn new_dense_array() DenseArray {
unsafe{
return DenseArray{
cap: 8
size: 0
deletes: 0
data: &KeyValue(malloc(8 * sizeof(KeyValue)))
}
}
}
// Push element to array and return index
// The growth-factor is roughly 12.5 `(x + (x >> 3))`
[inline]
fn (d mut DenseArray) push(kv KeyValue) u32 {
if d.cap == d.size {
d.cap += d.cap>>3
d.data = &KeyValue(C.realloc(d.data, sizeof(KeyValue) * d.cap))
}
push_index := d.size
d.data[push_index] = kv
d.size++
return push_index
}
// Move all zeros to the end of the array
// and resize array
fn (d mut DenseArray) zeros_to_end() {
mut count := u32(0)
for i in 0 .. d.size {
if d.data[i].key.str != 0 {
tmp := d.data[count]
d.data[count] = d.data[i]
d.data[i] = tmp
count++
}
}
d.deletes = 0
d.size = count
d.cap = if count < 8 { 8 } else { count }
d.data = &KeyValue(C.realloc(d.data, sizeof(KeyValue) * d.cap))
}
pub struct map {
// Byte size of value
value_bytes int
mut:
// Index of the highest index in the hashtable
cap u32
// Number of cached hashbits left for rehasing
window byte
// Used for right-shifting out used hashbits
shift byte
// Array storing key-values (ordered)
key_values DenseArray
// Pointer to meta-data:
// Odd indices stores index in `key_values`.
// Even indices stores 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
size int
}
fn new_map(n, value_bytes int) map {
return map{
value_bytes: value_bytes
cap: init_cap
window: cached_hashbits
shift: init_log_capicity
key_values: new_dense_array()
metas: &u32(vcalloc(sizeof(u32) * (init_capicity + extra_metas_inc)))
extra_metas: extra_metas_inc
size: 0
}
}
fn new_map_init(n, value_bytes int, keys &string, values voidptr) map {
mut out := new_map(n, value_bytes)
for i in 0 .. n {
out.set(keys[i], byteptr(values) + i * value_bytes)
}
return out
}
[inline]
fn (m map) key_to_index(key string) (u32,u32) {
hash := u32(wyhash.wyhash_c(key.str, u64(key.len), 0))
index := hash & m.cap
meta := ((hash>>m.shift) & hash_mask) | probe_inc
return index,meta
}
[inline]
fn meta_less(metas &u32, i u32, m u32) (u32,u32) {
mut index := i
mut meta := m
for meta < metas[index] {
index += 2
meta += probe_inc
}
return index,meta
}
[inline]
fn (m mut map) meta_greater(ms &u32, i u32, me u32, kvi u32) &u32 {
mut metas := ms
mut meta := me
mut index := i
mut kv_index := kvi
for metas[index] != 0 {
if meta > metas[index] {
tmp_meta := metas[index]
metas[index] = meta
meta = tmp_meta
tmp_index := metas[index + 1]
metas[index + 1] = kv_index
kv_index = tmp_index
}
index += 2
meta += probe_inc
}
metas[index] = meta
metas[index + 1] = kv_index
probe_count := (meta>>hashbits) - 1
if (probe_count<<1) == m.extra_metas {
m.extra_metas += extra_metas_inc
mem_size := (m.cap + 2 + m.extra_metas)
metas = &u32(C.realloc(metas, sizeof(u32) * mem_size))
C.memset(metas + mem_size - extra_metas_inc, 0, sizeof(u32) * extra_metas_inc)
// Should almost never happen
if probe_count == 252 {
panic('Probe overflow')
}
}
return metas
}
fn (m mut map) set(key string, value voidptr) {
load_factor := f32(m.size<<1) / f32(m.cap)
if load_factor > max_load_factor {
m.expand()
}
mut index,mut meta := m.key_to_index(key)
index,meta = meta_less(m.metas, index, meta)
// While we might have a match
for meta == m.metas[index] {
kv_index := m.metas[index + 1]
if key == m.key_values.data[kv_index].key {
C.memcpy(m.key_values.data[kv_index].value, value, m.value_bytes)
return
}
index += 2
meta += probe_inc
}
// Match not possible anymore
kv := KeyValue{
key: key
value: malloc(m.value_bytes)
}
C.memcpy(kv.value, value, m.value_bytes)
kv_index := m.key_values.push(kv)
m.metas = m.meta_greater(m.metas, index, meta, kv_index)
m.size++
}
// Doubles the size of the hashmap
fn (m mut map) expand() {
old_cap := m.cap
m.cap = ((m.cap + 2)<<1) - 2
// Check if any hashbits are left
if m.window == 0 {
m.shift += cached_hashbits
m.window = cached_hashbits
m.rehash()
}
else {
m.cached_rehash(old_cap)
}
m.window--
}
fn (m mut map) rehash() {
meta_bytes := sizeof(u32) * (m.cap + 2 + m.extra_metas)
m.metas = &u32(C.realloc(m.metas, meta_bytes))
C.memset(m.metas, 0, meta_bytes)
for i := u32(0); i < m.key_values.size; i++ {
if m.key_values.data[i].key.str == 0 {
continue
}
kv := m.key_values.data[i]
mut index,mut meta := m.key_to_index(kv.key)
index,meta = meta_less(m.metas, index, meta)
m.metas = m.meta_greater(m.metas, index, meta, i)
}
}
fn (m mut map) cached_rehash(old_cap u32) {
mut new_meta := &u32(vcalloc(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 m.metas[i] == 0 {
continue
}
old_meta := m.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 = meta_less(new_meta, index, meta)
kv_index := m.metas[i + 1]
new_meta = m.meta_greater(new_meta, index, meta, kv_index)
}
unsafe{
free(m.metas)
}
m.metas = new_meta
}
fn (m map) get3(key string, zero voidptr) voidptr {
mut index,mut meta := m.key_to_index(key)
index,meta = meta_less(m.metas, index, meta)
for meta == m.metas[index] {
kv_index := m.metas[index + 1]
if key == m.key_values.data[kv_index].key {
out := malloc(m.value_bytes)
C.memcpy(out, m.key_values.data[kv_index].value, m.value_bytes)
return out
}
index += 2
meta += probe_inc
}
return zero
//return voidptr(0)
}
fn (m map) exists(key string) bool {
if m.value_bytes == 0 {
return false
}
mut index,mut meta := m.key_to_index(key)
index,meta = meta_less(m.metas, index, meta)
for meta == m.metas[index] {
kv_index := m.metas[index + 1]
if key == m.key_values.data[kv_index].key {
return true
}
index += 2
meta += probe_inc
}
return false
}
pub fn (m mut map) delete(key string) {
mut index,mut meta := m.key_to_index(key)
index,meta = meta_less(m.metas, index, meta)
// Perform backwards shifting
for meta == m.metas[index] {
kv_index := m.metas[index + 1]
if key == m.key_values.data[kv_index].key {
C.memset(&m.key_values.data[kv_index], 0, sizeof(KeyValue))
for (m.metas[index + 2]>>hashbits) > 1 {
m.metas[index] = m.metas[index + 2] - probe_inc
m.metas[index + 1] = m.metas[index + 3]
index += 2
}
m.size--
m.metas[index] = 0
m.key_values.deletes++
if m.key_values.size <= 32 {
return
}
if (f32(m.key_values.size) / f32(m.key_values.deletes)) < 1 {
m.key_values.zeros_to_end()
m.rehash()
}
return
}
index += 2
meta += probe_inc
}
}
pub fn (m &map) keys() []string {
mut keys := [''].repeat(m.size)
if m.value_bytes == 0 {
return keys
}
mut j := 0
for i := u32(0); i < m.key_values.size; i++ {
if m.key_values.data[i].key.str == 0 {
continue
}
keys[j] = m.key_values.data[i].key
j++
}
return keys
}
pub fn (m map) free() {
unsafe{
free(m.metas)
free(m.key_values.data)
}
}
pub fn (m map) print() {
println('TODO')
}
pub fn (m map_string) str() string {
if m.size == 0 {
return '{}'
}
mut sb := strings.new_builder(50)
sb.writeln('{')
for key, val in m {
sb.writeln(' "$key" => "$val"')
}
sb.writeln('}')
return sb.str()
}