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map: small cleanup

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ka-weihe 2020-03-21 13:55:07 +01:00 committed by GitHub
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vlib/builtin/map.v
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@ -1,14 +1,12 @@
// 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.
@ -17,7 +15,7 @@ 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
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.
@ -26,50 +24,48 @@ way such that all keys stay reasonably close to the slot they originally hash to
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 "metas" (meta-data). Each Key-value has a corresponding meta. A
"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.
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 you can use hash & (SIZE -1) instead
of abs(hash) % SIZE. Modulo is extremely expensive so using '&' is a big perfor-
mance improvement. The general concern with this is that you only use the lower
bits of the hash and can cause many collisions. This is solved by using very go-
od hash-function.
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-
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
index the last index.
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 complete, it simply uses
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 rehasing
// 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
// Initial max load-factor
init_max_load_factor = 0.8
// Minimum Load-factor.
// Number is picked to make delete O(1) amortized
min_load_factor = 0.3
// Initial range cap
// 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
@ -78,8 +74,6 @@ const (
hash_mask = u32(0x00FFFFFF)
// Used for incrementing the probe-count
probe_inc = u32(0x01000000)
// Bitmask for maximum probe count
max_probe = u32(0xFF000000)
)
struct KeyValue {
@ -91,20 +85,20 @@ mut:
// Dynamic array with very low growth factor
struct DenseArray {
mut:
data &KeyValue
cap u32
size u32
deletes u32
data &KeyValue
}
[inline]
fn new_dense_array() DenseArray {
unsafe{
return DenseArray{
data: &KeyValue(malloc(8 * sizeof(KeyValue)))
cap: 8
size: 0
deletes: 0
data: &KeyValue(malloc(8 * sizeof(KeyValue)))
}
}
}
@ -135,7 +129,7 @@ fn (d mut DenseArray) zeros_to_end() {
count++
}
}
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))
@ -151,12 +145,12 @@ mut:
window byte
// Used for right-shifting out used hashbits
shift byte
// Pointer to Key-value memory
// Array storing key-values (ordered)
key_values DenseArray
// Pointer to meta-data
// Pointer to meta-data:
// Odd indices stores index in `key_values`.
// Even indices stores probe_count and hashbits.
metas &u32
// Measure that decides when to increase the capacity
max_load_factor f32
// Extra metas that allows for no ranging when incrementing
// index in the hashmap
extra_metas u32
@ -173,7 +167,6 @@ fn new_map(n, value_bytes int) map {
shift: init_log_capicity
key_values: new_dense_array()
metas: &u32(vcalloc(sizeof(u32) * (init_capicity + extra_metas_inc)))
max_load_factor: init_max_load_factor
extra_metas: extra_metas_inc
size: 0
}
@ -188,15 +181,15 @@ fn new_map_init(n, value_bytes int, keys &string, values voidptr) map {
}
[inline]
fn (m map) key_to_index(key string) (u64, u32) {
hash := wyhash.wyhash_c(key.str, u64(key.len), 0)
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 := u32(((hash>>m.shift) & hash_mask) | probe_inc)
meta := ((hash>>m.shift) & hash_mask) | probe_inc
return index,meta
}
[inline]
fn meta_less(metas &u32, i u64, m u32) (u64, u32){
fn meta_less(metas &u32, i u32, m u32) (u32,u32) {
mut index := i
mut meta := m
for meta < metas[index] {
@ -207,7 +200,7 @@ fn meta_less(metas &u32, i u64, m u32) (u64, u32){
}
[inline]
fn (m mut map) meta_greater(ms &u32, i u64, me u32, kvi u32) &u32 {
fn (m mut map) meta_greater(ms &u32, i u32, me u32, kvi u32) &u32 {
mut metas := ms
mut meta := me
mut index := i
@ -234,7 +227,7 @@ fn (m mut map) meta_greater(ms &u32, i u64, me u32, kvi u32) &u32 {
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")
panic('Probe overflow')
}
}
return metas
@ -242,7 +235,7 @@ fn (m mut map) meta_greater(ms &u32, i u64, me u32, kvi u32) &u32 {
fn (m mut map) set(key string, value voidptr) {
load_factor := f32(m.size<<1) / f32(m.cap)
if load_factor > m.max_load_factor {
if load_factor > max_load_factor {
m.expand()
}
mut index,mut meta := m.key_to_index(key)
@ -275,8 +268,8 @@ fn (m mut map) expand() {
// Check if any hashbits are left
if m.window == 0 {
m.shift += cached_hashbits
m.rehash()
m.window = cached_hashbits
m.rehash()
}
else {
m.cached_rehash(old_cap)
@ -302,14 +295,14 @@ fn (m mut map) rehash() {
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 := 0; i <= old_cap + old_extra_metas; i += 2 {
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 := u64((old_meta>>hashbits) - 1) << 1
old_probe_count := ((old_meta>>hashbits) - 1)<<1
old_index := (i - old_probe_count) & (m.cap>>1)
mut index := u64(old_index) | (old_meta << m.shift) & m.cap
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]
@ -321,7 +314,6 @@ fn (m mut map) cached_rehash(old_cap u32) {
m.metas = new_meta
}
[inline]
fn (m map) get(key string, out voidptr) bool {
mut index,mut meta := m.key_to_index(key)
index,meta = meta_less(m.metas, index, meta)
@ -337,7 +329,6 @@ fn (m map) get(key string, out voidptr) bool {
return false
}
[inline]
fn (m map) get2(key string) voidptr {
mut index,mut meta := m.key_to_index(key)
index,meta = meta_less(m.metas, index, meta)
@ -354,7 +345,6 @@ fn (m map) get2(key string) voidptr {
return voidptr(0)
}
[inline]
fn (m map) exists(key string) bool {
if m.value_bytes == 0 {
return false
@ -380,22 +370,17 @@ pub fn (m mut map) delete(key string) {
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))
mut old_index := index
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
mut cur_meta := m.metas[index]
mut cur_index := m.metas[index + 1]
for (cur_meta >> hashbits) > 1 {
m.metas[old_index] = cur_meta - probe_inc
m.metas[old_index + 1] = cur_index
old_index = index
index += 2
cur_meta = m.metas[index]
cur_index = m.metas[index + 1]
}
m.metas[old_index] = 0
m.size--
m.metas[index] = 0
m.key_values.deletes++
if m.key_values.size <= 32 {return}
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()