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/
db.go
161 lines (125 loc) · 2.88 KB
/
db.go
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// Copyright 2015 The XC Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package xc
import (
"encoding/binary"
"sync"
"github.com/cznic/mathutil"
)
const (
maxUvarint = (64 + 7) / 7
dbPageShift = 20 //TODO bench tune.
dbPageSize = 1 << dbPageShift
dbPageMask = dbPageSize - 1
)
// MemDB stores data accessible using a numeric identifier.
type MemDB struct {
b256 [256]byte
mu sync.RWMutex // Access guard.
nextID int
pages [][]byte
}
// NewMemDB returns a newly create *MemDB.
func NewMemDB() *MemDB {
d := &MemDB{}
for i := 0; i < 256; i++ {
d.b256[i] = byte(i)
}
return d
}
// Bytes returns the byte slice associated with id. Passing id not obtained by
// PutBytes has undefined behavior. Bytes RLocks the DB to obtain the result.
//
// NOTE: The result refers to the DB backing storage directly without copying
// and is thus strictly R/O.
func (d *MemDB) Bytes(id int) []byte {
if id == 0 {
return nil
}
if id < 257 {
return d.b256[id-1 : id:id]
}
id -= 257
off := id & dbPageMask
d.mu.RLock() // R+
page := d.pages[id>>dbPageShift]
n, l := binary.Uvarint(page[off:])
off += l
h := off + int(n)
r := page[off:h:h]
d.mu.RUnlock() // R-
return r
}
// bytesUnlocked is like Bytes but it does not RLock the DB.
func (d *MemDB) bytesUnlocked(id int) []byte {
if id == 0 {
return nil
}
if id < 257 {
return d.b256[id-1 : id:id]
}
id -= 257
off := id & dbPageMask
page := d.pages[id>>dbPageShift]
n, l := binary.Uvarint(page[off:])
off += l
r := page[off : off+int(n)]
return r
}
// Cap reports the current DB capacity.
func (d *MemDB) Cap() int {
d.mu.RLock() // R+
r := 0
for _, v := range d.pages {
r += cap(v)
}
d.mu.RUnlock() // R-
return r
}
// Len reports the size of the DB used by data.
func (d *MemDB) Len() int {
d.mu.RLock() // R+
r := 0
for _, v := range d.pages {
r += len(v)
}
d.mu.RUnlock() // R-
return r
}
// PutBytes stores b in the DB and returns its id. Zero length byte slices are
// guaranteed to return zero ID. PutBytes Locks the DB before updating it.
func (d *MemDB) PutBytes(b []byte) int {
if len(b) == 0 {
return 0
}
if len(b) == 1 {
return int(b[0]) + 1
}
d.mu.Lock() // W+
id := d.nextID
pi := id >> dbPageShift
if pi < len(d.pages) {
p := d.pages[pi]
off := id & dbPageMask
if n := cap(p) - off - maxUvarint; n >= len(b) {
p = p[:cap(p)]
l := binary.PutUvarint(p[off:], uint64(len(b)))
copy(p[off+l:], b)
n = l + len(b)
d.pages[pi] = p[:off+n]
d.nextID += n
d.mu.Unlock() // W-
return id + 257
}
pi++
}
p := make([]byte, mathutil.Max(dbPageSize, maxUvarint+len(b)))
p = p[:binary.PutUvarint(p, uint64(len(b)))]
p = append(p, b...)
d.pages = append(d.pages, p)
id = pi << dbPageShift
d.nextID = id + mathutil.Min(dbPageSize, len(p))
d.mu.Unlock() // W-
return id + 257
}