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			126 lines
		
	
	
		
			3.5 KiB
		
	
	
	
		
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			126 lines
		
	
	
		
			3.5 KiB
		
	
	
	
		
			Go
		
	
	
	
	
	
/*
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 * Copyright 2017 Dgraph Labs, Inc. and Contributors
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 * Modifications copyright (C) 2017 Andy Kimball and Contributors
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 *
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 * Licensed under the Apache License, Version 2.0 (the "License");
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 * you may not use this file except in compliance with the License.
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 * You may obtain a copy of the License at
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 *
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 *     http://www.apache.org/licenses/LICENSE-2.0
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 *
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 * Unless required by applicable law or agreed to in writing, software
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 * distributed under the License is distributed on an "AS IS" BASIS,
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 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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 * See the License for the specific language governing permissions and
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 * limitations under the License.
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 */
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package arenaskl
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import (
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	"sync/atomic"
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	"unsafe"
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	"github.com/cockroachdb/errors"
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	"github.com/cockroachdb/pebble/internal/constants"
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	"github.com/cockroachdb/pebble/internal/invariants"
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)
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// Arena is lock-free.
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type Arena struct {
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	n   atomic.Uint64
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	buf []byte
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}
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const nodeAlignment = 4
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var (
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	// ErrArenaFull indicates that the arena is full and cannot perform any more
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	// allocations.
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	ErrArenaFull = errors.New("allocation failed because arena is full")
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)
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// NewArena allocates a new arena using the specified buffer as the backing
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// store.
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func NewArena(buf []byte) *Arena {
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	if len(buf) > constants.MaxUint32OrInt {
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		if invariants.Enabled {
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			panic(errors.AssertionFailedf("attempting to create arena of size %d", len(buf)))
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		}
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		buf = buf[:constants.MaxUint32OrInt]
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	}
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	a := &Arena{
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		buf: buf,
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	}
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	// We don't store data at position 0 in order to reserve offset=0 as a kind of
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	// nil pointer.
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	a.n.Store(1)
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	return a
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}
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// Size returns the number of bytes allocated by the arena.
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func (a *Arena) Size() uint32 {
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	s := a.n.Load()
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	if s > constants.MaxUint32OrInt {
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		// The last failed allocation can push the size higher than len(a.buf).
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		// Saturate at the maximum representable offset.
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		return constants.MaxUint32OrInt
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	}
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	return uint32(s)
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}
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// Capacity returns the capacity of the arena.
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func (a *Arena) Capacity() uint32 {
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	return uint32(len(a.buf))
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}
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// alloc allocates a buffer of the given size and with the given alignment
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// (which must be a power of 2).
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//
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// If overflow is not 0, it also ensures that many bytes after the buffer are
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// inside the arena (this is used for structures that are larger than the
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// requested size but don't use those extra bytes).
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func (a *Arena) alloc(size, alignment, overflow uint32) (uint32, uint32, error) {
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	if invariants.Enabled && (alignment&(alignment-1)) != 0 {
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		panic(errors.AssertionFailedf("invalid alignment %d", alignment))
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	}
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	// Verify that the arena isn't already full.
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	origSize := a.n.Load()
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	if int(origSize) > len(a.buf) {
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		return 0, 0, ErrArenaFull
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	}
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	// Pad the allocation with enough bytes to ensure the requested alignment.
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	padded := uint64(size) + uint64(alignment) - 1
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	newSize := a.n.Add(padded)
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	if newSize+uint64(overflow) > uint64(len(a.buf)) {
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		return 0, 0, ErrArenaFull
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	}
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	// Return the aligned offset.
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	offset := (uint32(newSize) - size) & ^(alignment - 1)
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	return offset, uint32(padded), nil
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}
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func (a *Arena) getBytes(offset uint32, size uint32) []byte {
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	if offset == 0 {
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		return nil
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	}
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	return a.buf[offset : offset+size : offset+size]
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}
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func (a *Arena) getPointer(offset uint32) unsafe.Pointer {
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	if offset == 0 {
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		return nil
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	}
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	return unsafe.Pointer(&a.buf[offset])
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}
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func (a *Arena) getPointerOffset(ptr unsafe.Pointer) uint32 {
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	if ptr == nil {
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		return 0
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	}
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	return uint32(uintptr(ptr) - uintptr(unsafe.Pointer(&a.buf[0])))
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}
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