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vendor/github.com/trivago/tgo/tcontainer/bytepool.go

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+// Copyright 2015-2016 trivago GmbH
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tcontainer
+
+import (
+	"reflect"
+	"runtime"
+	"sync/atomic"
+	"unsafe"
+)
+
+const (
+	tiny   = 64
+	small  = 512
+	medium = 1024
+	large  = 1024 * 10
+	huge   = 1024 * 100
+
+	tinyCount   = 16384 // 1 MB
+	smallCount  = 2048  // 1 MB
+	mediumCount = 1024  // 1 MB
+	largeCount  = 102   // ~1 MB
+	hugeCount   = 10    // ~1 MB
+)
+
+type byteSlab struct {
+	buffer     []byte
+	bufferSize uintptr
+	stride     uintptr
+	basePtr    *uintptr
+	nextPtr    *uintptr
+}
+
+// BytePool is a fragmentation friendly way to allocated byte slices.
+type BytePool struct {
+	tinySlab   byteSlab
+	smallSlab  byteSlab
+	mediumSlab byteSlab
+	largeSlab  byteSlab
+	hugeSlab   byteSlab
+}
+
+func newByteSlab(size, count int) byteSlab {
+	bufferSize := count * size
+	buffer := make([]byte, bufferSize)
+	basePtr := (*reflect.SliceHeader)(unsafe.Pointer(&buffer)).Data
+	nextPtr := basePtr + uintptr(bufferSize)
+
+	return byteSlab{
+		buffer:     buffer,
+		bufferSize: uintptr(bufferSize),
+		stride:     uintptr(size),
+		basePtr:    &basePtr,
+		nextPtr:    &nextPtr,
+	}
+}
+
+func (slab *byteSlab) getSlice(size int) (chunk []byte) {
+	chunkHeader := (*reflect.SliceHeader)(unsafe.Pointer(&chunk))
+	chunkHeader.Len = size
+	chunkHeader.Cap = int(slab.stride)
+
+	for {
+		// WARNING: The following two lines are order sensitive
+		basePtr := atomic.LoadUintptr(slab.basePtr)
+		nextPtr := atomic.AddUintptr(slab.nextPtr, -slab.stride)
+		lastPtr := basePtr + slab.bufferSize
+
+		switch {
+		case nextPtr < basePtr || nextPtr >= lastPtr:
+			// out of range either means alloc while realloc or race between
+			// base and next during realloc. In the latter case we lose a chunk.
+			runtime.Gosched()
+
+		case nextPtr == basePtr:
+			// Last item: realloc
+			slab.buffer = make([]byte, slab.bufferSize)
+			dataPtr := (*reflect.SliceHeader)(unsafe.Pointer(&slab.buffer)).Data
+
+			// WARNING: The following two lines are order sensitive
+			atomic.StoreUintptr(slab.nextPtr, dataPtr+slab.bufferSize)
+			atomic.StoreUintptr(slab.basePtr, dataPtr)
+			fallthrough
+
+		default:
+			chunkHeader.Data = nextPtr
+			return
+		}
+	}
+}
+
+// NewBytePool creates a new BytePool with each slab using 1 MB of storage.
+// The pool contains 5 slabs of different sizes: 64B, 512B, 1KB, 10KB and 100KB.
+// Allocations above 100KB will be allocated directly.
+func NewBytePool() BytePool {
+	return BytePool{
+		tinySlab:   newByteSlab(tiny, tinyCount),
+		smallSlab:  newByteSlab(small, smallCount),
+		mediumSlab: newByteSlab(medium, mediumCount),
+		largeSlab:  newByteSlab(large, largeCount),
+		hugeSlab:   newByteSlab(huge, hugeCount),
+	}
+}
+
+// NewBytePoolWithSize creates a new BytePool with each slab size using n MB of
+// storage. See NewBytePool() for slab size details.
+func NewBytePoolWithSize(n int) BytePool {
+	if n <= 0 {
+		n = 1
+	}
+	return BytePool{
+		tinySlab:   newByteSlab(tiny, tinyCount*n),
+		smallSlab:  newByteSlab(small, smallCount*n),
+		mediumSlab: newByteSlab(medium, mediumCount*n),
+		largeSlab:  newByteSlab(large, largeCount*n),
+		hugeSlab:   newByteSlab(huge, hugeCount*n),
+	}
+}
+
+// Get returns a slice allocated to a normalized size.
+// Sizes are organized in evenly sized buckets so that fragmentation is kept low.
+func (b *BytePool) Get(size int) []byte {
+	switch {
+	case size == 0:
+		return []byte{}
+
+	case size <= tiny:
+		return b.tinySlab.getSlice(size)
+
+	case size <= small:
+		return b.smallSlab.getSlice(size)
+
+	case size <= medium:
+		return b.mediumSlab.getSlice(size)
+
+	case size <= large:
+		return b.largeSlab.getSlice(size)
+
+	case size <= huge:
+		return b.hugeSlab.getSlice(size)
+
+	default:
+		return make([]byte, size)
+	}
+}