[WIP] Partial bot skeleton, non-functional though

vendor/github.com/cockroachdb/pebble/internal/arenaskl/arena.go

@@ -0,0 +1,109 @@
+/*
+ * Copyright 2017 Dgraph Labs, Inc. and Contributors
+ * Modifications copyright (C) 2017 Andy Kimball and Contributors
+ *
+ * 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 arenaskl
+
+import (
+	"math"
+	"sync/atomic"
+	"unsafe"
+
+	"github.com/cockroachdb/errors"
+)
+
+// Arena is lock-free.
+type Arena struct {
+	n   uint64
+	buf []byte
+}
+
+const (
+	align4 = 3
+)
+
+var (
+	// ErrArenaFull indicates that the arena is full and cannot perform any more
+	// allocations.
+	ErrArenaFull = errors.New("allocation failed because arena is full")
+)
+
+// NewArena allocates a new arena using the specified buffer as the backing
+// store.
+func NewArena(buf []byte) *Arena {
+	// Don't store data at position 0 in order to reserve offset=0 as a kind
+	// of nil pointer.
+	return &Arena{
+		n:   1,
+		buf: buf,
+	}
+}
+
+// Size returns the number of bytes allocated by the arena.
+func (a *Arena) Size() uint32 {
+	s := atomic.LoadUint64(&a.n)
+	if s > math.MaxUint32 {
+		// Saturate at MaxUint32.
+		return math.MaxUint32
+	}
+	return uint32(s)
+}
+
+// Capacity returns the capacity of the arena.
+func (a *Arena) Capacity() uint32 {
+	return uint32(len(a.buf))
+}
+
+func (a *Arena) alloc(size, align, overflow uint32) (uint32, uint32, error) {
+	// Verify that the arena isn't already full.
+	origSize := atomic.LoadUint64(&a.n)
+	if int(origSize) > len(a.buf) {
+		return 0, 0, ErrArenaFull
+	}
+
+	// Pad the allocation with enough bytes to ensure the requested alignment.
+	padded := uint32(size) + align
+
+	newSize := atomic.AddUint64(&a.n, uint64(padded))
+	if int(newSize)+int(overflow) > len(a.buf) {
+		return 0, 0, ErrArenaFull
+	}
+
+	// Return the aligned offset.
+	offset := (uint32(newSize) - padded + align) & ^align
+	return offset, padded, nil
+}
+
+func (a *Arena) getBytes(offset uint32, size uint32) []byte {
+	if offset == 0 {
+		return nil
+	}
+	return a.buf[offset : offset+size : offset+size]
+}
+
+func (a *Arena) getPointer(offset uint32) unsafe.Pointer {
+	if offset == 0 {
+		return nil
+	}
+	return unsafe.Pointer(&a.buf[offset])
+}
+
+func (a *Arena) getPointerOffset(ptr unsafe.Pointer) uint32 {
+	if ptr == nil {
+		return 0
+	}
+	return uint32(uintptr(ptr) - uintptr(unsafe.Pointer(&a.buf[0])))
+}