plumbing: add buffer cache and use it in packfile parser

It uses less memory and is faster as slices don't have to be converted
from/to MemoryObject and they are indexed by offset.

Signed-off-by: Javi Fontan <jfontan@gmail.com>

3 files changed, 239 insertions, 0 deletions

diff --git a/plumbing/cache/buffer_lru.go b/plumbing/cache/buffer_lru.go
new file mode 100644
index 0000000..f2c0f90
--- /dev/null
+++ b/plumbing/cache/buffer_lru.go
@@ -0,0 +1,98 @@
+package cache
+
+import (
+	"container/list"
+	"sync"
+)
+
+// BufferLRU implements an object cache with an LRU eviction policy and a
+// maximum size (measured in object size).
+type BufferLRU struct {
+	MaxSize FileSize
+
+	actualSize FileSize
+	ll         *list.List
+	cache      map[int64]*list.Element
+	mut        sync.Mutex
+}
+
+// NewBufferLRU creates a new BufferLRU with the given maximum size. The maximum
+// size will never be exceeded.
+func NewBufferLRU(maxSize FileSize) *BufferLRU {
+	return &BufferLRU{MaxSize: maxSize}
+}
+
+// NewBufferLRUDefault creates a new BufferLRU with the default cache size.
+func NewBufferLRUDefault() *BufferLRU {
+	return &BufferLRU{MaxSize: DefaultMaxSize}
+}
+
+type buffer struct {
+	Key   int64
+	Slice []byte
+}
+
+// Put puts a buffer into the cache. If the buffer is already in the cache, it
+// will be marked as used. Otherwise, it will be inserted. A buffers might
+// be evicted to make room for the new one.
+func (c *BufferLRU) Put(key int64, slice []byte) {
+	c.mut.Lock()
+	defer c.mut.Unlock()
+
+	if c.cache == nil {
+		c.actualSize = 0
+		c.cache = make(map[int64]*list.Element, 1000)
+		c.ll = list.New()
+	}
+
+	if ee, ok := c.cache[key]; ok {
+		c.ll.MoveToFront(ee)
+		ee.Value = buffer{key, slice}
+		return
+	}
+
+	objSize := FileSize(len(slice))
+
+	if objSize > c.MaxSize {
+		return
+	}
+
+	for c.actualSize+objSize > c.MaxSize {
+		last := c.ll.Back()
+		lastObj := last.Value.(buffer)
+		lastSize := FileSize(len(lastObj.Slice))
+
+		c.ll.Remove(last)
+		delete(c.cache, lastObj.Key)
+		c.actualSize -= lastSize
+	}
+
+	ee := c.ll.PushFront(buffer{key, slice})
+	c.cache[key] = ee
+	c.actualSize += objSize
+}
+
+// Get returns a buffer by its key. It marks the buffer as used. If the buffer
+// is not in the cache, (nil, false) will be returned.
+func (c *BufferLRU) Get(key int64) ([]byte, bool) {
+	c.mut.Lock()
+	defer c.mut.Unlock()
+
+	ee, ok := c.cache[key]
+	if !ok {
+		return nil, false
+	}
+
+	c.ll.MoveToFront(ee)
+	return ee.Value.(buffer).Slice, true
+}
+
+// Clear the content of this buffer cache.
+func (c *BufferLRU) Clear() {
+	c.mut.Lock()
+	defer c.mut.Unlock()
+
+	c.ll = nil
+	c.cache = nil
+	c.actualSize = 0
+}
diff --git a/plumbing/cache/buffer_test.go b/plumbing/cache/buffer_test.go
new file mode 100644
index 0000000..262138a
--- /dev/null
+++ b/plumbing/cache/buffer_test.go
@@ -0,0 +1,128 @@
+package cache
+
+import (
+	"sync"
+
+	. "gopkg.in/check.v1"
+)
+
+type BufferSuite struct {
+	c       map[string]Buffer
+	aBuffer []byte
+	bBuffer []byte
+	cBuffer []byte
+	dBuffer []byte
+	eBuffer []byte
+}
+
+var _ = Suite(&BufferSuite{})
+
+func (s *BufferSuite) SetUpTest(c *C) {
+	s.aBuffer = []byte("a")
+	s.bBuffer = []byte("bbb")
+	s.cBuffer = []byte("c")
+	s.dBuffer = []byte("d")
+	s.eBuffer = []byte("ee")
+
+	s.c = make(map[string]Buffer)
+	s.c["two_bytes"] = NewBufferLRU(2 * Byte)
+	s.c["default_lru"] = NewBufferLRUDefault()
+}
+
+func (s *BufferSuite) TestPutSameBuffer(c *C) {
+	for _, o := range s.c {
+		o.Put(1, s.aBuffer)
+		o.Put(1, s.aBuffer)
+		_, ok := o.Get(1)
+		c.Assert(ok, Equals, true)
+	}
+}
+
+func (s *BufferSuite) TestPutBigBuffer(c *C) {
+	for _, o := range s.c {
+		o.Put(1, s.bBuffer)
+		_, ok := o.Get(2)
+		c.Assert(ok, Equals, false)
+	}
+}
+
+func (s *BufferSuite) TestPutCacheOverflow(c *C) {
+	// this test only works with an specific size
+	o := s.c["two_bytes"]
+
+	o.Put(1, s.aBuffer)
+	o.Put(2, s.cBuffer)
+	o.Put(3, s.dBuffer)
+
+	obj, ok := o.Get(1)
+	c.Assert(ok, Equals, false)
+	c.Assert(obj, IsNil)
+	obj, ok = o.Get(2)
+	c.Assert(ok, Equals, true)
+	c.Assert(obj, NotNil)
+	obj, ok = o.Get(3)
+	c.Assert(ok, Equals, true)
+	c.Assert(obj, NotNil)
+}
+
+func (s *BufferSuite) TestEvictMultipleBuffers(c *C) {
+	o := s.c["two_bytes"]
+
+	o.Put(1, s.cBuffer)
+	o.Put(2, s.dBuffer) // now cache is full with two objects
+	o.Put(3, s.eBuffer) // this put should evict all previous objects
+
+	obj, ok := o.Get(1)
+	c.Assert(ok, Equals, false)
+	c.Assert(obj, IsNil)
+	obj, ok = o.Get(2)
+	c.Assert(ok, Equals, false)
+	c.Assert(obj, IsNil)
+	obj, ok = o.Get(3)
+	c.Assert(ok, Equals, true)
+	c.Assert(obj, NotNil)
+}
+
+func (s *BufferSuite) TestClear(c *C) {
+	for _, o := range s.c {
+		o.Put(1, s.aBuffer)
+		o.Clear()
+		obj, ok := o.Get(1)
+		c.Assert(ok, Equals, false)
+		c.Assert(obj, IsNil)
+	}
+}
+
+func (s *BufferSuite) TestConcurrentAccess(c *C) {
+	for _, o := range s.c {
+		var wg sync.WaitGroup
+
+		for i := 0; i < 1000; i++ {
+			wg.Add(3)
+			go func(i int) {
+				o.Put(int64(i), []byte{00})
+				wg.Done()
+			}(i)
+
+			go func(i int) {
+				if i%30 == 0 {
+					o.Clear()
+				}
+				wg.Done()
+			}(i)
+
+			go func(i int) {
+				o.Get(int64(i))
+				wg.Done()
+			}(i)
+		}
+
+		wg.Wait()
+	}
+}
+
+func (s *BufferSuite) TestDefaultLRU(c *C) {
+	defaultLRU := s.c["default_lru"].(*BufferLRU)
+
+	c.Assert(defaultLRU.MaxSize, Equals, DefaultMaxSize)
+}
diff --git a/plumbing/cache/common.go b/plumbing/cache/common.go
index e77baf0..2b7f36a 100644
--- a/plumbing/cache/common.go
+++ b/plumbing/cache/common.go
@@ -24,3 +24,16 @@ type Object interface {
 	// Clear clears every object from the cache.
 	Clear()
 }
+
+// Buffer is an interface to a buffer cache.
+type Buffer interface {
+	// Put puts a buffer into the cache. If the buffer is already in the cache,
+	// it will be marked as used. Otherwise, it will be inserted. Buffer might
+	// be evicted to make room for the new one.
+	Put(key int64, slice []byte)
+	// Get returns a buffer by its key. It marks the buffer as used. If the
+	// buffer is not in the cache, (nil, false) will be returned.
+	Get(key int64) ([]byte, bool)
+	// Clear clears every object from the cache.
+	Clear()
+}