Extend DocumentBuffer with apply interface

The existing DocumentBuffer interface provided no facility
to iterate over events in order without caching them all in memory
and pushing/popping them. This was deemed significantly unoptimal and
so Apply() was born.

When Apply() is called, DocumentBuffer will recieve a filter callable
(functor, lambda, or closure) and will call it on all documents in order
throughout the entire cache and store, without caching stored events
or modifying the cache either.

The filter Apply() uses will recieve one document and return true or
false: true to continue iterating and false to stop iterating.

Signed-off-by: Ava Affine <ava@sunnypup.io>

internal/docbuf/docbuf.go

@@ -48,6 +48,14 @@ type DocumentBuffer interface {
 	 * removing them from either data structure.
 	 */
 	Read(int) ([]string, error)
+	/* Apply
+	 * Takes a callable/closure/function that accepts a string (document)
+	 * and returns a bool. Apply returns an error, or nil.
+	 * Apply will begin applying this callable to each doc from most recent
+	 * to least recent so long as the callable continues to return true.
+	 * If the callable returns false, Apply ceases iteration and returns.
+	 */
+	Apply(func(string)bool) error
 	/* Close
 	 * flushes and then prepares backing store for closure
 	 */
@@ -86,7 +94,7 @@ func (b *DocBuf) promote(bounds bool) error {
 		return errors.New(PromoteToFullCacheError)
 	}
 
-	doc, err := b.readDocumentsFromDisk(1, true)
+	doc, err := b.readDocumentsFromDisk(1, true, false)
 	if err != nil {
 		return err
 	}
@@ -135,16 +143,25 @@ func (b *DocBuf) writeToBackingStore(doc string) error {
 func (b *DocBuf) readDocumentsFromDisk(
 	count int,
 	truncate bool,
+	continues bool,
 ) ([]string, error) {
 	docs   := []string{}
 	cursor := int64(0)
+	var err error
 
 	if count < 1 {
 		return docs, errors.New(BadReadRequestError)
 	}
 
+	if continues {
+		cursor, err = b.store.Seek(0, io.SeekCurrent)
+		if b.endIndex == 0 || err != nil || cursor == 0 {
+			return docs, err
+		}
+	
+	} else {
 		// catch store is empty
-	cursor, err := b.store.Seek(0, io.SeekEnd)
+		cursor, err = b.store.Seek(0, io.SeekEnd)
 		if b.endIndex == 0 || err != nil || cursor == 0 {
 			return docs, err
 		}
@@ -153,8 +170,8 @@ func (b *DocBuf) readDocumentsFromDisk(
 		if b.endIndex > cursor {
 			b.endIndex = cursor
 		}
-	
 		cursor = b.endIndex - 1
+	}
 
 	for len(docs) < count && cursor >= 0 {
 		doc     := ""
@@ -190,7 +207,7 @@ func (b *DocBuf) readDocumentsFromDisk(
 		 * each read will stop on the newline so make sure
 		 * we dont try to account for an empty document
 		 */
-		if (doc != "\n") {
+		if len(doc) > 0 && doc != "\n" {
 			str, err := base64.StdEncoding.DecodeString(doc)
 			if err != nil {
 				return docs, err
@@ -338,7 +355,7 @@ func (b *DocBuf) Peek() (string, error) {
 	if len(b.cache) > 0 {
 		return b.cache[0], nil
 	} else {
-		d, e := b.readDocumentsFromDisk(1, false)
+		d, e := b.readDocumentsFromDisk(1, false, false)
 		if len(d) < 1 {
 			return "", e
 		}
@@ -355,7 +372,7 @@ func (b *DocBuf) Read(count int) ([]string, error) {
 	candidates := b.cache[:count - (delta)]
 
 	if delta > 0 {
-		fromStorage, err := b.readDocumentsFromDisk(delta, false)
+		fromStorage, err := b.readDocumentsFromDisk(delta, false, false)
 		if err != nil {
 			return candidates, err
 		}
@@ -366,6 +383,61 @@ func (b *DocBuf) Read(count int) ([]string, error) {
 	return candidates, nil
 }
 
+/* Apply
+ * Takes a callable/closure/function that accepts a string (document)
+ * and returns a bool. Apply returns an error, or nil.
+ * Apply will begin applying this callable to each doc from most recent
+ * to least recent so long as the callable continues to return true.
+ * If the callable returns false, Apply ceases iteration and returns.
+ */
+func (b *DocBuf) Apply(f func(string)bool) error {
+	// iterate over internal cache applying function
+	for _, i := range b.cache {
+		if !f(i) {
+			return nil
+		}
+	}
+
+	// begin iterating with readDocumentsFromDisk
+	first := true
+	flag := true
+	for flag {
+		doc, err := b.readDocumentsFromDisk(1, false, !first)
+		if err != nil {
+			return err
+		}
+		if len(doc) == 0 {
+			return nil
+		} else if len(doc) != 1 {
+			return errors.New("improper read from buffer")
+		}
+
+		first = false
+		flag = f(doc[0])
+
+		if flag {
+			var c, d int64
+			var err error
+			// since continue is on we need to bring the cursor back one
+			if c, err = b.store.Seek(0, io.SeekCurrent); err != nil {
+				return err
+			// dont seek earlier than 0
+			} else if c < 1 {
+				break
+			}
+			if d, err = b.store.Seek(-1, io.SeekCurrent); err != nil {
+				return err
+			}
+
+			if c == d {
+				return errors.New("Seek failure!")
+			}
+		}
+	}
+
+	return nil
+}
+
 /* Close
  * flushes and then prepares backing store for closure.
  * returns the end index into the underlying stream...

internal/docbuf/docbuf_test.go

@@ -302,3 +302,36 @@ func TestInitialize(t *testing.T) {
 		t.Fatalf("error reading: %e", err)
 	}
 }
+
+func TestApply(t *testing.T) {
+	backingStore := NewReadWriteSeekString()
+	backingStore.Write([]byte("MQo=\nMgo=\nMwo=\nNAo=\nNQo=\nNgo="))
+	buf, e := NewDocumentBuffer(3, &backingStore)
+	if e != nil {
+		t.Fatalf("error making documentbuffer: %e", e)
+	}
+	
+	// test cached
+	if buf.Cached() != 3 {
+		t.Fatalf("expected 3 docs in cache")
+	}
+
+	count := 0
+	if err := buf.Apply(func(doc string) bool {
+		count += 1
+		return true
+	}); err != nil || count != 6 {
+		t.Fatalf("error applying: %e", err)
+	}
+
+	count = 0
+	if err := buf.Apply(func(doc string) bool {
+		if doc == "2\n" {
+			return false
+		}
+		count += 1
+		return true
+	}); err != nil || count != 4 {
+		t.Fatalf("error applying: %e", err)
+	}
+}