feat: add buffer truncation (#12)

pkg/btree/buffer.go

@@ -50,4 +50,15 @@ func (b *seekableBuffer) Read(p []byte) (int, error) {
 	return n, nil
 }
 
+func (b *seekableBuffer) Truncate(size int64) error {
+	if size < 0 {
+		return io.ErrShortBuffer
+	}
+	if size > int64(len(b.buf)) {
+		return io.ErrShortWrite
+	}
+	b.buf = b.buf[:size]
+	return nil
+}
+
 var _ io.ReadWriteSeeker = &seekableBuffer{}

pkg/btree/buffer_test.go

@@ -93,4 +93,17 @@ func TestSeekableBuffer(t *testing.T) {
 			t.Fatalf("expected to read 'llo', read %s", string(buf))
 		}
 	})
+
+	t.Run("truncate", func(t *testing.T) {
+		b := newSeekableBuffer()
+		if _, err := b.Write([]byte("hello")); err != nil {
+			t.Fatal(err)
+		}
+		if err := b.Truncate(3); err != nil {
+			t.Fatal(err)
+		}
+		if string(b.buf) != "hel" {
+			t.Fatalf("expected to truncate to 'hel', truncated to %s", string(b.buf))
+		}
+	})
 }