readerutil: add NewBufferingReaderAt

To get an io.ReaderAt from an io.Reader.

readerutil/bufreaderat.go

@@ -0,0 +1,48 @@
+/*
+Copyright 2018 The go4 Authors
+
+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 readerutil
+
+import "io"
+
+// NewBufferingReaderAt returns an io.ReaderAt that reads from r as
+// necessary and keeps a copy of all data read in memory.
+func NewBufferingReaderAt(r io.Reader) io.ReaderAt {
+	return &bufReaderAt{r: r}
+}
+
+type bufReaderAt struct {
+	r   io.Reader
+	buf []byte
+}
+
+func (br *bufReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
+	endOff := off + int64(len(p))
+	need := endOff - int64(len(br.buf))
+	if need > 0 {
+		buf := make([]byte, need)
+		var rn int
+		rn, err = io.ReadFull(br.r, buf)
+		br.buf = append(br.buf, buf[:rn]...)
+	}
+	if int64(len(br.buf)) >= off {
+		n = copy(p, br.buf[off:])
+	}
+	if n == len(p) {
+		err = nil
+	}
+	return
+}

readerutil/bufreaderat_test.go

@@ -0,0 +1,70 @@
+/*
+Copyright 2018 The go4 Authors
+
+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 readerutil
+
+import "testing"
+
+type trackingReader struct {
+	off       int
+	reads     int
+	readBytes int
+}
+
+func (t *trackingReader) Read(p []byte) (n int, err error) {
+	t.reads++
+	t.readBytes += len(p)
+	for len(p) > 0 {
+		p[0] = '0' + byte(t.off%10)
+		t.off++
+		p = p[1:]
+		n++
+	}
+	return
+
+}
+
+func TestBufferingReaderAt(t *testing.T) {
+	tr := new(trackingReader)
+	ra := NewBufferingReaderAt(tr)
+	for i, tt := range []struct {
+		off           int64
+		want          string
+		wantReads     int
+		wantReadBytes int
+	}{
+		{off: 0, want: "0123456789", wantReads: 1, wantReadBytes: 10},
+		{off: 5, want: "56789", wantReads: 1, wantReadBytes: 10},      // already buffered
+		{off: 6, want: "67890", wantReads: 2, wantReadBytes: 11},      // need 1 more byte
+		{off: 0, want: "0123456789", wantReads: 2, wantReadBytes: 11}, // already buffered
+	} {
+		got := make([]byte, len(tt.want))
+		n, err := ra.ReadAt(got, tt.off)
+		if err != nil || n != len(tt.want) {
+			t.Errorf("step %d: ReadAt = %v, %v; want %v, %v", i, n, err, len(tt.want), nil)
+			continue
+		}
+		if string(got) != tt.want {
+			t.Errorf("step %d: ReadAt read %q; want %q", i, got, tt.want)
+		}
+		if tr.reads != tt.wantReads {
+			t.Errorf("step %d: num reads = %d; want %d", i, tr.reads, tt.wantReads)
+		}
+		if tr.readBytes != tt.wantReadBytes {
+			t.Errorf("step %d: read bytes = %d; want %d", i, tr.reads, tt.wantReads)
+		}
+	}
+}