1 files changed, 80 insertions, 0 deletions
diff --git a/src/internal/coverage/slicewriter/slicewriter.go b/src/internal/coverage/slicewriter/slicewriter.go
new file mode 100644
index 0000000..3522bf5
--- /dev/null
+++ b/src/internal/coverage/slicewriter/slicewriter.go
@@ -0,0 +1,80 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slicewriter
+
+import (
+	"fmt"
+	"io"
+)
+
+// WriteSeeker is a helper object that implements the io.WriteSeeker
+// interface. Clients can create a WriteSeeker, make a series of Write
+// calls to add data to it (and possibly Seek calls to update
+// previously written portions), then finally invoke BytesWritten() to
+// get a pointer to the constructed byte slice.
+type WriteSeeker struct {
+	payload []byte
+	off     int64
+}
+
+func (sws *WriteSeeker) Write(p []byte) (n int, err error) {
+	amt := len(p)
+	towrite := sws.payload[sws.off:]
+	if len(towrite) < amt {
+		sws.payload = append(sws.payload, make([]byte, amt-len(towrite))...)
+		towrite = sws.payload[sws.off:]
+	}
+	copy(towrite, p)
+	sws.off += int64(amt)
+	return amt, nil
+}
+
+// Seek repositions the read/write position of the WriteSeeker within
+// its internally maintained slice. Note that it is not possible to
+// expand the size of the slice using SEEK_SET; trying to seek outside
+// the slice will result in an error.
+func (sws *WriteSeeker) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case io.SeekStart:
+		if sws.off != offset && (offset < 0 || offset >= int64(len(sws.payload))) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", offset, len(sws.payload))
+		}
+		sws.off = offset
+		return offset, nil
+	case io.SeekCurrent:
+		newoff := sws.off + offset
+		if newoff != sws.off && (newoff < 0 || newoff >= int64(len(sws.payload))) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(sws.payload))
+		}
+		sws.off += offset
+		return sws.off, nil
+	case io.SeekEnd:
+		newoff := int64(len(sws.payload)) + offset
+		if newoff != sws.off && (newoff < 0 || newoff >= int64(len(sws.payload))) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(sws.payload))
+		}
+		sws.off = newoff
+		return sws.off, nil
+	}
+	// other modes not supported
+	return 0, fmt.Errorf("unsupported seek mode %d", whence)
+}
+
+// BytesWritten returns the underlying byte slice for the WriteSeeker,
+// containing the data written to it via Write/Seek calls.
+func (sws *WriteSeeker) BytesWritten() []byte {
+	return sws.payload
+}
+
+func (sws *WriteSeeker) Read(p []byte) (n int, err error) {
+	amt := len(p)
+	toread := sws.payload[sws.off:]
+	if len(toread) < amt {
+		amt = len(toread)
+	}
+	copy(p, toread)
+	sws.off += int64(amt)
+	return amt, nil
+}