1 files changed, 85 insertions, 0 deletions
diff --git a/src/cmd/internal/cov/mreader.go b/src/cmd/internal/cov/mreader.go
new file mode 100644
index 0000000..30f53d6
--- /dev/null
+++ b/src/cmd/internal/cov/mreader.go
@@ -0,0 +1,85 @@
+// 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 cov
+
+import (
+	"cmd/internal/bio"
+	"io"
+	"os"
+)
+
+// This file contains the helper "MReader", a wrapper around bio plus
+// an "mmap'd read-only" view of the file obtained from bio.SliceRO().
+// MReader is designed to implement the io.ReaderSeeker interface.
+// Since bio.SliceOS() is not guaranteed to succeed, MReader falls back
+// on explicit reads + seeks provided by bio.Reader if needed.
+
+type MReader struct {
+	f        *os.File
+	rdr      *bio.Reader
+	fileView []byte
+	off      int64
+}
+
+func NewMreader(f *os.File) (*MReader, error) {
+	rdr := bio.NewReader(f)
+	fi, err := f.Stat()
+	if err != nil {
+		return nil, err
+	}
+	r := MReader{
+		f:        f,
+		rdr:      rdr,
+		fileView: rdr.SliceRO(uint64(fi.Size())),
+	}
+	return &r, nil
+}
+
+func (r *MReader) Read(p []byte) (int, error) {
+	if r.fileView != nil {
+		amt := len(p)
+		toread := r.fileView[r.off:]
+		if len(toread) < 1 {
+			return 0, io.EOF
+		}
+		if len(toread) < amt {
+			amt = len(toread)
+		}
+		copy(p, toread)
+		r.off += int64(amt)
+		return amt, nil
+	}
+	return io.ReadFull(r.rdr, p)
+}
+
+func (r *MReader) ReadByte() (byte, error) {
+	if r.fileView != nil {
+		toread := r.fileView[r.off:]
+		if len(toread) < 1 {
+			return 0, io.EOF
+		}
+		rv := toread[0]
+		r.off++
+		return rv, nil
+	}
+	return r.rdr.ReadByte()
+}
+
+func (r *MReader) Seek(offset int64, whence int) (int64, error) {
+	if r.fileView == nil {
+		return r.rdr.MustSeek(offset, whence), nil
+	}
+	switch whence {
+	case io.SeekStart:
+		r.off = offset
+		return offset, nil
+	case io.SeekCurrent:
+		return r.off, nil
+	case io.SeekEnd:
+		r.off = int64(len(r.fileView)) + offset
+		return r.off, nil
+	}
+	panic("other modes not implemented")
+}