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// 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")
}