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file.go
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file.go
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package pager
import (
"errors"
"io"
"os"
)
var (
_ RandomAccessFile = (*inMemory)(nil)
_ RandomAccessFile = (*os.File)(nil)
)
// RandomAccessFile represents a file-like object that can be read from and
// written to at any offset.
type RandomAccessFile interface {
io.ReaderAt
io.WriterAt
io.Closer
Truncate(size int64) error
Name() string
}
type sizedFile interface {
RandomAccessFile
Size() int64
}
// inMemory implements an in-memory random access file.
type inMemory struct {
closed bool
data []byte
readOnly bool
}
func (mem *inMemory) ReadAt(p []byte, off int64) (n int, err error) {
if len(p) == 0 {
return 0, nil
} else if off < 0 {
return 0, &os.PathError{
Op: "readat",
Path: InMemoryFileName,
Err: errors.New("negative offset"),
}
} else if err := mem.canMutate("readat"); err != nil {
return 0, err
} else if int(off) >= len(mem.data) {
return 0, io.EOF
}
return copy(p, mem.data[off:]), nil
}
func (mem *inMemory) WriteAt(p []byte, off int64) (n int, err error) {
if len(p) == 0 {
return 0, nil
} else if off < 0 {
return 0, &os.PathError{
Op: "writeat",
Path: InMemoryFileName,
Err: errors.New("negative offset"),
}
} else if err := mem.canMutate("writeat"); err != nil {
return 0, err
}
spaceRequired := off + int64(len(p))
if int(spaceRequired) > len(mem.data) {
_ = mem.Truncate(spaceRequired)
}
n = copy(mem.data[off:], p)
return n, nil
}
func (mem *inMemory) Close() error {
mem.closed = true
mem.data = nil
return nil
}
func (mem *inMemory) Truncate(size int64) error {
d := mem.data
mem.data = make([]byte, size)
copy(mem.data, d)
return nil
}
func (mem *inMemory) Size() int64 {
return int64(len(mem.data))
}
func (mem *inMemory) Name() string {
return InMemoryFileName
}
func (mem *inMemory) canMutate(op string) error {
if mem.readOnly {
return &os.PathError{
Op: op,
Path: InMemoryFileName,
Err: errors.New("read-only file"),
}
} else if mem.closed {
return errors.New("closed file")
}
return nil
}
func findSize(f RandomAccessFile) (int64, error) {
switch file := f.(type) {
case *os.File:
stat, err := file.Stat()
if err != nil {
return 0, err
}
return stat.Size(), nil
case sizedFile:
return file.Size(), nil
}
return 0, errors.New("failed to find file size")
}