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84 lines
2.7 KiB
Go
84 lines
2.7 KiB
Go
3 years ago
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/*
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Package fragment contains a Reader which can read Fragments which may be scattered around a volume (and perhaps even
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not in sequence). Typically these could be translated from MFT attribute DataRuns. To convert MFT attribute DataRuns
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to Fragments for use in the fragment Reader, use mft.DataRunsToFragments().
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Implementation notes
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When the fragment Reader is near the end of a fragment and a Read() call requests more data than what is left in
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the current fragment, the Reader will exhaust only the current fragment and return that data (which could be less
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than len(p)). A next Read() call will then seek to the next fragment and continue reading there. When the last
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fragment is exhausted by a Read(), it will return the remaining bytes read and a nil error. Any subsequent Read()
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calls after that will return 0, io.EOF.
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When accessing a new fragment, the Reader will seek using the absolute Length in the fragment from the start
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of the contained io.ReadSeeker (using io.SeekStart).
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*/
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package fragment
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import (
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"fmt"
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"io"
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"os"
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)
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// Fragment contains an absolute Offset in bytes from the start of a volume and a Length of the fragment, also in bytes.
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type Fragment struct {
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Offset int64
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Length int64
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}
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// A fragment Reader will read data from the fragments in order. When one fragment is depleted, it will seek to the
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// position of the next fragment and continue reading from there, until all fragments have been exhausted. When the last
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// fragment has been exhaused, each subsequent Read() will return io.EOF.
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type Reader struct {
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src io.ReadSeeker
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fragments []Fragment
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idx int
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remaining int64
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file *os.File
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}
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// NewReader initializes a new Reader from the io.ReaderSeeker and fragments and returns a pointer to. Note that
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// fragments may not be sequential in order, so the io.ReadSeeker should support seeking backwards (or rather, from the
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// start).
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func NewReader(src io.ReadSeeker, fragments []Fragment) *Reader {
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return &Reader{src: src, fragments: fragments, idx: -1, remaining: 0}
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}
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func (r *Reader) Read(p []byte) (n int, err error) {
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if r.idx >= len(r.fragments) {
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r.src.(*os.File).Close()
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return 0, io.EOF
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}
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if len(p) == 0 {
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return 0, nil
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}
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if r.remaining == 0 {
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r.idx++
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if r.idx >= len(r.fragments) {
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return 0, io.EOF
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}
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next := r.fragments[r.idx]
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r.remaining = next.Length
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seeked, err := r.src.Seek(next.Offset, io.SeekStart)
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if err != nil {
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return 0, fmt.Errorf("unable to seek to next offset %d: %v", next.Offset, err)
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}
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if seeked != next.Offset {
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return 0, fmt.Errorf("wanted to seek to %d but reached %d", next.Offset, seeked)
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}
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}
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target := p
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if int64(len(p)) > r.remaining {
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target = p[:r.remaining]
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}
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n, err = io.ReadFull(r.src, target)
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r.remaining -= int64(n)
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return n, err
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}
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