95 lines
2.2 KiB
Go
95 lines
2.2 KiB
Go
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package wrapio
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import (
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"encoding/gob"
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"io"
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"github.com/djherbis/buffer/limio"
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)
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// ReadWriterAt implements io.ReaderAt and io.WriterAt
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type ReadWriterAt interface {
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io.ReaderAt
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io.WriterAt
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}
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// Wrapper implements a io.ReadWriter and ReadWriterAt such that
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// when reading/writing goes past N bytes, it "wraps" back to the beginning.
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type Wrapper struct {
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// N is the offset at which to "wrap" back to the start
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N int64
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// L is the length of the data written
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L int64
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// O is our offset in the data
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O int64
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rwa ReadWriterAt
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}
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// NewWrapper creates a Wrapper based on ReadWriterAt rwa.
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// L is the current length, O is the current offset, and N is offset at which we "wrap".
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func NewWrapper(rwa ReadWriterAt, L, O, N int64) *Wrapper {
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return &Wrapper{
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L: L,
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O: O,
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N: N,
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rwa: rwa,
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}
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}
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// Len returns the # of bytes in the Wrapper
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func (wpr *Wrapper) Len() int64 {
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return wpr.L
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}
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// Cap returns the "wrap" offset (max # of bytes)
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func (wpr *Wrapper) Cap() int64 {
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return wpr.N
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}
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// Reset seeks to the start (0 offset), and sets the length to 0.
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func (wpr *Wrapper) Reset() {
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wpr.O = 0
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wpr.L = 0
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}
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// SetReadWriterAt lets you switch the underlying Read/WriterAt
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func (wpr *Wrapper) SetReadWriterAt(rwa ReadWriterAt) {
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wpr.rwa = rwa
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}
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// Read reads from the current offset into p, wrapping at Cap()
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func (wpr *Wrapper) Read(p []byte) (n int, err error) {
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n, err = wpr.ReadAt(p, 0)
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wpr.L -= int64(n)
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wpr.O += int64(n)
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wpr.O %= wpr.N
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return n, err
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}
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// ReadAt reads from the current offset+off into p, wrapping at Cap()
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func (wpr *Wrapper) ReadAt(p []byte, off int64) (n int, err error) {
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wrap := NewWrapReader(wpr.rwa, wpr.O+off, wpr.N)
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r := io.LimitReader(wrap, wpr.L-off)
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return r.Read(p)
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}
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// Write writes p to the end of the Wrapper (at Len()), wrapping at Cap()
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func (wpr *Wrapper) Write(p []byte) (n int, err error) {
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return wpr.WriteAt(p, wpr.L)
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}
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// WriteAt writes p at the current offset+off, wrapping at Cap()
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func (wpr *Wrapper) WriteAt(p []byte, off int64) (n int, err error) {
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wrap := NewWrapWriter(wpr.rwa, wpr.O+off, wpr.N)
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w := limio.LimitWriter(wrap, wpr.N-off)
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n, err = w.Write(p)
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if wpr.L < off+int64(n) {
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wpr.L = int64(n) + off
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}
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return n, err
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}
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func init() {
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gob.Register(&Wrapper{})
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}
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