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55 lines
1.8 KiB
Go
55 lines
1.8 KiB
Go
// Package transform provides a convenient utility for transforming one data
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// format to another.
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package transform
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// Transformer represents a transform reader.
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type Transformer struct {
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tfn func() ([]byte, error) // user-defined transform function
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buf []byte // read buffer
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idx int // read buffer index
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err error // last error
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}
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// NewTransformer returns an object that can be used for transforming one
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// data formant to another. The param is a function that performs the
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// conversion and returns the transformed data in chunks/messages.
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func NewTransformer(fn func() ([]byte, error)) *Transformer {
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return &Transformer{tfn: fn}
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}
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// ReadMessage allows for reading a one transformed message at a time.
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func (r *Transformer) ReadMessage() ([]byte, error) {
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return r.tfn()
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}
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// Read conforms to io.Reader
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func (r *Transformer) Read(p []byte) (n int, err error) {
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if len(r.buf)-r.idx > 0 {
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// There's data in the read buffer, return it prior to returning errors
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// or reading more messages.
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if len(r.buf)-r.idx > len(p) {
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// The input slice is smaller than the read buffer, copy a subslice
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// of the read buffer and increase the read index.
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copy(p, r.buf[r.idx:r.idx+len(p)])
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r.idx += len(p)
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return len(p), nil
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}
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// Copy the entire read buffer to the input slice.
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n = len(r.buf) - r.idx
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copy(p[:n], r.buf[r.idx:])
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r.buf = r.buf[:0] // reset the read buffer, keeping it's capacity
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r.idx = 0 // rewind the read buffer index
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return n, nil
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}
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if r.err != nil {
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return 0, r.err
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}
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var msg []byte
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msg, r.err = r.ReadMessage()
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// We should immediately append the incoming message to the read
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// buffer to allow for the implemented transformer to repurpose
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// it's own message space if needed.
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r.buf = append(r.buf, msg...)
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return r.Read(p)
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}
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