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package preimage
import (
"encoding/binary"
"fmt"
"io"
)
// HintWriter writes hints to an io.Writer (e.g. a special file descriptor, or a debug log),
// for a pre-image oracle service to prepare specific pre-images.
type HintWriter struct {
w io.Writer
}
var _ Hinter = (*HintWriter)(nil)
func NewHintWriter(w io.Writer) *HintWriter {
return &HintWriter{w: w}
}
func (hw *HintWriter) Hint(v Hint) {
hint := v.Hint()
var hintBytes []byte
hintBytes = binary.BigEndian.AppendUint32(hintBytes, uint32(len(hint)))
hintBytes = append(hintBytes, []byte(hint)...)
hintBytes = append(hintBytes, 0) // to block writing on
_, err := hw.w.Write(hintBytes)
if err != nil {
panic(fmt.Errorf("failed to write pre-image hint: %w", err))
}
}
// HintReader reads the hints of HintWriter and passes them to a router for preparation of the requested pre-images.
// Onchain the written hints are no-op.
type HintReader struct {
r io.Reader
}
func NewHintReader(r io.Reader) *HintReader {
return &HintReader{r: r}
}
func (hr *HintReader) NextHint(router func(hint string) error) error {
var length uint32
if err := binary.Read(hr.r, binary.BigEndian, &length); err != nil {
if err == io.EOF {
return io.EOF
}
return fmt.Errorf("failed to read hint length prefix: %w", err)
}
payload := make([]byte, length)
if length > 0 {
if _, err := io.ReadFull(hr.r, payload); err != nil {
return fmt.Errorf("failed to read hint payload (length %d): %w", length, err)
}
}
if err := router(string(payload)); err != nil {
// stream recovery
_, _ = hr.r.Read([]byte{0})
return fmt.Errorf("failed to handle hint: %w", err)
}
if _, err := hr.r.Read([]byte{0}); err != nil {
return fmt.Errorf("failed to read trailing no-op byte to unblock hint writer: %w", err)
}
return nil
}