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package readers
import (
"errors"
"io"
)
// This is the smallest prime less than 256
//
// Using a prime here means we are less likely to hit repeating patterns
const patternReaderModulo = 251
// NewPatternReader creates a reader, that returns a deterministic byte pattern.
// After length bytes are read
func NewPatternReader(length int64) io.ReadSeeker {
return &patternReader{
length: length,
}
}
type patternReader struct {
offset int64
length int64
c byte
}
func (r *patternReader) Read(p []byte) (n int, err error) {
for i := range p {
if r.offset >= r.length {
return n, io.EOF
}
p[i] = r.c
r.c = (r.c + 1) % patternReaderModulo
r.offset++
n++
}
return
}
// Seek implements the io.Seeker interface.
func (r *patternReader) Seek(offset int64, whence int) (abs int64, err error) {
switch whence {
case io.SeekStart:
abs = offset
case io.SeekCurrent:
abs = r.offset + offset
case io.SeekEnd:
abs = r.length + offset
default:
return 0, errors.New("patternReader: invalid whence")
}
if abs < 0 {
return 0, errors.New("patternReader: negative position")
}
r.offset = abs
r.c = byte(abs % patternReaderModulo)
return abs, nil
}
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