lib/readers: add Seek method to PatternReader

lib/readers/pattern_reader.go

@@ -1,29 +1,59 @@
 package readers
 
-import "io"
+import (
+	"io"
+
+	"github.com/pkg/errors"
+)
+
+// 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.Reader {
+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.length <= 0 {
+		if r.offset >= r.length {
 			return n, io.EOF
 		}
 		p[i] = r.c
-		r.c = (r.c + 1) % 253
-		r.length--
+		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
+}