test/mock: implement mock for crypto [rand.Reader]

The RandReader implement [io.Reader].
To provide predictable result, the RandReader is seeded with slice of
bytes.
A call to Read will fill the passed bytes with those seed.

For example, given seed as "abc" (length is three), calling Read with
bytes length five will return "abcab".

lib/test/mock/mock.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Package mock provide a mocking for standard input, standard output,
-// standard error, and io.ReadWriter.
+// standard error, io.ReadWriter, and [rand.Reader].
 package mock
 
 import (

lib/test/mock/rand_reader.go

@@ -0,0 +1,51 @@
+// Copyright 2024, Shulhan <[email protected]>. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mock
+
+// RandReader implement [io.Reader] for mocking crypto [rand.Reader].
+// To provide predictable result, the RandReader is seeded with the same
+// slice of bytes.
+// A call to Read will fill the passed bytes with those seed.
+type RandReader struct {
+	seed    []byte
+	counter int
+}
+
+// NewRandReader create new random reader using seed as generator.
+// The longer the seed, the longer the random values become unique.
+func NewRandReader(seed []byte) (r *RandReader) {
+	r = &RandReader{
+		seed: seed,
+	}
+	return r
+}
+
+// Read fill the raw bytes with seed.
+// If raw length larger than the seed, it will be filled with the same seed
+// until all bytes filled.
+//
+// For example, given seed as "abc" (length is three), and raw length is
+// five, then Read will return "abcab".
+func (rr *RandReader) Read(raw []byte) (n int, err error) {
+	var (
+		expn = len(raw)
+
+		nwrite int
+	)
+
+	for n < expn {
+		nwrite = copy(raw[n:], rr.seed[rr.counter:])
+		n += nwrite
+	}
+
+	// Increment the counter to make the seed start from next byte, so
+	// the next Read will return different result but still predictable.
+	rr.counter++
+	if rr.counter == len(rr.seed) {
+		rr.counter = 0
+	}
+
+	return n, nil
+}

lib/test/mock/rand_reader_example_test.go

@@ -0,0 +1,41 @@
+// Copyright 2024, Shulhan <[email protected]>. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mock_test
+
+import (
+	"crypto/rand"
+	"fmt"
+	"log"
+
+	"github.com/shuLhan/share/lib/test/mock"
+)
+
+func ExampleRandReader() {
+	var (
+		seed = []byte(`123`)
+		rr   = mock.NewRandReader(seed)
+		b    = make([]byte, 8)
+
+		x   int
+		n   int
+		err error
+	)
+
+	rand.Reader = rr
+
+	for x = 0; x <= len(seed); x++ {
+		n, err = rand.Read(b)
+		if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Println(n, string(b))
+	}
+
+	// Output:
+	// 8 12312312
+	// 8 23232323
+	// 8 33333333
+	// 8 12312312
+}