An implementation of rand.Source which reads from an io.Reader

readersource.go

package main

import (
	"fmt"
	"io"
	"math/rand"
	"strings"
)

type ReaderSource struct {
	io.Reader
	buf [8]byte
}

func (s ReaderSource) Seed(seed int64) {}
func (s ReaderSource) Int63() int64 {
	_, err := io.ReadFull(s.Reader, s.buf[:8])
	if err != nil {
		panic("ReaderSource: read failed")
	}
	v := int64(s.buf[0])<<0 + int64(s.buf[1])<<8 + int64(s.buf[2])<<16 + int64(s.buf[3])<<24 +
		int64(s.buf[4])<<32 + int64(s.buf[5])<<40 + int64(s.buf[6])<<48 + int64(s.buf[7]&^0x80)<<56
	return v
}

func main() {
	s := ReaderSource{Reader: strings.NewReader(strings.Repeat("\xff", 16))}
	r := rand.New(s)
	fmt.Println(r.Int63())
	fmt.Println(r.Int63())
	fmt.Println(r.Int63())
}