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    <p>This does about 200MBps on my box. There's obvious room for improvement.</p> <pre><code>type randomDataMaker struct { src rand.Source } func (r *randomDataMaker) Read(p []byte) (n int, err error) { for i := range p { p[i] = byte(r.src.Int63() &amp; 0xff) } return len(p), nil } </code></pre> <p>You'd just use <code>io.CopyN</code> to produce the string you want. Obviously you could adjust the character set on the way in or whatever.</p> <p>The nice thing about this model is that it's just an <code>io.Reader</code> so you can use it making anything.</p> <p>Test is below:</p> <pre><code>func BenchmarkRandomDataMaker(b *testing.B) { randomSrc := randomDataMaker{rand.NewSource(1028890720402726901)} for i := 0; i &lt; b.N; i++ { b.SetBytes(int64(i)) _, err := io.CopyN(ioutil.Discard, &amp;randomSrc, int64(i)) if err != nil { b.Fatalf("Error copying at %v: %v", i, err) } } } </code></pre> <p>On one core of my 2.2GHz i7:</p> <pre><code>BenchmarkRandomDataMaker 50000 246512 ns/op 202.83 MB/s </code></pre> <p><strong>EDIT</strong></p> <p>Since I wrote the benchmark, I figured I'd do the obvious improvement thing (call out to the random less frequently). With 1/8 the calls to rand, it runs about 4x faster, though it's a big uglier:</p> <p>New version:</p> <pre><code>func (r *randomDataMaker) Read(p []byte) (n int, err error) { todo := len(p) offset := 0 for { val := int64(r.src.Int63()) for i := 0; i &lt; 8; i++ { p[offset] = byte(val &amp; 0xff) todo-- if todo == 0 { return len(p), nil } offset++ val &gt;&gt;= 8 } } panic("unreachable") } </code></pre> <p>New benchmark:</p> <pre><code>BenchmarkRandomDataMaker 200000 251148 ns/op 796.34 MB/s </code></pre> <p><strong>EDIT 2</strong></p> <p>Took out the masking in the cast to byte since it was redundant. Got a good deal faster:</p> <pre><code>BenchmarkRandomDataMaker 200000 231843 ns/op 862.64 MB/s </code></pre> <p>(this is so much easier than real work <em>sigh</em>)</p> <p><strong>EDIT 3</strong></p> <p>This came up in irc today, so I released a library. Also, my actual benchmark tool, while useful for relative speed, isn't sufficiently accurate in its reporting.</p> <p>I created <a href="https://github.com/dustin/randbo">randbo</a> that you can reuse to produce random streams wherever you may need them.</p>
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