code review 95460049: math/rand: restore Go 1.2 value stream for Float32, Float64

math/rand: restore Go 1.2 value stream for Float32, Float64

CL 22730043 fixed a bug in these functions: they could
return 1.0 despite documentation saying otherwise.
But the fix changed the values returned in the non-buggy case too,
which might invalidate programs depending on a particular
stream when using rand.Seed(0) or when passing their own
Source to rand.New.

The example test says:
        // These tests serve as an example but also make sure we don't change
        // the output of the random number generator when given a fixed seed.
so I think there is some justification for thinking we have
promised not to change the values. In any case, there's no point in
changing the values gratuitously: we can easily fix this bug without
changing the values, and so we should.

That CL just changed the test values too, which defeats the
stated purpose, but it was just a comment.
Add an explicit regression test, which might be
a clearer signal next time that we don't want to change
the values.

Fixes issue 6721. (again)
Fixes issue 8013.

[rsc, 2014-05-19 13:36:00]

Hello iant, r (cc: golang-codereviews@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go/

[r, 2014-05-19 15:07:26]

LGTM although the test could be avoid one level of reflection with some
refactoring

[rsc, 2014-05-19 16:30:09]

noted, but i'm going to leave the test as is for now to move on to other
things.

[rsc, 2014-05-19 16:30:28]

*** Submitted as https://code.google.com/p/go/source/detail?r=0d7a1ac60a57 ***

math/rand: restore Go 1.2 value stream for Float32, Float64

CL 22730043 fixed a bug in these functions: they could
return 1.0 despite documentation saying otherwise.
But the fix changed the values returned in the non-buggy case too,
which might invalidate programs depending on a particular
stream when using rand.Seed(0) or when passing their own
Source to rand.New.

The example test says:
        // These tests serve as an example but also make sure we don't change
        // the output of the random number generator when given a fixed seed.
so I think there is some justification for thinking we have
promised not to change the values. In any case, there's no point in
changing the values gratuitously: we can easily fix this bug without
changing the values, and so we should.

That CL just changed the test values too, which defeats the
stated purpose, but it was just a comment.
Add an explicit regression test, which might be
a clearer signal next time that we don't want to change
the values.

Fixes issue 6721. (again)
Fixes issue 8013.

LGTM=r
R=iant, r
CC=golang-codereviews
https://codereview.appspot.com/95460049