sync-up gammavariate and expovariate code

[leodema]

BPO	30561
Nosy	@rhettinger, @leodema
PRs	bpo-30561: Sync-up expovariate() and gammavariate code #1934 bpo-30464: Fix comment in gammavariate #1798

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-12-24.06:55:58.854>
created_at = <Date 2017-06-03.20:16:30.394>
labels = ['performance']
title = 'sync-up gammavariate and expovariate code'
updated_at = <Date 2018-12-24.16:17:04.242>
user = 'https://github.com/leodema'

bugs.python.org fields:

activity = <Date 2018-12-24.16:17:04.242>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2018-12-24.06:55:58.854>
closer = 'rhettinger'
components = []
creation = <Date 2017-06-03.20:16:30.394>
creator = 'leodema'
dependencies = []
files = []
hgrepos = []
issue_num = 30561
keywords = []
message_count = 2.0
messages = ['295102', '332481']
nosy_count = 2.0
nosy_names = ['rhettinger', 'leodema']
pr_nums = ['1934', '1798']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue30561'
versions = []

[leodema]

As suggested by rhettinger in http://bugs.python.org/msg294438:

I agree that the comment should be changed. While we at it, perhaps sync-up with expovariate() code and eliminate the u <= 1e-7 test:

Instead of:

    elif alpha == 1.0:
        # expovariate(1)
        u = random()
        while u <= 1e-7:
            u = random()
        return -_log(u) * beta

Use this instead:

    elif alpha == 1.0:
        # expovariate(1.0 / beta)
        return -_log(1.0 - random()) * beta

[rhettinger]

New changeset 63d1522 by Raymond Hettinger (leodema) in branch 'master':
bpo-30561: Sync-up expovariate() and gammavariate code (GH-1934)
63d1522