[PATCH] more efficient mersenne_twister_engine::discard

[Ulrich Drepper <drepper at gmail dot com>]

The discard member function of the mersenne_twister_engine class is
unnecessarily inefficient.   It currently discard elements one-by-one.
 It is possible to discard with higher granularity by discarding the
entire internal buffer.  The attached patch implements this.  To avoid
duplication a new internal member function is introduced which
generates a new set of bits for the internal buffer.  The operator()
is changed to use it.

2012-08-22  Ulrich Drepper  <drepper@gmail.com>

	* include/bits/random.h (mersenne_twister_engine): Don't inline
	discard here.  New member function _M_gen_rand.
	* include/bits/random.tcc (mersenne_twister_engine<>::_M_gen_rand):
	New function.  Extracted from operator().
	(mersenne_twister_engine<>::discard): New implementation which
	skips in large steps.
	(mersenne_twister_engine<>::operator()): Use _M_gen_rand.

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