Commits on Feb 12, 2014

1. Remove a source of O(n^2) running time in bigints. …

   ::num::bigint, Remove a source of O(n^2) running time in `fn shr_bits`.
   
   I'll cut to the chase: On my laptop, this brings the running time on
   `pidigits 2000` (from src/test/bench/shootout-pidigits.rs) from this:
   ```
   % time ./pidigits 2000 > /dev/null
   
   real	0m7.695s
   user	0m7.690s
   sys	0m0.005s
   ```
   to this:
   ```
   % time ./pidigits 2000 > /dev/null
   
   real	0m0.322s
   user	0m0.318s
   sys	0m0.004s
   ```
   
   The previous code was building up a vector by repeatedly making a
   fresh copy for each element that was unshifted onto the front,
   yielding quadratic running time.  This fixes that by building up the
   vector in reverse order (pushing elements onto the end) and then
   reversing it.
   
   (Another option would be to build up a zero-initialized vector of the
   desired length and then installing all of the shifted result elements
   into their target index, but this was easier to hack up quickly, and
   yields the desired asymptotic improvement.  I have been thinking of
   adding a `vec::from_fn_rev` to handle this case, maybe I will try that
   this weekend.)

   

   pnkfelix committed Feb 12, 2014
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