Simplify hangs

[proy87]

simplify(65712362363534280139543*ln(49/50)/2441406250000000000) is hanging. I guess it is because of coeff near ln.

[smichr]

>>> simplify(S.Half*log(4))
log(2)

logombine is responsible for making this change. Here is a less fortuitous change:

>>> logcombine(65*ln(49/S(50))/24)
log(16807*2**(7/24)*5**(7/12)*7**(5/12)/125000)

In simplify, logcombine is attempted without any requirements other than that the expression has log in it. There may be a better way to test whether such a change should be attempted.

[oscarbenjamin]

This no longer hangs and simplify does not seem to call logcombine any more:

In [47]: e = simplify(65712362363534280139543*ln(S(49)/50)/2441406250000000000)

In [48]: e
Out[48]: 
                           ⎛49⎞
65712362363534280139543⋅log⎜──⎟
                           ⎝50⎠
───────────────────────────────
      2441406250000000000

Calling logcombine explicitly does create a large expression but it is still computed quickly:

In [1]: %time e = logcombine(65712362363534280139543*ln(S(49)/50)/2441406250000000000)
CPU times: user 65.6 ms, sys: 424 µs, total: 66 ms
Wall time: 64.9 ms

In [5]: %time len(str(e))
CPU times: user 91.4 ms, sys: 205 µs, total: 91.6 ms
Wall time: 91.1 ms
Out[5]: 91361

Possibly what was hanging before was converting the output to a string. By default this fails now (I had to use sys.set_int_max_str_digits(0)) so that would not hang any more.

Probably with large enough inputs logcombine would still hang on an expression like this.