digits: make it work for BigInts (by making ndigits0z work for them)

Also, add test for Project Euler problem #16.

base/gmp.jl

@@ -7,7 +7,7 @@ import Base: *, +, -, /, <, <<, >>, >>>, <=, ==, >, >=, ^, (~), (&), (|), ($),
              ndigits, promote_rule, rem, show, isqrt, string, isprime, powermod,
              widemul, sum, trailing_zeros, trailing_ones, count_ones, base, parseint,
              serialize, deserialize, bin, oct, dec, hex, isequal, invmod,
-             prevpow2, nextpow2
+             prevpow2, nextpow2, ndigits0z
 
 type BigInt <: Integer
     alloc::Cint
@@ -406,6 +406,8 @@ function base(b::Integer, n::BigInt)
 end
 
 ndigits(x::BigInt, base::Integer=10) = ccall((:__gmpz_sizeinbase,:libgmp), Culong, (Ptr{BigInt}, Int32), &x, base)
+ndigits0z(x::BigInt, base::Integer=10) = x == 0 ? 0 : ndigits(x)
+
 isprime(x::BigInt, reps=25) = ccall((:__gmpz_probab_prime_p,:libgmp), Cint, (Ptr{BigInt}, Cint), &x, reps) > 0
 
 widemul(x::BigInt, y::BigInt)   = x*y

test/bigint.jl

@@ -262,3 +262,5 @@ let
     seek(b,0)
     @test deserialize(b) == n
 end
+
+@test sum(digits(big(2)^1000)) == 1366