minor gamma cleanups.

[oscardssmith]

This should arguably should use Base.@assume_effects :terminates_locally but that only exists on 1.8+.

[codecov]

Codecov Report

Base: 94.70% // Head: 94.53% // Decreases project coverage by -0.16% ⚠️

Coverage data is based on head (c264d27) compared to base (6aa49b7).
Patch coverage: 57.89% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   94.70%   94.53%   -0.17%     
==========================================
  Files          18       18              
  Lines        1963     1957       -6     
==========================================
- Hits         1859     1850       -9     
- Misses        104      107       +3     

Impacted Files	Coverage Δ
src/gamma.jl	72.22% <57.89%> (-11.12%)	⬇️

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[heltonmc]

Awesome! I'm wondering if we should go ahead and solve #58 here.

Essentially, that issue can be solved by having some check if x is an integer and (x < 21) to just return the base factorial lookup table.... I'd honestly be fine just to have a different dispatch for it. So you would only hit that branch if you input an integer nu but I guess we could also check for it within the routines... but it seems way easier to do it here than update all of the functions. Something like

function gamma(x::Integer)
    if x < 21
        if x > 0
            return float(factorial(x - 1))
        elseif x < 0
            return DomainError...
        else
            return Float64(Inf)
        end
    else
        return gamma(Float64(x))
    end
end

[heltonmc]

Looking at how SpecialFunctions.jl handles it..

https://github.com/JuliaMath/SpecialFunctions.jl/blob/36c547b4a270b6089b1baf7bec05707e9fb8c7f9/src/gamma.jl#L567-L575

Which might be cleaner than what I just posted

[heltonmc]

Great - I'll merge and test if this solves that issue.