• Brian Kessler's avatar
    math/cmplx: use signed zero to correct branch cuts · 802a8f88
    Brian Kessler authored
    Branch cuts for the elementary complex functions along real or imaginary axes
    should be resolved in floating point calculations by one-sided continuity with
    signed zero as described in:
    
    "Branch Cuts for Complex Elementary Functions or Much Ado About Nothing's Sign Bit"
    W. Kahan
    
    Available at: https://people.freebsd.org/~das/kahan86branch.pdf
    
    And as described in the C99 standard which is claimed as the original cephes source.
    
    Sqrt did not return the correct branch when imag(x) == 0. The branch is now
    determined by sign(imag(x)).  This incorrect branch choice was affecting the behavior
    of the Trigonometric/Hyperbolic functions that use Sqrt in intermediate calculations.
    
    Asin, Asinh and Atan had spurious domain checks, whereas the functions should be valid
    over the whole complex plane with appropriate branch cuts.
    
    Fixes #6888
    
    Change-Id: I9b1278af54f54bfb4208276ae345bbd3ddf3ec83
    Reviewed-on: https://go-review.googlesource.com/46492
    Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: 's avatarRuss Cox <rsc@golang.org>
    802a8f88
sqrt.go 2.93 KB