function cgamma(z)
  ! sikinote (http://slpr.sakura.ne.jp/qp)
  ! author : sikino
  ! date   : 2017/01/09
  !          2017/01/10 optimize the number of a(M)
  !          2017/10/27 fix for positive integer
  
  implicit none
  complex(kind(0d0)),intent(in)::z
  complex(kind(0d0))::cgamma
 
  integer::i,n,M
  double precision,parameter::pi=3.14159265358979324d0
  double precision,parameter::e1=0.3678794411714423216d0
  double precision,parameter::ln2pi2=0.91893853320467274d0
  double precision,parameter::sq2pi=2.5066282746310005d0
  double precision::a(1:30),AB(1:14),dz,iz
  complex(kind(0d0))::q,s,w,r,q1,q2

  dz=dble(z)
  iz=imag(z)

  if(dz.eq.nint(dz).and.iz.eq.0d0)then
     if(dz.gt.0d0)then
        s=dcmplx(1d0,0d0)
        n=nint(dz)
        do i=2,n-1
           s=s*i
        enddo
     else
        s=1d+300
     endif
     cgamma=s
  else
     q=z
     if(dz.lt.0d0)q=1d0-z

     if(abs(iz).lt.1.45d0)then
        ! For x=arb, |y|\lt 1.45
        n=int(dble(q))
        s=1d0
        do i=1,n
           s=s*(q-i)
        enddo
        w=q-dble(n)

        a(1) = 1d0
        a(2) = 0.57721566490153286d0
        a(3) =-0.65587807152025388d0
        a(4) =-0.42002635034095236d-1
        a(5) = 0.16653861138229149d0
        a(6) =-0.42197734555544337d-1
        a(7) =-0.96219715278769736d-2
        a(8) = 0.72189432466630995d-2
        a(9) =-0.11651675918590651d-2
        a(10)=-0.21524167411495097d-3
        a(11)= 0.12805028238811619d-3
        a(12)=-0.20134854780788239d-4
        a(13)=-0.12504934821426707d-5
        a(14)= 0.11330272319816959d-5
        a(15)=-0.20563384169776071d-6
        a(16)= 0.61160951044814158d-8
        a(17)= 0.50020076444692229d-8
        a(18)=-0.11812745704870201d-8
        a(19)= 0.10434267116911005d-9
        a(20)= 0.77822634399050713d-11
        a(21)=-0.36968056186422057d-11
        a(22)= 0.51003702874544760d-12
        a(23)=-0.20583260535665068d-13
        a(24)=-0.53481225394230180d-14
        a(25)= 0.12267786282382608d-14
        a(26)=-0.11812593016974588d-15
        a(27)= 0.11866922547516003d-17
        a(28)= 0.14123806553180318d-17
        a(29)=-0.22987456844353702d-18
        a(30)= 0.17144063219273374d-19

        M=int(11.3*abs(w)+13)
        if(M.gt.30)M=30
        r=a(M)
        do i=M-1,1,-1
           r=r*w+a(i)
        enddo
        
        cgamma=s/(r*w)
     else
        ! For x=arb, |y|\gt 1.45
        s=1d0
        if(abs(q).lt.9d0)then
           do i=0,7
              s=s*(q+i)
           enddo
           s=1d0/s
           q=q+8d0
        endif

        AB(1) = 0.83333333333333333d-1
        AB(2) =-0.27777777777777778d-2
        AB(3) = 0.79365079365079365d-3
        AB(4) =-0.59523809523809524d-3
        AB(5) = 0.84175084175084175d-3
        AB(6) =-0.19175269175269175d-2
        AB(7) = 0.64102564102564103d-2
        AB(8) =-0.29550653594771242d-1

        q1=1d0/q
        q2=q1*q1
       
        r=AB(8)
        do i=7,1,-1
           r=r*q2+AB(i)
        enddo
        cgamma=s*exp((q-0.5d0)*log(q)-q+ln2pi2+r*q1)
     endif
     if(dz.lt.0d0)cgamma=pi/(cgamma*sin(pi*z))
  endif

  return
end function cgamma