xzlog.f

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      SUBROUTINE xzlog(AR, AI, BR, BI, IERR)
C***BEGIN PROLOGUE  XZLOG
C***REFER TO  ZBESH,ZBESI,ZBESJ,ZBESK,ZBESY,ZAIRY,ZBIRY
C
C     DOUBLE PRECISION COMPLEX LOGARITHM B=CLOG(A)
C     IERR=0,NORMAL RETURN      IERR=1, Z=CMPLX(0.0,0.0)
C***ROUTINES CALLED  XZABS
C***END PROLOGUE  XZLOG
      DOUBLE PRECISION ar, ai, br, bi, zm, dtheta, dpi, dhpi
      DOUBLE PRECISION xzabs
      DATA dpi , dhpi  / 3.141592653589793238462643383d+0,
     1                   1.570796326794896619231321696d+0/
C
      ierr=0
      IF (ar.EQ.0.0d+0) go to 10
      IF (ai.EQ.0.0d+0) go to 20
      dtheta = datan(ai/ar)
      IF (dtheta.LE.0.0d+0) go to 40
      IF (ar.LT.0.0d+0) dtheta = dtheta - dpi
      go to 50
   10 IF (ai.EQ.0.0d+0) go to 60
      bi = dhpi
      br = dlog(dabs(ai))
      IF (ai.LT.0.0d+0) bi = -bi
      RETURN
   20 IF (ar.GT.0.0d+0) go to 30
      br = dlog(dabs(ar))
      bi = dpi
      RETURN
   30 br = dlog(ar)
      bi = 0.0d+0
      RETURN
   40 IF (ar.LT.0.0d+0) dtheta = dtheta + dpi
   50 zm = xzabs(ar,ai)
      br = dlog(zm)
      bi = dtheta
      RETURN
   60 CONTINUE
      ierr=1
      RETURN
      END