SUBROUTINE iau_RV2M ( W, R )
*+
*  - - - - - - - - -
*   i a u _ R V 2 M
*  - - - - - - - - -
*
*  Form the r-matrix corresponding to a given r-vector.
*
*  This routine is part of the International Astronomical Union's
*  SOFA (Standards of Fundamental Astronomy) software collection.
*
*  Status:  vector/matrix support routine.
*
*  Given:
*     W        d(3)      rotation vector (Note 1)
*
*  Returned:
*     R        d(3,3)    rotation matrix
*
*  Notes:
*
*  1) A rotation matrix describes a rotation through some angle about
*     some arbitrary axis called the Euler axis.  The "rotation vector"
*     supplied to this routine has the same direction as the Euler axis,
*     and its magnitude is the angle in radians.
*
*  2) If W is null, the indentity matrix is returned.
*
*  3) The reference frame rotates clockwise as seen looking along the
*     rotation vector from the origin.
*
*  This revision:  2020 August 21
*
*  SOFA release 2023-10-11
*
*  Copyright (C) 2023 IAU SOFA Board.  See notes at end.
*
*-----------------------------------------------------------------------

      IMPLICIT NONE

      DOUBLE PRECISION W(3), R(3,3)

      DOUBLE PRECISION X, Y, Z, PHI, S, C, F

* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

*  Euler angle (magnitude of rotation vector) and functions.
      X = W(1)
      Y = W(2)
      Z = W(3)
      PHI = SQRT(X*X + Y*Y + Z*Z)
      S = SIN(PHI)
      C = COS(PHI)
      F = 1D0 - C

*  Euler axis (direction of rotation vector), perhaps null.
      IF ( PHI .GT. 0D0 ) THEN
         X = X / PHI
         Y = Y / PHI
         Z = Z / PHI
      END IF

*  Form the rotation matrix.
      R(1,1) = X*X*F + C
      R(1,2) = X*Y*F + Z*S
      R(1,3) = X*Z*F - Y*S
      R(2,1) = Y*X*F - Z*S
      R(2,2) = Y*Y*F + C
      R(2,3) = Y*Z*F + X*S
      R(3,1) = Z*X*F + Y*S
      R(3,2) = Z*Y*F - X*S
      R(3,3) = Z*Z*F + C

*  Finished.

*+----------------------------------------------------------------------
*
*  Copyright (C) 2023
*  Standards of Fundamental Astronomy Board
*  of the International Astronomical Union.
*
*  =====================
*  SOFA Software License
*  =====================
*
*  NOTICE TO USER:
*
*  BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING SIX TERMS AND
*  CONDITIONS WHICH APPLY TO ITS USE.
*
*  1. The Software is owned by the IAU SOFA Board ("SOFA").
*
*  2. Permission is granted to anyone to use the SOFA software for any
*     purpose, including commercial applications, free of charge and
*     without payment of royalties, subject to the conditions and
*     restrictions listed below.
*
*  3. You (the user) may copy and distribute SOFA source code to others,
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*     distribution that does not consist of intact and unchanged copies
*     of SOFA source code files is a "derived work" that must comply
*     with the following requirements:
*
*     a) Your work shall be marked or carry a statement that it
*        (i) uses routines and computations derived by you from
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*
*     b) The source code of your derived work must contain descriptions
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*
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*        thereof such as changes of case.
*
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*
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*
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*
*  In any published work or commercial product which uses the SOFA
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*
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*
*      By email:  sofa@ukho.gov.uk
*      By post:   IAU SOFA Center
*                 HM Nautical Almanac Office
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*
*-----------------------------------------------------------------------

      END