!***********************************************************************************************************************************
!
!                                                       M 2 2 I N V _ M A I N
!
!  Program:      M22INV_MAIN
!
!  Programmer:   David G. Simpson
!                NASA Goddard Space Flight Center
!                Greenbelt, Maryland  20771
!
!  Date:         July 22, 2005
!
!  Language:     Fortran-90
!
!  Version:      1.00b  (February 7, 2009)
!
!  Description:  This program is a short "driver" to call function M22INV, which inverts a 2x2 matrix.
!
!  Files:        Source files:
!
!                   m22inv.f90                   Main program
!
!***********************************************************************************************************************************

      PROGRAM M22INV_MAIN

      IMPLICIT NONE

      INTEGER :: I, J
      DOUBLE PRECISION, DIMENSION(2,2) :: MAT, MATINV
      LOGICAL :: OK_FLAG

      LOGICAL :: M22INV

!-----------------------------------------------------------------------------------------------------------------------------------

!
!     Get user input.
!

      WRITE (UNIT=*, FMT='(/A/)') ' Enter matrix:'

      DO I = 1, 2
         DO J = 1, 2
            WRITE (UNIT=*, FMT='(A,I1,1H,,I1,A)', ADVANCE='NO') ' A(', I, J, ') = '
            READ (UNIT=*, FMT=*) MAT(I,J)
         END DO
      END DO

!
!     Invert the input matrix.
!

      CALL M22INV (MAT, MATINV, OK_FLAG)

!
!     Print the result.
!

      IF (OK_FLAG) THEN
         WRITE (UNIT=*, FMT='(/A/)') ' Inverse:'
         WRITE (UNIT=*, FMT='(2ES25.15)') ((MATINV(I,J), J=1,2), I=1,2)
      ELSE
         WRITE (UNIT=*, FMT='(/A)') ' Singular matrix.'
      END IF

      STOP

      END PROGRAM M22INV_MAIN



!***********************************************************************************************************************************
!  M22INV  -  Compute the inverse of a 2x2 matrix.
!
!  A       = input 2x2 matrix to be inverted
!  AINV    = output 2x2 inverse of matrix A
!  OK_FLAG = (output) .TRUE. if the input matrix could be inverted, and .FALSE. if the input matrix is singular.
!***********************************************************************************************************************************

      SUBROUTINE M22INV (A, AINV, OK_FLAG)

      IMPLICIT NONE

      DOUBLE PRECISION, DIMENSION(2,2), INTENT(IN)  :: A
      DOUBLE PRECISION, DIMENSION(2,2), INTENT(OUT) :: AINV
      LOGICAL, INTENT(OUT) :: OK_FLAG

      DOUBLE PRECISION, PARAMETER :: EPS = 1.0D-10
      DOUBLE PRECISION :: DET
      DOUBLE PRECISION, DIMENSION(2,2) :: COFACTOR


      DET =   A(1,1)*A(2,2) - A(1,2)*A(2,1)

      IF (ABS(DET) .LE. EPS) THEN
         AINV = 0.0D0
         OK_FLAG = .FALSE.
         RETURN
      END IF

      COFACTOR(1,1) = +A(2,2)
      COFACTOR(1,2) = -A(2,1)
      COFACTOR(2,1) = -A(1,2)
      COFACTOR(2,2) = +A(1,1)

      AINV = TRANSPOSE(COFACTOR) / DET

      OK_FLAG = .TRUE.

      RETURN

      END SUBROUTINE M22INV