PROGRAM TRIANGLES
      IMPLICIT NONE
      REAL*4 A, B, ALPHA, MINIMALANGLE
      INTEGER ACTION
      COMMON /TR/A, B, ALPHA
      CALL MENU
      CONTAINS
       LOGICAL *1 FUNCTION TRCON()
        TRCON = A.GT.0 .AND. B.GT.0 .AND. ALPHA.GT.0 .AND. ALPHA.LT.180
       END
       SUBROUTINE MENU
        WRITE (*, *) ""
        WRITE (*,*) "--- MENU ---"
        WRITE (*,*) "1. INPUT TRIANGLE"
        WRITE (*,*) "2. FIND SPACE"
        WRITE (*,*) "3. FIND MINIMAL ANGLE"
        WRITE (*,*) "4. FIND COS(MINIMAL ANGLE)"
        WRITE (*,*) "5. EXIT"
        WRITE (*, *) ""
        READ (*, *) ACTION
        IF (ACTION.EQ.1) CALL MENU1
        IF (ACTION.EQ.2) CALL MENU2
        IF (ACTION.EQ.3) CALL MENU3
        IF (ACTION.EQ.4) CALL MENU4
        IF (ACTION.EQ.5) CALL MENU5
        IF (ACTION.LT.1 .OR. ACTION.GT.5) THEN
         WRITE (*, *) "WRONG COMMAND"
         CALL EXIT(-1)
        END IF
       END
       SUBROUTINE MENU1
        ! DATA INPUT
        WRITE (*,*) 'TYPE EDGES A, B AND ANGLE ALPHA:'
        READ (*,*) A, B, ALPHA
        IF (.NOT.TRCON()) THEN
         WRITE (*,*) 'ERROR: PROVIDED DATA IS NOT TRIANGLE'
         A = 0
         B = 0
         ALPHA = 180
        END IF
        CALL MENU
       END
       SUBROUTINE MENU2
        ! SPACE CALCULATION
        REAL*4 S
        IF (TRCON()) THEN
         S = 0.5 * A * B * SIND(ALPHA)
         WRITE (*,*) 'S  =', S
        ELSE
         WRITE (*, *) "INPUT VALID TRIANGLE (1) FIRST"
        END IF
        CALL MENU
       END
       SUBROUTINE MENU3
        ! MINIMAL ANGLE SEARCH (IN DEGREES)
        REAL*4 GAMMA, BETA, C
        IF (TRCON()) THEN
         C = SQRT(A**2 + B**2 - 2*A*B*COSD(ALPHA))
         WRITE (*, *) "--- ANGLES ---"
         WRITE (*, *) ALPHA
         BETA = ACOSD((B**2 + C**2 - A**2) / (2 * B * C))
         WRITE (*,*) BETA
         GAMMA = ACOSD((C**2 + A**2 - B**2) / (2 * C * A))
         WRITE (*,*) GAMMA
         MINIMALANGLE = ALPHA
         IF (BETA.LT.MINIMALANGLE) MINIMALANGLE = BETA
         IF (GAMMA.LT.MINIMALANGLE) MINIMALANGLE = GAMMA
         WRITE (*,*) "MINIMAL ANGLE EQUALS", MINIMALANGLE
        ELSE
         WRITE (*, *) "INPUT VALID TRIANGLE (1) FIRST"
        END IF
         CALL MENU
       END
       SUBROUTINE MENU4
        ! COS(MINIMAL ANGLE) CALCULATION
        REAL*4 RESULT
        IF (TRCON()) THEN
         RESULT = COSD(MINIMALANGLE)
         WRITE (*,*) "COS(MINIMAL ANGLE) =", RESULT
        ELSE
         WRITE (*, *) "INPUT VALID TRIANGLE (1) FIRST"
        END IF
        CALL MENU
       END
       SUBROUTINE MENU5
        ! PROGRAM EXIT
        CALL EXIT(0)
       END
      END