C     MAIN PROGRAM FOR LOGARITHMIC SPIRAL GENERATION
C     ALGORITHM/PROGRAM BY SK MISHRA, DEPT. OF ECONOMICS, NEHU,SHILLONG
      COMMON /RNDM/IU,IV
      DIMENSION X(200),Y(200),THETA(200),ANGLE(200),R(200)
C     ----------------------------------------
C     NOW WE GENERATE THE DATA
      WRITE(*,*)'INPUT THE SAMPLE SIZE = NPOINTS'
      READ(*,*) NPOINTS
      N=NPOINTS !RENAMING NPOINTS AS N
      WRITE(*,*)'INPUT HIGHEST ANGLE (UPTO 360 FOR K=0, UPTO 720 FOR K=1
     *, UPTO 1080 FOR K=2, ETC'
      READ(*,*) HANG
C     GENERATION OF ANGLES IN DEGREES
      WRITE(*,*)'INPUT RANDOM NUMBER SEED'
      READ(*,*) IU

      DO 1 I=1,N
      CALL RANDOM(RAND)
      THETA(I)=HANG*RAND
    1 CONTINUE
C     CONVERSION OF ANGLES INTO RADIANS : PI=4*ATAN(1.0)
      FACT=4.0*ATAN(1.0)/180.0
      DO 2 I=1,N
      ANGLE(I)=THETA(I)*FACT
    2 CONTINUE
      WRITE(*,*)'FEED A AND B IN R=A*EXP(B*THETA) : LOGARITHMIC SPIRAL'
      READ(*,*) A,B
C     CX AND CY DISPLACE CENTER OF SPIRAL (0,0) BY CX  AND CY
      WRITE(*,*)'DISPLACEMENT OF X --  CX ?'
      READ(*,*)CX
      WRITE(*,*)'DISPLACEMENT OF Y --  CY ?'
      READ(*,*)CY
C     GENERATION OF X AND Y
      DO 3 I=1,N
      R(I)=A*EXP(B*ANGLE(I))
      X(I)=R(I)*COS(ANGLE(I))
      Y(I)=R(I)*SIN(ANGLE(I))
    3 CONTINUE
C     WE SAVE DATA X AND Y IN 'XYDAT' FILE
      OPEN(7,FILE='XYDAT')
      WRITE(7,*) N
C     INTRODUCING DISPLACEMENT TO X BY CX AND TO Y BY CY
      DO 33 I=1,N
      call normal(rand)
      X(I)=X(I)+CX+RAND*1
      call normal(rand)
      Y(I)=Y(I)+CY+RAND*1
      WRITE(7,*) X(I),Y(I)
   33 CONTINUE
      CLOSE(7)
      WRITE(*,*)'SPIRAL DATA GENERATED AND SAVED IN FILE NAMED XYDAT'
      write(*,*)'program ends'
      END
C     -----------------------------------------------------------------
      SUBROUTINE NORMAL(R)
C     PROGRAM TO GENERATE N(0,1) FROM RECTANGULAR RANDOM NUMBERS
C     IT USES BOX-MULLER VARIATE TRANSFORMATION FOR THIS PURPOSE.
C     -----------------------------------------------------------------
C     ----- BOX-MULLER METHOD BY GEP BOX AND ME MULLER (1958) ---------
C     BOX, G. E. P. AND MULLER, M. E. "A NOTE ON THE GENERATION OF
C     RANDOM NORMAL DEVIATES." ANN. MATH. STAT. 29, 610-611, 1958.
C     IF U1 AND U2 ARE UNIFORMLY DISTRIBUTED RANDOM NUMBERS (0,1),
C     THEN X=[(-2*LN(U1))**.5]*(COS(2*PI*U2) IS N(0,1)
C     ALSO,  X=[(-2*LN(U1))**.5]*(SIN(2*PI*U2) IS N(0,1)
C     PI = 4*ARCTAN(1.0)= 3.1415926535897932384626433832795
C     2*PI = 6.283185307179586476925286766559
C     -----------------------------------------------------------------
      COMMON /RNDM/IU,IV
      INTEGER IU,IV
C     -----------------------------------------------------------------
      CALL RANDOM(RAND) ! INVOKES RANDOM TO GENERATE UNIFORM RAND [0, 1]
      U1=RAND ! U1 IS UNIFORMLY DISTRIBUTED [0, 1]
      CALL RANDOM(RAND) ! INVOKES RANDOM TO GENERATE UNIFORM RAND [0, 1]
      U2=RAND ! U1 IS UNIFORMLY DISTRIBUTED [0, 1]
      R=SQRT(-2.D0*ALOG(U1))
      R=R*COS(U2*6.28318531)
      RETURN
      END
      SUBROUTINE RANDOM(RAND)
      COMMON /RNDM/IU,IV
      IV=IU*65539
      IF(IV.LT.0) THEN
      IV=IV+2147483647+1
      ENDIF
      RAND=IV
      IU=IV
      RAND=RAND*0.4656613E-09
      RETURN
      END