| double precision function randlc(x, a) |
| |
| c--------------------------------------------------------------------- |
| c |
| c This routine returns a uniform pseudorandom double precision number in the |
| c range (0, 1) by using the linear congruential generator |
| c |
| c x_{k+1} = a x_k (mod 2^46) |
| c |
| c where 0 < x_k < 2^46 and 0 < a < 2^46. This scheme generates 2^44 numbers |
| c before repeating. The argument A is the same as 'a' in the above formula, |
| c and X is the same as x_0. A and X must be odd double precision integers |
| c in the range (1, 2^46). The returned value RANDLC is normalized to be |
| c between 0 and 1, i.e. RANDLC = 2^(-46) * x_1. X is updated to contain |
| c the new seed x_1, so that subsequent calls to RANDLC using the same |
| c arguments will generate a continuous sequence. |
| |
| implicit none |
| double precision x, a |
| integer*8 Lx, La, a1, a2, x1, x2, xa |
| double precision d2m46 |
| parameter(d2m46=0.5d0**46) |
| |
| Lx = x |
| La = A |
| a1 = ibits(La, 23, 23) |
| a2 = ibits(La, 0, 23) |
| x1 = ibits(Lx, 23, 23) |
| x2 = ibits(Lx, 0, 23) |
| xa = ishft(ibits(a1*x2+a2*x1, 0, 23), 23) + a2*x2 |
| Lx = ibits(xa,0, 46) |
| x = dble(Lx) |
| randlc = d2m46*x |
| return |
| end |
| |
| |
| c--------------------------------------------------------------------- |
| c--------------------------------------------------------------------- |
| |
| |
| SUBROUTINE VRANLC (N, X, A, Y) |
| implicit none |
| integer n, i |
| double precision x, a, y(*) |
| integer*8 Lx, La, a1, a2, x1, x2, xa |
| double precision d2m46 |
| parameter(d2m46=0.5d0**46) |
| |
| Lx = X |
| La = A |
| a1 = ibits(La, 23, 23) |
| a2 = ibits(La, 0, 23) |
| do i = 1, N |
| x1 = ibits(Lx, 23, 23) |
| x2 = ibits(Lx, 0, 23) |
| xa = ishft(ibits(a1*x2+a2*x1, 0, 23), 23) + a2*x2 |
| Lx = ibits(xa,0, 46) |
| y(i) = d2m46*dble(Lx) |
| end do |
| x = dble(Lx) |
| return |
| end |
| |