enhance consistency of prng

Author: Hyeoksu Lee

src/common/m_constants.fpp

@@ -66,9 +66,10 @@ module m_constants
 
     ! Pseudo-random number generator
     integer, parameter :: modulus = 2**30 - 1
-    integer, parameter :: multiplier = 1103515245
-    integer, parameter :: increment = 12345
+    integer, parameter :: multiplier = 1664525
+    integer, parameter :: increment = 1013904223
     integer, parameter :: amplifier = 3**13
+    real(wp), parameter :: decimal_trim = 1.e5_wp
 
     ! System constants
     integer, parameter :: CASE_FILE_ERROR_CODE = 22

src/pre_process/m_perturbation.fpp

@@ -286,12 +286,12 @@ contains
 
     function modmul(a) result(val)
         integer, intent(in) :: a
-        integer :: xint, val
-        real(wp) :: x
+        integer :: val
+        real(wp) :: x, y
 
         x = (multiplier/real(modulus, wp))*a + (increment/real(modulus, wp))
-        xint = floor(x)
-        val = nint((x - xint)*modulus)
+        y = nint((x - floor(x))*decimal_trim)/decimal_trim
+        val = nint(y*modulus)
 
     end function modmul