Added Chase's Fortran version of GenPrime

Signed-off-by: Eddie Ringle <eddie@eringle.net>
Signed-off-by: Steven Noonan <steven@uplinklabs.net>

genprime.f

@@ -0,0 +1,84 @@
+C  Genprime.f
+C  Copyright(c) 2008, ME
+C  
+C  You may use this file in any way you like.
+C  Print it out and use it as nesting material
+C    for your rabbit, I don't care.
+C
+C       I'm using integer here due to inconsistancies in
+C       Fortran LOGICAL datatype and Fortran FUNCTIONs.
+C
+        INTEGER FUNCTION ISPRIME(X)
+        INTEGER*8 LIM, Y, X
+        IF(X.lt.2) THEN
+            ISPRIME = 0
+            RETURN
+        ENDIF
+        IF(X.lt.4) THEN
+            ISPRIME = 1
+            RETURN
+        ENDIF
+        IF(X.eq.5) THEN
+            ISPRIME = 1
+            RETURN
+        ENDIF
+        IF(MOD(X,2).eq.0) THEN
+            ISPRIME = 0
+            RETURN
+        ENDIF
+        IF(MOD(X,5).eq.0) THEN
+            ISPRIME = 0
+            RETURN
+        ENDIF
+        IF(MOD(X + 1, 6) .NE. 0) THEN
+            IF(MOD(X - 1, 6) .NE. 0) THEN
+                ISPRIME = 0
+                RETURN
+            ENDIF
+        ENDIF
+        LIM = INT(SQRT(REAL(X)) + 1.0);
+        DO Y = 3, LIM, 2
+            IF(MOD(X,Y).eq.0) THEN
+                ISPRIME = 0
+                RETURN
+            ENDIF
+        ENDDO
+        ISPRIME = 1
+        RETURN
+        END
+
+        FUNCTION GENPRIME(MX)
+        INTEGER*8 CNT, CUR, MX
+        CUR = 0
+		CNT = 0
+        DO WHILE(CNT.lt.MX)
+            IF(ISPRIME(CUR).eq.1) THEN
+                CNT = CNT + 1
+            ENDIF
+            CUR = CUR + 1
+        ENDDO
+        GENPRIME = CUR - 1;
+        RETURN
+        END
+
+C       Doesn't have to be MAIN
+        PROGRAM MAIN
+          REAL TIMES, TIMET, TIME
+          INTEGER*8 STRT, STP, X
+          INTEGER Z
+
+          CHARACTER *20 BUFFER
+          CALL GETARG(1,BUFFER)
+          READ (BUFFER,*) STRT
+          CALL GETARG(2,BUFFER)
+          READ (BUFFER,*) STP
+
+          DO X = STRT, STP, STRT
+              TIMES = SECOND()
+              Z = GENPRIME(X)
+              TIMET = SECOND()
+              TIME = TIMET - TIMES
+              PRINT *,'Found ',X,' primes in ',TIME,' (last was',Z,')'
+          ENDDO
+
+        END