1: COMMENT: RANDOM NUMBER GENERATOR
2: ***********************
3:
4: PROCEDURE RANDOM RETURNS A LONG REAL RANDOM NUMBER UNIFORMLY
5: DISTRIBUTED IN (0,1) (INCLUDING 0 BUT NOT 1),
6: RANINIT(R) WITH R ANY INTEGER MUST BE CALLED FOR
7: INITIALIZATION BEFORE THE FIRST CALL TO RANDOM, AND THE
8: DECLARATIONS OF RAN1, RAN2 AND RAN3 MUST BE GLOBAL,
9: THE ALGORITHM RETURNS X(N)/2**56, WHERE
10: X(N) = X(N-1) + X(N-127) (MOO 2**56),
11: SINCE 1 + X + X**127 IS PRIMITIVE (MOD 2), THE PERIOD IS AT
12: LEAST 2**127 - 1 > 10**38, SEE KNUTH (1969), PP. 26, 34, 464,
13: X(N) IS STORED IN A LONG REAL WORD AS
14: RAN3 = X(N)/2**56 - 1/2, AND ALL FLOATING POINT ARITHMETIC
15: IS EXACT;
16:
17: LONG REAL PROCEDURE RANDOM(INTEGER VALUE NAUGHT);
18: BEGIN
19: LONG REAL RAN1; INTEGER RAN2; LONG REAL ARRAY RAN3 (0::126);
20: INTEGER R; LOGICAL INIT;
21: INIT := FALSE;
22: IF INIT THEN GO TO L3;
23: R := ABS(NAUGHT) REM 8190 + 1;
24: RAN2 := 127; WHILE RAN2 > 0 DO
25: BEGIN RAN2 := RAN2 - 1; RAN1 := -2L**55;
26: FOR I := 1 UNTIL 7 DO
27: BEGIN R := (1756*R) REM 8191;
28: RAN1 := (RAN1 + (R DIV 32) )*( 1/256) ;
29: END;
30: RAN3 (RAN2) := RAN1
31: END;
32: INIT := TRUE;
33: L3: RAN2 := IF RAN2 = 0 THEN 126 ELSE RAN2 - 1;
34: RAN1 := RAN1 + RAN3 (RAN2);
35: RAN3 (RAN2) := RAN1 := IF RAN1 < 0L THEN RAN1 + 0.5L
36: ELSE RAN1 - 0.5L;
37: RAN1 + 0.5L
38: END RANDOM.
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