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Summary: Random from Brent in Algol W

    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.