Clozure Common Lisp Version 1.12 (v1.12) LinuxX8664

For more information about CCL, please see http://ccl.clozure.com.

CCL is free software.  It is distributed under the terms of the Apache
Licence, Version 2.0.

;;; I shall redefine %FLOAT-RANDOM to signal an error in case the result is equal to the original number.
;;; Reference: https://github.com/Clozure/ccl/blob/8778079bd32e0ff9398d66d1b1da9c33641ddf1d/level-0/l0-numbers.lisp#L1970

? (in-package :ccl)
#<Package "CCL">

? (defun %float-random (number state)
    (let* ((old-state (make-random-state state))
           (random-int (random target::target-most-positive-fixnum state))
           (ratio (gvector :ratio random-int target::target-most-positive-fixnum)))
      (declare (dynamic-extent ratio))
      (let ((result (* number ratio)))
        (if (= number result)
            (error "%FLOAT-RANDOM failure: ~S ~S ~S" result random-int old-state)
            result))))
%FLOAT-RANDOM

;;; Triggering the bug:

? (dotimes (i 1000000000) (random 57.0))

> Error: %FLOAT-RANDOM failure: 57.0 1152921472459506616 #.(INITIALIZE-MRG31K3P-STATE 267351351 1658092716 1369862252 257820909 1071609802 1939985399)
> While executing: %FLOAT-RANDOM, in process listener(1).
> Type :POP to abort, :R for a list of available restarts.
> Type :? for other options.

;;; The error is because the random fixnum 1152921472459506616 was produced by the random state:

1 > (let ((*random-state* #.(INITIALIZE-MRG31K3P-STATE 267351351 1658092716 1369862252 257820909 1071609802 1939985399)))
      (random most-positive-fixnum))
1152921472459506616

;;; This number is dannnnnngerously close to MOST-POSITIVE-FIXNUM:

1 > most-positive-fixnum
1152921504606846975

1 > (/ 1152921472459506616 most-positive-fixnum)
1152921472459506616/1152921504606846975

1 > (float *)
1.0 ;; wow!

1 > (float ** 1.0d0)
0.999999972116627D0

;;; And so, finally, (* 1152921472459506616/1152921504606846975 57.0) == 57.0 because of floating point rounding:

1 > (* 1152921472459506616/1152921504606846975 57.0)
57.0