Commit f20785f0 authored by Nikodemus Siivola's avatar Nikodemus Siivola

more error-checking in RANDOM-ELT

 Check for empty sequences and invalid bounding index designators.
parent c9e15432
......@@ -104,13 +104,28 @@ error if SEQUENCE is not a proper sequence."
(defun random-elt (sequence &key (start 0) end)
"Returns a random element from SEQUENCE bounded by START and END. Signals an
error if the SEQUENCE is not a proper sequence."
error if the SEQUENCE is not a proper non-empty sequence, or if END and START
are not proper bounding index designators for SEQUENCE."
(declare (sequence sequence) (fixnum start) (type (or fixnum null) end))
(let ((i (+ start (random (- (or end (if (listp sequence)
(proper-list-length sequence)
(length sequence)))
start)))))
(elt sequence i)))
(let* ((size (if (listp sequence)
(proper-list-length sequence)
(length sequence)))
(end2 (or end size)))
(cond ((zerop size)
(error 'type-error
:datum sequence
:expected-type `(and sequence (not (satisfies emptyp)))))
((not (and (<= 0 start) (< start end2) (<= end2 size)))
(error 'simple-type-error
:datum (cons start end)
:expected-type `(cons (integer 0 (,end2))
(or null (integer (,start) ,size)))
:format-control "~@<~S and ~S are not valid bounding index designators for ~
a sequence of length ~S.~:@>"
:format-arguments (list start end size)))
(t
(let ((index (+ start (random (- end2 start)))))
(elt sequence index))))))
(declaim (inline remove/swapped-arguments))
(defun remove/swapped-arguments (sequence item &rest keyword-arguments)
......
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