From: keydana at gmx.de (keydana at gmx.de) Date: Thu Sep 10 13:49:43 EDT 2009 Previous message: [plt-scheme] valgrind advice / experience? Next message: [plt-scheme] please help with generating random permutation Messages sorted by: [date] [thread] [subject] [author]

```Hi,

I am in a rather uncomfortable situation after writing a script to
select people for the (not too popular) rotating job of "duty person"
in our department...
One colleague complains he is always chosen on Monday. Up till now the
sample is very small, which would make "proving" this hypothesis
impossible, but it's not a nice situation all the same, so I would
like to make "as sure as possible" that the distribution is random.

The part of the script in question works on a weekly basis, generating
a different "duty person" for every day of a week. It gets any known
"unavailability events" for the week from our calendar and filters the
input list accordingly. Then, for every day, it divides the list of
people who are available into two parts, those who did duty the week
before (list A) and those who did not (list B). It then uses the amb
operator to select a person for every day (separately), preferredly
from list B but if necessary also from list A. In the end, it checks
that the 5 people selected are unique, else it will backtrack.

Now because it was clear that amb would backtrack in a foreseeable
way, when the script starts it first shuffles the input list. For the
, at the end), which was said to have a good random distribution (I
also can confirm this when I run it e.g. 1000 times in a row).
However, in my script I run the shuffle only once per execution, and
when I e.g. run the whole script 20 times and look who is first in the
shuffled list, I get the following distribution (the numbers stand for
the position of the collegue in the input list):

0 xxxxx
1 x
2 xxx
3 xx
4 xx
5 xx
6 x
7
8 x

Unfortunately this looks like a correlation, and the aforementioned
colleague is the one above :-;

However, I do not understand this, as the mentioned shuffle function
uses random, and as I read in the documentation random is seeded with
current-milliseconds... So it really should get a different seed at
every execution.

I think that somewhere I must be making a mistake here... I would be
very grateful for any hints.
I attach a bit of the code, hopefully  as a clarification of how the
selection works, although it is not runnable without the rest and the
data provider of course.

Thanks a lot
Sigrid

(define *morning-duty-people* (shuffle '(AG MB SK MM AK RA AV CM HM)))

(define make-morning-duty-list
(lambda (start-date-string type)
(with-output-to-file (string-append logs-dir (duty-type-logfile
type)) #:exists 'append
(lambda ()
(printf "~nGoing to choose duty of type ~a for week starting
from ~a~n" (duty-type-name type) start-date-string)
(let ((from (string-to-date start-date-string)))
(if (not (= (date-week-day from) 1))
(error "Start date has to be a Monday.")
(let ((to (get-date-for-interval from 4))
(previous-from (get-date-for-interval from -7)))
(let ((previous-to (get-date-for-interval previous-
from 4)))
(let ((known-events-list (get-events from to))
(previous-week-list (get-events previous-from
previous-to)))
known-events-list from to))
(unavailables (extract-unavailables known-
events-list from to))
(previous-week-victims (flatten (extract-
morning-duties previous-week-list previous-from previous-to))))
(if (all-true? (map (lambda (days-list) (not
(let ((week-of-year (get-week-of-year from)))
(printf "Already chosen for morning duty:
(send-email *SERVER* *MAIL-TO* *SENDER*
(if dry-run (format "DRY RUN: ~a ~a" (duty-type-mail-subject type)
week-of-year) (format "~a ~a" (duty-type-mail-subject type) week-of-
year)) (insert-variables (duty-type-mail-body-exists type) week-of-
year (date-to-string from 'short) (date-to-string to 'short) (map car
(let ((availables (map (lambda (lst)
(complement *morning-duty-people* lst)) unavailables))
(previous-week-spared (complement
*morning-duty-people* previous-week-victims)))
(when debug (printf "Going to determine
morning duties for period from ~a to ~a:~nUnavailable are: ~s~nAlready
entered are: ~s~nChoosable are: ~s~nPrevious week's victims were:
~s~n" (date->string from "~Y-~m-~d") (date->string to "~Y-~m-~d")
(let ((possible-duties (choose-possible-
(when debug (printf "Choose-possible-
duties output: ~s~n" possible-duties))
(check-unique possible-duties)
(when debug (printf "Unique duties are:
~s~n" possible-duties))
(if dry-run
(printf "Not sending events to
calendar 'cause dry-run~n")
(let ((result (build-morning-duty-
events possible-duties from)))
(let ((new-only (filter (lambda
(event) (not (event-exists? known-events-list event))) result)))
(store-events new-only))))
(let ((week-of-year (get-week-of-year
from)))
(send-email *SERVER* *MAIL-TO*
*SENDER* (if dry-run (format "DRY RUN: ~a ~a" (duty-type-mail-subject
type) week-of-year) (format "~a ~a" (duty-type-mail-subject type) week-
of-year)) (insert-variables (duty-type-mail-body type) week-of-year
(date-to-string from 'short) (date-to-string to 'short) possible-
duties))))))))))))))))

(define choose-possible-duties
(when debug (printf "Choose-possible-duties called with
input:~nAvailable are: ~s~nAlready registered are: ~s~n" availables-
(let loop ((availables availables-list) (selected already-selected-
list) (result '()))
(if (null? availables)
(reverse result)
(let ((current-date-availables (car availables)) (current-
date-selected (car selected)))
(if (not (null? current-date-selected))
(loop (cdr availables) (cdr selected) (cons already-
entered result)))
(let-values (((preferred&available
availablebutrathernot) (partition (lambda (person) (member person
preferred-list)) current-date-availables)))
(let ((amb-result (grouped-list-amb
preferred&available availablebutrathernot)))
(loop (cdr availables) (cdr selected) (cons amb-
result result))))))))))

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