Source code in the rexx programming language

/*REXX program displays dates of last Sundays of each month for any year*/
parse arg yyyy
                   do j=1 for  12
                   _ = lastDOW('Sunday', j, yyyy)
                   say right(_,4)'-'right(j,2,0)"-"left(word(_,2),2)
                   end  /*j*/
exit                                   /*stick a fork in it, we're done.*/
/*┌────────────────────────────────────────────────────────────────────┐
  │ lastDOW:  procedure to return the date of the  last day-of-week of │
  │           any particular month  of any particular year.            │
  │                                                                    │
  │ The  day-of-week  must be specified (it can be in any case,        │
  │ (lower-/mixed-/upper-case)  as an English name of the spelled day  │
  │ of the week,   with a minimum length that causes no ambiguity.     │
  │ I.E.:   W  for Wednesday,   Sa  for Saturday,   Su  for Sunday ... │
  │                                                                    │
  │ The month can be specified as an integer   1 ──► 12                │
  │    1=January     2=February     3=March     ...     12=December    │
  │ or the English  name  of the month,  with a minimum length that    │
  │ causes no ambiguity.    I.E.:  Jun  for June,   D  for December.   │
  │ If omitted  [or an asterisk(*)],  the current month is used.       │
  │                                                                    │
  │ The year is specified as an integer or just the last two digits    │
  │ (two digit years are assumed to be in the current century,  and    │
  │ there is no windowing for a two-digit year).                       │
  │ If omitted  [or an asterisk(*)],  the current year is used.        │
  │ Years < 100   must be specified with  (at least 2)  leading zeroes.│
  │                                                                    │
  │ Method used: find the "day number" of the 1st of the next month,   │
  │ then subtract one  (this gives the "day number" of the last day of │
  │ the month,  bypassing the leapday mess).   The last day-of-week is │
  │ then obtained straightforwardly,   or  via subtraction.            │
  └────────────────────────────────────────────────────────────────────┘*/
lastdow: procedure; arg dow .,mm .,yy .              /*DOW = day of week*/
parse arg a.1,a.2,a.3                                /*orig args, errmsg*/
if mm=='' | mm=='*' then mm=left(date('U'),2)        /*use default month*/
if yy=='' | yy=='*' then yy=left(date('S'),4)        /*use default year */
if length(yy)==2 then yy=left(date('S'),2)yy         /*append century.  */
                   /*Note mandatory leading blank in strings below.*/
$=" Monday TUesday Wednesday THursday Friday SAturday SUnday"
!=" JAnuary February MARch APril MAY JUNe JULy AUgust September",
  " October November December"
upper $ !                                            /*uppercase strings*/
if dow==''                 then call .er "wasn't specified",1
if arg()>3                 then call .er 'arguments specified',4

  do j=1 for 3                                       /*any plural args ?*/
  if words(arg(j))>1       then call .er 'is illegal:',j
  end

dw=pos(' 'dow,$)                                     /*find  day-of-week*/
if dw==0                   then call .er 'is invalid:',1
if dw\==lastpos(' 'dow,$)  then call .er 'is ambigious:',1

if datatype(mm,'month') then                         /*if MM is alpha...*/
  do
  m=pos(' 'mm,!)                                     /*maybe its good...*/
  if m==0                  then call .er 'is invalid:',1
  if m\==lastpos(' 'mm,!)  then call .er 'is ambigious:',2
  mm=wordpos(word(substr(!,m),1),!)-1                /*now, use true Mon*/
  end

if \datatype(mm,'W')       then call .er "isn't an integer:",2
if \datatype(yy,'W')       then call .er "isn't an integer:",3
if mm<1 | mm>12            then call .er "isn't in range 1──►12:",2
if yy=0                    then call .er "can't be 0 (zero):",3
if yy<0                    then call .er "can't be negative:",3
if yy>9999                 then call .er "can't be > 9999:",3

tdow=wordpos(word(substr($,dw),1),$)-1               /*target DOW, 0──►6*/
                                                     /*day# of last dom.*/
_=date('B',right(yy+(mm=12),4)right(mm//12+1,2,0)"01",'S')-1
?=_//7                                               /*calc. DOW,  0──►6*/
if ?\==tdow then _=_-?-7+tdow+7*(?>tdow)             /*not DOW?  Adjust.*/
return date('weekday',_,"B") date(,_,'B')            /*return the answer*/

.er: arg ,_;say; say '***error!*** (in LASTDOW)';say /*tell error,  and */
  say word('day-of-week month year excess',arg(2)) arg(1) a._
  say; exit 13                                       /*... then exit.   */