scheme48/scheme/srfi/srfi-19.scm

;; SRFI-19: Time Data Types and Procedures.
;; 
;; Copyright (C) I/NET, Inc. (2000, 2002, 2003). All Rights Reserved. 
;; 
;; Permission is hereby granted, free of charge, to any person obtaining
;; a copy of this software and associated documentation files (the
;; "Software"), to deal in the Software without restriction, including
;; without limitation the rights to use, copy, modify, merge, publish,
;; distribute, sublicense, and/or sell copies of the Software, and to
;; permit persons to whom the Software is furnished to do so, subject to
;; the following conditions:
;; 
;; The above copyright notice and this permission notice shall be
;; included in all copies or substantial portions of the Software.
;; 
;; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
;; EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
;; MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
;; NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
;; LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
;; OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
;; WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

;; -- Bug fixes.
;;
;; MAKE-TIME had parameters seconds and nanoseconds reversed; change all
;;           references in file to match.  Will F: 2002-10-15
;;
;; DATE-YEAR-DAY returned the wrong day; tm:year-day fixed to do the right
;;               thing. Will F: 2002-10-15
;;               It also called an undefined error procedure.
;;
;; DISPLAYING procedure removed. Will F: 2002-10-15.
;;
;; TM:NANO constant corrected. 2002-11-04.
;;
;; The following fixes by Will Fitzgerald, February, 2003.
;;  -- Thanks to Steven Ma and others.
;;
;; (CURRENT-TIME 'TIME-THREAD) added.
;;
;; TIME-RESOLUTION for TIME-PROCESS added. 
;;
;; TIME comparison procedures (time=?, etc. fixed. 
;;
;; Corrected errors in converting between TAI and UTC time.
;;
;; TAI and UTC date converters no longer look at leap seconds,
;; which was an error.
;;
;; corrections to calls to tm:time-error
;;
;; timezone offset not used in date->time-utc and date->julian-day
;;
;; typos in tm:integer-reader-exact, tm:string->date,
;; time-monotonic->time-utc!, tm:char->int fixed
;;
;; corrected "~k", "~f" formatting for date->string (includes fix for
;; "~4"
;;
;; 'split-real' fixed.
;;
;; fixed julian-day->time-utc and variants. 
;; --------------------------------------------------------------

;;; SCSH portability: import receive from srfi-8

;;; -- we want receive later on for a couple of small things
;; 

;; :OPTIONAL is nice, too

(define-syntax :optional
  (syntax-rules ()
    ((_ val default-value)
     (if (null? val) default-value (car val)))))

(define (seconds+nanoseconds)
  (let ((time (os-time:current-utc-time)))
    (values (os-time:time-seconds time) (* (os-time:time-microseconds time) 1000))))

(define (ticks/sec) (expt 10 6))
(define (cpu-ticks/sec) 1) ;; we don't suppport sub-second precision yet

(define time-tai 'time-tai)
(define time-utc 'time-utc)
(define time-monotonic 'time-monotonic)
(define time-thread 'time-thread)
(define time-process 'time-process)
(define time-duration 'time-duration)

;; example of extension (MZScheme specific)
(define time-gc 'time-gc)

;;-- LOCALE dependent constants

(define tm:locale-number-separator ".")

(define tm:locale-abbr-weekday-vector (vector "Sun" "Mon" "Tue" "Wed"
                                              "Thu" "Fri" "Sat")) 
(define tm:locale-long-weekday-vector (vector "Sunday" "Monday"
                                              "Tuesday" "Wednesday"
                                              "Thursday" "Friday"
                                              "Saturday"))
;; note empty string in 0th place. 
(define tm:locale-abbr-month-vector   (vector "" "Jan" "Feb" "Mar"
                                              "Apr" "May" "Jun" "Jul"
                                              "Aug" "Sep" "Oct" "Nov"
                                              "Dec")) 
(define tm:locale-long-month-vector   (vector "" "January" "February"
                                              "March" "April" "May"
                                              "June" "July" "August"
                                              "September" "October"
                                              "November" "December")) 

(define tm:locale-pm "PM")
(define tm:locale-am "AM")

;; See date->string
(define tm:locale-date-time-format "~a ~b ~d ~H:~M:~S~z ~Y")
(define tm:locale-short-date-format "~m/~d/~y")
(define tm:locale-time-format "~H:~M:~S")
(define tm:iso-8601-date-time-format "~Y-~m-~dT~H:~M:~S~z")
;;-- Miscellaneous Constants.
;;-- only the tm:tai-epoch-in-jd might need changing if
;;   a different epoch is used.

(define tm:nano (expt 10 9))
(define tm:sid  86400)    ; seconds in a day
(define tm:sihd 43200)    ; seconds in a half day
(define tm:tai-epoch-in-jd 4881175/2) ; julian day number for 'the epoch'
;; SCSH portability
(define tm:ns/quantum (/ tm:nano (ticks/sec)))


;;; A Very simple Error system for the time procedures
;;; 
(define tm:time-error-types
  '(invalid-clock-type
    unsupported-clock-type
    incompatible-time-types
    not-duration
    dates-are-immutable
    bad-date-format-string
    bad-date-template-string
    invalid-month-specification
    ))

(define (tm:time-error caller type value)
  (if (member type tm:time-error-types)
      (if value
	  (error caller "TIME-ERROR type" type value)
	  (error caller "TIME-ERROR type" type))
      (error caller "TIME-ERROR unsupported error type" type)))


;; A table of leap seconds
;; See ftp://maia.usno.navy.mil/ser7/tai-utc.dat
;; and update as necessary.
;; this procedures reads the file in the abover
;; format and creates the leap second table
;; it also calls the almost standard, but not R5 procedures read-line 
;; & open-input-string
;; ie (set! tm:leap-second-table (tm:read-tai-utc-date "tai-utc.dat"))

; (define (tm:read-tai-utc-data filename)
;   (define (convert-jd jd)
;     (* (- (inexact->exact jd) tm:tai-epoch-in-jd) tm:sid))
;   (define (convert-sec sec)
;     (inexact->exact sec))
;   (let ( (port (open-input-file filename))
; 	 (table '()) )
;     (let loop ((line (read-line port)))
;       (if (not (eof-object? line))
; 	  (begin
; 	    (let* ( (data (read (open-input-string (string-append "(" line ")")))) 
; 		    (year (car data))
; 		    (jd   (cadddr (cdr data)))
; 		    (secs (cadddr (cdddr data))) )
; 	      (if (>= year 1972)
; 		  (set! table (cons (cons (convert-jd jd) (convert-sec secs)) table)))
; 	      (loop (read-line port))))))
;     table))

;; each entry is ( utc seconds since epoch . # seconds to add for tai )
;; note they go higher to lower, and end in 1972.
(define tm:leap-second-table
 '((1230768000 . 34)
   (1136073600 . 33)
    (915148800 . 32)
    (867715200 . 31)
    (820454400 . 30)
    (773020800 . 29)
    (741484800 . 28)
    (709948800 . 27)
    (662688000 . 26)
    (631152000 . 25)
    (567993600 . 24)
    (489024000 . 23)
    (425865600 . 22)
    (394329600 . 21)
    (362793600 . 20)
    (315532800 . 19)
    (283996800 . 18)
    (252460800 . 17)
    (220924800 . 16)
    (189302400 . 15)
    (157766400 . 14)
    (126230400 . 13)
    (94694400  . 12)
    (78796800  . 11)
    (63072000  . 10)))

; (define (read-leap-second-table filename)
;   (set! tm:leap-second-table (tm:read-tai-utc-data filename))
;   (values))


(define (tm:leap-second-delta utc-seconds)
  (letrec ( (lsd (lambda (table) 
		   (cond
		    ((>= utc-seconds (caar table))
		     (cdar table))
		    (else (lsd (cdr table)))))) )
    (if (< utc-seconds  (* (- 1972 1970) 365 tm:sid)) 0
	(lsd  tm:leap-second-table))))

;; going from tai seconds to utc seconds ... 
(define (tm:leap-second-neg-delta tai-seconds)
  (letrec ( (lsd (lambda (table)
		   (cond ((null? table) 0)
			 ((<= (cdar table) (- tai-seconds (caar table)))
			  (cdar table))
			 (else (lsd (cdr table)))))) )
    (if (< tai-seconds  (* (- 1972 1970) 365 tm:sid)) 0
	(lsd  tm:leap-second-table))))


;;; the time structure; creates the accessors, too.
;;; wf: changed to match srfi documentation. uses mzscheme structures & inspectors

(define-record-type :time 
  (make-time type nanosecond second)
  time?
  (type time-type set-time-type!)
  (nanosecond time-nanosecond set-time-nanosecond!)
  (second time-second set-time-second!))

(define (copy-time time)
  (let ((ntime (make-time #f #f #f)))
    (set-time-type! ntime (time-type time))
    (set-time-second! ntime (time-second time))
    (set-time-nanosecond! ntime (time-nanosecond time))
    ntime))

;;; current-time

;;; specific time getters.

(define (tm:current-time-utc)
  (receive (seconds nanoseconds) (seconds+nanoseconds)
	   (make-time  time-utc 
		       nanoseconds
		       seconds)))

(define (tm:current-time-tai)
  (receive (seconds nanoseconds) (seconds+nanoseconds)
	   (make-time time-tai
                      nanoseconds
		      (+ seconds (tm:leap-second-delta seconds)))))

;; SCSH portability: use tm:ns/quantum
(define (tm:current-time-ms-time time-type proc)
  (let ((current-ms (proc)))
    (make-time time-type 
	       (* (remainder current-ms 1000) tm:ns/quantum)
	       (quotient current-ms 10000)
	       )))

;; -- we define it to be the same as tai.
;;    a different implemation of current-time-montonic
;;    will require rewriting all of the time-monotonic converters,
;;    of course.

(define (tm:current-time-monotonic)
  (receive (seconds nanoseconds) (seconds+nanoseconds)
	   (make-time time-monotonic
		      nanoseconds
		      (+ seconds (tm:leap-second-delta seconds))
		      )))

;; SCSH portability: thread time not available in scsh
(define (tm:current-time-thread)
  (tm:time-error 'current-time 'unsupported-clock-type 'time-thread))

;; Scheme48 portability: no process time in Scheme48 (regeression from SCSH)
(define (tm:current-time-process)
  (tm:time-error 'current-time 'unsupported-clock-type 'time-gc))

;; SCSH portability: GC time not available in scsh
(define (tm:current-time-gc)
  (tm:time-error 'current-time 'unsupported-clock-type 'time-gc))


(define (current-time . clock-type)
  (let ( (clock-type (:optional clock-type time-utc)) )
    (cond
      ((eq? clock-type time-tai) (tm:current-time-tai))
      ((eq? clock-type time-utc) (tm:current-time-utc))
      ((eq? clock-type time-monotonic) (tm:current-time-monotonic))
      ((eq? clock-type time-thread) (tm:current-time-thread))
      ((eq? clock-type time-process) (tm:current-time-process))
      ((eq? clock-type time-gc) (tm:current-time-gc))
      (else (tm:time-error 'current-time 'invalid-clock-type clock-type)))))


;; -- time resolution
;; this is the resolution of the clock in nanoseconds.
;; this will be implementation specific.

;; SCSH portability: use tm:ns/quantum and tm:nano
(define (time-resolution . clock-type)
  (let ((clock-type (:optional clock-type time-utc)))
    (cond
      ((eq? clock-type time-tai)  tm:ns/quantum)
      ((eq? clock-type time-utc)  tm:ns/quantum)
      ((eq? clock-type time-monotonic) tm:ns/quantum)
      ((eq? clock-type time-thread) tm:ns/quantum)
      ((eq? clock-type time-process) (/ tm:nano (cpu-ticks/sec)))
      (else (tm:time-error 'time-resolution 'invalid-clock-type clock-type)))))

;; -- time comparisons

(define (tm:time-compare-check time1 time2 caller)
  (if (or (not (and (time? time1) (time? time2)))
	  (not (eq? (time-type time1) (time-type time2))))
      (tm:time-error caller 'incompatible-time-types #f)
      #t))

(define (time=? time1 time2)
  (tm:time-compare-check time1 time2 'time=?)
  (and (= (time-second time1) (time-second time2))
       (= (time-nanosecond time1) (time-nanosecond time2))))

(define (time>? time1 time2)
  (tm:time-compare-check time1 time2 'time>?)
  (or (> (time-second time1) (time-second time2))
      (and (= (time-second time1) (time-second time2))
	   (> (time-nanosecond time1) (time-nanosecond time2)))))

(define (time<? time1 time2)
  (tm:time-compare-check time1 time2 'time<?)
  (or (< (time-second time1) (time-second time2))
      (and (= (time-second time1) (time-second time2))
	   (< (time-nanosecond time1) (time-nanosecond time2)))))

(define (time>=? time1 time2)
  (tm:time-compare-check time1 time2 'time>=?)
  (or (> (time-second time1) (time-second time2))
      (and (= (time-second time1) (time-second time2))
	   (>= (time-nanosecond time1) (time-nanosecond time2)))))

(define (time<=? time1 time2)
  (tm:time-compare-check time1 time2 'time<=?)
  (or (< (time-second time1) (time-second time2))
      (and (= (time-second time1) (time-second time2))
	   (<= (time-nanosecond time1) (time-nanosecond time2)))))

;; -- time arithmetic

(define (tm:time->nanoseconds time)
  (define (sign1 n)
    (if (negative? n) -1 1))
  (+ (* (time-second time) tm:nano)
      (time-nanosecond time)))

(define (tm:nanoseconds->time time-type nanoseconds)
  (make-time time-type
             (remainder nanoseconds tm:nano)
             (quotient nanoseconds tm:nano)))

(define (tm:nanoseconds->values nanoseconds)
  (values (abs (remainder nanoseconds tm:nano))
          (quotient nanoseconds tm:nano)))

(define (tm:time-difference time1 time2 time3)
  (if (or (not (and (time? time1) (time? time2)))
	  (not (eq? (time-type time1) (time-type time2))))
      (tm:time-error 'time-difference 'incompatible-time-types #f))
  (set-time-type! time3 time-duration)
  (if (time=? time1 time2)
      (begin
	(set-time-second! time3 0)
	(set-time-nanosecond! time3 0))
      (receive 
       (nanos secs)
       (tm:nanoseconds->values (- (tm:time->nanoseconds time1)
                                  (tm:time->nanoseconds time2)))
       (set-time-second! time3 secs)
       (set-time-nanosecond! time3 nanos)))
  time3)

(define (time-difference time1 time2)
  (tm:time-difference time1 time2 (make-time #f #f #f)))

(define (time-difference! time1 time2)
  (tm:time-difference time1 time2 time1))

(define (tm:add-duration time1 duration time3)
  (if (not (and (time? time1) (time? duration)))
      (tm:time-error 'add-duration 'incompatible-time-types #f))
  (if (not (eq? (time-type duration) time-duration))
      (tm:time-error 'add-duration 'not-duration duration)
      (let ( (sec-plus (+ (time-second time1) (time-second duration)))
	     (nsec-plus (+ (time-nanosecond time1) (time-nanosecond duration))) )
	(let ((r (remainder nsec-plus tm:nano))
	      (q (quotient nsec-plus tm:nano)))
          ; (set-time-type! time3 (time-type time1))
	  (if (negative? r)
	      (begin
		(set-time-second! time3 (+ sec-plus q -1))
		(set-time-nanosecond! time3 (+ tm:nano r)))
	      (begin
		(set-time-second! time3 (+ sec-plus q))
		(set-time-nanosecond! time3 r)))
	  time3))))

(define (add-duration time1 duration)
  (tm:add-duration time1 duration (make-time (time-type time1) #f #f)))

(define (add-duration! time1 duration)
  (tm:add-duration time1 duration time1))

(define (tm:subtract-duration time1 duration time3)
  (if (not (and (time? time1) (time? duration)))
      (tm:time-error 'add-duration 'incompatible-time-types #f))
  (if (not (eq? (time-type duration) time-duration))
      (tm:time-error 'tm:subtract-duration 'not-duration duration)
      (let ( (sec-minus  (- (time-second time1) (time-second duration)))
	     (nsec-minus (- (time-nanosecond time1) (time-nanosecond duration))) )
	(let ((r (remainder nsec-minus tm:nano))
	      (q (quotient nsec-minus tm:nano)))
	  (if (negative? r)
	      (begin
		;;; SCSH portability: use binary -
		(set-time-second! time3 (- (- sec-minus q) 1))
		(set-time-nanosecond! time3 (+ tm:nano r)))
	      (begin
		(set-time-second! time3 (- sec-minus q))
		(set-time-nanosecond! time3 r)))
	  time3))))

(define (subtract-duration time1 duration)
  (tm:subtract-duration time1 duration (make-time (time-type time1) #f #f)))

(define (subtract-duration! time1 duration)
  (tm:subtract-duration time1 duration time1))


;; -- converters between types.

(define (tm:time-tai->time-utc! time-in time-out caller)
  (if (not (eq? (time-type time-in) time-tai))
      (tm:time-error caller 'incompatible-time-types time-in))
  (set-time-type! time-out time-utc)
  (set-time-nanosecond! time-out (time-nanosecond time-in))
  (set-time-second!     time-out (- (time-second time-in)
				    (tm:leap-second-neg-delta 
				     (time-second time-in))))
  time-out)

(define (time-tai->time-utc time-in)
  (tm:time-tai->time-utc! time-in (make-time #f #f #f) 'time-tai->time-utc))


(define (time-tai->time-utc! time-in)
  (tm:time-tai->time-utc! time-in time-in 'time-tai->time-utc!))


(define (tm:time-utc->time-tai! time-in time-out caller)
  (if (not (eq? (time-type time-in) time-utc))
      (tm:time-error caller 'incompatible-time-types time-in))
  (set-time-type! time-out time-tai)
  (set-time-nanosecond! time-out (time-nanosecond time-in))
  (set-time-second!     time-out (+ (time-second time-in)
				    (tm:leap-second-delta 
				     (time-second time-in))))
  time-out)


(define (time-utc->time-tai time-in)
  (tm:time-utc->time-tai! time-in (make-time #f #f #f) 'time-utc->time-tai))

(define (time-utc->time-tai! time-in)
  (tm:time-utc->time-tai! time-in time-in 'time-utc->time-tai!))

;; -- these depend on time-monotonic having the same definition as time-tai!
(define (time-monotonic->time-utc time-in)
  (if (not (eq? (time-type time-in) time-monotonic))
      (tm:time-error 'time-monotoinc->time-utc 'incompatible-time-types time-in))
  (let ((ntime (copy-time time-in)))
    (set-time-type! ntime time-tai)
    (tm:time-tai->time-utc! ntime ntime 'time-monotonic->time-utc)))

(define (time-monotonic->time-utc! time-in)
  (if (not (eq? (time-type time-in) time-monotonic))
      (tm:time-error 'time-monotonic->time-utc! 'incompatible-time-types time-in))
  (set-time-type! time-in time-tai)
  (tm:time-tai->time-utc! time-in time-in 'time-monotonic->time-utc))

(define (time-monotonic->time-tai time-in)
  (if (not (eq? (time-type time-in) time-monotonic))
      (tm:time-error 'time-monotonic->time-tai 'incompatible-time-types time-in))
  (let ((ntime (copy-time time-in)))
    (set-time-type! ntime time-tai)
    ntime))

(define (time-monotonic->time-tai! time-in)
  (if (not (eq? (time-type time-in) time-monotonic))
      (tm:time-error 'time-monotonic->time-tai! 'incompatible-time-types time-in))
  (set-time-type! time-in time-tai)
  time-in)

(define (time-utc->time-monotonic time-in)
  (if (not (eq? (time-type time-in) time-utc))
      (tm:time-error 'time-utc->time-monotonic 'incompatible-time-types time-in))
  (let ((ntime (tm:time-utc->time-tai! time-in (make-time #f #f #f)
				       'time-utc->time-monotonic)))
    (set-time-type! ntime time-monotonic)
    ntime))


(define (time-utc->time-monotonic! time-in)
  (if (not (eq? (time-type time-in) time-utc))
      (tm:time-error 'time-utc->time-montonic! 'incompatible-time-types time-in))
  (let ((ntime (tm:time-utc->time-tai! time-in time-in
				       'time-utc->time-monotonic!)))
    (set-time-type! ntime time-monotonic)
    ntime))


(define (time-tai->time-monotonic time-in)
  (if (not (eq? (time-type time-in) time-tai))
      (tm:time-error 'time-tai->time-monotonic 'incompatible-time-types time-in))
  (let ((ntime (copy-time time-in)))
    (set-time-type! ntime time-monotonic)
    ntime))

(define (time-tai->time-monotonic! time-in)
  (if (not (eq? (time-type time-in) time-tai))
      (tm:time-error 'time-tai->time-monotonic!  'incompatible-time-types time-in))
  (set-time-type! time-in time-monotonic)
  time-in)


;; -- date structures

(define-record-type :date
  (make-date nanosecond second minute hour day month year zone-offset)
  date?
  (nanosecond date-nanosecond set-date-nanosecond!)
  (second date-second set-date-second!)
  (minute date-minute set-date-minute!) 
  (hour date-hour set-date-hour!)
  (day date-day set-date-day!)
  (month date-month set-date-month!)
  (year date-year set-date-year!)
  (zone-offset date-zone-offset set-date-zone-offset!))

;; redefine setters

(define tm:set-date-nanosecond! set-date-nanosecond!)
(define tm:set-date-second! set-date-second!)
(define tm:set-date-minute! set-date-minute!)
(define tm:set-date-hour! set-date-hour!)
(define tm:set-date-day! set-date-day!)
(define tm:set-date-month! set-date-month!)
(define tm:set-date-year! set-date-year!)
(define tm:set-date-zone-offset! set-date-zone-offset!)

(define (set-date-second! date val)
  (tm:time-error 'set-date-second! 'dates-are-immutable date))

(define (set-date-minute! date val)
  (tm:time-error 'set-date-minute! 'dates-are-immutable date))

(define (set-date-day! date val)
  (tm:time-error 'set-date-day! 'dates-are-immutable date))

(define (set-date-month! date val)
  (tm:time-error 'set-date-month! 'dates-are-immutable date))

(define (set-date-year! date val)
  (tm:time-error 'set-date-year! 'dates-are-immutable date))

(define (set-date-zone-offset! date val)
  (tm:time-error 'set-date-zone-offset! 'dates-are-immutable date))

;; gives the julian day which starts at noon.
(define (tm:encode-julian-day-number day month year)
  (let* ((a (quotient (- 14 month) 12))
	 ;; SCSH portability: use binary -
	 (y (- (- (+ year 4800) a) (if (negative? year) -1  0)))
	 (m (- (+ month (* 12 a)) 3)))
    (+ day
       (quotient (+ (* 153 m) 2) 5)
       (* 365 y)
       (quotient y 4)
       (- (quotient y 100))
       (quotient y 400)
       -32045)))

(define (tm:char-pos char str index len)
  (cond
   ((>= index len) #f)
   ((char=? (string-ref str index) char)
    index)
   (else
    (tm:char-pos char str (+ index 1) len))))
  
;; return a string representing the decimal expansion of the fractional 
;; portion of a number, limited by a specified precision
(define (tm:decimal-expansion r precision)
  (let loop ((num (- r (round r)))
             (p precision))
    (if (or (= p 0) (= num 0))
        ""
        (let* ((num-times-10 (* 10 num))
               (round-num-times-10 (round num-times-10))) 
          (string-append (number->string (inexact->exact round-num-times-10))
                         (loop (- num-times-10 round-num-times-10) (- p 1)))))))

;; gives the seconds/date/month/year 
(define (tm:decode-julian-day-number jdn)
  (let* ((days (truncate jdn))
	 (a (+ days 32044))
	 (b (quotient (+ (* 4 a) 3) 146097))
	 (c (- a (quotient (* 146097 b) 4)))
	 (d (quotient (+ (* 4 c) 3) 1461))
	 (e (- c (quotient (* 1461 d) 4)))
	 (m (quotient (+ (* 5 e) 2) 153))
	 (y (+ (* 100 b) d -4800 (quotient m 10))))
    (values ; seconds date month year
     (* (- jdn days) tm:sid)
     (+ e (- (quotient (+ (* 153 m) 2) 5)) 1)
     (+ m 3 (* -12 (quotient m 10)))
     (if (>= 0 y) (- y 1) y))
    ))

(define (tm:local-tz-offset)
  (os-time:timezone-offset))

;; special thing -- ignores nanos
(define (tm:time->julian-day-number seconds tz-offset)
  (+ (/ (+ seconds
	   tz-offset
	   tm:sihd)
	tm:sid)
     tm:tai-epoch-in-jd))

(define (tm:find proc l)
  (if (null? l)
      #f
      (if (proc (car l))
	  #t
	  (tm:find proc (cdr l)))))

(define (tm:tai-before-leap-second? second)
  (tm:find (lambda (x)
	     (= second (- (+ (car x) (cdr x)) 1)))
	   tm:leap-second-table))

(define (tm:time->date time tz-offset ttype)
  (if (not (eq? (time-type time) ttype))
      (tm:time-error 'time->date 'incompatible-time-types  time))
  (let* ( (offset (:optional tz-offset (tm:local-tz-offset))) )
    (receive (secs date month year)
	     (tm:decode-julian-day-number
	      (tm:time->julian-day-number (time-second time) offset))
	     (let* ( (hours    (quotient secs (* 60 60)))
		     (rem      (remainder secs (* 60 60)))
		     (minutes  (quotient rem 60))
		     (seconds  (remainder rem 60)) )
	       (make-date (time-nanosecond time)
			  seconds
			  minutes
			  hours
			  date
			  month
			  year
			  offset)))))

(define (time-tai->date time . tz-offset)
  (if (tm:tai-before-leap-second? (time-second time))
      ;; if it's *right* before the leap, we need to pretend to subtract a second ...
      (let ((d (tm:time->date (subtract-duration! (time-tai->time-utc time) (make-time time-duration 0 1)) tz-offset time-utc)))
	(tm:set-date-second! d 60)
	d)
      (tm:time->date (time-tai->time-utc time) tz-offset time-utc)))

(define (time-utc->date time . tz-offset)
  (tm:time->date time tz-offset time-utc))

;; again, time-monotonic is the same as time tai
(define (time-monotonic->date time . tz-offset)
  (tm:time->date time tz-offset time-monotonic))

(define (date->time-utc date)
  (let ( (nanosecond (date-nanosecond date))
	 (second (date-second date))
	 (minute (date-minute date))
	 (hour (date-hour date))
	 (day (date-day date))
	 (month (date-month date))
	 (year (date-year date))
	 (offset (date-zone-offset date)) )
    (let ( (jdays (- (tm:encode-julian-day-number day month year)
		     tm:tai-epoch-in-jd)) )
      (make-time 
       time-utc
       nanosecond
       (+ (* (- jdays 1/2) 24 60 60)
	  (* hour 60 60)
	  (* minute 60)
	  second
	  (- offset))
       ))))

(define (date->time-tai d)
  (if (= (date-second d) 60)
      (subtract-duration! (time-utc->time-tai! (date->time-utc d)) (make-time time-duration 0 1))
      (time-utc->time-tai! (date->time-utc d))))

(define (date->time-monotonic date)
  (time-utc->time-monotonic! (date->time-utc date)))


(define (tm:leap-year? year)
  (or (= (modulo year 400) 0)
      (and (= (modulo year 4) 0) (not (= (modulo year 100) 0)))))

(define (leap-year? date)
  (tm:leap-year? (date-year date)))

;; tm:year-day fixed: adding wrong number of days.
(define  tm:month-assoc '((0 . 0) (1 . 31)  (2 . 59)   (3 . 90)   (4 . 120) 
			  (5 . 151) (6 . 181)  (7 . 212)  (8 . 243)
			  (9 . 273) (10 . 304) (11 . 334)))

(define (tm:year-day day month year)
  (let ((days-pr (assoc (- month 1) tm:month-assoc)))
    (if (not days-pr)
	(tm:time-error 'date-year-day 'invalid-month-specification month))
    (if (and (tm:leap-year? year) (> month 2))
	(+ day (cdr days-pr) 1)
	(+ day (cdr days-pr)))))

(define (date-year-day date)
  (tm:year-day (date-day date) (date-month date) (date-year date)))

;; from calendar faq 
(define (tm:week-day day month year)
  (let* ((a (quotient (- 14 month) 12))
	 (y (- year a))
	 (m (+ month (* 12 a) -2)))
    (modulo (+ day y (quotient y 4) (- (quotient y 100))
	       (quotient y 400) (quotient (* 31 m) 12))
	    7)))

(define (date-week-day date)
  (tm:week-day (date-day date) (date-month date) (date-year date)))

(define (tm:days-before-first-week date day-of-week-starting-week)
  (let* ( (first-day (make-date 0 0 0 0
                                1
                                1
                                (date-year date)
                                #f))
          (fdweek-day (date-week-day first-day))  )
    (modulo (- day-of-week-starting-week fdweek-day)
            7)))

(define (date-week-number date day-of-week-starting-week)
  (quotient (- (date-year-day date)
	       (tm:days-before-first-week  date day-of-week-starting-week))
	    7))

(define (current-date . tz-offset) 
  (time-utc->date (current-time time-utc)
		  (:optional tz-offset (tm:local-tz-offset))))

;; given a 'two digit' number, find the year within 50 years +/-
(define (tm:natural-year n)
  (let* ( (current-year (date-year (current-date)))
	  (current-century (* (quotient current-year 100) 100)) )
    (cond
      ((>= n 100) n)
      ((<  n 0) n)
      ((<=  (- (+ current-century n) current-year) 50)
       (+ current-century n))
      (else
       (+ (- current-century 100) n)))))

(define (date->julian-day date)
  (let ( (nanosecond (date-nanosecond date))
         (second (date-second date))
         (minute (date-minute date))
         (hour (date-hour date))
         (day (date-day date))
         (month (date-month date))
         (year (date-year date))
         (offset (date-zone-offset date)) )
    (+ (tm:encode-julian-day-number day month year)
       (- 1/2)
       (+ (/ (+ (* hour 60 60)
                (* minute 60)
                second
                (/ nanosecond tm:nano)
                (- offset))
             tm:sid)))))

(define (date->modified-julian-day date)
  (- (date->julian-day date)
     4800001/2))


(define (time-utc->julian-day time)
  (if (not (eq? (time-type time) time-utc))
      (tm:time-error 'time-utc->julian-day 'incompatible-time-types  time))
  (+ (/ (+ (time-second time) (/ (time-nanosecond time) tm:nano))
	tm:sid)
     tm:tai-epoch-in-jd))

(define (time-utc->modified-julian-day time)
  (- (time-utc->julian-day time)
     4800001/2))

(define (time-tai->julian-day time)
  (if (not (eq? (time-type time) time-tai))
      (tm:time-error 'time-tai->julian-day 'incompatible-time-types  time))
  (+ (/ (+ (- (time-second time) 
	      (tm:leap-second-delta (time-second time)))
	   (/ (time-nanosecond time) tm:nano))
	tm:sid)
     tm:tai-epoch-in-jd))

(define (time-tai->modified-julian-day time)
  (- (time-tai->julian-day time)
     4800001/2))

;; this is the same as time-tai->julian-day
(define (time-monotonic->julian-day time)
  (if (not (eq? (time-type time) time-monotonic))
      (tm:time-error 'time-monotonic->julian-day 'incompatible-time-types  time))
  (+ (/ (+ (- (time-second time) 
	      (tm:leap-second-delta (time-second time)))
	   (/ (time-nanosecond time) tm:nano))
	tm:sid)
     tm:tai-epoch-in-jd))


(define (time-monotonic->modified-julian-day time)
  (- (time-monotonic->julian-day time)
     4800001/2))


(define (julian-day->time-utc jdn)
  (let ( (nanosecs (* tm:nano tm:sid (- jdn tm:tai-epoch-in-jd))) )
    (make-time time-utc
	       (remainder nanosecs tm:nano)
	       (floor (/ nanosecs tm:nano)))))

(define (julian-day->time-tai jdn)
  (time-utc->time-tai! (julian-day->time-utc jdn)))

(define (julian-day->time-monotonic jdn)
  (time-utc->time-monotonic! (julian-day->time-utc jdn)))

(define (julian-day->date jdn . tz-offset)
  (let ((offset (:optional tz-offset (tm:local-tz-offset))))
    (time-utc->date (julian-day->time-utc jdn) offset)))

(define (modified-julian-day->date jdn . tz-offset)
  (let ((offset (:optional tz-offset (tm:local-tz-offset))))
    (julian-day->date (+ jdn 4800001/2) offset)))

(define (modified-julian-day->time-utc jdn)
  (julian-day->time-utc (+ jdn 4800001/2)))

(define (modified-julian-day->time-tai jdn)
  (julian-day->time-tai (+ jdn 4800001/2)))

(define (modified-julian-day->time-monotonic jdn)
  (julian-day->time-monotonic (+ jdn 4800001/2)))

(define (current-julian-day)
  (time-utc->julian-day (current-time time-utc)))

(define (current-modified-julian-day)
  (time-utc->modified-julian-day (current-time time-utc)))

;; returns a string rep. of number N, of minimum LENGTH,
;; padded with character PAD-WITH. If PAD-WITH if #f, 
;; no padding is done, and it's as if number->string was used.
;; if string is longer than LENGTH, it's as if number->string was used.

(define (tm:padding n pad-with length)
  (let* ( (str (number->string n))
	  (str-len (string-length str)) )
    (if (or (> str-len length)
            (not pad-with))
	str
	(let* ( (new-str (make-string length pad-with))
		(new-str-offset (- (string-length new-str)
				   str-len)) )
	  (do ((i 0 (+ i 1)))
            ((>= i (string-length str)))
            (string-set! new-str (+ new-str-offset i) 
                         (string-ref str i)))
	  new-str))))

(define (tm:last-n-digits i n)
  (abs (remainder i (expt 10 n))))

(define (tm:locale-abbr-weekday n) 
  (vector-ref tm:locale-abbr-weekday-vector n))

(define (tm:locale-long-weekday n)
  (vector-ref tm:locale-long-weekday-vector n))

(define (tm:locale-abbr-month n)
  (vector-ref tm:locale-abbr-month-vector n))

(define (tm:locale-long-month n)
  (vector-ref tm:locale-long-month-vector n))

(define (tm:vector-find needle haystack comparator)
  (let ((len (vector-length haystack)))
    (define (tm:vector-find-int index)
      (cond
        ((>= index len) #f)
        ((comparator needle (vector-ref haystack index)) index)
        (else (tm:vector-find-int (+ index 1)))))
    (tm:vector-find-int 0)))

(define (tm:locale-abbr-weekday->index string)
  (tm:vector-find string tm:locale-abbr-weekday-vector string=?))

(define (tm:locale-long-weekday->index string)
  (tm:vector-find string tm:locale-long-weekday-vector string=?))

(define (tm:locale-abbr-month->index string)
  (tm:vector-find string tm:locale-abbr-month-vector string=?))

(define (tm:locale-long-month->index string)
  (tm:vector-find string tm:locale-long-month-vector string=?))



;; do nothing. 
;; Your implementation might want to do something...
;; 
(define (tm:locale-print-time-zone date port)
  (values))

;; Again, locale specific.
(define (tm:locale-am/pm hr)
  (if (> hr 11) tm:locale-pm tm:locale-am))

(define (tm:tz-printer offset port)
  (cond
    ((= offset 0) (display "Z" port))
    ((negative? offset) (display "-" port))
    (else (display "+" port)))
  (if (not (= offset 0))
      (let ( (hours   (abs (quotient offset (* 60 60))))
	     (minutes (abs (quotient (remainder offset (* 60 60)) 60))) )
	(display (tm:padding hours #\0 2) port)
	(display (tm:padding minutes #\0 2) port))))

;; A table of output formatting directives.
;; the first time is the format char.
;; the second is a procedure that takes the date, a padding character
;; (which might be #f), and the output port.
;;

;; SCSH portability: use #\space instead of #\Space and 
;;                   (ascii->char 9) instead of #\Tab

(define char-tab (ascii->char 9))

(define tm:directives 
  (list
   (cons #\~ (lambda (date pad-with port) (display #\~ port)))
   
   (cons #\a (lambda (date pad-with port)
	       (display (tm:locale-abbr-weekday (date-week-day date))
			port)))
   (cons #\A (lambda (date pad-with port)
	       (display (tm:locale-long-weekday (date-week-day date))
			port)))
   (cons #\b (lambda (date pad-with port)
	       (display (tm:locale-abbr-month (date-month date))
			port)))
   (cons #\B (lambda (date pad-with port)
	       (display (tm:locale-long-month (date-month date))
			port)))
   (cons #\c (lambda (date pad-with port)
	       (display (date->string date tm:locale-date-time-format) port)))
   (cons #\d (lambda (date pad-with port)
	       (display (tm:padding (date-day date)
				    #\0 2)
                        port)))
   (cons #\D (lambda (date pad-with port)
	       (display (date->string date "~m/~d/~y") port)))
   (cons #\e (lambda (date pad-with port)
	       (display (tm:padding (date-day date)
				    #\space 2)
			port)))
   (cons #\f (lambda (date pad-with port)
	       (if (> (date-nanosecond date)
		      tm:nano)
		   (display (tm:padding (+ (date-second date) 1)
					pad-with 2)
			    port)
		   (display (tm:padding (date-second date)
					pad-with 2)
			    port))
	       (let* ((f (tm:decimal-expansion (/ (date-nanosecond date) tm:nano) 9)))
                 (if (> (string-length f) 0)
                       (begin
                         (display tm:locale-number-separator port)
                         (display f port))))))
   (cons #\h (lambda (date pad-with port)
	       (display (date->string date "~b") port)))
   (cons #\H (lambda (date pad-with port)
	       (display (tm:padding (date-hour date)
				    pad-with 2)
			port)))
   (cons #\I (lambda (date pad-with port)
	       (let ((hr (date-hour date)))
		 (if (> hr 12)
		     (display (tm:padding (- hr 12)
					  pad-with 2)
			      port)
		     (display (tm:padding hr
					  pad-with 2)
			      port)))))
   (cons #\j (lambda (date pad-with port)
	       (display (tm:padding (date-year-day date)
				    pad-with 3)
			port)))
   (cons #\k (lambda (date pad-with port)
	       (display (tm:padding (date-hour date)
				    #\0 2)
                        port)))
   (cons #\l (lambda (date pad-with port)
	       (let ((hr (if (> (date-hour date) 12)
			     (- (date-hour date) 12) (date-hour date))))
		 (display (tm:padding hr  #\space 2)
			  port))))
   (cons #\m (lambda (date pad-with port)
	       (display (tm:padding (date-month date)
				    pad-with 2)
			port)))
   (cons #\M (lambda (date pad-with port)
	       (display (tm:padding (date-minute date)
				    pad-with 2)
			port)))
   (cons #\n (lambda (date pad-with port)
	       (newline port)))
   (cons #\N (lambda (date pad-with port)
	       (display (tm:padding (date-nanosecond date)
				    pad-with 9)
			port)))
   (cons #\p (lambda (date pad-with port)
	       (display (tm:locale-am/pm (date-hour date)) port)))
   (cons #\r (lambda (date pad-with port)
	       (display (date->string date "~I:~M:~S ~p") port)))
   (cons #\s (lambda (date pad-with port)
	       (display (time-second (date->time-utc date)) port)))
   (cons #\S (lambda (date pad-with port)
	       (if (> (date-nanosecond date)
		      tm:nano)
                   (display (tm:padding (+ (date-second date) 1)
                                        pad-with 2)
                            port)
                   (display (tm:padding (date-second date)
                                        pad-with 2)
                            port))))
   (cons #\t (lambda (date pad-with port)
	       (display char-tab port)))
   (cons #\T (lambda (date pad-with port)
	       (display (date->string date "~H:~M:~S") port)))
   (cons #\U (lambda (date pad-with port)
	       (if (> (tm:days-before-first-week date 0) 0)
		   (display (tm:padding (+ (date-week-number date 0) 1)
					#\0 2) port)
		   (display (tm:padding (date-week-number date 0)
					#\0 2) port))))
   (cons #\V (lambda (date pad-with port)
	       (display (tm:padding (date-week-number date 1)
				    #\0 2) port)))
   (cons #\w (lambda (date pad-with port)
	       (display (date-week-day date) port)))
   (cons #\x (lambda (date pad-with port)
	       (display (date->string date tm:locale-short-date-format) port)))
   (cons #\X (lambda (date pad-with port)
	       (display (date->string date tm:locale-time-format) port)))
   (cons #\W (lambda (date pad-with port)
	       (if (> (tm:days-before-first-week date 1) 0)
		   (display (tm:padding (+ (date-week-number date 1) 1)
					#\0 2) port)
		   (display (tm:padding (date-week-number date 1)
					#\0 2) port))))
   (cons #\y (lambda (date pad-with port)
	       (display (tm:padding (tm:last-n-digits 
				     (date-year date) 2)
				    pad-with
				    2)
			port)))
   (cons #\Y (lambda (date pad-with port)
	       (display (tm:padding (date-year date)
				    pad-with
				    4)
			port)))
   (cons #\z (lambda (date pad-with port)
	       (tm:tz-printer (date-zone-offset date) port)))
   (cons #\Z (lambda (date pad-with port)
	       (tm:locale-print-time-zone date port)))
   (cons #\1 (lambda (date pad-with port)
	       (display (date->string date "~Y-~m-~d") port)))
   (cons #\2 (lambda (date pad-with port)
	       (display (date->string date "~k:~M:~S~z") port)))
   (cons #\3 (lambda (date pad-with port)
	       (display (date->string date "~k:~M:~S") port)))
   (cons #\4 (lambda (date pad-with port)
	       (display (date->string date "~Y-~m-~dT~k:~M:~S~z") port)))
   (cons #\5 (lambda (date pad-with port)
	       (display (date->string date "~Y-~m-~dT~k:~M:~S") port)))
   ))


(define (tm:get-formatter char)
  (let ( (associated (assoc char tm:directives)) )
    (if associated (cdr associated) #f)))

(define (tm:date-printer date index format-string str-len port)
  (if (>= index str-len)
      (values)
      (let ( (current-char (string-ref format-string index)) )
	(if (not (char=? current-char #\~))
	    (begin
	      (display current-char port)
	      (tm:date-printer date (+ index 1) format-string str-len port))
	    (if (= (+ index 1) str-len) ; bad format string.
		(tm:time-error 'tm:date-printer 'bad-date-format-string 
			       format-string)
                (let ( (pad-char? (string-ref format-string (+ index 1))) )
                  (cond
                    ((char=? pad-char? #\-)
                     (if (= (+ index 2) str-len) ; bad format string.
                         (tm:time-error 'tm:date-printer 'bad-date-format-string 
                                        format-string)
                         (let ( (formatter (tm:get-formatter 
                                            (string-ref format-string
                                                        (+ index 2)))) )
                           (if (not formatter)
                               (tm:time-error 'tm:date-printer 'bad-date-format-string 
                                              format-string)
                               (begin
                                 (formatter date #f port)
                                 (tm:date-printer date (+ index 3)
                                                  format-string str-len port))))))
                    
                    ((char=? pad-char? #\_)
                     (if (= (+ index 2) str-len) ; bad format string.
                         (tm:time-error 'tm:date-printer 'bad-date-format-string 
                                        format-string)
                         (let ( (formatter (tm:get-formatter 
                                            (string-ref format-string
                                                        (+ index 2)))) )
                           (if (not formatter)
                               (tm:time-error 'tm:date-printer 'bad-date-format-string 
                                              format-string)
                               (begin
                                 (formatter date #\space port)
                                 (tm:date-printer date (+ index 3)
                                                  format-string str-len port))))))
                    (else
                     (let ( (formatter (tm:get-formatter 
                                        (string-ref format-string
                                                    (+ index 1)))) )
                       (if (not formatter)
                           (tm:time-error 'tm:date-printer 'bad-date-format-string 
                                          format-string)
                           (begin
                             (formatter date #\0 port)
                             (tm:date-printer date (+ index 2)
                                              format-string str-len port))))))))))))


(define (date->string date .  format-string)
  (let ( (str-port (open-output-string))
	 (fmt-str (:optional format-string "~c")) )
    (tm:date-printer date 0 fmt-str (string-length fmt-str) str-port)
    (get-output-string str-port)))

(define (tm:char->int ch)
  (cond
    ((char=? ch #\0) 0)
    ((char=? ch #\1) 1)
    ((char=? ch #\2) 2)
    ((char=? ch #\3) 3)
    ((char=? ch #\4) 4)
    ((char=? ch #\5) 5)
    ((char=? ch #\6) 6)
    ((char=? ch #\7) 7)
    ((char=? ch #\8) 8)
    ((char=? ch #\9) 9)
    (else (tm:time-error 'string->date 'bad-date-template-string
                         (list "Non-integer character" ch )))))

;; read an integer upto n characters long on port; upto -> #f if any length
(define (tm:integer-reader upto port)
  (define (accum-int port accum nchars)
    (let ((ch (peek-char port)))
      (if (or (eof-object? ch)
              (not (char-numeric? ch))
              (and upto (>= nchars  upto )))
          accum
          (accum-int port (+ (* accum 10) (tm:char->int (read-char
                                                         port))) (+
                                                                  nchars 1)))))
  (accum-int port 0 0))

(define (tm:make-integer-reader upto)
  (lambda (port)
    (tm:integer-reader upto port)))

;; read an fractional integer upto n characters long on port; upto -> #f if any length
;;
;; The return value is normalized to upto decimal places. For example, if upto is 9 and 
;; the string read is "123", the return value is 123000000.
(define (tm:fractional-integer-reader upto port)
  (define (accum-int port accum nchars)
    (let ((ch (peek-char port)))
     (if (or (eof-object? ch)
     	(not (char-numeric? ch))
     	(and upto (>= nchars  upto )))
         (* accum (expt 10 (- upto nchars)))
         (accum-int port (+ (* accum 10) (tm:char->int (read-char port))) (+ nchars 1)))))
  (accum-int port 0 0))

(define (tm:make-fractional-integer-reader upto)
  (lambda (port)
    (tm:fractional-integer-reader upto port)))


;; read *exactly* n characters and convert to integer; could be padded
(define (tm:integer-reader-exact n port)
  (let ( (padding-ok #t) )
    (define (accum-int port accum nchars)
      (let ((ch (peek-char port)))
	(cond
          ((>= nchars n) accum)
          ((eof-object? ch) 
           (tm:time-error 'string->date 'bad-date-template-string 
                          "Premature ending to integer read."))
          ((char-numeric? ch)
           (set! padding-ok #f)
           (accum-int port (+ (* accum 10) (tm:char->int (read-char
                                                          port)))
                      (+ nchars 1)))
          (padding-ok
           (read-char port) ; consume padding
           (accum-int port accum (+ nchars 1)))
          (else ; padding where it shouldn't be
           (tm:time-error 'string->date 'bad-date-template-string 
			  "Non-numeric characters in integer read.")))))
    (accum-int port 0 0)))


(define (tm:make-integer-exact-reader n)
  (lambda (port)
    (tm:integer-reader-exact n port)))

(define (tm:zone-reader port) 
  (let ( (offset 0) 
	 (positive? #f) )
    (let ( (ch (read-char port)) )
      (if (eof-object? ch)
	  (tm:time-error 'string->date 'bad-date-template-string
			 (list "Invalid time zone +/-" ch)))
      (if (or (char=? ch #\Z) (char=? ch #\z))
	  0
	  (begin
	    (cond
              ((char=? ch #\+) (set! positive? #t))
              ((char=? ch #\-) (set! positive? #f))
              (else
               (tm:time-error 'string->date 'bad-date-template-string
                              (list "Invalid time zone +/-" ch))))
	    (let ((ch (read-char port)))
	      (if (eof-object? ch)
		  (tm:time-error 'string->date 'bad-date-template-string
                                 (list "Invalid time zone number" ch)))
	      (set! offset (* (tm:char->int ch)
			      10 60 60)))
	    (let ((ch (read-char port)))
	      (if (eof-object? ch)
		  (tm:time-error 'string->date 'bad-date-template-string
				 (list "Invalid time zone number" ch)))
	      (set! offset (+ offset (* (tm:char->int ch)
					60 60))))
	    (let ((ch (read-char port)))
	      (if (eof-object? ch)
		  (tm:time-error 'string->date 'bad-date-template-string
				 (list "Invalid time zone number" ch)))
	      (set! offset (+ offset (* (tm:char->int ch)
					10 60))))
	    (let ((ch (read-char port)))
	      (if (eof-object? ch)
		  (tm:time-error 'string->date 'bad-date-template-string
				 (list "Invalid time zone number" ch)))
	      (set! offset (+ offset (* (tm:char->int ch)
					60))))
	    (if positive? offset (- offset)))))))

;; looking at a char, read the char string, run thru indexer, return index
(define (tm:locale-reader port indexer)
  (let ( (string-port (open-output-string)) )
    (define (read-char-string)
      (let ((ch (peek-char port)))
	(if (char-alphabetic? ch)
	    (begin (write-char (read-char port) string-port) 
		   (read-char-string))
	    (get-output-string string-port))))
    (let* ( (str (read-char-string)) 
	    (index (indexer str)) )
      (if index index (tm:time-error 'string->date
				     'bad-date-template-string
				     (list "Invalid string for " indexer))))))

(define (tm:make-locale-reader indexer)
  (lambda (port)
    (tm:locale-reader port indexer)))

(define (tm:make-char-id-reader char)
  (lambda (port)
    (if (char=? char (read-char port))
	char
	(tm:time-error 'string->date
		       'bad-date-template-string
		       "Invalid character match."))))

;; A List of formatted read directives.
;; Each entry is a list.
;; 1. the character directive; 
;; a procedure, which takes a character as input & returns
;; 2. #t as soon as a character on the input port is acceptable
;; for input,
;; 3. a port reader procedure that knows how to read the current port
;; for a value. Its one parameter is the port.
;; 4. a action procedure, that takes the value (from 3.) and some
;; object (here, always the date) and (probably) side-effects it.
;; In some cases (e.g., ~A) the action is to do nothing

(define tm:read-directives 
  (let ( (ireader4 (tm:make-integer-reader 4))
	 (ireader2 (tm:make-integer-reader 2))
	 (fireader9 (tm:make-fractional-integer-reader 9))
	 (ireaderf (tm:make-integer-reader #f))
	 (eireader2 (tm:make-integer-exact-reader 2))
	 (eireader4 (tm:make-integer-exact-reader 4))
	 (locale-reader-abbr-weekday (tm:make-locale-reader
				      tm:locale-abbr-weekday->index))
	 (locale-reader-long-weekday (tm:make-locale-reader
				      tm:locale-long-weekday->index))
	 (locale-reader-abbr-month   (tm:make-locale-reader
				      tm:locale-abbr-month->index))
	 (locale-reader-long-month   (tm:make-locale-reader
				      tm:locale-long-month->index))
	 (char-fail (lambda (ch) #t))
	 (do-nothing (lambda (val object) (values)))
	 )
    
    (list
     (list #\~ char-fail (tm:make-char-id-reader #\~) do-nothing)
     (list #\a char-alphabetic? locale-reader-abbr-weekday do-nothing)
     (list #\A char-alphabetic? locale-reader-long-weekday do-nothing)
     (list #\b char-alphabetic? locale-reader-abbr-month
           (lambda (val object)
             (tm:set-date-month! object val)))
     (list #\B char-alphabetic? locale-reader-long-month
           (lambda (val object)
             (tm:set-date-month! object val)))
     (list #\d char-numeric? ireader2 (lambda (val object)
                                        (tm:set-date-day!
                                         object val)))
     (list #\e char-fail eireader2 (lambda (val object)
                                     (tm:set-date-day! object val)))
     (list #\h char-alphabetic? locale-reader-abbr-month
           (lambda (val object)
             (tm:set-date-month! object val)))
     (list #\H char-numeric? ireader2 (lambda (val object)
                                        (tm:set-date-hour! object val)))
     (list #\k char-fail eireader2 (lambda (val object)
                                     (tm:set-date-hour! object val)))
     (list #\m char-numeric? ireader2 (lambda (val object)
                                        (tm:set-date-month! object val)))
     (list #\M char-numeric? ireader2 (lambda (val object)
                                        (tm:set-date-minute!
                                         object val)))
     (list #\N char-numeric? fireader9 (lambda (val object)
					 (tm:set-date-nanosecond! object val)))
     (list #\S char-numeric? ireader2 (lambda (val object)
                                        (tm:set-date-second! object val)))
     (list #\y char-fail eireader2 
           (lambda (val object)
             (tm:set-date-year! object (tm:natural-year val))))
     (list #\Y char-numeric? ireader4 (lambda (val object)
                                        (tm:set-date-year! object val)))
     (list #\z (lambda (c)
                 (or (char=? c #\Z)
                     (char=? c #\z)
                     (char=? c #\+)
                     (char=? c #\-)))
           tm:zone-reader (lambda (val object)
                            (tm:set-date-zone-offset! object val)))
     )))

(define (tm:string->date date index format-string str-len port template-string)
  (define (skip-until port skipper)
    (let ((ch (peek-char port)))
      (if (eof-object? ch)
	  (tm:time-error 'string->date 'bad-date-format-string template-string)
	  (if (not (skipper ch))
	      (begin (read-char port) (skip-until port skipper))))))
  (if (>= index str-len)
      (begin 
	(values))
      (let ( (current-char (string-ref format-string index)) )
	(if (not (char=? current-char #\~))
	    (let ((port-char (read-char port)))
	      (if (or (eof-object? port-char)
		      (not (char=? current-char port-char)))
		  (tm:time-error 'string->date 'bad-date-format-string template-string))
	      (tm:string->date date (+ index 1) format-string str-len port template-string))
	    ;; otherwise, it's an escape, we hope
	    (if (> (+ index 1) str-len)
		(tm:time-error 'string->date 'bad-date-format-string template-string)
		(let* ( (format-char (string-ref format-string (+ index 1)))
			(format-info (assoc format-char tm:read-directives)) )
		  (if (not format-info)
		      (tm:time-error 'string->date 'bad-date-format-string template-string)
		      (begin
			(let ((skipper (cadr format-info))
			      (reader  (caddr format-info))
			      (actor   (cadddr format-info)))
			  (skip-until port skipper)
			  (let ((val (reader port)))
			    (if (eof-object? val)
				(tm:time-error 'string->date 'bad-date-format-string template-string)
				(actor val date)))
			  (tm:string->date date (+ index 2) format-string  str-len port template-string))))))))))

(define (string->date input-string template-string)
  (define (tm:date-ok? date)
    (and (date-nanosecond date)
	 (date-second date)
	 (date-minute date)
	 (date-hour date)
	 (date-day date)
	 (date-month date)
	 (date-year date)
	 (date-zone-offset date)))
  (let ( (newdate (make-date 0 0 0 0 #f #f #f (tm:local-tz-offset))) )
    (tm:string->date newdate
		     0
		     template-string
		     (string-length template-string)
		     (open-input-string input-string)
		     template-string)
    (if (tm:date-ok? newdate)
	newdate
	(tm:time-error 'string->date 'bad-date-format-string (list "Incomplete date read. " newdate template-string)))))