;;
;; SRFI-19: Time Data Types and Procedures.
;;
;; Copyright (C) I/NET, Inc. (2000, 2002, 2003). All Rights Reserved.
;; Copyright (C) 2008 Per Eckerdal
;; Copyright (C) 2009, 2012, 2013 Mikael More
;;
;; ## History
;; 2008      : This file is modified by Per Eckerdal to work in Gambit and the Black
;;             Hole module system. The other change I did (which is rather large) is
;;             that I renamed current-time to current-time-tc to avoid name clash
;;             with the built-in Gambit function with the same name.
;; 2009-10-02: define-type:s extended with ID:s.
;;             Implemented (:optional), needed for a lot of procedures to work.
;; 2012-11-03: Improved code formatting a little bit, replaced use of tabs with use of spaces.
;; 2013-08-19: Some code formatting improvements
;; 2016-01-28: [vyzo] imported to Gerbil, dropped bh specifics and some comment cosmetics
;; 2017-07-08: [vyzo] use ##current-time-point with ns accuracy for get-time-of-day
;; 2017-09-15: [fare] make make-time stricter, update leap second database.
;; 2018-03-21: [vyzo] stop neutering the mutators, it makes no sense.
;;
;; ## TODO
;;  * The exception handling presumes user code to be written in a very specific style using exception
;;    handlers, that I think there are purer alternatives of. If development and use of this SRFI is
;;    to be pursued, those would be things to address.
;;


;; This document and translations of it may be copied and furnished to others,
;; and derivative works that comment on or otherwise explain it or assist in its
;; implementation may be prepared, copied, published and distributed, in whole or
;; in part, without restriction of any kind, provided that the above copyright
;; notice and this paragraph are included on all such copies and derivative works.
;; However, this document itself may not be modified in any way, such as by
;; removing the copyright notice or references to the Scheme Request For
;; Implementation process or editors, except as needed for the purpose of
;; developing SRFIs in which case the procedures for copyrights defined in the SRFI
;; process must be followed, or as required to translate it into languages other
;; than English.
;;
;; The limited permissions granted above are perpetual and will not be revoked
;; by the authors or their successors or assigns.
;;
;; This document and the information contained herein is provided on an "AS IS"
;; basis and THE AUTHOR AND THE SRFI EDITORS DISCLAIM ALL WARRANTIES, EXPRESS OR
;; IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
;; INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
;; MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


;; -- 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.
;;
;; changes 2003-02-26, based on comments by Martin Gasbichler.
;;
;; moronic, overly complicated COPY-TIME procedure changed
;; to simple version suggested by Martin Gasbichler.
;;
;; To provide more portability, changed #\Space to #\space
;; and #\tab to #\Tab to (integer->char 9)
;;
;; changed arity-3 calls to / and - to arity 2 calls (again,
;; for more general portability).
;;
;; split-real fixed again -- by removing it, and using
;; 'fractional part'. Will Fitzgerald 5/16/2003.
;; --------------------------------------------------------------

(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)

;;-- 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'


;;; 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 "TIME-ERROR type" caller type value)
      (error "TIME-ERROR type" caller type))
    (error "TIME-ERROR unsupported error type" caller 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 (previous one is in 1968 before the Unix epoch).
(define tm:leap-second-table
  '((1483228800 . 37)
    (1435708800 . 36)
    (1341100800 . 35)
    (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))
  (void))


(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
;;; vyzo: change to use gerbil defstruct
;;; fare: added checking in make-time
(defstruct time (type nanosecond second))

;; Safer variant of make-time
(define (make-time% type nanosecond second)
  (case type
    ((time-tai time-utc time-monotonic time-thread time-process time-duration) #t)
    (else (error "Wrong time type" type)))
  (unless (exact-integer? second)
    (error "Invalid time second" second))
  (unless (and (exact-integer? nanosecond)
               (<= (if (eq? type time-duration) -999999999 0) nanosecond 999999999))
    (error "Invalid time nanosecond" nanosecond))
  (make-time type nanosecond second))


;; thanks, Martin Gasbichler ...

(define (copy-time time)
  (make-time (time-type time)
             (time-nanosecond time)
             (time-second time)))


;;; current-time

;;; specific time getters.
;;; these should be rewritten to be os specific.
;;
;; -- using gnu gettimeofday() would be useful here -- gets
;;    second + millisecond
;;    let's pretend we do, using mzscheme's current-seconds & current-milliseconds
;;    this is supposed to return utc.
;;

;; vyzo: use current-time-point, ns resolution
(define (current-seconds)
  (inexact->exact (floor (##current-time-point))))

(define (current-milliseconds)
  (inexact->exact (floor (fl* 1e3 (##current-time-point)))))

#;(define (tm:get-time-of-day)
  (values (current-seconds)
          (abs (remainder (current-milliseconds) 1000))))

(define (tm:get-time-of-day)
  (let* ((now (##current-time-point))
         (sec (floor now)))
    (values (inexact->exact sec)
            (inexact->exact
             (floor (fl* (fl- now sec) 1e9))))))

(define (tm:current-time-utc)
  (receive (seconds ns) (tm:get-time-of-day)
	   (make-time time-utc ns seconds)))

(define (tm:current-time-tai)
  (receive (seconds ns) (tm:get-time-of-day)
	   (make-time time-tai ns (+ seconds (tm:leap-second-delta seconds)))))

;; -- 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 ns) (tm:get-time-of-day)
       (make-time time-monotonic ns (+ seconds (tm:leap-second-delta seconds)))))


(define (tm:current-time-thread)
  (error "Unimplemented"))

(define (tm:current-time-process)
  (error "Unimplemented"))

(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))
     (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.

(define (time-resolution . clock-type)
  (let ((clock-type (:optional clock-type time-utc)))
    (cond
     ((eq? clock-type time-tai) 1)
     ((eq? clock-type time-utc) 1)
     ((eq? clock-type time-monotonic) 1)
     ((eq? clock-type time-thread) 1000)
     ((eq? clock-type time-process) 1000)
     (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))
  (time-type-set! time3 time-duration)
  (if (time=? time1 time2)
      (begin
        (time-second-set! time3 0)
        (time-nanosecond-set! time3 0))
      (receive
          (nanos secs)
          (tm:nanoseconds->values (- (tm:time->nanoseconds time1)
                                     (tm:time->nanoseconds time2)))
        (time-second-set! time3 secs)
        (time-nanosecond-set! 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
                (time-second-set! time3 (+ sec-plus q -1))
                (time-nanosecond-set! time3 (+ tm:nano r)))
              (begin
                (time-second-set! time3 (+ sec-plus q))
                (time-nanosecond-set! 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
                (time-second-set! time3 (- sec-minus q 1))
                (time-nanosecond-set! time3 (+ tm:nano r)))
              (begin
                (time-second-set! time3 (- sec-minus q))
                (time-nanosecond-set! 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))
  (time-type-set! time-out time-utc)
  (time-nanosecond-set! time-out (time-nanosecond time-in))
  (time-second-set!     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))
  (time-type-set! time-out time-tai)
  (time-nanosecond-set! time-out (time-nanosecond time-in))
  (time-second-set!     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)))
    (time-type-set! 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))
  (time-type-set! 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)))
    (time-type-set! 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))
  (time-type-set! 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)))
    (time-type-set! 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!)))
    (time-type-set! 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)))
    (time-type-set! 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))
  (time-type-set! time-in time-monotonic)
  time-in)


;; -- date structures

;; vyzo: defstruct for gerbil, with equality defined
(defstruct date (nanosecond second minute hour day month year zone-offset)
  equal: #t)

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

;; redefine setters -- vyzo: why? PEBKAC if the user breaks contract
;;                     we don't even export them.
;; (define tm:set-date-nanosecond! (values date-nanosecond-set!))
;; (define tm:set-date-second! (values date-second-set!))
;; (define tm:set-date-minute! (values date-minute-set!))
;; (define tm:set-date-hour! (values date-hour-set!))
;; (define tm:set-date-day! (values date-day-set!))
;; (define tm:set-date-month! (values date-month-set!))
;; (define tm:set-date-year! (values date-year-set!))
;; (define tm:set-date-zone-offset! (values date-zone-offset-set!))

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

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

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

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

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

;; (set! date-zone-offset-set! (lambda (date val)
;;                               (tm:time-error 'date-zone-offset-set! '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))
         (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))))


(define (tm:fractional-part r)
  (if (integer? r) "0"
      (let ((str (number->string (exact->inexact r))))
        (let ((ppos (tm:char-pos #\. str 0 (string-length str))))
          (substring str  (+ ppos 1) (string-length str))))))


;; 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))
    ))


;; relies on the fact that we named our time zone accessor
;; differently from MzScheme's....
;; This should be written to be OS specific.

(define (tm:local-tz-offset)
  ;(date-time-zone-offset ;(seconds->date
  ;                        (current-seconds)
  ;                       ; )
  ;                       )
  0
  )

;; 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)) tm:sid)
             (- offset))))))

(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)
  (void))

;; 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.
;;
(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* ((ns (tm:fractional-part (/
                                               (date-nanosecond date)
                                               tm:nano 1.0)))
                      (le (string-length ns)))
                 (if (> le 2)
                     (begin
                       (display tm:locale-number-separator port)
                       (display (substring ns 2 le) 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 (integer->char 9) 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 (date-year date) 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)
      (void)
      (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) (void)))
        )

    (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
        (void))
      (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 0; (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)))))
