#!r6rs
(library (rsc3 ntp ntp)
(export ntp->utc. utc->ntp utc->ntpr ntpr->ntp)
(import (rnrs)
	(only (rsc3 math exact) round-exact))

;; NTP is the Network Time Protocol.  NTP time is represented by a 64
;; bit fixed point number. The first 32 bits specify the number of
;; seconds since midnight on January 1, 1900, and the last 32 bits
;; specify fractional parts of a second to a precision of about 200
;; picoseconds. This is the representation used by Internet NTP
;; timestamps.

;; The number of seconds from the start of 1900 to the start of 1970.
;; NTP is measured from the former, UTC from the latter.  There are 17
;; leap years in this period.

;; 2^32 = 4294967296

(define seconds-from-1900-to-1970 (+ (* 70 365 24 60 60) (* 17 24 60 60)))
(define (ntpr->ntp i)             (round-exact (* i 4294967296)))
(define (ntp-to-seconds i)        (/ i 4294967296))
(define (ntp-to-seconds. i)       (/ i 4294967296.0))
(define (nanoseconds-to-ntp i)    (round-exact (* i (/ 4294967296 (expt 10 9)))))
(define (ntp-to-nanoseconds i)    (* i (/ (expt 10 9) 4294967296)))

;; Convert between time intervals in seconds and NTP intervals.

(define (time-interval->ntp-interval interval)
  (ntpr->ntp interval))

(define (ntp-interval->time-interval ntp-interval)
  (ntp-to-seconds ntp-interval))

(define (ntp-interval->time-interval. ntp-interval)
  (ntp-to-seconds. ntp-interval))

;; Evaluate to an <real> representing the NTP time of the UTC time of
;; the <real> number `n'.

(define (utc->ntpr n)
  (+ n seconds-from-1900-to-1970))

;; Evaluate to an <integer> representing the NTP time of the UTC time
;; of the <real> number `n'.

(define (utc->ntp n)
  (ntpr->ntp (+ n seconds-from-1900-to-1970)))

;; Evaluate to a <real> number representing the UTC time of the
;; <integer> NTP time `ntp'.

(define (ntp->utc ntp)
  (- (ntp-to-seconds ntp) seconds-from-1900-to-1970))

(define (ntp->utc. ntp)
  (- (ntp-to-seconds. ntp) seconds-from-1900-to-1970))

)