~psmay/+junk/coffee-clock-face-resolver : revision 1

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MS_PER_MINUTE = 60 * 1000

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MS_PER_DAY = 24 * 60 * MS_PER_MINUTE

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# Copy the UTC face of a `Date` object to a new object's local face, or vice versa.

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# The special cases ensure that any year is treated as non-rolling; e.g. 99 is

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# 0099 and not 1999. (The fact that the shape of a year is congruent modulo 400

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# years is used to help minimize the necessary changes.)

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# Returns a `Date` object with its UTC face (the face returned by `.getUTC*()`)

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# set to the local face (the face returned by the local methods `.get*()`

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# corresponding to `.getUTC*()`) suggested by the `Date` `from`.

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#

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# The conversion is lossless in this direction.

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dateLocalToUtc = (from) ->

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y = from.getFullYear()

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mo = from.getMonth()

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d = from.getDate()

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h = from.getHours()

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min = from.getMinutes()

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s = from.getSeconds()

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ms = from.getMilliseconds()

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to = new Date Date.UTC y, mo, d, h, min, s, ms

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if to.getUTCFullYear() isnt y

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to.setTime Date.UTC y + 400, mo, d, h, min, s, ms

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to.setUTCFullYear y

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to

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# Returns a `Date` object with its local face set to the UTC face suggested by

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# of the `Date` object `from`.

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#

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# This conversion is generally lossy if the face falls within a daylight/summer

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# time ambiguity; the actual results are dependent on the JavaScript

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# implementation.

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dateUtcToLocalRaw = (from) ->

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y = from.getUTCFullYear()

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mo = from.getUTCMonth()

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d = from.getUTCDate()

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h = from.getUTCHours()

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min = from.getUTCMinutes()

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s = from.getUTCSeconds()

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ms = from.getUTCMilliseconds()

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to = new Date y, mo, d, h, min, s, ms

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if to.getFullYear() isnt y

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to = new Date y + 400, mo, d, h, min, s, ms

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to.setFullYear y

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to

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# `faceMs` is an interpretation of an ms date as if the epoch were not

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# strictly UTC but entirely timezone-ignorant instead (i.e., the number of ms

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# since `1970-01-01T00:00:00` with no specified timezone). If a `Date` object

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# is set to `faceMs`, the intended face appears on its UTC face.

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# Returns a `Date` object with the local face set as suggested by `faceMs`.

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#

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# This conversion is generally lossy if the face falls within a daylight/summer

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# time ambiguity; the actual results are dependent on the JavaScript

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# implementation.

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faceMsToLocalRaw = (faceMs) -> dateUtcToLocalRaw new Date faceMs

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# Returns a `Date` object with its local face set to the UTC face suggested by

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# of the `Date` object `from`. This conversion is generally lossy if the face

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# falls within a daylight/summer time ambiguity; such times are adjusted

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# according to the following rules:

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#

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# - If the face date is skipped by a DST skip forward, the time is resolved to

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# the first minute following the skip (e.g., if 01:59 is directly followed by

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# 03:00, 02:30 would be mapped to 03:00).

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# - If two events are scheduled at different times, the event scheduled

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# earlier will occur no later than the event scheduled later. If one or

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# both events are scheduled for a skipped time, they may occur at the same

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# time.

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# - If the face date happens twice due a DST skip backward, the time is

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# resolved to the earlier instance (e.g. if 01:00-01:59 daylight time is

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# directly followed by 01:00-01:59 standard time, 01:30 would be mapped to

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# 01:30 daylight time).

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# - An event scheduled for a time that is doubled will still occur only once,

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# at the earlier instance of the time.

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# - A face cannot be resolved to the later instance of a doubled time;

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# specifically, even if the target application has a concept of *current

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# time* and the later instance of the time is in the future but the earlier

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# instance is in the past, the time itself is considered past.

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dateUtcToLocalAdjusted = (dateWithUtcFace) ->

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faceMs = dateWithUtcFace.getTime()

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date = dateUtcToLocalRaw dateWithUtcFace

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dateOffset = date.getTimezoneOffset()

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previousDateOffset = (faceMsToLocalRaw faceMs - MS_PER_DAY).getTimezoneOffset()

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if faceMs is dateLocalToUtc(date).getTime()

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# Face matches

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if dateOffset is previousDateOffset

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# Matches earliest (or only) instance.

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date

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else

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# If there are two instances, matches the later one and needs adjustment.

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# Otherwise, matches the only instance.

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offsetChangeMinutes = dateOffset - previousDateOffset

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if offsetChangeMinutes <= 0

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# This is a skipping rather than a doubling adjustment; the

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# offset change is the wrong direction

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date

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else

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# The offset change is subtracted from the date's epoch time

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# rather than its face.

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adjustedDate = new Date date.getTime() - (offsetChangeMinutes * MS_PER_MINUTE)

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# If the offsets match, the recent past contains no discontinuity,

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# so there is only one instance. Otherwise, adjust the time to be

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# before the discontinuity.

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if adjustedDate.getTimezoneOffset() is dateOffset then date else adjustedDate

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else

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# Face refers to a skipped time. Resolve to the first minute the end of

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# the discontinuity. The discontinuity is located by binary search;

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# minutes are differentiated based on whether a given time's local

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# offset matches the previous day's offset (before the discontinuity)

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# or not (after the discontinuity).

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followingDateOffset = (faceMsToLocalRaw faceMs + MS_PER_DAY).getTimezoneOffset()

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offsetChangeMinutes = previousDateOffset - followingDateOffset

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faceMinute = faceMs / MS_PER_MINUTE

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before = Math.floor( faceMinute - offsetChangeMinutes )

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after = Math.ceil( faceMinute + offsetChangeMinutes )

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while before + 1 < after

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mid = Math.floor( (before + after) / 2 )

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midOffset = (faceMsToLocalRaw mid * MS_PER_MINUTE).getTimezoneOffset()

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if midOffset is previousDateOffset

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before = mid

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else

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after = mid

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faceMsToLocalRaw after * MS_PER_MINUTE