    {-# LANGUAGE CPP #-}
    {-# LANGUAGE NoMonoLocalBinds #-}
    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE TypeFamilies #-}
    {-# LANGUAGE OverloadedStrings #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE PackageImports #-}
    {-|

    Date parsing and utilities for hledger.

    For date and time values, we use the standard Day and UTCTime types.

    A 'SmartDate' is a date which may be partially-specified or relative.
    Eg 2008\/12\/31, but also 2008\/12, 12\/31, tomorrow, last week, next year.
    We represent these as a triple of strings like (\"2008\",\"12\",\"\"),
    (\"\",\"\",\"tomorrow\"), (\"\",\"last\",\"week\").

    A 'DateSpan' is the span of time between two specific calendar dates, or
    an open-ended span where one or both dates are unspecified. (A date span
    with both ends unspecified matches all dates.)

    An 'Interval' is ledger's \"reporting interval\" - weekly, monthly,
    quarterly, etc.

    'Period' will probably replace DateSpan in due course.

    -}

    -- XXX fromGregorian silently clips bad dates, use fromGregorianValid instead ?

    module Hledger.Data.Dates (
      -- * Misc date handling utilities
      getCurrentDay,
      getCurrentMonth,
      getCurrentYear,
      nulldate,
      spanContainsDate,
      periodContainsDate,
      parsedateM,
      parsedate,
      showDate,
      showDateSpan,
      showDateSpanMonthAbbrev,
      elapsedSeconds,
      prevday,
      periodexprp,
      parsePeriodExpr,
      parsePeriodExpr',
      nulldatespan,
      emptydatespan,
      failIfInvalidYear,
      failIfInvalidMonth,
      failIfInvalidDay,
      datesepchar,
      datesepchars,
      isDateSepChar,
      spanStart,
      spanEnd,
      spansSpan,
      spanIntersect,
      spansIntersect,
      spanIntervalIntersect,
      spanDefaultsFrom,
      spanUnion,
      spansUnion,
      smartdate,
      splitSpan,
      fixSmartDate,
      fixSmartDateStr,
      fixSmartDateStrEither,
      fixSmartDateStrEither',
      daysInSpan,
      maybePeriod,
      mkdatespan,
    )
    where

    import Prelude ()
    import "base-compat-batteries" Prelude.Compat
    import Control.Applicative.Permutations
    import Control.Monad
    import "base-compat-batteries" Data.List.Compat
    import Data.Default
    import Data.Maybe
    import Data.Text (Text)
    import qualified Data.Text as T
    #if MIN_VERSION_time(1,5,0)
    import Data.Time.Format hiding (months)
    #else
    import Data.Time.Format
    import System.Locale (TimeLocale, defaultTimeLocale)
    #endif
    import Data.Time.Calendar
    import Data.Time.Calendar.OrdinalDate
    import Data.Time.Clock
    import Data.Time.LocalTime
    import Safe (headMay, lastMay, readMay)
    import Text.Megaparsec
    import Text.Megaparsec.Char
    import Text.Megaparsec.Custom
    import Text.Printf

    import Hledger.Data.Types
    import Hledger.Data.Period
    import Hledger.Utils

    -- Help ppShow parse and line-wrap DateSpans better in debug output.
    instance Show DateSpan where
        show s = "DateSpan " ++ showDateSpan s
        -- show s = "DateSpan \"" ++ showDateSpan s ++ "\"" -- quotes to help pretty-show

    showDate :: Day -> String
    showDate = formatTime defaultTimeLocale "%0C%y/%m/%d"

    -- | Render a datespan as a display string, abbreviating into a
    -- compact form if possible.
    showDateSpan :: DateSpan -> String
    showDateSpan = showPeriod . dateSpanAsPeriod

    -- | Like showDateSpan, but show month spans as just the abbreviated month name
    -- in the current locale.
    showDateSpanMonthAbbrev :: DateSpan -> String
    showDateSpanMonthAbbrev = showPeriodMonthAbbrev . dateSpanAsPeriod

    -- | Get the current local date.
    getCurrentDay :: IO Day
    getCurrentDay = do
        t <- getZonedTime
        return $ localDay (zonedTimeToLocalTime t)

    -- | Get the current local month number.
    getCurrentMonth :: IO Int
    getCurrentMonth = do
      (_,m,_) <- toGregorian `fmap` getCurrentDay
      return m

    -- | Get the current local year.
    getCurrentYear :: IO Integer
    getCurrentYear = do
      (y,_,_) <- toGregorian `fmap` getCurrentDay
      return y

    elapsedSeconds :: Fractional a => UTCTime -> UTCTime -> a
    elapsedSeconds t1 = realToFrac . diffUTCTime t1

    spanStart :: DateSpan -> Maybe Day
    spanStart (DateSpan d _) = d

    spanEnd :: DateSpan -> Maybe Day
    spanEnd (DateSpan _ d) = d

    -- might be useful later: http://en.wikipedia.org/wiki/Allen%27s_interval_algebra

    -- | Get overall span enclosing multiple sequentially ordered spans.
    spansSpan :: [DateSpan] -> DateSpan
    spansSpan spans = DateSpan (maybe Nothing spanStart $ headMay spans) (maybe Nothing spanEnd $ lastMay spans)

    -- | Split a DateSpan into consecutive whole spans of the specified interval
    -- which fully encompass the original span (and a little more when necessary).
    -- If no interval is specified, the original span is returned.
    -- If the original span is the null date span, ie unbounded, the null date span is returned.
    -- If the original span is empty, eg if the end date is <= the start date, no spans are returned.
    --
    --
    -- ==== Examples:
    -- >>> let t i d1 d2 = splitSpan i $ mkdatespan d1 d2
    -- >>> t NoInterval "2008/01/01" "2009/01/01"
    -- [DateSpan 2008]
    -- >>> t (Quarters 1) "2008/01/01" "2009/01/01"
    -- [DateSpan 2008q1,DateSpan 2008q2,DateSpan 2008q3,DateSpan 2008q4]
    -- >>> splitSpan (Quarters 1) nulldatespan
    -- [DateSpan -]
    -- >>> t (Days 1) "2008/01/01" "2008/01/01"  -- an empty datespan
    -- []
    -- >>> t (Quarters 1) "2008/01/01" "2008/01/01"
    -- []
    -- >>> t (Months 1) "2008/01/01" "2008/04/01"
    -- [DateSpan 2008/01,DateSpan 2008/02,DateSpan 2008/03]
    -- >>> t (Months 2) "2008/01/01" "2008/04/01"
    -- [DateSpan 2008/01/01-2008/02/29,DateSpan 2008/03/01-2008/04/30]
    -- >>> t (Weeks 1) "2008/01/01" "2008/01/15"
    -- [DateSpan 2007/12/31w01,DateSpan 2008/01/07w02,DateSpan 2008/01/14w03]
    -- >>> t (Weeks 2) "2008/01/01" "2008/01/15"
    -- [DateSpan 2007/12/31-2008/01/13,DateSpan 2008/01/14-2008/01/27]
    -- >>> t (DayOfMonth 2) "2008/01/01" "2008/04/01"
    -- [DateSpan 2007/12/02-2008/01/01,DateSpan 2008/01/02-2008/02/01,DateSpan 2008/02/02-2008/03/01,DateSpan 2008/03/02-2008/04/01]
    -- >>> t (WeekdayOfMonth 2 4) "2011/01/01" "2011/02/15"
    -- [DateSpan 2010/12/09-2011/01/12,DateSpan 2011/01/13-2011/02/09,DateSpan 2011/02/10-2011/03/09]
    -- >>> t (DayOfWeek 2) "2011/01/01" "2011/01/15"
    -- [DateSpan 2010/12/28-2011/01/03,DateSpan 2011/01/04-2011/01/10,DateSpan 2011/01/11-2011/01/17]
    -- >>> t (DayOfYear 11 29) "2011/10/01" "2011/10/15"
    -- [DateSpan 2010/11/29-2011/11/28]
    -- >>> t (DayOfYear 11 29) "2011/12/01" "2012/12/15"
    -- [DateSpan 2011/11/29-2012/11/28,DateSpan 2012/11/29-2013/11/28]
    --
    splitSpan :: Interval -> DateSpan -> [DateSpan]
    splitSpan _ (DateSpan Nothing Nothing) = [DateSpan Nothing Nothing]
    splitSpan _ s | isEmptySpan s = []
    splitSpan NoInterval     s = [s]
    splitSpan (Days n)       s = splitspan startofday     (applyN n nextday)     s
    splitSpan (Weeks n)      s = splitspan startofweek    (applyN n nextweek)    s
    splitSpan (Months n)     s = splitspan startofmonth   (applyN n nextmonth)   s
    splitSpan (Quarters n)   s = splitspan startofquarter (applyN n nextquarter) s
    splitSpan (Years n)      s = splitspan startofyear    (applyN n nextyear)    s
    splitSpan (DayOfMonth n) s = splitspan (nthdayofmonthcontaining n) (nthdayofmonth n . nextmonth) s
    splitSpan (WeekdayOfMonth n wd) s = splitspan (nthweekdayofmonthcontaining n wd) (advancetonthweekday n wd . nextmonth) s
    splitSpan (DayOfWeek n)  s = splitspan (nthdayofweekcontaining n)  (applyN (n-1) nextday . nextweek)  s
    splitSpan (DayOfYear m n) s = splitspan (nthdayofyearcontaining m n) (applyN (n-1) nextday . applyN (m-1) nextmonth . nextyear) s
    -- splitSpan (WeekOfYear n)    s = splitspan startofweek    (applyN n nextweek)    s
    -- splitSpan (MonthOfYear n)   s = splitspan startofmonth   (applyN n nextmonth)   s
    -- splitSpan (QuarterOfYear n) s = splitspan startofquarter (applyN n nextquarter) s

    -- Split the given span using the provided helper functions:
    -- start is applied to the span's start date to get the first sub-span's start date
    -- next is applied to a sub-span's start date to get the next sub-span's start date
    splitspan :: (Day -> Day) -> (Day -> Day) -> DateSpan -> [DateSpan]
    splitspan _ _ (DateSpan Nothing Nothing) = []
    splitspan start next (DateSpan Nothing (Just e)) = splitspan start next (DateSpan (Just $ start e) (Just $ next $ start e))
    splitspan start next (DateSpan (Just s) Nothing) = splitspan start next (DateSpan (Just $ start s) (Just $ next $ start s))
    splitspan start next span@(DateSpan (Just s) (Just e))
        | s == e = [span]
        | otherwise = splitspan' start next span
        where
          splitspan' start next (DateSpan (Just s) (Just e))
              | s >= e = []
              | otherwise = DateSpan (Just subs) (Just sube) : splitspan' start next (DateSpan (Just sube) (Just e))
              where subs = start s
                    sube = next subs
          splitspan' _ _ _ = error' "won't happen, avoids warnings"

    -- | Count the days in a DateSpan, or if it is open-ended return Nothing.
    daysInSpan :: DateSpan -> Maybe Integer
    daysInSpan (DateSpan (Just d1) (Just d2)) = Just $ diffDays d2 d1
    daysInSpan _ = Nothing

    -- | Is this an empty span, ie closed with the end date on or before the start date ?
    isEmptySpan :: DateSpan -> Bool
    isEmptySpan s = case daysInSpan s of
                      Just n  -> n < 1
                      Nothing -> False

    -- | Does the span include the given date ?
    spanContainsDate :: DateSpan -> Day -> Bool
    spanContainsDate (DateSpan Nothing Nothing)   _ = True
    spanContainsDate (DateSpan Nothing (Just e))  d = d < e
    spanContainsDate (DateSpan (Just b) Nothing)  d = d >= b
    spanContainsDate (DateSpan (Just b) (Just e)) d = d >= b && d < e

    -- | Does the period include the given date ?
    -- (Here to avoid import cycle).
    periodContainsDate :: Period -> Day -> Bool
    periodContainsDate p = spanContainsDate (periodAsDateSpan p)

    -- | Calculate the intersection of a number of datespans.
    spansIntersect [] = nulldatespan
    spansIntersect [d] = d
    spansIntersect (d:ds) = d `spanIntersect` (spansIntersect ds)

    -- | Calculate the intersection of two datespans.
    --
    -- For non-intersecting spans, gives an empty span beginning on the second's start date:
    -- >>> mkdatespan "2018-01-01" "2018-01-03" `spanIntersect` mkdatespan "2018-01-03" "2018-01-05"
    -- DateSpan 2018/01/03-2018/01/02
    spanIntersect (DateSpan b1 e1) (DateSpan b2 e2) = DateSpan b e
        where
          b = latest b1 b2
          e = earliest e1 e2

    -- | Calculate the intersection of two DateSpans, adjusting the start date so
    -- the interval is preserved.
    --
    -- >>> let intervalIntersect = spanIntervalIntersect (Days 3)
    -- >>> mkdatespan "2018-01-01" "2018-01-03" `intervalIntersect` mkdatespan "2018-01-01" "2018-01-05"
    -- DateSpan 2018/01/01-2018/01/02
    -- >>> mkdatespan "2018-01-01" "2018-01-05" `intervalIntersect` mkdatespan "2018-01-02" "2018-01-05"
    -- DateSpan 2018/01/04
    -- >>> mkdatespan "2018-01-01" "2018-01-05" `intervalIntersect` mkdatespan "2018-01-03" "2018-01-05"
    -- DateSpan 2018/01/04
    -- >>> mkdatespan "2018-01-01" "2018-01-05" `intervalIntersect` mkdatespan "2018-01-04" "2018-01-05"
    -- DateSpan 2018/01/04
    -- >>> mkdatespan "2018-01-01" "2018-01-05" `intervalIntersect` mkdatespan "2017-12-01" "2018-01-05"
    -- DateSpan 2018/01/01-2018/01/04
    spanIntervalIntersect :: Interval -> DateSpan -> DateSpan -> DateSpan
    spanIntervalIntersect (Days n) (DateSpan (Just b1) e1) sp2@(DateSpan (Just b2) _) =
          DateSpan (Just b) e1 `spanIntersect` sp2
        where
          b = if b1 < b2 then addDays (diffDays b1 b2 `mod` toInteger n) b2 else b1
    spanIntervalIntersect _ sp1 sp2 = sp1 `spanIntersect` sp2

    -- | Fill any unspecified dates in the first span with the dates from
    -- the second one. Sort of a one-way spanIntersect.
    spanDefaultsFrom (DateSpan a1 b1) (DateSpan a2 b2) = DateSpan a b
        where a = if isJust a1 then a1 else a2
              b = if isJust b1 then b1 else b2

    -- | Calculate the union of a number of datespans.
    spansUnion [] = nulldatespan
    spansUnion [d] = d
    spansUnion (d:ds) = d `spanUnion` (spansUnion ds)

    -- | Calculate the union of two datespans.
    spanUnion (DateSpan b1 e1) (DateSpan b2 e2) = DateSpan b e
        where
          b = earliest b1 b2
          e = latest e1 e2

    latest d Nothing = d
    latest Nothing d = d
    latest (Just d1) (Just d2) = Just $ max d1 d2

    earliest d Nothing = d
    earliest Nothing d = d
    earliest (Just d1) (Just d2) = Just $ min d1 d2

    -- | Parse a period expression to an Interval and overall DateSpan using
    -- the provided reference date, or return a parse error.
    parsePeriodExpr
      :: Day -> Text -> Either (ParseErrorBundle Text CustomErr) (Interval, DateSpan)
    parsePeriodExpr refdate s = parsewith (periodexprp refdate <* eof) (T.toLower s)

    -- | Like parsePeriodExpr, but call error' on failure.
    parsePeriodExpr' :: Day -> Text -> (Interval, DateSpan)
    parsePeriodExpr' refdate s =
      either (error' . ("failed to parse:" ++) . customErrorBundlePretty) id $
      parsePeriodExpr refdate s

    maybePeriod :: Day -> Text -> Maybe (Interval,DateSpan)
    maybePeriod refdate = either (const Nothing) Just . parsePeriodExpr refdate

    -- | Show a DateSpan as a human-readable pseudo-period-expression string.
    -- dateSpanAsText :: DateSpan -> String
    -- dateSpanAsText (DateSpan Nothing Nothing)   = "all"
    -- dateSpanAsText (DateSpan Nothing (Just e))  = printf "to %s" (show e)
    -- dateSpanAsText (DateSpan (Just b) Nothing)  = printf "from %s" (show b)
    -- dateSpanAsText (DateSpan (Just b) (Just e)) = printf "%s to %s" (show b) (show e)

    -- | Convert a single smart date string to a date span using the provided
    -- reference date, or raise an error.
    -- spanFromSmartDateString :: Day -> String -> DateSpan
    -- spanFromSmartDateString refdate s = spanFromSmartDate refdate sdate
    --     where
    --       sdate = fromparse $ parsewith smartdateonly s

    spanFromSmartDate :: Day -> SmartDate -> DateSpan
    spanFromSmartDate refdate sdate = DateSpan (Just b) (Just e)
        where
          (ry,rm,_) = toGregorian refdate
          (b,e) = span sdate
          span :: SmartDate -> (Day,Day)
          span ("","","today")       = (refdate, nextday refdate)
          span ("","this","day")     = (refdate, nextday refdate)
          span ("","","yesterday")   = (prevday refdate, refdate)
          span ("","last","day")     = (prevday refdate, refdate)
          span ("","","tomorrow")    = (nextday refdate, addDays 2 refdate)
          span ("","next","day")     = (nextday refdate, addDays 2 refdate)
          span ("","last","week")    = (prevweek refdate, thisweek refdate)
          span ("","this","week")    = (thisweek refdate, nextweek refdate)
          span ("","next","week")    = (nextweek refdate, startofweek $ addDays 14 refdate)
          span ("","last","month")   = (prevmonth refdate, thismonth refdate)
          span ("","this","month")   = (thismonth refdate, nextmonth refdate)
          span ("","next","month")   = (nextmonth refdate, startofmonth $ addGregorianMonthsClip 2 refdate)
          span ("","last","quarter") = (prevquarter refdate, thisquarter refdate)
          span ("","this","quarter") = (thisquarter refdate, nextquarter refdate)
          span ("","next","quarter") = (nextquarter refdate, startofquarter $ addGregorianMonthsClip 6 refdate)
          span ("","last","year")    = (prevyear refdate, thisyear refdate)
          span ("","this","year")    = (thisyear refdate, nextyear refdate)
          span ("","next","year")    = (nextyear refdate, startofyear $ addGregorianYearsClip 2 refdate)
          span ("","",d)             = (day, nextday day) where day = fromGregorian ry rm (read d)
          span ("",m,"")             = (startofmonth day, nextmonth day) where day = fromGregorian ry (read m) 1
          span ("",m,d)              = (day, nextday day) where day = fromGregorian ry (read m) (read d)
          span (y,"","")             = (startofyear day, nextyear day) where day = fromGregorian (read y) 1 1
          span (y,m,"")              = (startofmonth day, nextmonth day) where day = fromGregorian (read y) (read m) 1
          span (y,m,d)               = (day, nextday day) where day = fromGregorian (read y) (read m) (read d)

    -- showDay :: Day -> String
    -- showDay day = printf "%04d/%02d/%02d" y m d where (y,m,d) = toGregorian day

    -- | Convert a smart date string to an explicit yyyy\/mm\/dd string using
    -- the provided reference date, or raise an error.
    fixSmartDateStr :: Day -> Text -> String
    fixSmartDateStr d s = either
                           (\e->error' $ printf "could not parse date %s %s" (show s) (show e))
                           id
                           $ (fixSmartDateStrEither d s :: Either (ParseErrorBundle Text CustomErr) String)

    -- | A safe version of fixSmartDateStr.
    fixSmartDateStrEither :: Day -> Text -> Either (ParseErrorBundle Text CustomErr) String
    fixSmartDateStrEither d = either Left (Right . showDate) . fixSmartDateStrEither' d

    fixSmartDateStrEither'
      :: Day -> Text -> Either (ParseErrorBundle Text CustomErr) Day
    fixSmartDateStrEither' d s = case parsewith smartdateonly (T.toLower s) of
                                   Right sd -> Right $ fixSmartDate d sd
                                   Left e -> Left e

    -- | Convert a SmartDate to an absolute date using the provided reference date.
    --
    -- ==== Examples:
    -- >>> :set -XOverloadedStrings
    -- >>> let t = fixSmartDateStr (parsedate "2008/11/26")
    -- >>> t "0000-01-01"
    -- "0000/01/01"
    -- >>> t "1999-12-02"
    -- "1999/12/02"
    -- >>> t "1999.12.02"
    -- "1999/12/02"
    -- >>> t "1999/3/2"
    -- "1999/03/02"
    -- >>> t "19990302"
    -- "1999/03/02"
    -- >>> t "2008/2"
    -- "2008/02/01"
    -- >>> t "0020/2"
    -- "0020/02/01"
    -- >>> t "1000"
    -- "1000/01/01"
    -- >>> t "4/2"
    -- "2008/04/02"
    -- >>> t "2"
    -- "2008/11/02"
    -- >>> t "January"
    -- "2008/01/01"
    -- >>> t "feb"
    -- "2008/02/01"
    -- >>> t "today"
    -- "2008/11/26"
    -- >>> t "yesterday"
    -- "2008/11/25"
    -- >>> t "tomorrow"
    -- "2008/11/27"
    -- >>> t "this day"
    -- "2008/11/26"
    -- >>> t "last day"
    -- "2008/11/25"
    -- >>> t "next day"
    -- "2008/11/27"
    -- >>> t "this week"  -- last monday
    -- "2008/11/24"
    -- >>> t "last week"  -- previous monday
    -- "2008/11/17"
    -- >>> t "next week"  -- next monday
    -- "2008/12/01"
    -- >>> t "this month"
    -- "2008/11/01"
    -- >>> t "last month"
    -- "2008/10/01"
    -- >>> t "next month"
    -- "2008/12/01"
    -- >>> t "this quarter"
    -- "2008/10/01"
    -- >>> t "last quarter"
    -- "2008/07/01"
    -- >>> t "next quarter"
    -- "2009/01/01"
    -- >>> t "this year"
    -- "2008/01/01"
    -- >>> t "last year"
    -- "2007/01/01"
    -- >>> t "next year"
    -- "2009/01/01"
    --
    -- t "last wed"
    -- "2008/11/19"
    -- t "next friday"
    -- "2008/11/28"
    -- t "next january"
    -- "2009/01/01"
    --
    fixSmartDate :: Day -> SmartDate -> Day
    fixSmartDate refdate sdate = fix sdate
        where
          fix :: SmartDate -> Day
          fix ("","","today")       = fromGregorian ry rm rd
          fix ("","this","day")     = fromGregorian ry rm rd
          fix ("","","yesterday")   = prevday refdate
          fix ("","last","day")     = prevday refdate
          fix ("","","tomorrow")    = nextday refdate
          fix ("","next","day")     = nextday refdate
          fix ("","last","week")    = prevweek refdate
          fix ("","this","week")    = thisweek refdate
          fix ("","next","week")    = nextweek refdate
          fix ("","last","month")   = prevmonth refdate
          fix ("","this","month")   = thismonth refdate
          fix ("","next","month")   = nextmonth refdate
          fix ("","last","quarter") = prevquarter refdate
          fix ("","this","quarter") = thisquarter refdate
          fix ("","next","quarter") = nextquarter refdate
          fix ("","last","year")    = prevyear refdate
          fix ("","this","year")    = thisyear refdate
          fix ("","next","year")    = nextyear refdate
          fix ("","",d)             = fromGregorian ry rm (read d)
          fix ("",m,"")             = fromGregorian ry (read m) 1
          fix ("",m,d)              = fromGregorian ry (read m) (read d)
          fix (y,"","")             = fromGregorian (read y) 1 1
          fix (y,m,"")              = fromGregorian (read y) (read m) 1
          fix (y,m,d)               = fromGregorian (read y) (read m) (read d)
          (ry,rm,rd) = toGregorian refdate

    prevday :: Day -> Day
    prevday = addDays (-1)
    nextday = addDays 1
    startofday = id

    thisweek = startofweek
    prevweek = startofweek . addDays (-7)
    nextweek = startofweek . addDays 7
    startofweek day = fromMondayStartWeek y w 1
        where
          (y,_,_) = toGregorian day
          (w,_) = mondayStartWeek day

    thismonth = startofmonth
    prevmonth = startofmonth . addGregorianMonthsClip (-1)
    nextmonth = startofmonth . addGregorianMonthsClip 1
    startofmonth day = fromGregorian y m 1 where (y,m,_) = toGregorian day
    nthdayofmonth d day = fromGregorian y m d where (y,m,_) = toGregorian day

    thisquarter = startofquarter
    prevquarter = startofquarter . addGregorianMonthsClip (-3)
    nextquarter = startofquarter . addGregorianMonthsClip 3
    startofquarter day = fromGregorian y (firstmonthofquarter m) 1
        where
          (y,m,_) = toGregorian day
          firstmonthofquarter m = ((m-1) `div` 3) * 3 + 1

    thisyear = startofyear
    prevyear = startofyear . addGregorianYearsClip (-1)
    nextyear = startofyear . addGregorianYearsClip 1
    startofyear day = fromGregorian y 1 1 where (y,_,_) = toGregorian day

    -- | For given date d find year-long interval that starts on given
    -- MM/DD of year and covers it.
    -- The given MM and DD should be basically valid (1-12 & 1-31),
    -- or an error is raised.
    --
    -- Examples: lets take 2017-11-22. Year-long intervals covering it that
    -- starts before Nov 22 will start in 2017. However
    -- intervals that start after Nov 23rd should start in 2016:
    -- >>> let wed22nd = parsedate "2017-11-22"
    -- >>> nthdayofyearcontaining 11 21 wed22nd
    -- 2017-11-21
    -- >>> nthdayofyearcontaining 11 22 wed22nd
    -- 2017-11-22
    -- >>> nthdayofyearcontaining 11 23 wed22nd
    -- 2016-11-23
    -- >>> nthdayofyearcontaining 12 02 wed22nd
    -- 2016-12-02
    -- >>> nthdayofyearcontaining 12 31 wed22nd
    -- 2016-12-31
    -- >>> nthdayofyearcontaining 1 1 wed22nd
    -- 2017-01-01
    nthdayofyearcontaining :: Month -> MonthDay -> Day -> Day
    nthdayofyearcontaining m md date
      | not (validMonth $ show m)  = error' $ "nthdayofyearcontaining: invalid month "++show m
      | not (validDay   $ show md) = error' $ "nthdayofyearcontaining: invalid day "  ++show md
      | mmddOfSameYear <= date = mmddOfSameYear
      | otherwise = mmddOfPrevYear
      where mmddOfSameYear = addDays (fromIntegral md-1) $ applyN (m-1) nextmonth s
            mmddOfPrevYear = addDays (fromIntegral md-1) $ applyN (m-1) nextmonth $ prevyear s
            s = startofyear date

    -- | For given date d find month-long interval that starts on nth day of month
    -- and covers it.
    -- The given day of month should be basically valid (1-31), or an error is raised.
    --
    -- Examples: lets take 2017-11-22. Month-long intervals covering it that
    -- start on 1st-22nd of month will start in Nov. However
    -- intervals that start on 23rd-30th of month should start in Oct:
    -- >>> let wed22nd = parsedate "2017-11-22"
    -- >>> nthdayofmonthcontaining 1 wed22nd
    -- 2017-11-01
    -- >>> nthdayofmonthcontaining 12 wed22nd
    -- 2017-11-12
    -- >>> nthdayofmonthcontaining 22 wed22nd
    -- 2017-11-22
    -- >>> nthdayofmonthcontaining 23 wed22nd
    -- 2017-10-23
    -- >>> nthdayofmonthcontaining 30 wed22nd
    -- 2017-10-30
    nthdayofmonthcontaining :: MonthDay -> Day -> Day
    nthdayofmonthcontaining md date
      | not (validDay $ show md) = error' $ "nthdayofmonthcontaining: invalid day "  ++show md
      | nthOfSameMonth <= date = nthOfSameMonth
      | otherwise = nthOfPrevMonth
      where nthOfSameMonth = nthdayofmonth md s
            nthOfPrevMonth = nthdayofmonth md $ prevmonth s
            s = startofmonth date

    -- | For given date d find week-long interval that starts on nth day of week
    -- and covers it.
    --
    -- Examples: 2017-11-22 is Wed. Week-long intervals that cover it and
    -- start on Mon, Tue or Wed will start in the same week. However
    -- intervals that start on Thu or Fri should start in prev week:
    -- >>> let wed22nd = parsedate "2017-11-22"
    -- >>> nthdayofweekcontaining 1 wed22nd
    -- 2017-11-20
    -- >>> nthdayofweekcontaining 2 wed22nd
    -- 2017-11-21
    -- >>> nthdayofweekcontaining 3 wed22nd
    -- 2017-11-22
    -- >>> nthdayofweekcontaining 4 wed22nd
    -- 2017-11-16
    -- >>> nthdayofweekcontaining 5 wed22nd
    -- 2017-11-17
    nthdayofweekcontaining :: WeekDay -> Day -> Day
    nthdayofweekcontaining n d | nthOfSameWeek <= d = nthOfSameWeek
                               | otherwise = nthOfPrevWeek
        where nthOfSameWeek = addDays (fromIntegral n-1) s
              nthOfPrevWeek = addDays (fromIntegral n-1) $ prevweek s
              s = startofweek d

    -- | For given date d find month-long interval that starts on nth weekday of month
    -- and covers it.
    --
    -- Examples: 2017-11-22 is 3rd Wed of Nov. Month-long intervals that cover it and
    -- start on 1st-4th Wed will start in Nov. However
    -- intervals that start on 4th Thu or Fri or later should start in Oct:
    -- >>> let wed22nd = parsedate "2017-11-22"
    -- >>> nthweekdayofmonthcontaining 1 3 wed22nd
    -- 2017-11-01
    -- >>> nthweekdayofmonthcontaining 3 2 wed22nd
    -- 2017-11-21
    -- >>> nthweekdayofmonthcontaining 4 3 wed22nd
    -- 2017-11-22
    -- >>> nthweekdayofmonthcontaining 4 4 wed22nd
    -- 2017-10-26
    -- >>> nthweekdayofmonthcontaining 4 5 wed22nd
    -- 2017-10-27
    nthweekdayofmonthcontaining :: Int -> WeekDay -> Day -> Day
    nthweekdayofmonthcontaining n wd d | nthWeekdaySameMonth <= d  = nthWeekdaySameMonth
                                       | otherwise = nthWeekdayPrevMonth
        where nthWeekdaySameMonth = advancetonthweekday n wd $ startofmonth d
              nthWeekdayPrevMonth = advancetonthweekday n wd $ prevmonth d

    -- | Advance to nth weekday wd after given start day s
    advancetonthweekday :: Int -> WeekDay -> Day -> Day
    advancetonthweekday n wd s =
      maybe err (addWeeks (n-1)) $ firstMatch (>=s) $ iterate (addWeeks 1) $ firstweekday s
      where
        err = error' "advancetonthweekday: should not happen"
        addWeeks k = addDays (7 * fromIntegral k)
        firstMatch p = headMay . dropWhile (not . p)
        firstweekday = addDays (fromIntegral wd-1) . startofweek

    ----------------------------------------------------------------------
    -- parsing

    -- -- | Parse a couple of date-time string formats to a time type.
    -- parsedatetimeM :: String -> Maybe LocalTime
    -- parsedatetimeM s = firstJust [
    --     parseTime defaultTimeLocale "%Y/%m/%d %H:%M:%S" s,
    --     parseTime defaultTimeLocale "%Y-%m-%d %H:%M:%S" s
    --     ]

    parsetime :: ParseTime t => TimeLocale -> String -> String -> Maybe t
    parsetime =
    #if MIN_VERSION_time(1,5,0)
         parseTimeM True
    #else
         parseTime
    #endif

    -- | Parse a couple of date string formats to a time type.
    parsedateM :: String -> Maybe Day
    parsedateM s = firstJust [
         parsetime defaultTimeLocale "%Y/%m/%d" s,
         parsetime defaultTimeLocale "%Y-%m-%d" s
         ]

    -- -- | Parse a date-time string to a time type, or raise an error.
    -- parsedatetime :: String -> LocalTime
    -- parsedatetime s = fromMaybe (error' $ "could not parse timestamp \"" ++ s ++ "\"")
    --                             (parsedatetimeM s)

    -- | Parse a YYYY-MM-DD or YYYY/MM/DD date string to a Day, or raise an error. For testing/debugging.
    --
    -- >>> parsedate "2008/02/03"
    -- 2008-02-03
    parsedate :: String -> Day
    parsedate s =  fromMaybe (error' $ "could not parse date \"" ++ s ++ "\"")
                             (parsedateM s)
    -- doctests I haven't been able to make compatible with both GHC 7 and 8
    -- -- >>> parsedate "2008/02/03/"
    -- -- *** Exception: could not parse date "2008/02/03/"
    -- #if MIN_VERSION_base(4,9,0)
    -- -- ...
    -- #endif
    -- #if MIN_VERSION_time(1,6,0)
    -- -- >>> parsedate "2008/02/30"  -- with time >= 1.6, invalid dates are rejected
    -- -- *** Exception: could not parse date "2008/02/30"
    -- #if MIN_VERSION_base(4,9,0)
    -- -- ...
    -- #endif
    -- #else
    -- -- >>> parsedate "2008/02/30"  -- with time < 1.6, they are silently adjusted
    -- -- 2008-02-29
    -- #endif

    {-|
    Parse a date in any of the formats allowed in Ledger's period expressions, and some others.
    Assumes any text in the parse stream has been lowercased.
    Returns a SmartDate, to be converted to a full date later (see fixSmartDate).

    Examples:

    > 2004                                        (start of year, which must have 4+ digits)
    > 2004/10                                     (start of month, which must be 1-12)
    > 2004/10/1                                   (exact date, day must be 1-31)
    > 10/1                                        (month and day in current year)
    > 21                                          (day in current month)
    > october, oct                                (start of month in current year)
    > yesterday, today, tomorrow                  (-1, 0, 1 days from today)
    > last/this/next day/week/month/quarter/year  (-1, 0, 1 periods from the current period)
    > 20181201                                    (8 digit YYYYMMDD with valid year month and day)
    > 201812                                      (6 digit YYYYMM with valid year and month)

    Note malformed digit sequences might give surprising results:

    > 201813                                      (6 digits with an invalid month is parsed as start of 6-digit year)
    > 20181301                                    (8 digits with an invalid month is parsed as start of 8-digit year)
    > 20181232                                    (8 digits with an invalid day gives an error)
    > 201801012                                   (9+ digits beginning with a valid YYYYMMDD gives an error)

    Eg:

    YYYYMMDD is parsed as year-month-date if those parts are valid
    (>=4 digits, 1-12, and 1-31 respectively):
    >>> parsewith (smartdate <* eof) "20181201"
    Right ("2018","12","01")

    YYYYMM is parsed as year-month-01 if year and month are valid:
    >>> parsewith (smartdate <* eof) "201804"
    Right ("2018","04","01")

    With an invalid month, it's parsed as a year:
    >>> parsewith (smartdate <* eof) "201813"
    Right ("201813","","")

    A 9+ digit number beginning with valid YYYYMMDD gives an error:
    >>> parsewith (smartdate <* eof) "201801012"
    Left (...)

    Big numbers not beginning with a valid YYYYMMDD are parsed as a year:
    >>> parsewith (smartdate <* eof) "201813012"
    Right ("201813012","","")

    -}
    smartdate :: TextParser m SmartDate
    smartdate = do
      -- XXX maybe obscures date errors ? see ledgerdate
      (y,m,d) <- choice' [yyyymmdd, yyyymm, ymd, ym, md, y, d, month, mon, today, yesterday, tomorrow, lastthisnextthing]
      return (y,m,d)

    -- | Like smartdate, but there must be nothing other than whitespace after the date.
    smartdateonly :: TextParser m SmartDate
    smartdateonly = do
      d <- smartdate
      skipMany spacenonewline
      eof
      return d

    datesepchars :: [Char]
    datesepchars = "/-."

    datesepchar :: TextParser m Char
    datesepchar = satisfy isDateSepChar

    isDateSepChar :: Char -> Bool
    isDateSepChar c = c == '/' || c == '-' || c == '.'

    validYear, validMonth, validDay :: String -> Bool
    validYear s = length s >= 4 && isJust (readMay s :: Maybe Year)
    validMonth s = maybe False (\n -> n>=1 && n<=12) $ readMay s
    validDay s = maybe False (\n -> n>=1 && n<=31) $ readMay s

    failIfInvalidYear, failIfInvalidMonth, failIfInvalidDay :: (Monad m) => String -> m ()
    failIfInvalidYear s  = unless (validYear s)  $ fail $ "bad year number: " ++ s
    failIfInvalidMonth s = unless (validMonth s) $ fail $ "bad month number: " ++ s
    failIfInvalidDay s   = unless (validDay s)   $ fail $ "bad day number: " ++ s

    yyyymmdd :: TextParser m SmartDate
    yyyymmdd = do
      y <- count 4 digitChar
      m <- count 2 digitChar
      failIfInvalidMonth m
      d <- count 2 digitChar
      failIfInvalidDay d
      return (y,m,d)

    yyyymm :: TextParser m SmartDate
    yyyymm = do
      y <- count 4 digitChar
      m <- count 2 digitChar
      failIfInvalidMonth m
      return (y,m,"01")

    ymd :: TextParser m SmartDate
    ymd = do
      y <- some digitChar
      failIfInvalidYear y
      sep <- datesepchar
      m <- some digitChar
      failIfInvalidMonth m
      char sep
      d <- some digitChar
      failIfInvalidDay d
      return $ (y,m,d)

    ym :: TextParser m SmartDate
    ym = do
      y <- some digitChar
      failIfInvalidYear y
      datesepchar
      m <- some digitChar
      failIfInvalidMonth m
      return (y,m,"")

    y :: TextParser m SmartDate
    y = do
      y <- some digitChar
      failIfInvalidYear y
      return (y,"","")

    d :: TextParser m SmartDate
    d = do
      d <- some digitChar
      failIfInvalidDay d
      return ("","",d)

    md :: TextParser m SmartDate
    md = do
      m <- some digitChar
      failIfInvalidMonth m
      datesepchar
      d <- some digitChar
      failIfInvalidDay d
      return ("",m,d)

    -- These are compared case insensitively, and should all be kept lower case.
    months         = ["january","february","march","april","may","june",
                      "july","august","september","october","november","december"]
    monthabbrevs   = ["jan","feb","mar","apr","may","jun","jul","aug","sep","oct","nov","dec"]
    weekdays       = ["monday","tuesday","wednesday","thursday","friday","saturday","sunday"]
    weekdayabbrevs = ["mon","tue","wed","thu","fri","sat","sun"]

    -- | Convert a case insensitive english month name to a month number.
    monthIndex name = maybe 0 (+1) $ T.toLower name `elemIndex` months

    -- | Convert a case insensitive english three-letter month abbreviation to a month number.
    monIndex   name = maybe 0 (+1) $ T.toLower name `elemIndex` monthabbrevs

    month :: TextParser m SmartDate
    month = do
      m <- choice $ map (try . string') months
      let i = monthIndex m
      return ("",show i,"")

    mon :: TextParser m SmartDate
    mon = do
      m <- choice $ map (try . string') monthabbrevs
      let i = monIndex m
      return ("",show i,"")

    weekday :: TextParser m Int
    weekday = do
      wday <- T.toLower <$> (choice . map string' $ weekdays ++ weekdayabbrevs)
      case catMaybes $ [wday `elemIndex` weekdays, wday `elemIndex` weekdayabbrevs] of
        (i:_) -> return (i+1)
        []    -> fail  $ "weekday: should not happen: attempted to find " <>
                          show wday <> " in " <> show (weekdays ++ weekdayabbrevs)

    today,yesterday,tomorrow :: TextParser m SmartDate
    today     = string' "today"     >> return ("","","today")
    yesterday = string' "yesterday" >> return ("","","yesterday")
    tomorrow  = string' "tomorrow"  >> return ("","","tomorrow")

    lastthisnextthing :: TextParser m SmartDate
    lastthisnextthing = do
      r <- choice $ map string' [
            "last"
           ,"this"
           ,"next"
          ]
      skipMany spacenonewline  -- make the space optional for easier scripting
      p <- choice $ map string' [
            "day"
           ,"week"
           ,"month"
           ,"quarter"
           ,"year"
          ]
    -- XXX support these in fixSmartDate
    --       ++ (map string' $ months ++ monthabbrevs ++ weekdays ++ weekdayabbrevs)

      return ("", T.unpack r, T.unpack p)

    -- |
    -- >>> let p = parsePeriodExpr (parsedate "2008/11/26")
    -- >>> p "from Aug to Oct"
    -- Right (NoInterval,DateSpan 2008/08/01-2008/09/30)
    -- >>> p "aug to oct"
    -- Right (NoInterval,DateSpan 2008/08/01-2008/09/30)
    -- >>> p "every 3 days in Aug"
    -- Right (Days 3,DateSpan 2008/08)
    -- >>> p "daily from aug"
    -- Right (Days 1,DateSpan 2008/08/01-)
    -- >>> p "every week to 2009"
    -- Right (Weeks 1,DateSpan -2008/12/31)
    -- >>> p "every 2nd day of month"
    -- Right (DayOfMonth 2,DateSpan -)
    -- >>> p "every 2nd day"
    -- Right (DayOfMonth 2,DateSpan -)
    -- >>> p "every 2nd day 2009-"
    -- Right (DayOfMonth 2,DateSpan 2009/01/01-)
    -- >>> p "every 29th Nov"
    -- Right (DayOfYear 11 29,DateSpan -)
    -- >>> p "every 29th nov -2009"
    -- Right (DayOfYear 11 29,DateSpan -2008/12/31)
    -- >>> p "every nov 29th"
    -- Right (DayOfYear 11 29,DateSpan -)
    -- >>> p "every Nov 29th 2009-"
    -- Right (DayOfYear 11 29,DateSpan 2009/01/01-)
    -- >>> p "every 11/29 from 2009"
    -- Right (DayOfYear 11 29,DateSpan 2009/01/01-)
    -- >>> p "every 2nd Thursday of month to 2009"
    -- Right (WeekdayOfMonth 2 4,DateSpan -2008/12/31)
    -- >>> p "every 1st monday of month to 2009"
    -- Right (WeekdayOfMonth 1 1,DateSpan -2008/12/31)
    -- >>> p "every tue"
    -- Right (DayOfWeek 2,DateSpan -)
    -- >>> p "every 2nd day of week"
    -- Right (DayOfWeek 2,DateSpan -)
    -- >>> p "every 2nd day of month"
    -- Right (DayOfMonth 2,DateSpan -)
    -- >>> p "every 2nd day"
    -- Right (DayOfMonth 2,DateSpan -)
    -- >>> p "every 2nd day 2009-"
    -- Right (DayOfMonth 2,DateSpan 2009/01/01-)
    -- >>> p "every 2nd day of month 2009-"
    -- Right (DayOfMonth 2,DateSpan 2009/01/01-)
    periodexprp :: Day -> TextParser m (Interval, DateSpan)
    periodexprp rdate = do
      skipMany spacenonewline
      choice $ map try [
                        intervalanddateperiodexprp rdate,
                        (,) NoInterval <$> periodexprdatespanp rdate
                       ]

    intervalanddateperiodexprp :: Day -> TextParser m (Interval, DateSpan)
    intervalanddateperiodexprp rdate = do
      i <- reportingintervalp
      s <- option def . try $ do
          skipMany spacenonewline
          periodexprdatespanp rdate
      return (i,s)

    -- Parse a reporting interval.
    reportingintervalp :: TextParser m Interval
    reportingintervalp = choice' [
                           tryinterval "day"     "daily"     Days,
                           tryinterval "week"    "weekly"    Weeks,
                           tryinterval "month"   "monthly"   Months,
                           tryinterval "quarter" "quarterly" Quarters,
                           tryinterval "year"    "yearly"    Years,
                           do string' "biweekly"
                              return $ Weeks 2,
                           do string' "bimonthly"
                              return $ Months 2,
                           do string' "every"
                              skipMany spacenonewline
                              n <- nth
                              skipMany spacenonewline
                              string' "day"
                              of_ "week"
                              return $ DayOfWeek n,
                           do string' "every"
                              skipMany spacenonewline
                              n <- weekday
                              return $ DayOfWeek n,
                           do string' "every"
                              skipMany spacenonewline
                              n <- nth
                              skipMany spacenonewline
                              string' "day"
                              optOf_ "month"
                              return $ DayOfMonth n,
                           do string' "every"
                              let mnth = choice' [month, mon] >>= \(_,m,_) -> return (read m)
                              d_o_y <- runPermutation $
                                DayOfYear <$> toPermutation (try (skipMany spacenonewline *> mnth))
                                          <*> toPermutation (try (skipMany spacenonewline *> nth))
                              optOf_ "year"
                              return d_o_y,
                           do string' "every"
                              skipMany spacenonewline
                              ("",m,d) <- md
                              optOf_ "year"
                              return $ DayOfYear (read m) (read d),
                           do string' "every"
                              skipMany spacenonewline
                              n <- nth
                              skipMany spacenonewline
                              wd <- weekday
                              optOf_ "month"
                              return $ WeekdayOfMonth n wd
                        ]
        where
          of_ period = do
            skipMany spacenonewline
            string' "of"
            skipMany spacenonewline
            string' period

          optOf_ period = optional $ try $ of_ period

          nth = do n <- some digitChar
                   choice' $ map string' ["st","nd","rd","th"]
                   return $ read n

          -- Parse any of several variants of a basic interval, eg "daily", "every day", "every N days".
          tryinterval :: String -> String -> (Int -> Interval) -> TextParser m Interval
          tryinterval singular compact intcons =
            choice' [
              do string' compact'
                 return $ intcons 1,
              do string' "every"
                 skipMany spacenonewline
                 string' singular'
                 return $ intcons 1,
              do string' "every"
                 skipMany spacenonewline
                 n <- fmap read $ some digitChar
                 skipMany spacenonewline
                 string' plural'
                 return $ intcons n
              ]
            where
              compact'  = T.pack compact
              singular' = T.pack singular
              plural'   = T.pack $ singular ++ "s"

    periodexprdatespanp :: Day -> TextParser m DateSpan
    periodexprdatespanp rdate = choice $ map try [
                                doubledatespanp rdate,
                                fromdatespanp rdate,
                                todatespanp rdate,
                                justdatespanp rdate
                               ]

    -- |
    -- -- >>> parsewith (doubledatespan (parsedate "2018/01/01") <* eof) "20180101-201804"
    -- Right DateSpan 2018/01/01-2018/04/01
    doubledatespanp :: Day -> TextParser m DateSpan
    doubledatespanp rdate = do
      optional (string' "from" >> skipMany spacenonewline)
      b <- smartdate
      skipMany spacenonewline
      optional (choice [string' "to", string' "-"] >> skipMany spacenonewline)
      e <- smartdate
      return $ DateSpan (Just $ fixSmartDate rdate b) (Just $ fixSmartDate rdate e)

    fromdatespanp :: Day -> TextParser m DateSpan
    fromdatespanp rdate = do
      b <- choice [
        do
          string' "from" >> skipMany spacenonewline
          smartdate
        ,
        do
          d <- smartdate
          string' "-"
          return d
        ]
      return $ DateSpan (Just $ fixSmartDate rdate b) Nothing

    todatespanp :: Day -> TextParser m DateSpan
    todatespanp rdate = do
      choice [string' "to", string' "-"] >> skipMany spacenonewline
      e <- smartdate
      return $ DateSpan Nothing (Just $ fixSmartDate rdate e)

    justdatespanp :: Day -> TextParser m DateSpan
    justdatespanp rdate = do
      optional (string' "in" >> skipMany spacenonewline)
      d <- smartdate
      return $ spanFromSmartDate rdate d

    -- | Make a datespan from two valid date strings parseable by parsedate
    -- (or raise an error). Eg: mkdatespan \"2011/1/1\" \"2011/12/31\".
    mkdatespan :: String -> String -> DateSpan
    mkdatespan b = DateSpan (Just $ parsedate b) . Just . parsedate

    nulldatespan :: DateSpan
    nulldatespan = DateSpan Nothing Nothing

    -- | A datespan of zero length, that matches no date.
    emptydatespan :: DateSpan
    emptydatespan = DateSpan (Just $ addDays 1 nulldate) (Just nulldate)

    nulldate :: Day
    nulldate = fromGregorian 0 1 1