From acd9c66c663ea6b0b2fb9dd0a563897c2fae45eb Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom@tomsmeding.com>
Date: Fri, 3 Apr 2026 22:10:56 +0200
Subject: Calendar view

---
 src/Calendar.hs | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 src/Calendar.hs

(limited to 'src/Calendar.hs')

diff --git a/src/Calendar.hs b/src/Calendar.hs
new file mode 100644
index 0000000..337e4ea
--- /dev/null
+++ b/src/Calendar.hs
@@ -0,0 +1,69 @@
+module Calendar where
+
+import Control.Monad (forM)
+import Control.Monad.Trans.State.Strict (evalState, state)
+import Data.Maybe (isNothing)
+import Data.Time
+
+import Util
+
+
+-- | The list of @a@ values must be as long as the number of days between the
+-- two bounds, which are inclusive. Values are returned with the appropriate
+-- day in the table.
+--
+-- * Returns a list of years with:
+--   * a list of rows, with in each row
+--     * a list of months (empty spacer if Nothing), with inside each month
+--       * a list of weeks (rows), with in each week
+--         * a list of dates in the week (empty spacer if Nothing).
+--
+-- >>> calendarLayout (fromGregorian 2025 12 16) (fromGregorian 2026 2 3) (cycle ['A'..'Z'])
+calendarLayout :: Day -> Day -> [a] -> [(Year, [[Maybe (MonthOfYear, [[Maybe (DayOfMonth, a)]])]])]
+calendarLayout startDay endDay values =
+  let (startYear, startMonth, startDate) = toGregorian startDay
+      (endYear, endMonth, endDate) = toGregorian endDay
+      takeValue = state $ \case val:vals -> (val, vals)
+                                [] -> error "calendarLayout: too few values"
+  in flip evalState values $
+       forM [startYear .. endYear] $ \year -> do
+         let row1 | year == startYear = (startMonth - 1) `div` 4
+                  | otherwise = 0
+             row2 | year == endYear = (endMonth - 1) `div` 4
+                  | otherwise = 2
+         fmap (year,) . forM [row1..row2] $ \monRowIdx -> do
+           let mon1 | year == startYear && monRowIdx == row1 = startMonth
+                    | otherwise = 4 * monRowIdx + 1
+               mon2 | year == endYear && monRowIdx == row2 = endMonth
+                    | otherwise = 4 * monRowIdx + 4
+           forM [4 * monRowIdx + 1 .. 4 * monRowIdx + 4] $ \month ->
+             if month < mon1 || month > mon2 then return Nothing else fmap Just $ do
+               let cal = monthCalendar year month
+                   filterCal predicate = map (map (\md -> if maybe False predicate md then md else Nothing))
+                   cal' | year == startYear && monRowIdx == row1 && month == mon1 = filterCal (>= startDate) cal
+                        | year == endYear   && monRowIdx == row2 && month == mon2 = filterCal (<= endDate)   cal
+                        | otherwise = cal
+                   cal'' = dropWhileEnd null $ map (\week -> if all isNothing week then [] else week) cal'
+               fmap (month,) . forM cal'' $ \week ->
+                 forM week $ \mdate ->
+                   traverse (\d -> (d,) <$> takeValue) mdate
+
+-- >>> import Text.Printf
+-- >>> error $ unlines . map (unwords . map (maybe "  " (printf "%2d"))) $ monthCalendar 2026 4
+--        1  2  3  4  5
+--  6  7  8  9 10 11 12
+-- 13 14 15 16 17 18 19
+-- 20 21 22 23 24 25 26
+-- 27 28 29 30
+monthCalendar :: Year -> MonthOfYear -> [[Maybe DayOfMonth]]
+monthCalendar year month
+  | [1, 7] <- map fromEnum [Monday, Sunday] =
+      let wkday1 = dayOfWeek (fromGregorian year month 1)
+          nskipsRow1 = fromEnum wkday1 - 1
+          lastDate = gregorianMonthLength year month
+          genRows date =
+            map Just [date .. min (date + 6) lastDate] : (if lastDate <= date + 6 then [] else genRows (date + 7))
+      in (replicate nskipsRow1 Nothing ++ map Just [1 .. 7 - nskipsRow1])
+         : genRows (7 - nskipsRow1 + 1)
+
+  | otherwise = error "DayOfWeek enum is unexpected"
-- 
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