added date.lua (from github) and date from isoweek

.luacheckrc

@@ -23,4 +23,5 @@ globals = {
 -- Global objects defined by the C code
 read_globals = {
   "vim",
+  "os",
 }

lua/taglinks/dateutils.lua

@@ -0,0 +1,115 @@
+local M = {}
+local date = require("taglinks.date")
+
+--- returns the day of week (1..Monday, ..., 7..Sunday) for Dec, 31st of year
+--- see https://webspace.science.uu.nl/~gent0113/calendar/isocalendar.htm
+--- see https://en.wikipedia.org/wiki/ISO_week_date
+M.dow_for_year = function(year)
+    return (
+            year
+            + math.floor(year / 4)
+            - math.floor(year / 100)
+            + math.floor(year / 400)
+        ) % 7
+end
+
+M.weeks_in_year = function(year)
+    local d = 0
+    local dy = M.dow_for_year(year) == 4 -- current year ends Thursday
+    local dyy = M.dow_for_year(year - 1) == 3 -- previous year ends Wednesday
+    if dy or dyy then
+        d = 1
+    end
+    return 52 + d
+end
+
+M.days_in_year = function(year)
+    local t = os.time({ year = year, month = 12, day = 31 })
+    return os.date("*t", t).yday
+end
+
+M.date_from_doy = function(year, doy)
+    local ret = {
+        year = year,
+        month = 1,
+        day = doy,
+    }
+    -- january is clear immediately
+    if doy < 32 then
+        return ret
+    end
+
+    local dmap = {
+        [1] = 31,
+        [2] = 28, -- will be fixed further down
+        [3] = 31,
+        [4] = 30,
+        [5] = 31,
+        [6] = 30,
+        [7] = 31,
+        [8] = 31,
+        [9] = 30,
+        [10] = 31,
+        [11] = 30,
+        [12] = 31,
+    }
+    if M.days_in_year(year) == 366 then
+        dmap[2] = 29
+    end
+
+    for month, d in pairs(dmap) do
+        doy = doy - d
+        if doy < 0 then
+            ret.day = doy + d
+            ret.month = month
+            return ret
+        end
+    end
+    return ret -- unreachable if input values are sane
+end
+
+M.isoweek_to_date = function(year, isoweek)
+    local ret = date(year .. "-W" .. string.format("%02d", isoweek) .. "-1")
+    return {
+        year = ret:getyear(),
+        month = ret:getmonth(),
+        day = ret:getday(),
+    }
+end
+
+local function check_isoweek(year, isoweek, ydate)
+    print("***********   KW " .. isoweek .. " " .. year .. ": ")
+    -- local ret = M.weeknumber_to_date(year, isoweek)
+    local ret = M.isoweek_to_date(year, isoweek)
+    local result = ret.year == ydate.year
+        and ret.month == ydate.month
+        and ret.day == ydate.day
+    print(
+        ret.year
+            .. "-"
+            .. ret.month
+            .. "-"
+            .. ret.day
+            .. " == "
+            .. ydate.year
+            .. "-"
+            .. ydate.month
+            .. "-"
+            .. ydate.day
+            .. " : "
+            .. tostring(result)
+    )
+end
+
+M.run_tests = function()
+    print(check_isoweek(2020, 1, { year = 2019, month = 12, day = 30 })) -- 30.12.2019
+    print(check_isoweek(2020, 52, { year = 2020, month = 12, day = 21 })) -- 21.12.2020
+    print(check_isoweek(2020, 53, { year = 2020, month = 12, day = 28 })) -- 28.12.2020
+    print(check_isoweek(2021, 1, { year = 2021, month = 1, day = 4 })) -- 4.1.2020
+    print(check_isoweek(2021, 52, { year = 2021, month = 12, day = 27 })) -- 27.12.2021
+    print(check_isoweek(2022, 1, { year = 2022, month = 1, day = 3 })) -- 3.1.2022
+end
+
+-- M.run_tests()
+
+return M