extra timestamp functions

utils/timestamp.lua

@@ -4,6 +4,7 @@
 local Public = {}
 
 local floor = math.floor
+local strformat = string.format
 
 local function borrow(tens, units, base)
     local frac = tens % 1
@@ -35,6 +36,12 @@ end
 
 local mon_lengths = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
 
+-- Number of days in year until start of month; not corrected for leap years
+local months_to_days_cumulative = {0}
+for i = 2, 12 do
+    months_to_days_cumulative[i] = months_to_days_cumulative[i - 1] + mon_lengths[i - 1]
+end
+
 local function month_length(m, y)
     if m == 2 then
         return is_leap(y) and 29 or 28
@@ -43,12 +50,21 @@ local function month_length(m, y)
     end
 end
 
-local function normalise(sec)
-    local year = 1970
-    local month = 1
-    local day = 1
-    local hour = 0
-    local min = 0
+local function day_of_year(day, month, year)
+    local yday = months_to_days_cumulative[month]
+    if month > 2 and is_leap(year) then
+        yday = yday + 1
+    end
+    return yday + day
+end
+
+local function leap_years_since(year)
+    return floor(year / 4) - floor(year / 100) + floor(year / 400)
+end
+
+local leap_years_since_1970 = leap_years_since(1970)
+
+local function normalise(year, month, day, hour, min, sec)
     -- `month` and `day` start from 1, need -1 and +1 so it works modulo
     month, day = month - 1, day - 1
 
@@ -105,14 +121,32 @@ end
 --- Converts unix epoch timestamp into table {year: number, month: number, day: number, hour: number, min: number, sec: number}
 -- @param  sec<number> unix epoch timestamp
 -- @return {year: number, month: number, day: number, hour: number, min: number, sec: number}
-Public.to_timetable = normalise
+function Public.to_timetable(secs)
+    return normalise(1970, 1, 1, 0, 0, secs)
+end
+
+--- Converts timetable into unix epoch timestamp
+-- @param  timetable<table> {year: number, month: number, day: number, hour: number, min: number, sec: number}
+-- @return number
+function Public.from_timetable(timetable)
+    local tt = normalise(timetable.year, timetable.month, timetable.day, timetable.hour, timetable.min, timetable.sec)
+
+    local year, month, day, hour, min, sec = tt.year, tt.month, tt.day, tt.hour, tt.min, tt.sec
+
+    local days_since_epoch =
+        day_of_year(day, month, year) + 365 * (year - 1970) + -- Each leap year adds one day
+        (leap_years_since(year - 1) - leap_years_since_1970) -
+        1
+
+    return days_since_epoch * (60 * 60 * 24) + hour * (60 * 60) + min * 60 + sec
+end
 
 --- Converts unix epoch timestamp into human readable string.
 -- @param  secs<type> unix epoch timestamp
 -- @return string
 function Public.to_string(secs)
     local tt = normalise(secs)
-    return table.concat({tt.year, '-', tt.month, '-', tt.day, ' ', tt.hour, ':', tt.min, ':', tt.sec})
+    return strformat('%04u-%02u-%02u %02u:%02u:%02d', tt.year, tt.month, tt.day, tt.hour, tt.min, tt.sec)
 end
 
 return Public