doc/timetable: Document `normalise` fractional behaviour

[etc/awesome.git] / luatz / timetable.lua

diff --git a/luatz/timetable.lua b/luatz/timetable.lua
index 879f0a226fe6839b8c5424112d0945a91899063c..fc89fcaec5c7271d6756ed4c3ff0429e01ff7280 100644 (file)
--- a/luatz/timetable.lua
+++ b/luatz/timetable.lua
@@ -56,6 +56,13 @@ local function day_of_week ( day , month , year )
        return ( year + leap_years_since ( year ) + sakamoto[month] + day ) % 7 + 1
 end
 
+local function borrow ( tens , units , base )
+       local frac = tens % 1
+       units = units + frac * base
+       tens = tens - frac
+       return tens , units
+end
+
 local function carry ( tens , units , base )
        if units >= base then
                tens  = tens + idiv ( units , base )
@@ -69,34 +76,50 @@ end
 
 -- Modify parameters so they all fit within the "normal" range
 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
+
+       -- Convert everything (except seconds) to an integer
+       -- by propagating fractional components down.
+       year  , month = borrow ( year  , month , 12 )
+       -- Carry from month to year first, so we get month length correct in next line around leap years
+       year  , month = carry ( year , month , 12 )
+       month , day   = borrow ( month , day   , month_length ( floor ( month + 1 ) , year ) )
+       day   , hour  = borrow ( day   , hour  , 24 )
+       hour  , min   = borrow ( hour  , min   , 60 )
+       min   , sec   = borrow ( min   , sec   , 60 )
+
        -- Propagate out of range values up
        -- e.g. if `min` is 70, `hour` increments by 1 and `min` becomes 10
+       -- This has to happen for all columns after borrowing, as lower radixes may be pushed out of range
        min   , sec   = carry ( min   , sec   , 60 ) -- TODO: consider leap seconds?
        hour  , min   = carry ( hour  , min   , 60 )
        day   , hour  = carry ( day   , hour  , 24 )
-
-       while day <= 0 do
+       -- Ensure `day` is not underflowed
+       -- Add a whole year of days at a time, this is later resolved by adding months
+       -- TODO[OPTIMIZE]: This could be slow if `day` is far out of range
+       while day < 0 do
                year = year - 1
                day  = day + year_length ( year )
        end
-
-       -- Lua months start from 1, need -1 and +1 around this increment
-       month = month - 1
        year , month = carry ( year , month , 12 )
-       month = month + 1
 
-       -- This could potentially be slow if `day` is very large
+       -- TODO[OPTIMIZE]: This could potentially be slow if `day` is very large
        while true do
-               local i = month_length ( month , year )
-               if day <= i then break end
+               local i = month_length ( month + 1 , year )
+               if day < i then break end
                day = day - i
                month = month + 1
-               if month > 12 then
-                       month = 1
+               if month >= 12 then
+                       month = 0
                        year = year + 1
                end
        end
 
+       -- Now we can place `day` and `month` back in their normal ranges
+       -- e.g. month as 1-12 instead of 0-11
+       month , day = month + 1 , day + 1
+
        return year , month , day , hour , min , sec
 end
 
@@ -204,7 +227,7 @@ local function new_from_timestamp ( ts )
        if type ( ts ) ~= "number" then
                error ( "bad argument #1 to 'new_from_timestamp' (number expected, got " .. type ( ts ) .. ")" , 2 )
        end
-       return new_timetable ( 1970 , 1 , 1 , 0 , 0 , ts )
+       return new_timetable ( 1970 , 1 , 1 , 0 , 0 , ts ):normalise ( )
 end
 
 return {