doc/timetable: Document `normalise` fractional behaviour

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

diff --git a/luatz/timetable.lua b/luatz/timetable.lua
index e1649a0239517c4c15bb31ca3b3191216110be1e..fc89fcaec5c7271d6756ed4c3ff0429e01ff7280 100644 (file)
--- a/luatz/timetable.lua
+++ b/luatz/timetable.lua
@@ -1,3 +1,4 @@
+local strftime = require "luatz.strftime".strftime
 local strformat = string.format
 local floor = math.floor
 local function idiv ( n , d )
@@ -15,7 +16,13 @@ end
 local sakamoto = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
 
 local function is_leap ( y )
-       return (y % 4) == 0 and (y % 100) ~= 0 or (y % 400) == 0
+       if (y % 4) ~= 0 then
+               return false
+       elseif (y % 100) ~= 0 then
+               return true
+       else
+               return (y % 400) == 0
+       end
 end
 
 local function year_length ( y )
@@ -49,7 +56,14 @@ local function day_of_week ( day , month , year )
        return ( year + leap_years_since ( year ) + sakamoto[month] + day ) % 7 + 1
 end
 
-local function increment ( tens , units , base )
+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 )
                units = units % base
@@ -62,32 +76,50 @@ end
 
 -- Modify parameters so they all fit within the "normal" range
 local function normalise ( year , month , day , hour , min , sec )
-       min  , sec  = increment ( min  , sec  , 60 ) -- TODO: consider leap seconds?
-       hour , min  = increment ( hour , min  , 60 )
-       day  , hour = increment ( day  , hour , 24 )
-
-       while day <= 0 do
+       -- `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 )
+       -- 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
+       year , month = carry ( year , month , 12 )
 
-       -- Lua months start from 1, need -1 and +1 around this increment
-       month = month - 1
-       year , month = increment ( 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
 
@@ -143,6 +175,10 @@ function timetable_methods:rfc_3339 ( )
        return strformat ( "%04u-%02u-%02uT%02u:%02u:%06.3f" , self:unpack ( ) )
 end
 
+function timetable_methods:strftime ( format_string )
+       return strftime ( format_string , self )
+end
+
 local timetable_mt
 
 local function coerce_arg ( t )
@@ -156,10 +192,13 @@ timetable_mt = {
        __index    = timetable_methods ;
        __tostring = timetable_methods.rfc_3339 ;
        __eq = function ( a , b )
-               return coerce_arg ( a ) == coerce_arg ( b )
+               return a:timestamp ( ) == b:timestamp ( )
        end ;
        __lt = function ( a , b )
-               return coerce_arg ( a ) < coerce_arg ( b )
+               return a:timestamp ( ) < b:timestamp ( )
+       end ;
+       __sub = function ( a , b )
+               return coerce_arg ( a ) - coerce_arg ( b )
        end ;
 }
 
@@ -185,10 +224,14 @@ function timetable_methods:clone ( )
 end
 
 local function new_from_timestamp ( ts )
-       return new_timetable ( 1970 , 1 , 1 , 0 , 0 , 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 ):normalise ( )
 end
 
 return {
+       is_leap = is_leap ;
        day_of_year = day_of_year ;
        day_of_week = day_of_week ;
        normalise = normalise ;