From 6c4658848c01ae0862a2fcad7f6565efd9aa948d Mon Sep 17 00:00:00 2001
From: daurnimator <quae@daurnimator.com>
Date: Fri, 29 Aug 2014 16:03:47 -0400
Subject: [PATCH] luatz/timetable: Add support to `normalise` for fractional
 columns

---
 luatz/timetable.lua     | 45 +++++++++++++++++++++++++++++++----------
 spec/timetable_spec.lua | 20 ++++++++++++++++++
 2 files changed, 54 insertions(+), 11 deletions(-)

diff --git a/luatz/timetable.lua b/luatz/timetable.lua
index 879f0a2..3e80242 100644
--- 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
 
diff --git a/spec/timetable_spec.lua b/spec/timetable_spec.lua
index 34c0529..569864f 100644
--- a/spec/timetable_spec.lua
+++ b/spec/timetable_spec.lua
@@ -84,4 +84,24 @@ describe ( "Timetable library" , function ( )
 		assert.same ( { timetable.normalise(2013,42,52,0,0,0) } , { 2016,7,22,0,0,0 } )
 		assert.same ( { timetable.normalise(2013,42,52,50,0,0) } , { 2016,7,24,2,0,0 } )
 	end )
+
+	it ( "#normalise handles fractional #month" , function ( )
+		assert.same ( { timetable.normalise(2014,14.5,1,0,0,0) } , { 2015,2,15,0,0,0 } )
+		assert.same ( { timetable.normalise(2015,14.5,1,0,0,0) } , { 2016,2,15,12,0,0 } ) -- leap year, so hours is 12
+		assert.same ( { timetable.normalise(2016,14.5,1,0,0,0) } , { 2017,2,15,0,0,0 } )
+	end )
+
+	local function round_trip_add(t, field, x)
+		local before = t:clone()
+		t[field]=t[field]+x;
+		t:normalise();
+		t[field]=t[field]-x;
+		t:normalise();
+		assert.same(0, t-before)
+	end
+	it ( "#normalise round trips" , function ( )
+		round_trip_add(timetable.new(2000,2,28,0,0,0), "month", 0.5)
+		round_trip_add(timetable.new(2014,8,28,19,23,0), "month", 0.4)
+		round_trip_add(timetable.new(2014,14.5,28,0,0,0), "month", 0.4)
+	end )
 end )
-- 
2.39.2