From cdaad9ec35371fcfc3b398b7302f1e8fe313b4f6 Mon Sep 17 00:00:00 2001
From: daurnimator <quae@daurnimator.com>
Date: Wed, 6 Dec 2017 17:12:47 +1100
Subject: [PATCH] luatz/timetable.lua: Fix incorrect normalisation logic for
 negative day field

Fixes #13
---
 NEWS                    |  2 +-
 luatz/timetable.lua     |  8 ++++++--
 spec/timetable_spec.lua | 10 ++++++++--
 3 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/NEWS b/NEWS
index d49c342..b4cc02d 100644
--- a/NEWS
+++ b/NEWS
@@ -1,6 +1,6 @@
 UNRELEASED
 
-  - Fix timetable normalisation carry bug (#10)
+  - Fix timetable normalisation carry bugs (#10, #13)
   - Clean up of docs
   - No longer throw errors in parse module on error (now return nil, err)
   - Support version 3 tzfiles
diff --git a/luatz/timetable.lua b/luatz/timetable.lua
index 0dbf4c8..6bae913 100644
--- a/luatz/timetable.lua
+++ b/luatz/timetable.lua
@@ -107,8 +107,12 @@ local function normalise ( year , month , day , hour , min , sec )
 	-- 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 )
+		month = month - 1
+		if month < 0 then
+			year = year - 1
+			month = 11
+		end
+		day = day + month_length ( month + 1 , year )
 	end
 	year , month = carry ( year , month , 12 )
 
diff --git a/spec/timetable_spec.lua b/spec/timetable_spec.lua
index 930d36b..6c9617e 100644
--- a/spec/timetable_spec.lua
+++ b/spec/timetable_spec.lua
@@ -77,8 +77,8 @@ describe ( "Timetable library" , function ( )
 		assert.same({ 2012,3,23,0,0,0 }, { timetable.normalise(2012,2,52,0,0,0) })
 		assert.same({ 2013,3,24,0,0,0 }, { timetable.normalise(2013,2,52,0,0,0) })
 
-		assert.same({ 2012,2,26,0,0,0 }, { timetable.normalise(2012,3,-2,0,0,0) })
-		assert.same({ 2013,2,27,0,0,0 }, { timetable.normalise(2013,3,-2,0,0,0) })
+		assert.same({ 2012,2,27,0,0,0 }, { timetable.normalise(2012,3,-2,0,0,0) })
+		assert.same({ 2013,2,26,0,0,0 }, { timetable.normalise(2013,3,-2,0,0,0) })
 
 		-- Also when more fields are out of range
 		assert.same({ 2016,7,22,0,0,0 }, { timetable.normalise(2013,42,52,0,0,0) })
@@ -98,6 +98,12 @@ describe ( "Timetable library" , function ( )
 		assert.same({ 2017,02,3,0,0,0 }, { timetable.normalise(2017,02,13,0,-14400,0) })
 	end )
 
+	it ( "#normalise handles negative day carry (issue #13)", function()
+		assert.same({ 2016,11,30,00,00,00 }, { timetable.normalise(2016,12,0,0,0,0) })
+		assert.same({ 2017,11,30,00,00,00 }, { timetable.normalise(2017,12,0,0,0,0) })
+		assert.same({ 2018,11,30,00,00,00 }, { timetable.normalise(2018,12,0,0,0,0) })
+	end )
+
 	local function round_trip_add(t, field, x)
 		local before = t:clone()
 		t[field]=t[field]+x;
-- 
2.39.2