From 820f9eddfe36813309cf4dc7893bdac2a16bc26e Mon Sep 17 00:00:00 2001
From: daurnimator <quae@daurnimator.com>
Date: Fri, 27 Jan 2017 11:46:24 +1100
Subject: [PATCH] luatz/timetable: Fix incorrect math for negative carry
 operations

Closes #10
---
 luatz/timetable.lua     | 4 ++--
 spec/timetable_spec.lua | 7 +++++++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/luatz/timetable.lua b/luatz/timetable.lua
index e2930d8..0dbf4c8 100644
--- a/luatz/timetable.lua
+++ b/luatz/timetable.lua
@@ -76,8 +76,8 @@ local function carry ( tens , units , base )
 		tens  = tens + idiv ( units , base )
 		units = units % base
 	elseif units < 0 then
-		tens  = tens - 1 + idiv ( -units , base )
-		units = base - ( -units % base )
+		tens  = tens + idiv ( units , base )
+		units = ( base + units ) % base
 	end
 	return tens , units
 end
diff --git a/spec/timetable_spec.lua b/spec/timetable_spec.lua
index ddc05c6..930d36b 100644
--- a/spec/timetable_spec.lua
+++ b/spec/timetable_spec.lua
@@ -91,6 +91,13 @@ describe ( "Timetable library" , function ( )
 		assert.same({ 2017,2,15,0,0,0 } , { timetable.normalise(2016,14.5,1,0,0,0) })
 	end )
 
+	it ( "#normalise handles negative carry (issue #10)", function()
+		assert.same({ 1970,01,01,00,59,00 }, { timetable.normalise(1970,01,01,01,00,-60) })
+		assert.same({ 1970,01,01,00,58,58 }, { timetable.normalise(1970,01,01,01,00,-62) })
+		assert.same({ 1969,12,31,23,55,58 }, { timetable.normalise(1970,01,01,01,-63,-62) })
+		assert.same({ 2017,02,3,0,0,0 }, { timetable.normalise(2017,02,13,0,-14400,0) })
+	end )
+
 	local function round_trip_add(t, field, x)
 		local before = t:clone()
 		t[field]=t[field]+x;
-- 
2.39.2