luatz/timetable: Fix incorrect math for negative carry operations

Closes #10

diff --git a/luatz/timetable.lua b/luatz/timetable.lua
index e2930d8dcc287cdaef8aeb92aab6272cffc5c52e..0dbf4c8eedb5d6fdce95d0f80187ef08e1363de0 100644 (file)
--- 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 ddc05c6d6fb5825cc81e02b92fecc66aa34d30aa..930d36b32b94f398f260580352ee980a6e4497d8 100644 (file)
--- 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;