tilldayofweek

function tilldayofweek( greg, target, dir ) --> days

Description

Calculates the days from a greg till a defined target weekday.

Parameters

greg

A table containing the Gregorian date values year, month and day.

  • The year as a number.

  • The month as a number from 1(January) to 12(December).

  • The day as a number from 1 to 31

target

The weekday as integer ranging from 1(Monday) to 7(Sunday).

dir

Indicates the direction(forward/backward) and distance of target weekday.

  • dir < 0 means that we are looking backward for the previous target weekday

  • dir > 0 means that we are looking forward for the next target weekday

  • |dir| defines the distance we are looking forward or backward

Return Values

days

The number of days till the target weekday

Code

--ZFUNC-tilldayofweek-v1
local function tilldayofweek( greg, target, dir ) --> days

   --ZFUNC-dayofweek-v1
   local dayofweek = function( year, month, day )
      local century = function( year ) return math.floor( year / 100 ) end

      --ZFUNC-isleapyear-v1
      local isleapyear = function( year )
         if( year % 4 == 0 and year % 100 ~= 0 ) or ( year % 400 == 0 ) then
            return true
         else
            return false
         end
      end

      local month_number_for_month = { 0, 3, 3, 6, 1, 4, 6, 2, 5, 0, 3, 5 }

      local day_num = day % 7
      local month_num = month_number_for_month[ month ]

      local y = year - century( year ) * 100
      local year_num = ( y + math.floor( y / 4 ) ) % 7

      local century_num = ( 3 - ( century( year ) % 4 ) ) * 2

      local leap_num = 0
      if ( isleapyear( year ) and ( month < 3 ) ) then leap_num = 6 end

      local res = day_num + month_num + year_num + century_num + leap_num - 1
      return ( res % 7 ) + 1
   end

   local last_dayofweek = function( current, target )
      if target < current then
         return target - current
      else
         return -7 + ( target - current )
      end
   end

   local next_dayofweek = function( current, target )
      if target > current then
         return target - current
      else
         return 7 + ( target - current )
      end
   end

   local dow = dayofweek( greg.year, greg.month, greg.day )
   if dir < 0 then
      local diff = last_dayofweek( dow, target )
      return 7 * ( dir + 1 ) + diff
   else
      local diff = next_dayofweek( dow, target )
      return 7 * ( dir - 1 ) + diff
   end
end

return tilldayofweek

Examples

local t = require( "taptest" )
local tilldayofweek = require( "tilldayofweek" )

local d = { year = 2014, month = 3, day = 22 } -- saturday

t( tilldayofweek( d, 7, 1 ), 1 ) -- next sunday
t( tilldayofweek( d, 3, 1 ), 4 ) -- next wednesday
t( tilldayofweek( d, 6, 1 ), 7 ) -- next saturday

-- next next wednesday
t( tilldayofweek( d, 3, 2 ), 11 )


t( tilldayofweek( d, 6, -1 ), -7 ) -- last saturday
t( tilldayofweek( d, 1, -1 ), -5 ) -- last monday
t( tilldayofweek( d, 5, -1 ), -1 ) -- last friday

-- last last last monday
t( -19, tilldayofweek( d, 1, -3 ) )

t()

See also