create reduce_right method

Author: EvandroLG

array.lua

@@ -224,6 +224,29 @@ array = {
     return _memo
   end,
 
+  -- This function is like reduce except that it interates over table's elements from right to left
+  -- @obj {table}
+  -- @callback {function}
+  -- @memo {*}
+  -- @returns {*}
+  reduce_right = function(obj, callback, memo)
+    raises_error(array, obj, 'reduce_right')
+
+    local initialIndex = #obj
+    local _memo = memo
+
+    if _memo == nil then
+      initialIndex = initialIndex - 1
+      _memo = obj[#obj]
+    end
+
+    for i=initialIndex, 1, -1 do
+      _memo = callback(_memo, obj[i], i)
+    end
+
+    return _memo
+  end,
+
   -- Return the sum of the values in table
   -- @obj {table}
   -- @callback {function}

test.lua

@@ -146,6 +146,14 @@ test('reduce should concatenate all items', function(a)
   a.equal(result, 'abcde')
 end)
 
+test('reduce_right should concatenate all items starting from right to left', function(a)
+  local result = array.reduce_right({ 'a', 'b', 'c', 'd', 'e' }, function(memo, value)
+    return memo .. value
+  end)
+
+  a.equal(result, 'edcba')
+end)
+
 test('sum should return sum of the values in table', function(a)
   local result = array.sum({10, 20, 30, 40, 50})
   a.equal(result, 150)