-- Postfix notation is an alternative to the normal "infix" notation
-- used in mathematical expressions.  Instead of writing the operator
-- between the operands, the operator comes after them.  For example,
-- "1 + 2" is notated as "1 2 +".  This removes the need for operator
-- precedence and parentheses: rather than having to resolve the
-- ambiguity of an expression like "1 + 2 * 3" with convention or
-- parentheses, the evaluation order is explicit -- multiplication-
-- first is "1 2 3 * +" and addition-first is "1 2 + 3 *".
--
-- As well as this notational convenience, postfix can be easily
-- evaluated using a stack.  Whenever an operand is encountered, the
-- value is pushed onto the stack, and whenever an operator is
-- encountered, two values are popped from the stack and the result of
-- the operation is pushed on in their place.  Finally, the value left
-- on the top of the stack is the result of the operation.
--
-- Write a function that will evaluate a postfix expression and return
-- the result.  You can assume that the expression is valid, and not
-- worry about handling error cases.  The operators will be "+", "-",
-- "*" and "/".
--
-- Solution --------------------------------------------------------------------
function evaluate_postfix(expression)
	-- Your implementation here
end

-- Tests -----------------------------------------------------------------------

local luaunit = require("luaunit.luaunit")

function test_1_2_plus_3_times_is_9()
	luaunit.assertEquals(evaluate_postfix("1 2 + 3 *"), 9)
end

function test_1_2_3_times_plus_is_7()
	luaunit.assertEquals(evaluate_postfix("1 2 3 * +"), 7)
end

function test_4_6_plus_5_divide_2_1_minus_minus_is_1()
	luaunit.assertEquals(evaluate_postfix("4 6 + 5 / 2 1 - -"), 1)
end

function test_12_2_times_6_divide_10_times_2_plus_is_42()
	luaunit.assertEquals(evaluate_postfix("12 2 * 6 / 10 * 2 +"), 42)
end

os.exit(luaunit.LuaUnit.run())