RosettaCodeData/Task/Prime-decomposition/Eiffel/prime-decomposition.e

class
	PRIME_DECOMPOSITION

feature

	factor (p: INTEGER): ARRAY [INTEGER]
			-- Prime decomposition of 'p'.
		require
			p_positive: p > 0
		local
			div, i, next, rest: INTEGER
		do
			create Result.make_empty
			if p = 1 then
				Result.force (1, 1)
			end
			div := 2
			next := 3
			rest := p
			from
				i := 1
			until
				rest = 1
			loop
				from
				until
					rest \\ div /= 0
				loop
					Result.force (div, i)
					rest := (rest / div).floor
					i := i + 1
				end
				div := next
				next := next + 2
			end
		ensure
			is_divisor: across Result as r all p \\ r.item = 0 end
			is_prime: across Result as r all prime (r.item) end
		end