strstr() lua implementation

strstr.lua

-- works like c strstr()
-- retorn index if found pattern =)

function strstr(haystack, needle)
	if (not haystack) then return nil; end
	if (not needle) then return nil; end

    local h, n = #haystack, #needle;

    for i = 1, h - n + 1 do
        if haystack:sub(i, i + n - 1) == needle then
            return i;
        end
    end

    return nil;
end