def reverse_string(s)
  ret = ''
  for i in s.length-1 .. -1 ## -1 so ends at 0.
    ret += s[i]
  end
  return ret  
end

def is_palindrome(s)
  return reverse_string(s) == s
end

assert(reverse_string('abc') == 'cba')
assert(reverse_string('123456') == '654321')
assert(is_palindrome('racecar'))
assert(!is_palindrome('foobar'))


## An un-efficient function to check substrings.
def string_find(s, sub)

  ## Edge case.
  if sub.length == 0 then return 0 end
  if sub.length > s.length then return -1 end
  if s.length == 0 then
    if sub.length == 0 then return 0 end
    return -1
  end
  
  for i in 0..s.length
    if s[i] == sub[0]
      found = true
      for j in 1..sub.length
        if s[i+j] != sub[j]
          found = false
        end
      end
      if found then return i end
    end
  end
  
  return -1 ## Not found
end

assert(string_find("something", "thing") == 4)
assert(string_find("a long string", "string") == 7)