input: Write a function to compress a given string by replacing all repeating characters with a single instance and its count. If the given string is empty then return an empty string.
  
  def compress(s):
      if not s:
          return ""
      
      output = ""
      count = 1
      
      for i in range(len(s) - 1):
          if s[i] == s[i + 1]:
              count += 1
          else:
              output += str(count) + s[i]
              count = 1
      
      output += str(count) + s[-1]
      return output
  
  assert compress("aabbbbbbcccc") == "2a6b4c"
  assert compress("aaabbbcccaaa") == "3a3b3c3a"
  assert compress("abc") == "1a1b1c"
  assert compress("a") == "1a"
  assert compress("") == ""
output: def compress(s):
      if not s:
          return ""
      
      output = ""
      count = 1
      
      for i in range(len(s) - 1):
          if s[i] == s[i + 1]:
              count += 1
          else:
              output += str(count) + s[i]
              count = 1
      
      output += str(count) + s[-1]
      return output