substr() like in PHP

# substr() like in PHP

proc substr*(str: string, start: int, length: int): string =
  var o = start
  var n = length

  if n < 0 and -n > str.len:
    return "" # or nil?
  elif n > str.len:
    n = str.len

  if o > str.len:
    return "" # or nil?
  elif o < 0 and -o > str.len:
    o = 0

  if n < 0 and n + str.len - o < 0:
    return "" # or nil?

  # offset from end of string
  if o < 0:
    o = str.len + o;
    if o < 0:
      o = 0

  # negative length cuts the string
  if n < 0:
    n = str.len - o + n
    if n < 0:
      n = 0

  if o + n > str.len:
    n = str.len - o

  str[o ..< o + n]

proc substr*(str: string, start: int): string =
  substr(str, start, str.len)

when isMainModule:
  var test = "Test.xml"
  assert(test.substr(0) == "Test.xml")
  assert(test.substr(0,8) == "Test.xml")
  assert(test.substr(0,0) == "")
  assert(test.substr(0,1) == "T")
  assert(test.substr(1) == "est.xml")
  assert(test.substr(-1) == "l")
  assert(test.substr(0,-4) == "Test")
  assert(test.substr(7,1) == "l")
  assert(test.substr(-3) == "xml")
  assert(test.substr(-4,1) == ".")
  assert(test.substr(-100,2) == "Te")
  assert(test.substr(8) == "")