import std/parseutils

proc fastSubstr(s: string; token: var string; start, length: int) =
  token.setLen length
  for i in 0 ..< length: token[i] = s[i+start]

proc parseUntilBackwards*(s: string, token: var string, until: string,
                 start = 0): int {.inline.} =
  ## Parses a token and stores it in ``token``. Returns
  ## the number of the parsed characters or 0 in case of an error. A token
  ## consists of any character that comes before the `until`  token.
  runnableExamples:
    var myToken: string
    doAssert parseUntil("Hello World", myToken, "Wor") == 6
    doAssert myToken == "Hello "
    doAssert parseUntil("Hello World", myToken, "Wor", 2) == 4
    doAssert myToken == "llo "
  var i = s.len
  while i > 0:
    if until.len > 0 and s[i] == until[until.len - 1]:
      var u = 1
      while i+u < s.len and u < until.len and s[i+u] == until[u]:
        inc u
      if u >= until.len: break
    dec(i)
  result = i-start
  fastSubstr(s, token, start, result)
  #token = substr(s, start, i-1)