import std/[
  options,
  strutils,
  strformat,
  collections/sequtils,
  sugar,
]
import fp/[
  maybe,
  resultM,
]
import ../utils/str
import ./parser_types

# -- Types

type parserFnT = proc(t0: Parser): ParserResult

# -- Utilities

proc isStreamCompleted*(parser: Parser): bool =
  ## Check if the `parser` index is at/over the stream length.
  parser.state.position >= parser.state.stream.len - 1

proc isStreamCompleted*(parserResult: ParserResult): bool =
  ## Check if the `parserResult.state` index is at/over the stream length.
  parserResult.fold(
    err => false,
    isStreamCompleted,
  )

# -- Parsing functions

proc ch*(expectedChars: set[char]): parserFnT {.inline.} =
  ## Create parser function with set of `expectedChar`.
  ## When the parser has the character set at the following index return `ParserResult.ok`.
  return proc(parser: Parser): ParserResult =
    let state = parser.state
    let newIndex = state.position + 1

    if newIndex > (state.stream.len - 1):
      return err(ParserError(
        kind: endOfStringErr,
        expected: expectedChars.prettyExpectedSet(),
        index: newIndex,
        parser: parser,
      ))
    else:
      let foundChar = state.stream[newIndex]
      if foundChar in expectedChars:
        return Parser(
          state: ParserState(
            stream: state.stream,
            position: newIndex,
            lastPosition: parser.state.position,
          ),
          tokens: parser.tokens & initParserToken(foundChar)
        ).ok()
      else:
        return err(ParserError(
          kind: charMismatchErr,
          unexpected: $foundChar,
          expected: expectedChars.prettyExpectedSet(),
          index: newIndex,
          parser: parser,
        ))

proc ch*(expectedChar: char): parserFnT {.inline.} =
  ## Creates parser function with `expectedChar`
  ## When the parser has the character at the following index return `ParserResult.ok`
  return proc(parser: Parser): ParserResult =
    let state = parser.state
    let newIndex = state.position + 1

    if newIndex > (state.stream.len - 1):
      return err(ParserError(
        kind: endOfStringErr,
        expected: &"{expectedChar}",
        index: newIndex,
        parser: parser,
      ))
    else:
      let foundChar = state.stream[newIndex]
      if expectedChar == foundChar:
        return Parser(
          state: ParserState(
            stream: state.stream,
            position: newIndex,
            lastPosition: parser.state.position,
          ),
          tokens: parser.tokens & initParserToken(foundChar)
        ).ok()
      else:
        return err(ParserError(
          kind: charMismatchErr,
          unexpected: &"{foundChar}",
          expected: &"{expectedChar}",
          index: newIndex,
          parser: parser,
        ))

let anyCh* = ch(AllChars)
let digit* = ch(Digits)

proc str*(expectedString: string): parserFnT {.inline.} =
  ## Creates parser function with `expectedString`
  ## When the parser has the string at the following index return `ParserResult.ok`
  return proc(parser: Parser): ParserResult =
    var res: ParserResult = parser.ok()
    for c in expectedString.items:
      if res.isErr: break
      res = res.flatMap(ch(c))
    return res

# -- Parsing API

proc optional*(parserFn: parserFnT): parserFnT {.inline.} =
  ## Creates parser function with a nested `parserFn`:
  ## Continues on succesful parser
  ## Ignores failing parsers
  return proc(parser: Parser): ParserResult =
    let newParser = parserFn(parser)
    if newParser.isOk():
      newParser
    else:
      parser.ok()

proc ignore*(parserFn: parserFnT): parserFnT {.inline.} =
  ## Creates parser function with a nested `parserFn`:
  ## Parses using the `parserFn` but dont capture the resulting tokens.
  return proc(parser: Parser): ParserResult =
    return parserFn(parser)
    .map((x: Parser) => Parser(
      state: x.state,
      tokens: parser.tokens,
    ))

proc manyUntil*(acceptFn: parserFnT, stopFn: parserFnT): parserFnT {.inline.} =
  ## Creates parser function with a nested `acceptFn` parser function until the `stopFn` parserFunction is met:
  ## Parses until the `stopFn` is reached or on an errror.
  return proc(parser: Parser): ParserResult =
    var res: ParserResult = parser.ok()
    while res.isOk() and res.flatMap(stopFn).isErr():
      res = res.flatMap(acceptFn)
    return res

proc anyUntil*(stopFn: parserFnT): parserFnT {.inline.} =
  manyUntil(anyCh, stopFn)

proc choice*(parsers: seq[parserFnT]): parserFnT {.inline} =
  return proc(parser: Parser): ParserResult {.closure.} =
    var errors: seq[ParserResult] = newSeq[ParserResult]()
    var found = Nothing[ParserResult]()

    for fn in parsers:
      let fnResult: ParserResult = fn(parser)

      if fnResult.isOk():
        found = fnResult.just
        break
      else:
        errors = errors & fnResult

    return found
    .fold(
      proc(): ParserResult =
        let prettyErrors = errors.map((x: ParserResult) => x.error().expected)
        err(ParserError(
          kind: choiceMismatchErr,
          index: parser.state.position + 1,
          expected: &"Choice ({prettyErrors})",
          unexpected: errors[0].error().unexpected,
          parser: parser,
        )),
      proc(x: ParserResult): ParserResult = x,
    )


proc `+`*(parserFnA: parserFnT, parserFnB: parserFnT): parserFnT {.inline.} =
  ## Parse characters and ignore failure
  return proc(parser: Parser): ParserResult =
    parserFnA(parser).flatMap(parserFnB)

proc parseSeq*(parser: ParserResult, xs: seq[parserFnT]): ParserResult {.inline.} =
  xs.foldl(a.flatMap(b), parser)

# -- Parsing Aliases

proc endOfStream*(parser: Parser): ParserResult =
  let index = parser.state.position + 1
  if index == parser.state.stream.len:
    ok(parser)
  else:
    err(ParserError(
      kind: endOfStringErr,
      expected: &"EndOfString",
      index: index,
      parser: parser,
    ))

let newlineParser = choice(@[
  ch(NewLines),
  endOfStream,
])

proc newline*(parser: Parser): ParserResult =
  newlineParser(parser)
  .mapErr((x: ParserError) => x.setErrorExpectedField("Newline"))

let whitespaceParser = choice(@[
  ch(Whitespace),
  newlineParser,
])
proc whitespace*(parser: Parser): ParserResult =
  whitespaceParser(parser)
  .mapErr((x: ParserError) => x.setErrorExpectedField("Whitespace"))

# -- Parsing Helpers

let parseBetweenDelimiter* = proc(start: parserFnT, stop: parserFnT): parserFnT {.closure.} =
  ignore(start) + anyUntil(stop + whitespace) + ignore(start)

let parseBetweenPair* = proc(delimiterParser: parserFnT): parserFnT {.closure.} =
  parseBetweenDelimiter(delimiterParser, delimiterParser)

# -- Tests

when isMainModule:
  let testParser123 = initParserResult("123")

  block testParsingFunctions:
    let ch1 = ch('1')

    # Success
    assert testParser123.flatMap(ch1).tokensToString() == "1"
    assert testParser123.flatMap(anyCh).tokensToString() == "1"
    assert testParser123.flatMap(str("123")).tokensToString() == "123"

    # Mismatch
    assert testParser123.flatMap(ch('2')).error().kind == charMismatchErr
    assert testParser123.flatMap(ch(Letters)).error().kind == charMismatchErr
    assert testParser123.flatMap(str("1234")).error().kind == endOfStringErr
    assert testParser123.flatMap(str("456")).error().kind == charMismatchErr

    # Out of bounds
    # assert initParserResult("").flatMap(ch1).error().kind == endOfStringErr
    assert initParserResult("1").flatMap(ch1).flatMap(ch1).error().kind == endOfStringErr

    # Stream end reached
    assert initParserResult("1").flatMap(ch1).isStreamCompleted() == true
    assert initParserResult("12").flatMap(ch1).isStreamCompleted() == false
    assert initParserResult("").flatMap(ch1).isStreamCompleted() == false
    assert testParser123.flatMap(str("123")).isStreamCompleted() == true

  block testParsingApi:
    # Optional
    assert testParser123.flatMap(optional(ch('1'))).tokensToString() == "1"
    assert testParser123.flatMap(optional(ch('2'))).tokensToString() == ""

    # Ignore
    assert testParser123.flatMap(ignore(ch('1'))).tokensToString() == ""