use uwl::Stream;

use std::borrow::Cow;
use std::cell::Cell;
use std::error::Error as StdError;
use std::marker::PhantomData;
use std::{fmt, str::FromStr};

/// Defines how an operation on an `Args` method failed.
#[derive(Debug)]
pub enum Error<E> {
    /// "END-OF-STRING". We reached the end. There's nothing to parse anymore.
    Eos,
    /// Parsing operation failed. Contains how it did.
    Parse(E),
    #[doc(hidden)]
    __Nonexhaustive,
}

impl<E> From<E> for Error<E> {
    fn from(e: E) -> Self {
        Error::Parse(e)
    }
}

impl<E: fmt::Display> fmt::Display for Error<E> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        use self::Error::*;

        match *self {
            Eos => write!(f, "ArgError(\"end of string\")"),
            Parse(ref e) => write!(f, "ArgError(\"{}\")", e),
            __Nonexhaustive => unreachable!(),
        }
    }
}

impl<E: fmt::Debug + fmt::Display> StdError for Error<E> {
    fn description(&self) -> &str {
        match self {
            Error::Eos => "end-of-string",
            Error::Parse(_) => "parse-failure",
            Error::__Nonexhaustive => unreachable!(),
        }
    }
}

type Result<T, E> = ::std::result::Result<T, Error<E>>;

/// Dictates how `Args` should split arguments, if by one character, or a string.
#[derive(Debug, Clone)]
pub enum Delimiter {
    Single(char),
    Multiple(String),
}

impl Delimiter {
    #[inline]
    fn to_str(&self) -> Cow<'_, str> {
        match self {
            Delimiter::Single(c) => Cow::Owned(c.to_string()),
            Delimiter::Multiple(s) => Cow::Borrowed(s),
        }
    }
}

impl From<char> for Delimiter {
    #[inline]
    fn from(c: char) -> Delimiter {
        Delimiter::Single(c)
    }
}

impl From<String> for Delimiter {
    #[inline]
    fn from(s: String) -> Delimiter {
        Delimiter::Multiple(s)
    }
}

impl<'a> From<&'a String> for Delimiter {
    #[inline]
    fn from(s: &'a String) -> Delimiter {
        Delimiter::Multiple(s.clone())
    }
}

impl<'a> From<&'a str> for Delimiter {
    #[inline]
    fn from(s: &'a str) -> Delimiter {
        Delimiter::Multiple(s.to_string())
    }
}

#[derive(Debug, Clone, Copy, PartialEq)]
enum TokenKind {
    Argument,
    QuotedArgument,
}

#[derive(Debug, Clone, Copy)]
struct Token {
    kind: TokenKind,
    span: (usize, usize),
}

impl Token {
    #[inline]
    fn new(kind: TokenKind, start: usize, end: usize) -> Self {
        Token { kind, span: (start, end) }
    }
}

fn lex(stream: &mut Stream<'_>, delims: &[Cow<'_, str>]) -> Option<Token> {
    if stream.is_empty() {
        return None;
    }

    let start = stream.offset();
    if stream.current()? == b'"' {
        stream.next();

        stream.take_until(|b| b == b'"');

        let is_quote = stream.current().map_or(false, |b| b == b'"');
        stream.next();

        let end = stream.offset();

        // Remove possible delimiters after the quoted argument.
        for delim in delims {
            stream.eat(delim);
        }

        return Some(if is_quote {
            Token::new(TokenKind::QuotedArgument, start, end)
        } else {
            // We're missing an end quote. View this as a normal argument.
            Token::new(TokenKind::Argument, start, stream.len())
        });
    }

    let mut end = start;

    'outer: while !stream.is_empty() {
        for delim in delims {
            end = stream.offset();

            if stream.eat(&delim) {
                break 'outer;
            }
        }

        stream.next_char();
        end = stream.offset();
    }

    Some(Token::new(TokenKind::Argument, start, end))
}

fn remove_quotes(s: &str) -> &str {
    if s.starts_with('"') && s.ends_with('"') {
        return &s[1..s.len() - 1];
    }

    s
}

#[derive(Debug, Clone, Copy)]
enum State {
    None,
    Quoted,
    Trimmed,
    // Preserve the order they were called.
    QuotedTrimmed,
    TrimmedQuoted,
}

/// A utility struct for handling "arguments" of a command.
///
/// An "argument" is a part of the message up that ends at one of the specified delimiters, or the end of the message.
///
/// # Example
///
/// ```rust
/// use serenity::framework::standard::{Args, Delimiter};
///
/// let mut args = Args::new("hello world!", &[Delimiter::Single(' ')]); // A space is our delimiter.
///
/// // Parse our argument as a `String` and assert that it's the "hello" part of the message.
/// assert_eq!(args.single::<String>().unwrap(), "hello");
/// // Same here.
/// assert_eq!(args.single::<String>().unwrap(), "world!");
///
/// ```
///
/// We can also parse "quoted arguments" (no pun intended):
///
/// ```rust
/// use serenity::framework::standard::{Args, Delimiter};
///
/// // Let us imagine this scenario:
/// // You have a `photo` command that grabs the avatar url of a user. This command accepts names only.
/// // Now, one of your users wants the avatar of a user named Princess Zelda.
/// // Problem is, her name contains a space; our delimiter. This would result in two arguments, "Princess" and "Zelda".
/// // So how shall we get around this? Through quotes! By surrounding her name in them we can perceive it as one single argument.
/// let mut args = Args::new(r#""Princess Zelda""#, &[Delimiter::Single(' ')]);
///
/// // Hooray!
/// assert_eq!(args.single_quoted::<String>().unwrap(), "Princess Zelda");
/// ```
///
/// In case of a mistake, we can go back in time... er I mean, one step (or entirely):
///
/// ```rust
/// use serenity::framework::standard::{Args, Delimiter};
///
/// let mut args = Args::new("4 2", &[Delimiter::Single(' ')]);
///
/// assert_eq!(args.single::<u32>().unwrap(), 4);
///
/// // Oh wait, oops, meant to double the 4.
/// // But I won't able to access it now...
/// // oh wait, I can `rewind`.
/// args.rewind();
///
/// assert_eq!(args.single::<u32>().unwrap() * 2, 8);
///
/// // And the same for the 2
/// assert_eq!(args.single::<u32>().unwrap() * 2, 4);
///
/// // WAIT, NO. I wanted to concatenate them into a "42" string...
/// // Argh, what should I do now????
/// // ....
/// // oh, `restore`
/// args.restore();
///
/// let res = format!("{}{}", args.single::<String>().unwrap(), args.single::<String>().unwrap());
///
/// // Yay.
/// assert_eq!(res, "42");
/// ```
///
/// Hmm, taking a glance at the prior example, it seems we have an issue with reading the same argument over and over.
/// Is there a more sensible solution than rewinding...? Actually, there is! The `current` and `parse` methods:
///
/// ```rust
/// use serenity::framework::standard::{Args, Delimiter};
///
/// let mut args = Args::new("trois cinq quatre six", &[Delimiter::Single(' ')]);
///
/// assert_eq!(args.parse::<String>().unwrap(), "trois");
///
/// // It might suggest we've lost the `trois`. But in fact, we didn't! And not only that, we can do it an infinite amount of times!
/// assert_eq!(args.parse::<String>().unwrap(), "trois");
/// assert_eq!(args.current(), Some("trois"));
/// assert_eq!(args.parse::<String>().unwrap(), "trois");
/// assert_eq!(args.current(), Some("trois"));
///
/// // Only if we use its brother method we'll then lose it.
/// assert_eq!(args.single::<String>().unwrap(), "trois");
/// assert_eq!(args.single::<String>().unwrap(), "cinq");
/// assert_eq!(args.single::<String>().unwrap(), "quatre");
/// assert_eq!(args.single::<String>().unwrap(), "six");
/// ```
#[derive(Clone, Debug)]
pub struct Args {
    message: String,
    args: Vec<Token>,
    offset: usize,
    state: Cell<State>,
}

impl Args {
    /// Create a new instance of `Args` for parsing arguments.
    ///
    /// For more reference, look at [`Args`]'s struct documentation.
    ///
    /// # Example
    ///
    /// ```rust
    /// use serenity::framework::standard::{Args, Delimiter};
    ///
    /// let mut args = Args::new(
    /// // Our message from which we'll parse over.
    /// "the quick brown fox jumps over the lazy",
    ///
    /// // The "delimiters", or aka the separators. They denote how we distinguish arguments as their own.
    /// // For this example, we'll use one delimiter, the space (`0x20`), which will separate the message.
    /// &[Delimiter::Single(' ')],
    /// );
    ///
    /// assert_eq!(args.single::<String>().unwrap(), "the");
    /// assert_eq!(args.single::<String>().unwrap(), "quick");
    /// assert_eq!(args.single::<String>().unwrap(), "brown");
    ///
    /// // We shall not see `the quick brown` again.
    /// assert_eq!(args.rest(), "fox jumps over the lazy");
    /// ```
    ///
    /// [`Args`]: #struct.Args.html
    pub fn new(message: &str, possible_delimiters: &[Delimiter]) -> Self {
        let delims = possible_delimiters
            .iter()
            .filter(|d| match d {
                Delimiter::Single(c) => message.contains(*c),
                Delimiter::Multiple(s) => message.contains(s),
            })
            .map(|delim| delim.to_str())
            .collect::<Vec<_>>();

        let args = if delims.is_empty() && !message.is_empty() {
            let kind = if message.starts_with('"') && message.ends_with('"') {
                TokenKind::QuotedArgument
            } else {
                TokenKind::Argument
            };

            // If there are no delimiters, then the only possible argument is the whole message.
            vec![Token::new(kind, 0, message.len())]
        } else {
            let mut args = Vec::new();
            let mut stream = Stream::new(message);

            while let Some(token) = lex(&mut stream, &delims) {
                args.push(token);
            }

            args
        };

        Args {
            args,
            message: message.to_string(),
            offset: 0,
            state: Cell::new(State::None),
        }
    }

    #[inline]
    fn span(&self) -> (usize, usize) {
        self.args[self.offset].span
    }

    #[inline]
    fn slice(&self) -> &str {
        let (start, end) = self.span();

        &self.message[start..end]
    }

    /// Move to the next argument.
    /// This increments the offset pointer.
    ///
    /// Does nothing if the message is empty.
    pub fn advance(&mut self) -> &mut Self {
        if self.is_empty() {
            return self;
        }

        self.offset += 1;

        self
    }

    /// Go one step behind.
    /// This decrements the offset pointer.
    ///
    /// Does nothing if the offset pointer is `0`.
    #[inline]
    pub fn rewind(&mut self) -> &mut Self {
        if self.offset == 0 {
            return self;
        }

        self.offset -= 1;

        self
    }

    /// Go back to the starting point.
    #[inline]
    pub fn restore(&mut self) {
        self.offset = 0;
    }

    fn apply<'a>(&self, s: &'a str) -> &'a str {
        fn trim(s: &str) -> &str {
            let trimmed = s.trim();

            // Search where the argument starts and ends between the whitespace.
            let start = s.find(trimmed).unwrap_or(0);
            let end = start + trimmed.len();

            &s[start..end]
        }

        let mut s = s;

        match self.state.get() {
            State::None => {}
            State::Quoted => {
                s = remove_quotes(s);
            }
            State::Trimmed => {
                s = trim(s);
            }
            State::QuotedTrimmed => {
                s = remove_quotes(s);
                s = trim(s);
            }
            State::TrimmedQuoted => {
                s = trim(s);
                s = remove_quotes(s);
            }
        }

        self.state.set(State::None);

        s
    }

    /// Retrieve the current argument.
    ///
    /// Applies modifications set by [`trimmed`] and [`quoted`].
    ///
    /// # Note
    ///
    /// This borrows `Args` for the entire lifetime of the returned argument.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use serenity::framework::standard::{Args, Delimiter};
    ///
    /// let mut args = Args::new("4 2", &[Delimiter::Single(' ')]);
    ///
    /// assert_eq!(args.current(), Some("4"));
    /// args.advance();
    /// assert_eq!(args.current(), Some("2"));
    /// args.advance();
    /// assert_eq!(args.current(), None);
    /// ```
    ///
    /// [`trimmed`]: #method.trimmed
    /// [`quoted`]: #method.quoted
    #[inline]
    pub fn current(&self) -> Option<&str> {
        if self.is_empty() {
            return None;
        }

        let mut s = self.slice();
        s = self.apply(s);

        Some(s)
    }

    /// Apply trimming the next time the current argument is accessed.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use serenity::framework::standard::Args;
    ///
    /// let mut args = Args::new("     42     ", &[]);
    ///
    /// assert_eq!(args.trimmed().current(), Some("42"));
    /// // `trimmed`'s effect on argument retrieval diminishes after a call to `current`
    /// assert_eq!(args.current(), Some("     42     "));
    /// assert_eq!(args.message(), "     42     ");
    /// ```
    pub fn trimmed(&mut self) -> &mut Self {
        if self.is_empty() {
            return self;
        }

        match self.state.get() {
            State::None => self.state.set(State::Trimmed),
            State::Quoted => self.state.set(State::QuotedTrimmed),
            _ => {}
        }

        self
    }

    /// Remove quotations surrounding the current argument the next time it is accessed.
    ///
    /// Note that only the quotes of the argument are taken into account.
    /// The quotes in the message are preserved.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use serenity::framework::standard::Args;
    ///
    /// let mut args = Args::new("\"42\"", &[]);
    ///
    /// assert_eq!(args.quoted().current(), Some("42"));
    /// assert_eq!(args.current(), Some("\"42\""));
    /// assert_eq!(args.message(), "\"42\"");
    /// ```
    pub fn quoted(&mut self) -> &mut Self {
        if self.is_empty() {
            return self;
        }

        let is_quoted = self.args[self.offset].kind == TokenKind::QuotedArgument;

        if is_quoted {
            match self.state.get() {
                State::None => self.state.set(State::Quoted),
                State::Trimmed => self.state.set(State::TrimmedQuoted),
                _ => {}
            }
        }

        self
    }

    /// Parse the current argument.
    ///
    /// Modifications of [`trimmed`] and [`quoted`] are also applied if they were called.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use serenity::framework::standard::{Args, Delimiter};
    ///
    /// let mut args = Args::new("4 2", &[Delimiter::Single(' ')]);
    ///
    /// assert_eq!(args.parse::<u32>().unwrap(), 4);
    /// assert_eq!(args.current(), Some("4"));
    /// ```
    ///
    /// [`trimmed`]: #method.trimmed
    /// [`quoted`]: #method.quoted
    #[inline]
    pub fn parse<T: FromStr>(&self) -> Result<T, T::Err> {
        T::from_str(self.current().ok_or(Error::Eos)?).map_err(Error::Parse)
    }

    /// Parse the current argument and advance.
    ///
    /// Shorthand for calling [`parse`], storing the result,
    /// calling [`next`] and returning the result.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use serenity::framework::standard::{Args, Delimiter};
    ///
    /// let mut args = Args::new("4 2", &[Delimiter::Single(' ')]);
    ///
    /// assert_eq!(args.single::<u32>().unwrap(), 4);
    ///
    /// // `4` is now out of the way. Next we have `2`
    /// assert_eq!(args.single::<u32>().unwrap(), 2);
    /// assert!(args.is_empty());
    /// ```
    ///
    /// [`parse`]: #method.parse
    /// [`next`]: #method.next
    #[inline]
    pub fn single<T: FromStr>(&mut self) -> Result<T, T::Err> {
        let p = self.parse::<T>()?;
        self.advance();
        Ok(p)
    }

    /// Remove surrounding quotations, if present, from the argument; parse it and advance.
    ///
    /// Shorthand for `.quoted().single::<T>()`
    ///
    /// # Examples
    ///
    /// ```rust
    /// use serenity::framework::standard::{Args, Delimiter};
    ///
    /// let mut args = Args::new(r#""4" "2""#, &[Delimiter::Single(' ')]);
    ///
    /// assert_eq!(args.single_quoted::<String>().unwrap(), "4");
    /// assert_eq!(args.single_quoted::<u32>().unwrap(), 2);
    /// assert!(args.is_empty());
    /// ```
    ///
    #[inline]
    pub fn single_quoted<T: FromStr>(&mut self) -> Result<T, T::Err> {
        let p = self.quoted().parse::<T>()?;
        self.advance();
        Ok(p)
    }

    /// By starting from the current offset, iterate over
    /// any available arguments until there are none.
    ///
    /// Modifications of [`trimmed`] and [`quoted`] are also applied to all arguments if they were called.
    ///
    /// # Examples
    ///
    /// Assert that all of the numbers in the message are even.
    ///
    /// ```rust
    /// use serenity::framework::standard::{Args, Delimiter};
    ///
    /// let mut args = Args::new("4 2", &[Delimiter::Single(' ')]);
    ///
    /// for arg in args.iter::<u32>() {
    ///     // Zero troubles, zero worries.
    ///     let arg = arg.unwrap_or(0);
    ///     assert!(arg % 2 == 0);
    /// }
    ///
    /// assert!(args.is_empty());
    /// ```
    ///
    /// [`trimmed`]: struct.Iter.html#method.trimmed
    /// [`quoted`]: struct.Iter.html#method.quoted
    #[inline]
    pub fn iter<T: FromStr>(&mut self) -> Iter<'_, T> {
        Iter {
            args: self,
            state: State::None,
            _marker: PhantomData,
        }
    }

    /// Return an iterator over all unmodified arguments.
    ///
    /// # Examples
    ///
    /// Join the arguments by a comma and a space.
    ///
    /// ```rust
    /// use serenity::framework::standard::{Args, Delimiter};
    ///
    /// let args = Args::new("Harry Hermione Ronald", &[Delimiter::Single(' ')]);
    ///
    /// let protagonists = args.raw().collect::<Vec<&str>>().join(", ");
    ///
    /// assert_eq!(protagonists, "Harry, Hermione, Ronald");
    /// ```
    #[inline]
    pub fn raw(&self) -> RawArguments<'_> {
        RawArguments {
            tokens: &self.args,
            msg: &self.message,
            quoted: false,
        }
    }

    /// Return an iterator over all arguments, stripped of their quotations if any were present.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use serenity::framework::standard::{Args, Delimiter};
    ///
    /// let args = Args::new("Saw \"The Mist\" \"A Quiet Place\"", &[Delimiter::Single(' ')]);
    ///
    /// let horror_movies = args.raw_quoted().collect::<Vec<&str>>();
    ///
    /// assert_eq!(&*horror_movies, &["Saw", "The Mist", "A Quiet Place"]);
    /// ```
    #[inline]
    pub fn raw_quoted(&self) -> RawArguments<'_> {
        let mut raw = self.raw();
        raw.quoted = true;
        raw
    }

    /// Search for any available argument that can be parsed, and remove it from the "arguments queue".
    ///
    /// # Note
    /// The removal is irreversible. And happens after the search *and* the parse were successful.
    ///
    /// # Note 2
    /// "Arguments queue" is the list which contains all arguments that were deemed unique as defined by quotations and delimiters.
    /// The 'removed' argument can be, likewise, still accessed via `message`.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use serenity::framework::standard::{Args, Delimiter};
    ///
    /// let mut args = Args::new("c4 2", &[Delimiter::Single(' ')]);
    ///
    /// assert_eq!(args.find::<u32>().unwrap(), 2);
    /// assert_eq!(args.single::<String>().unwrap(), "c4");
    /// assert!(args.is_empty());
    /// ```
    pub fn find<T: FromStr>(&mut self) -> Result<T, T::Err> {
        if self.is_empty() {
            return Err(Error::Eos);
        }

        let before = self.offset;
        self.restore();

        let pos = match self.iter::<T>().quoted().position(|res| res.is_ok()) {
            Some(p) => p,
            None => {
                self.offset = before;
                return Err(Error::Eos);
            },
        };

        self.offset = pos;

        let parsed = self.single_quoted::<T>()?;

        self.args.remove(pos);
        self.offset = before;
        self.rewind();

        Ok(parsed)
    }

    /// Search for any available argument that can be parsed.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use serenity::framework::standard::{Args, Delimiter};
    ///
    /// let mut args = Args::new("c4 2", &[Delimiter::Single(' ')]);
    ///
    /// assert_eq!(args.find_n::<u32>().unwrap(), 2);
    ///
    /// // The `2` is still here, so let's parse it again.
    /// assert_eq!(args.single::<String>().unwrap(), "c4");
    /// assert_eq!(args.single::<u32>().unwrap(), 2);
    /// assert!(args.is_empty());
    /// ```
    pub fn find_n<T: FromStr>(&mut self) -> Result<T, T::Err> {
        if self.is_empty() {
            return Err(Error::Eos);
        }

        let before = self.offset;
        self.restore();

        let pos = match self.iter::<T>().quoted().position(|res| res.is_ok()) {
            Some(p) => p,
            None => {
                self.offset = before;
                return Err(Error::Eos);
            },
        };

        self.offset = pos;

        let parsed = self.quoted().parse::<T>()?;

        self.offset = before;

        Ok(parsed)
    }

    /// Get the original, unmodified message passed to the command.
    #[inline]
    pub fn message(&self) -> &str {
        &self.message
    }

    /// Starting from the offset, return the remainder of available arguments.
    #[inline]
    pub fn rest(&self) -> &str {
        self.remains().unwrap_or_default()
    }

    /// Starting from the offset, return the remainder of available arguments.
    ///
    /// Returns `None` if there are no remaining arguments.
    #[inline]
    pub fn remains(&self) -> Option<&str> {
        if self.is_empty() {
            return None;
        }

        let (start, _) = self.span();

        Some(&self.message[start..])
    }

    /// Return the full amount of recognised arguments.
    /// The length of the "arguments queue".
    ///
    /// # Note
    ///
    /// The value returned is to be assumed to stay static.
    /// However, if `find` was called previously, and was successful, then the value is substracted by one.
    #[inline]
    pub fn len(&self) -> usize {
        self.args.len()
    }

    /// Assert that there are no more arguments left.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.offset >= self.len()
    }

    /// Return the amount of arguments still available.
    #[inline]
    pub fn remaining(&self) -> usize {
        if self.is_empty() {
            return 0;
        }

        self.len() - self.offset
    }
}

/// Parse each argument individually, as an iterator.
pub struct Iter<'a, T: FromStr> {
    args: &'a mut Args,
    state: State,
    _marker: PhantomData<T>,
}

impl<'a, T: FromStr> Iter<'a, T> {
    /// Retrieve the current argument.
    pub fn current(&self) -> Option<&str> {
        self.args.state.set(self.state);
        self.args.current()
    }

    /// Parse the current argument independently.
    pub fn parse(&self) -> Result<T, T::Err> {
        self.args.state.set(self.state);
        self.args.parse::<T>()
    }

    /// Remove surrounding quotation marks from all of the arguments.
    #[inline]
    pub fn quoted(&mut self) -> &mut Self {
        match self.state {
            State::None => self.state = State::Quoted,
            State::Trimmed => self.state = State::TrimmedQuoted,
            _ => {}
        }

        self
    }

    /// Trim leading and trailling whitespace off all arguments.
    #[inline]
    pub fn trimmed(&mut self) -> &mut Self {
        match self.state {
            State::None => self.state = State::Trimmed,
            State::Quoted => self.state = State::QuotedTrimmed,
            _ => {}
        }

        self
    }
}

impl<'a, T: FromStr> Iterator for Iter<'a, T> {
    type Item = Result<T, T::Err>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.args.is_empty() {
            None
        } else {
            let arg = self.parse();
            self.args.advance();
            Some(arg)
        }
    }
}

/// Access to all of the arguments, as an iterator.
#[derive(Debug)]
pub struct RawArguments<'a> {
    msg: &'a str,
    tokens: &'a [Token],
    quoted: bool,
}

impl<'a> Iterator for RawArguments<'a> {
    type Item = &'a str;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        let (start, end) = self.tokens.get(0)?.span;

        self.tokens = &self.tokens[1..];

        let mut s = &self.msg[start..end];

        if self.quoted {
            s = remove_quotes(s);
        }

        Some(s)
    }
}