use std::fmt;
use std::iter::FusedIterator;
use std::ops::Range;

use line_span::{find_line_range, find_next_line_start};

use crate::syntax::SyntaxRule;

/// Events contain [`raw`] and [`text`].
///
/// Text is the contents of the comment, while raw includes additional
/// characters based on the type of comment, such as the comment
/// delimiters or "start and end symbols" of the comment.
///
/// - `LineComment`'s `raw` includes the whole line.
/// - `BlockComment`'s `raw` includes only the block comment delimiters.
///
/// *The above is only true, for events parsed by [`CommentParser`].*
///
/// [`text`]: enum.Event.html#method.text
/// [`raw`]: enum.Event.html#method.raw
/// [`CommentParser`]: struct.CommentParser.html
///
/// # Example
///
/// ```rust
/// # use comment_parser::Event;
/// let line = Event::LineComment("  // Foo Bar", " Foo Bar");
/// assert_eq!(line.text(), " Foo Bar");
/// assert_eq!(line.raw(),  "  // Foo Bar");
///
/// let block = Event::BlockComment("/* Foo\n  Bar */", " Foo\n  Bar ");
/// assert_eq!(block.text(), " Foo\n  Bar ");
/// assert_eq!(block.raw(),  "/* Foo\n  Bar */");
///
/// # use comment_parser::{get_syntax, CommentParser};
/// #
/// # let code = "  \n  // Foo Bar\r\n foo /* Foo\n  Bar */ foo\n";
/// #
/// # let mut parser = CommentParser::new(code, get_syntax("rust").unwrap());
/// # assert_eq!(parser.next(), Some(line));
/// # assert_eq!(parser.next(), Some(block));
/// # assert_eq!(parser.next(), None);
/// ```
#[derive(PartialEq, Clone)]
pub enum Event<'a> {
    /// `LineComment(raw, text)`
    LineComment(&'a str, &'a str),
    /// `BlockComment(raw, text)`
    BlockComment(&'a str, &'a str),
}

impl<'a> Event<'a> {
    /// Returns the raw part of an `Event`.
    #[inline]
    pub fn raw(&self) -> &str {
        use Event::*;
        match self {
            LineComment(raw, _) | BlockComment(raw, _) => raw,
        }
    }

    /// Returns the text part of an `Event`.
    #[inline]
    pub fn text(&self) -> &str {
        use Event::*;
        match self {
            LineComment(_, text) | BlockComment(_, text) => text,
        }
    }
}

impl<'a> fmt::Debug for Event<'a> {
    /// Renders [`raw`] as `_` as both [`raw`] and
    /// [`text`] are similar.
    ///
    /// [`text`]: enum.Event.html#method.text
    /// [`raw`]: enum.Event.html#method.raw
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        use Event::*;
        let name = match self {
            LineComment(..) => "LineComment",
            BlockComment(..) => "BlockComment",
        };
        fmt.debug_tuple(name)
            .field(&format_args!("_"))
            .field(&self.text())
            .finish()
    }
}

#[derive(Clone, Debug)]
enum RawEvent<'a> {
    LineComment(&'a str, &'a str),
    BlockComment(&'a str, &'a str),
    String(&'a str, &'a str),
}

impl<'a> RawEvent<'a> {
    #[inline]
    fn into_event(self) -> Option<Event<'a>> {
        use RawEvent::*;
        match self {
            LineComment(raw, text) => Some(Event::LineComment(raw, text)),
            BlockComment(raw, text) => Some(Event::BlockComment(raw, text)),
            String(..) => None,
        }
    }
}

/// `CommentParser` parses `text` and produces [`Event`]s.
///
/// [`Event`]: enum.Event.html
#[allow(missing_debug_implementations)]
#[derive(Clone)]
pub struct CommentParser<'a> {
    text: &'a str,
    index: usize,
    rules: &'a [SyntaxRule<'a>],
    max_rule_len: usize,
}

impl<'a> CommentParser<'a> {
    /// Creates a `CommentParser` which parses `text` based on
    /// `rules` and produces [`Event`]s.
    ///
    /// # Panics
    ///
    /// Note that `CommentParser` panics immediately upon calling `new`,
    /// if any [`SyntaxRule`] contains an empty `&[u8]`.
    ///
    /// [`SyntaxRule`]: enum.SyntaxRule.html
    #[inline]
    pub fn new(text: &'a str, rules: &'a [SyntaxRule]) -> Self {
        assert!(SyntaxRule::check_rules(rules), "empty syntax rule");

        Self {
            text,
            index: 0,
            rules,
            max_rule_len: SyntaxRule::max_rule_len(rules),
        }
    }

    fn next_event(&mut self) -> Option<RawEvent<'a>> {
        let bytes = self.text.as_bytes();

        let rule = bytes[self.index..]
            .windows(self.max_rule_len)
            .enumerate()
            .filter_map(|(i, w)| {
                let rule = self
                    .rules
                    .iter()
                    .position(|rule| w.starts_with(rule.start()))?;
                Some((self.index + i, &self.rules[rule]))
            })
            .next();

        if let Some((start, rule)) = rule {
            Some(match rule.parse_rule() {
                ParseRule::LineComment => self.parse_line_comment(start, rule),
                ParseRule::BlockComment => self.parse_block_comment(start, rule),
                ParseRule::String => self.parse_string(start, rule),
            })
        } else {
            self.index = bytes.len();
            None
        }
    }

    fn parse_line_comment(&mut self, start: usize, rule: &SyntaxRule) -> RawEvent<'a> {
        let after_start = start + rule.start().len();
        let Range { start, end } = find_line_range(self.text, start);

        self.index = find_next_line_start(self.text, end).unwrap_or_else(|| self.text.len());

        let line = &self.text[start..end];
        let comment = &self.text[after_start..end];

        RawEvent::LineComment(line, comment)
    }

    fn parse_block_comment(&mut self, start: usize, rule: &SyntaxRule) -> RawEvent<'a> {
        let after_start = start + rule.start().len();

        let rule_end = rule.end();

        let (before_end, end) = self.text.as_bytes()[after_start..]
            .windows(rule_end.len())
            .position(|w| w == rule_end)
            .map(|i| {
                let i = after_start + i;
                (i, i + rule_end.len())
            })
            .unwrap_or_else(|| {
                let i = self.text.len();
                (i, i)
            });

        self.index = end;

        let lines = &self.text[start..end];
        let comment = &self.text[after_start..before_end];

        RawEvent::BlockComment(lines, comment)
    }

    fn parse_string(&mut self, start: usize, rule: &SyntaxRule) -> RawEvent<'a> {
        let after_start = start + rule.start().len();
        let rule_end = rule.start();

        let mut skip = false;

        let (before_end, end) = self.text.as_bytes()[after_start..]
            .windows(rule_end.len())
            .position(|w| {
                if skip {
                    skip = false;
                    false
                // TODO: This should be part of SyntaxRule
                } else if w[0] == b'\\' {
                    skip = true;
                    false
                } else {
                    w == rule_end
                }
            })
            .map(|i| {
                let i = after_start + i;
                (i, i + rule_end.len())
            })
            .unwrap_or_else(|| {
                let i = self.text.len();
                (i, i)
            });

        self.index = end;

        let lines = &self.text[start..end];
        let string = &self.text[after_start..before_end];

        RawEvent::String(lines, string)
    }
}

impl<'a> Iterator for CommentParser<'a> {
    type Item = Event<'a>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.index == self.text.len() {
            return None;
        }

        while let Some(event) = self.next_event() {
            let event = event.into_event();
            if event.is_some() {
                return event;
            }
        }

        None
    }
}

impl<'a> FusedIterator for CommentParser<'a> {}

enum ParseRule {
    LineComment,
    BlockComment,
    String,
}

impl<'a> SyntaxRule<'a> {
    #[inline]
    fn parse_rule(&self) -> ParseRule {
        use SyntaxRule::*;
        match self {
            LineComment(..) => ParseRule::LineComment,
            BlockComment(..) => ParseRule::BlockComment,
            String(..) => ParseRule::String,
        }
    }

    #[inline]
    fn start(&self) -> &[u8] {
        use SyntaxRule::*;
        match self {
            LineComment(start) | BlockComment(start, _) | String(start) => start,
        }
    }

    #[inline]
    fn end(&self) -> &[u8] {
        use SyntaxRule::*;
        match self {
            BlockComment(_, end) => end,
            _ => unreachable!(),
        }
    }

    #[inline]
    fn max_rule_len(rules: &[Self]) -> usize {
        rules
            .iter()
            .map(Self::start)
            .map(<[u8]>::len)
            .max()
            .unwrap_or(0)
    }

    /// Returns `true` if the rules are valid.
    #[inline]
    fn check_rules(rules: &[Self]) -> bool {
        !rules.iter().any(|rule| {
            use SyntaxRule::*;
            match rule {
                LineComment(start) | String(start) => start.is_empty(),
                BlockComment(start, end) => start.is_empty() || end.is_empty(),
            }
        })
    }
}