use ruff_python_ast::whitespace::is_python_whitespace;
use ruff_text_size::{TextLen, TextRange, TextSize};
use std::str::Chars;

/// Searches for the first non-trivia character in `range`.
///
/// The search skips over any whitespace and comments.
///
/// Returns `Some` if the range contains any non-trivia character. The first item is the absolute offset
/// of the character, the second item the non-trivia character.
///
/// Returns `None` if the range is empty or only contains trivia (whitespace or comments).
pub(crate) fn first_non_trivia_token(offset: TextSize, code: &str) -> Option<Token> {
    SimpleTokenizer::starts_at(offset, code)
        .skip_trivia()
        .next()
}

/// Returns the first non-trivia token right before `offset` or `None` if at the start of the file
/// or all preceding tokens are trivia tokens.
///
/// ## Notes
///
/// Prefer [`first_non_trivia_token`] whenever possible because reverse lookup is expensive because of comments.
pub(crate) fn first_non_trivia_token_rev(offset: TextSize, code: &str) -> Option<Token> {
    SimpleTokenizer::up_to(offset, code)
        .skip_trivia()
        .next_back()
}

/// Returns the number of newlines between `offset` and the first non whitespace character in the source code.
pub(crate) fn lines_before(offset: TextSize, code: &str) -> u32 {
    let tokens = SimpleTokenizer::up_to(offset, code);
    let mut newlines = 0u32;

    for token in tokens.rev() {
        match token.kind() {
            TokenKind::Newline => {
                newlines += 1;
            }
            TokenKind::Whitespace => {
                // ignore
            }
            _ => {
                break;
            }
        }
    }

    newlines
}

/// Counts the empty lines between `offset` and the first non-whitespace character.
pub(crate) fn lines_after(offset: TextSize, code: &str) -> u32 {
    let tokens = SimpleTokenizer::starts_at(offset, code);
    let mut newlines = 0u32;

    for token in tokens {
        match token.kind() {
            TokenKind::Newline => {
                newlines += 1;
            }
            TokenKind::Whitespace => {
                // ignore
            }
            _ => {
                break;
            }
        }
    }

    newlines
}

/// Returns the position after skipping any trailing trivia up to, but not including the newline character.
pub(crate) fn skip_trailing_trivia(offset: TextSize, code: &str) -> TextSize {
    let tokenizer = SimpleTokenizer::starts_at(offset, code);

    for token in tokenizer {
        match token.kind() {
            TokenKind::Whitespace | TokenKind::Comment | TokenKind::Continuation => {
                // No op
            }
            _ => {
                return token.start();
            }
        }
    }

    offset
}

#[derive(Clone, Debug, Eq, PartialEq, Hash)]
pub(crate) struct Token {
    pub(crate) kind: TokenKind,
    pub(crate) range: TextRange,
}

impl Token {
    pub(crate) const fn kind(&self) -> TokenKind {
        self.kind
    }

    #[allow(unused)]
    pub(crate) const fn range(&self) -> TextRange {
        self.range
    }

    pub(crate) const fn start(&self) -> TextSize {
        self.range.start()
    }

    #[allow(unused)]
    pub(crate) const fn end(&self) -> TextSize {
        self.range.start()
    }
}

#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
pub(crate) enum TokenKind {
    /// A comment, not including the trailing new line.
    Comment,

    /// Sequence of ' ' or '\t'
    Whitespace,

    /// Start or end of the file
    EndOfFile,

    /// `\\`
    Continuation,

    /// `\n` or `\r` or `\r\n`
    Newline,

    /// `(`
    LParen,

    /// `)`
    RParen,

    /// `{`
    LBrace,

    /// `}`
    RBrace,

    /// `[`
    LBracket,

    /// `]`
    RBracket,

    /// `,`
    Comma,

    /// `:`
    Colon,

    /// '/'
    Slash,

    /// Any other non trivia token. Always has a length of 1
    Other,

    /// Returned for each character after [`TokenKind::Other`] has been returned once.
    Bogus,
}

impl TokenKind {
    const fn from_non_trivia_char(c: char) -> TokenKind {
        match c {
            '(' => TokenKind::LParen,
            ')' => TokenKind::RParen,
            '[' => TokenKind::LBracket,
            ']' => TokenKind::RBracket,
            '{' => TokenKind::LBrace,
            '}' => TokenKind::RBrace,
            ',' => TokenKind::Comma,
            ':' => TokenKind::Colon,
            '/' => TokenKind::Slash,
            _ => TokenKind::Other,
        }
    }

    const fn is_trivia(self) -> bool {
        matches!(
            self,
            TokenKind::Whitespace
                | TokenKind::Newline
                | TokenKind::Comment
                | TokenKind::Continuation
        )
    }
}

/// Simple zero allocation tokenizer for tokenizing trivia (and some tokens).
///
/// The tokenizer must start at an offset that is trivia (e.g. not inside of a multiline string).
///
/// The tokenizer doesn't guarantee any correctness after it returned a [`TokenKind::Other`]. That's why it
/// will return [`TokenKind::Bogus`] for every character after until it reaches the end of the file.
pub(crate) struct SimpleTokenizer<'a> {
    offset: TextSize,
    back_offset: TextSize,
    /// `true` when it is known that the current `back` line has no comment for sure.
    back_line_has_no_comment: bool,
    bogus: bool,
    cursor: Cursor<'a>,
}

impl<'a> SimpleTokenizer<'a> {
    pub(crate) fn new(source: &'a str, range: TextRange) -> Self {
        Self {
            offset: range.start(),
            back_offset: range.end(),
            back_line_has_no_comment: false,
            bogus: false,
            cursor: Cursor::new(&source[range]),
        }
    }

    pub(crate) fn starts_at(offset: TextSize, source: &'a str) -> Self {
        let range = TextRange::new(offset, source.text_len());
        Self::new(source, range)
    }

    pub(crate) fn up_to(offset: TextSize, source: &'a str) -> Self {
        Self::new(source, TextRange::up_to(offset))
    }

    fn next_token(&mut self) -> Token {
        self.cursor.start_token();

        let Some(first) = self.cursor.bump() else {
            return Token {
                kind: TokenKind::EndOfFile,
                range: TextRange::empty(self.offset),
            }
        };

        if self.bogus {
            let token = Token {
                kind: TokenKind::Bogus,
                range: TextRange::at(self.offset, first.text_len()),
            };

            self.offset += first.text_len();
            return token;
        }

        let kind = match first {
            ' ' | '\t' => {
                self.cursor.eat_while(|c| matches!(c, ' ' | '\t'));
                TokenKind::Whitespace
            }

            '\n' => TokenKind::Newline,

            '\r' => {
                self.cursor.eat_char('\n');
                TokenKind::Newline
            }

            '#' => {
                self.cursor.eat_while(|c| !matches!(c, '\n' | '\r'));
                TokenKind::Comment
            }

            '\\' => TokenKind::Continuation,

            c => {
                let kind = TokenKind::from_non_trivia_char(c);

                if kind == TokenKind::Other {
                    self.bogus = true;
                }

                kind
            }
        };

        let token_len = self.cursor.token_len();

        let token = Token {
            kind,
            range: TextRange::at(self.offset, token_len),
        };

        self.offset += token_len;

        token
    }

    /// Returns the next token from the back. Prefer iterating forwards. Iterating backwards is significantly more expensive
    /// because it needs to check if the line has any comments when encountering any non-trivia token.
    pub(crate) fn next_token_back(&mut self) -> Token {
        self.cursor.start_token();

        let Some(last) = self.cursor.bump_back() else {
            return Token {
                kind: TokenKind::EndOfFile,
                range: TextRange::empty(self.back_offset),
            }
        };

        if self.bogus {
            let token = Token {
                kind: TokenKind::Bogus,
                range: TextRange::at(self.back_offset - last.text_len(), last.text_len()),
            };

            self.back_offset -= last.text_len();
            return token;
        }

        let kind = match last {
            // This may not be 100% correct because it will lex-out trailing whitespace from a comment
            // as whitespace rather than being part of the token. This shouldn't matter for what we use the lexer for.
            ' ' | '\t' => {
                self.cursor.eat_back_while(|c| matches!(c, ' ' | '\t'));
                TokenKind::Whitespace
            }

            '\r' => {
                self.back_line_has_no_comment = false;
                TokenKind::Newline
            }

            '\n' => {
                self.back_line_has_no_comment = false;
                self.cursor.eat_char_back('\r');
                TokenKind::Newline
            }

            // Empty comment (could also be a comment nested in another comment, but this shouldn't matter for what we use the lexer for)
            '#' => TokenKind::Comment,

            // For all other tokens, test if the character isn't part of a comment.
            c => {
                let mut comment_offset = None;

                // Skip the test whether there's a preceding comment if it has been performed before.
                if !self.back_line_has_no_comment {
                    let rest = self.cursor.chars.as_str();

                    for (back_index, c) in rest.chars().rev().enumerate() {
                        match c {
                            '#' => {
                                // Potentially a comment
                                comment_offset = Some(back_index + 1);
                            }
                            '\r' | '\n' | '\\' => {
                                break;
                            }
                            c => {
                                if !is_python_whitespace(c)
                                    && TokenKind::from_non_trivia_char(c) == TokenKind::Other
                                {
                                    comment_offset = None;
                                }
                            }
                        }
                    }
                }

                // From here on it is guaranteed that this line has no other comment.
                self.back_line_has_no_comment = true;

                if let Some(comment_offset) = comment_offset {
                    // It is a comment, bump all tokens
                    for _ in 0..comment_offset {
                        self.cursor.bump_back().unwrap();
                    }

                    TokenKind::Comment
                } else if c == '\\' {
                    TokenKind::Continuation
                } else {
                    let kind = TokenKind::from_non_trivia_char(c);

                    if kind == TokenKind::Other {
                        self.bogus = true;
                    }

                    kind
                }
            }
        };

        let token_len = self.cursor.token_len();

        let start = self.back_offset - token_len;

        let token = Token {
            kind,
            range: TextRange::at(start, token_len),
        };

        self.back_offset = start;

        token
    }

    pub(crate) fn skip_trivia(self) -> impl Iterator<Item = Token> + DoubleEndedIterator + 'a {
        self.filter(|t| !t.kind().is_trivia())
    }
}

impl Iterator for SimpleTokenizer<'_> {
    type Item = Token;

    fn next(&mut self) -> Option<Self::Item> {
        let token = self.next_token();

        if token.kind == TokenKind::EndOfFile {
            None
        } else {
            Some(token)
        }
    }
}

impl DoubleEndedIterator for SimpleTokenizer<'_> {
    fn next_back(&mut self) -> Option<Self::Item> {
        let token = self.next_token_back();

        if token.kind == TokenKind::EndOfFile {
            None
        } else {
            Some(token)
        }
    }
}

const EOF_CHAR: char = '\0';

#[derive(Debug, Clone)]
struct Cursor<'a> {
    chars: Chars<'a>,
    source_length: TextSize,
}

impl<'a> Cursor<'a> {
    fn new(source: &'a str) -> Self {
        Self {
            source_length: source.text_len(),
            chars: source.chars(),
        }
    }

    /// Peeks the next character from the input stream without consuming it.
    /// Returns [`EOF_CHAR`] if the file is at the end of the file.
    fn first(&self) -> char {
        self.chars.clone().next().unwrap_or(EOF_CHAR)
    }

    /// Peeks the next character from the input stream without consuming it.
    /// Returns [`EOF_CHAR`] if the file is at the end of the file.
    fn last(&self) -> char {
        self.chars.clone().next_back().unwrap_or(EOF_CHAR)
    }

    // SAFETY: THe `source.text_len` call in `new` would panic if the string length is larger than a `u32`.
    #[allow(clippy::cast_possible_truncation)]
    fn text_len(&self) -> TextSize {
        TextSize::new(self.chars.as_str().len() as u32)
    }

    fn token_len(&self) -> TextSize {
        self.source_length - self.text_len()
    }

    fn start_token(&mut self) {
        self.source_length = self.text_len();
    }

    fn is_eof(&self) -> bool {
        self.chars.as_str().is_empty()
    }

    /// Consumes the next character
    fn bump(&mut self) -> Option<char> {
        self.chars.next()
    }

    /// Consumes the next character from the back
    fn bump_back(&mut self) -> Option<char> {
        self.chars.next_back()
    }

    fn eat_char(&mut self, c: char) -> bool {
        if self.first() == c {
            self.bump();
            true
        } else {
            false
        }
    }

    fn eat_char_back(&mut self, c: char) -> bool {
        if self.last() == c {
            self.bump_back();
            true
        } else {
            false
        }
    }

    /// Eats symbols while predicate returns true or until the end of file is reached.
    fn eat_while(&mut self, mut predicate: impl FnMut(char) -> bool) {
        // It was tried making optimized version of this for eg. line comments, but
        // LLVM can inline all of this and compile it down to fast iteration over bytes.
        while predicate(self.first()) && !self.is_eof() {
            self.bump();
        }
    }

    /// Eats symbols from the back while predicate returns true or until the beginning of file is reached.
    fn eat_back_while(&mut self, mut predicate: impl FnMut(char) -> bool) {
        // It was tried making optimized version of this for eg. line comments, but
        // LLVM can inline all of this and compile it down to fast iteration over bytes.
        while predicate(self.last()) && !self.is_eof() {
            self.bump_back();
        }
    }
}

#[cfg(test)]
mod tests {
    use crate::trivia::{lines_after, lines_before, SimpleTokenizer, Token};
    use insta::assert_debug_snapshot;
    use ruff_text_size::{TextLen, TextRange, TextSize};

    struct TokenizationTestCase {
        source: &'static str,
        range: TextRange,
        tokens: Vec<Token>,
    }

    impl TokenizationTestCase {
        fn assert_reverse_tokenization(&self) {
            let mut backwards = self.tokenize_reverse();

            // Re-reverse to get the tokens in forward order.
            backwards.reverse();

            assert_eq!(&backwards, &self.tokens);
        }

        fn tokenize_reverse(&self) -> Vec<Token> {
            SimpleTokenizer::new(self.source, self.range)
                .rev()
                .collect()
        }

        fn tokens(&self) -> &[Token] {
            &self.tokens
        }
    }

    fn tokenize_range(source: &'static str, range: TextRange) -> TokenizationTestCase {
        let tokens: Vec<_> = SimpleTokenizer::new(source, range).collect();

        TokenizationTestCase {
            source,
            range,
            tokens,
        }
    }

    fn tokenize(source: &'static str) -> TokenizationTestCase {
        tokenize_range(source, TextRange::new(TextSize::new(0), source.text_len()))
    }

    #[test]
    fn tokenize_trivia() {
        let source = "# comment\n    # comment";

        let test_case = tokenize(source);

        assert_debug_snapshot!(test_case.tokens());
        test_case.assert_reverse_tokenization();
    }

    #[test]
    fn tokenize_parentheses() {
        let source = "([{}])";

        let test_case = tokenize(source);

        assert_debug_snapshot!(test_case.tokens());
        test_case.assert_reverse_tokenization();
    }

    #[test]
    fn tokenize_comma() {
        let source = ",,,,";

        let test_case = tokenize(source);

        assert_debug_snapshot!(test_case.tokens());
        test_case.assert_reverse_tokenization();
    }

    #[test]
    fn tokenize_continuation() {
        let source = "( \\\n )";

        let test_case = tokenize(source);

        assert_debug_snapshot!(test_case.tokens());
        test_case.assert_reverse_tokenization();
    }

    #[test]
    fn tokenize_substring() {
        let source = "('some string') # comment";

        let test_case =
            tokenize_range(source, TextRange::new(TextSize::new(14), source.text_len()));

        assert_debug_snapshot!(test_case.tokens());
        test_case.assert_reverse_tokenization();
    }

    #[test]
    fn tokenize_slash() {
        let source = r#" # trailing positional comment
        # Positional arguments only after here
        ,/"#;

        let test_case = tokenize(source);

        assert_debug_snapshot!(test_case.tokens());
        test_case.assert_reverse_tokenization();
    }

    #[test]
    fn tokenize_bogus() {
        let source = r#"# leading comment
        "a string"
        a = (10)"#;

        let test_case = tokenize(source);

        assert_debug_snapshot!(test_case.tokens());
        assert_debug_snapshot!("Reverse", test_case.tokenize_reverse());
    }

    #[test]
    fn lines_before_empty_string() {
        assert_eq!(lines_before(TextSize::new(0), ""), 0);
    }

    #[test]
    fn lines_before_in_the_middle_of_a_line() {
        assert_eq!(lines_before(TextSize::new(4), "a = 20"), 0);
    }

    #[test]
    fn lines_before_on_a_new_line() {
        assert_eq!(lines_before(TextSize::new(7), "a = 20\nb = 10"), 1);
    }

    #[test]
    fn lines_before_multiple_leading_newlines() {
        assert_eq!(lines_before(TextSize::new(9), "a = 20\n\r\nb = 10"), 2);
    }

    #[test]
    fn lines_before_with_comment_offset() {
        assert_eq!(lines_before(TextSize::new(8), "a = 20\n# a comment"), 0);
    }

    #[test]
    fn lines_before_with_trailing_comment() {
        assert_eq!(
            lines_before(TextSize::new(22), "a = 20 # some comment\nb = 10"),
            1
        );
    }

    #[test]
    fn lines_before_with_comment_only_line() {
        assert_eq!(
            lines_before(TextSize::new(22), "a = 20\n# some comment\nb = 10"),
            1
        );
    }

    #[test]
    fn lines_after_empty_string() {
        assert_eq!(lines_after(TextSize::new(0), ""), 0);
    }

    #[test]
    fn lines_after_in_the_middle_of_a_line() {
        assert_eq!(lines_after(TextSize::new(4), "a = 20"), 0);
    }

    #[test]
    fn lines_after_before_a_new_line() {
        assert_eq!(lines_after(TextSize::new(6), "a = 20\nb = 10"), 1);
    }

    #[test]
    fn lines_after_multiple_newlines() {
        assert_eq!(lines_after(TextSize::new(6), "a = 20\n\r\nb = 10"), 2);
    }

    #[test]
    fn lines_after_before_comment_offset() {
        assert_eq!(lines_after(TextSize::new(7), "a = 20 # a comment\n"), 0);
    }

    #[test]
    fn lines_after_with_comment_only_line() {
        assert_eq!(
            lines_after(TextSize::new(6), "a = 20\n# some comment\nb = 10"),
            1
        );
    }
}