//! Cursor utilities for a token stream.
//!
//! Provides memoized lookahead over significant tokens and records trivia in
//! [`TriviaIndexBuilder`]. Significant tokens exclude whitespace and comments;
//! see [`parser::TriviaKind`].

use std::collections::VecDeque;

use crate::{
    ast::AstSpan,
    lexer::{self, Keyword, Token, TokenPosition},
    parser::{self, ParseResult, Parser, ResultRecoveryExt, trivia::TriviaIndexBuilder},
};

/// Cursor over a token stream with memoized lookahead and trivia attachment.
#[derive(Clone, Debug)]
pub(crate) struct Cursor<'file, 'src> {
    tokens: lexer::Tokens<'file, 'src>,
    lookahead_buffer: VecDeque<(TokenPosition, lexer::TokenData<'src>)>,
    last_consumed_position: Option<TokenPosition>,
}

impl<'file, 'src> Cursor<'file, 'src> {
    /// Creates a [`Cursor`] over `tokenized_file`.
    pub(crate) const fn new(tokenized_file: &'file lexer::TokenizedFile<'src>) -> Self {
        Self {
            tokens: tokenized_file.iter(),
            lookahead_buffer: VecDeque::new(),
            last_consumed_position: None,
        }
    }

    /// Ensures that the lookahead buffer contains at least `lookahead + 1`
    /// significant tokens, if available.
    ///
    /// May consume trivia from the underlying stream without consuming
    /// significant tokens.
    fn ensure_lookahead_available(
        &mut self,
        lookahead: usize,
        trivia: &mut TriviaIndexBuilder<'src>,
    ) {
        while self.lookahead_buffer.len() <= lookahead {
            if !self.buffer_next_significant_token(trivia) {
                break;
            }
        }
    }

    /// Buffers the next significant token and records any preceding trivia.
    ///
    /// Returns `true` if a significant token was buffered, or `false` if the
    /// stream is exhausted.
    fn buffer_next_significant_token(&mut self, trivia: &mut TriviaIndexBuilder<'src>) -> bool {
        for (token_position, token_data) in self.tokens.by_ref() {
            if let Ok(trivia_kind) = parser::TriviaKind::try_from(token_data.token) {
                trivia.record_trivia(parser::TriviaToken {
                    kind: trivia_kind,
                    text: token_data.lexeme,
                    position: token_position,
                });
            } else {
                trivia.record_significant_token(token_position);
                self.lookahead_buffer
                    .push_back((token_position, token_data));
                return true;
            }
        }
        false
    }
}

impl<'src, 'arena> Parser<'src, 'arena> {
    fn peek_buffered_token(&mut self) -> Option<&(TokenPosition, lexer::TokenData<'src>)> {
        self.cursor.ensure_lookahead_available(0, &mut self.trivia);
        self.cursor.lookahead_buffer.front()
    }

    /// Returns the next significant token without consuming it.
    ///
    /// May buffer additional tokens and record skipped trivia, but does not
    /// consume any significant token.
    ///
    /// Returns [`None`] if no tokens remain.
    #[must_use]
    pub(crate) fn peek_token(&mut self) -> Option<Token> {
        self.peek_buffered_token()
            .map(|(_, token_data)| token_data.token)
    }

    /// Returns the next keyword without consuming it.
    ///
    /// May buffer additional tokens and record skipped trivia, but does not
    /// consume any significant token.
    ///
    /// Returns [`None`] if no tokens remain or if the next token is not
    /// a keyword.
    #[must_use]
    pub(crate) fn peek_keyword(&mut self) -> Option<Keyword> {
        match self.peek_token() {
            Some(Token::Keyword(keyword)) => Some(keyword),
            _ => None,
        }
    }

    /// Returns the position of the next significant token without consuming it.
    ///
    /// May buffer additional tokens and record skipped trivia, but does not
    /// consume any significant token.
    ///
    /// Returns [`None`] if no tokens remain.
    #[must_use]
    pub(crate) fn peek_position(&mut self) -> Option<TokenPosition> {
        self.peek_buffered_token()
            .map(|(token_position, _)| *token_position)
    }

    /// Returns the next significant token and its lexeme without consuming it.
    ///
    /// May buffer additional tokens and record skipped trivia, but does not
    /// consume any significant token.
    ///
    /// Returns [`None`] if no tokens remain.
    #[must_use]
    pub(crate) fn peek_token_and_lexeme(&mut self) -> Option<(Token, &'src str)> {
        self.peek_buffered_token()
            .map(|(_, token_data)| (token_data.token, token_data.lexeme))
    }

    /// Returns the next significant token and its position without consuming
    /// it.
    ///
    /// May buffer additional tokens and record skipped trivia, but does not
    /// consume any significant token.
    ///
    /// Returns [`None`] if no tokens remain.
    #[must_use]
    pub(crate) fn peek_token_and_position(&mut self) -> Option<(Token, TokenPosition)> {
        self.peek_buffered_token()
            .map(|(token_position, token_data)| (token_data.token, *token_position))
    }

    /// Returns the next keyword and its position without consuming it.
    ///
    /// May buffer additional tokens and record skipped trivia, but does not
    /// consume any significant token.
    ///
    /// Returns [`None`] if next token isn't keyword or no tokens remain.
    #[must_use]
    pub(crate) fn peek_keyword_and_position(&mut self) -> Option<(Keyword, TokenPosition)> {
        let Some((Token::Keyword(keyword), keyword_position)) = self.peek_token_and_position()
        else {
            return None;
        };
        Some((keyword, keyword_position))
    }

    /// Returns the next significant token, its lexeme, and its position
    /// without consuming them.
    ///
    /// May buffer additional tokens and record skipped trivia, but does not
    /// consume any significant token.
    ///
    /// Returns [`None`] if no tokens remain.
    #[must_use]
    pub(crate) fn peek_token_lexeme_and_position(
        &mut self,
    ) -> Option<(Token, &'src str, TokenPosition)> {
        self.peek_buffered_token()
            .map(|(token_position, token_data)| {
                (token_data.token, token_data.lexeme, *token_position)
            })
    }

    /// Returns the next significant token at `lookahead` without consuming it.
    ///
    /// `lookahead` counts significant tokens, with `0` referring to the next
    /// significant token.
    ///
    /// May buffer additional tokens and record skipped trivia, but does not
    /// consume any significant token.
    ///
    /// Returns [`None`] if no tokens remain.
    #[must_use]
    pub(crate) fn peek_token_at(&mut self, lookahead: usize) -> Option<Token> {
        self.cursor
            .ensure_lookahead_available(lookahead, &mut self.trivia);
        self.cursor
            .lookahead_buffer
            .get(lookahead)
            .map(|(_, token_data)| token_data.token)
    }

    /// Returns the keyword at `lookahead` without consuming it.
    ///
    /// `lookahead` counts significant tokens, with `0` referring to the next
    /// significant token.
    ///
    /// May buffer additional tokens and record skipped trivia, but does not
    /// consume any significant token.
    ///
    /// Returns [`None`] if the token at that position is not a keyword or if
    /// the stream ends before that position.
    #[must_use]
    pub(crate) fn peek_keyword_at(&mut self, lookahead: usize) -> Option<Keyword> {
        match self.peek_token_at(lookahead) {
            Some(Token::Keyword(keyword)) => Some(keyword),
            _ => None,
        }
    }

    /// Returns the position of the next significant token without consuming it.
    ///
    /// Generates an error with `error_kind` if no tokens remain.
    pub(crate) fn require_position(
        &mut self,
        error_kind: parser::ParseErrorKind,
    ) -> ParseResult<'src, 'arena, TokenPosition> {
        self.peek_position()
            .ok_or_else(|| self.make_error_here(error_kind))
    }

    /// Returns the next significant token and its position without consuming
    /// it.
    ///
    /// Generates an error with `error_kind` if no tokens remain.
    pub(crate) fn require_token_and_position(
        &mut self,
        error_kind: parser::ParseErrorKind,
    ) -> ParseResult<'src, 'arena, (Token, TokenPosition)> {
        self.peek_token_and_position()
            .ok_or_else(|| self.make_error_here(error_kind))
    }

    /// Returns the next significant token, its lexeme, and its position
    /// without consuming them.
    ///
    /// Generates an error with `error_kind` if no tokens remain.
    pub(crate) fn require_token_lexeme_and_position(
        &mut self,
        error_kind: parser::ParseErrorKind,
    ) -> ParseResult<'src, 'arena, (Token, &'src str, TokenPosition)> {
        self.peek_token_lexeme_and_position()
            .ok_or_else(|| self.make_error_here(error_kind))
    }

    /// Advances by one significant token.
    ///
    /// Records any skipped trivia and returns the consumed token position.
    /// Returns [`None`] if no significant tokens remain.
    pub(crate) fn advance(&mut self) -> Option<TokenPosition> {
        self.cursor.ensure_lookahead_available(0, &mut self.trivia);
        if let Some((token_position, _)) = self.cursor.lookahead_buffer.pop_front() {
            self.cursor.last_consumed_position = Some(token_position);
            Some(token_position)
        } else {
            None
        }
    }

    /// If the next significant token equals `token`, consumes it and
    /// returns `true`.
    ///
    /// Otherwise leaves the cursor unchanged and returns `false`.
    #[must_use]
    pub(crate) fn eat(&mut self, token: Token) -> bool {
        if self.peek_token() == Some(token) {
            self.advance();
            true
        } else {
            false
        }
    }

    /// If the next significant token corresponds to the given keyword,
    /// consumes it and returns `true`.
    ///
    /// Otherwise leaves the cursor unchanged and returns `false`.
    #[must_use]
    pub(crate) fn eat_keyword(&mut self, keyword: Keyword) -> bool {
        self.eat(Token::Keyword(keyword))
    }

    /// Expects `expected` token as the next significant one.
    ///
    /// On match consumes the token and returns its [`TokenPosition`].
    /// Otherwise returns an error of `error_kind` anchored at
    /// the current token, or at the last consumed token if the stream is
    /// exhausted. That error also gets set a blame span that contains exactly
    /// that anchor point.
    pub(crate) fn expect(
        &mut self,
        expected: Token,
        error_kind: parser::ParseErrorKind,
    ) -> ParseResult<'src, 'arena, TokenPosition> {
        // Anchors EOF diagnostics at the last consumed token
        // when no current token exists.
        let anchor = self
            .peek_position()
            .unwrap_or_else(|| self.last_consumed_position_or_start());
        // `Token` equality is enough here because lexeme and position
        // are stored separately.
        if self.peek_token() == Some(expected) {
            self.advance();
            Ok(anchor)
        } else {
            Err(self
                .make_error_at(error_kind, anchor)
                .blame(AstSpan::new(anchor)))
        }
    }

    /// Expects `expected` keyword as the next significant token.
    ///
    /// On match consumes the keyword and returns its [`TokenPosition`].
    /// Otherwise returns an error of `error_kind` anchored at the current
    /// token, or at the last consumed token if the stream is exhausted.
    pub(crate) fn expect_keyword(
        &mut self,
        expected: Keyword,
        error_kind: parser::ParseErrorKind,
    ) -> ParseResult<'src, 'arena, TokenPosition> {
        self.expect(Token::Keyword(expected), error_kind)
    }

    /// Returns position of the last significant token that was actually
    /// consumed by [`parser::Parser::advance`].
    ///
    /// Returns [`None`] if no tokens have been consumed yet.
    #[must_use]
    pub(crate) const fn last_consumed_position(&self) -> Option<TokenPosition> {
        self.cursor.last_consumed_position
    }

    /// Returns the position of the last significant token consumed by
    /// [`parser::Parser::advance`], or the start of the stream if no token has
    /// been consumed yet.
    ///
    /// Useful when diagnostics need a stable anchor even at the beginning of
    /// input.
    #[must_use]
    pub(crate) fn last_consumed_position_or_start(&self) -> TokenPosition {
        self.cursor
            .last_consumed_position
            .unwrap_or(TokenPosition(0))
    }

    /// Ensures that parsing has advanced past `old_position`.
    ///
    /// This is intended as a safeguard against infinite-loop bugs while
    /// recovering from invalid input. In debug builds it asserts that progress
    /// was made; in release builds it consumes one significant token when
    /// the parser stalls.
    #[track_caller]
    pub(crate) fn ensure_forward_progress(&mut self, old_position: TokenPosition) {
        if let Some(peeked_position) = self.peek_position() {
            debug_assert!(
                peeked_position > old_position,
                "parser made no forward progress"
            );
            if peeked_position <= old_position {
                self.advance();
            }
        }
    }
}