//! Statement parsing for the language front-end.
//!
//! Implements a simple recursive-descent parser for
//! *Fermented UnrealScript statements*.

use crate::ast::{AstSpan, Statement, StatementRef};
use crate::lexer::Token;
use crate::parser::{ParseErrorKind, ResultRecoveryExt, SyncLevel};

impl<'src, 'arena> crate::parser::Parser<'src, 'arena> {
    /// Parses a single statement.
    ///
    /// Does not consume a trailing `;` except for [`Statement::Empty`].
    /// The caller handles semicolons. Returns [`Some`] if a statement is
    /// recognized; otherwise [`None`].
    #[must_use]
    pub(crate) fn parse_statement(&mut self) -> Option<StatementRef<'src, 'arena>> {
        let Some((token, lexeme, location)) = self.peek_token_lexeme_and_location() else {
            self.report_error_here(ParseErrorKind::UnexpectedEndOfFile);
            return None;
        };
        match token {
            // Empty statement
            Token::Semicolon => {
                self.advance(); // `;`
                Some(self.arena.alloc(Statement::Empty, AstSpan::new(location)))
            }
            // UnrealScript's standard `local` variable declaration
            Token::Local => {
                self.advance(); // `local`
                Some(
                    self.parse_local_variable_declaration_cont()
                        .widen_error_span_from(location)
                        .sync_error_until(self, SyncLevel::Statement)
                        .unwrap_or_fallback(self),
                )
            }
            // Label definition
            Token::Identifier if matches!(self.peek_token_at(1), Some(Token::Colon)) => {
                self.advance(); // `Token::Identifier`
                self.advance(); // `:`
                Some(self.arena.alloc(
                    Statement::Label(self.arena.string(lexeme)),
                    AstSpan::range(location, self.last_visited_location()),
                ))
            }
            // C-like variable declaration
            token
                if token.is_valid_type_name_token()
                    && Some(Token::Identifier) == self.peek_token_at(1) =>
            {
                self.advance(); // `TYPE_NAME`
                // Next token is guaranteed to exist by the arm condition
                Some(self.parse_variable_declaration_cont(lexeme))
            }
            // Not a statement
            _ => None,
        }
    }

    /// Parses a local variable declaration after `local` has been consumed.
    ///
    /// Requires the next token to be a type name. Initializers are not allowed.
    /// Reports and recovers from errors; the identifier list may be empty if
    /// recovery fails.
    fn parse_local_variable_declaration_cont(
        &mut self,
    ) -> crate::parser::ParseResult<'src, 'arena, StatementRef<'src, 'arena>> {
        let Some((type_token, type_name)) = self.peek_token_and_lexeme() else {
            return Err(self.make_error_here(ParseErrorKind::UnexpectedEndOfFile));
        };
        if !type_token.is_valid_type_name_token() {
            return Err(self.make_error_here(ParseErrorKind::LocalInvalidTypeName));
        }
        let declaration_start_location = self.last_visited_location();
        self.advance(); // `TYPE_NAME`

        let type_name = self.arena.string(type_name);
        let identifiers = self.parse_local_identifier_list();
        if identifiers.is_empty() {
            self.make_error_here(ParseErrorKind::LocalMissingIdentifier)
                .widen_error_span_from(declaration_start_location)
                .report_error(self);
        }
        Ok(self.arena.alloc(
            Statement::LocalVariableDeclaration {
                type_name,
                identifiers,
            },
            AstSpan::range(declaration_start_location, self.last_visited_location()),
        ))
    }

    /// Parses a comma-separated list of identifiers for a local declaration.
    ///
    /// Best-effort recovery from errors. Returns an empty list if no valid
    /// identifiers are found.
    fn parse_local_identifier_list(
        &mut self,
    ) -> crate::arena::ArenaVec<'arena, crate::arena::ArenaString<'arena>> {
        let mut identifiers = self.arena.vec();
        while let Some((token, next_variable_name)) = self.peek_token_and_lexeme() {
            if token == Token::Identifier {
                identifiers.push(self.arena.string(next_variable_name));
                self.advance(); // `Token::Identifier`
            } else {
                self.report_error_here(ParseErrorKind::LocalBadVariableIdentifier);
                // Try to recover to the next variable name
                self.recover_until(SyncLevel::ListSeparator);
            }

            // Disallow initializers in `local`.
            if let Some(Token::Assign) = self.peek_token() {
                self.report_error_here(ParseErrorKind::LocalInitializerNotAllowed);
                self.recover_until(SyncLevel::ListSeparator);
            }

            // Can the list continue?
            // Loop cannot stall: each iteration consumes a token or breaks
            if !self.eat(Token::Comma) {
                break;
            }
        }
        // End-of-file branch
        identifiers
    }

    /// Parses a non-local variable declaration after the type name token
    /// has been consumed.
    ///
    /// The caller must guarantee that at least one declarator follows.
    /// Optional initializers are allowed.
    fn parse_variable_declaration_cont(
        &mut self,
        type_name: &'src str,
    ) -> StatementRef<'src, 'arena> {
        let declaration_start_location = self.last_visited_location();
        let type_name = self.arena.string(type_name);
        let declarations = self.parse_variable_declaration_list();
        // An identifier required by method's condition
        debug_assert!(!declarations.is_empty());
        self.arena.alloc(
            Statement::VariableDeclaration {
                type_name,
                declarations,
            },
            AstSpan::range(declaration_start_location, self.last_visited_location()),
        )
    }

    /// Parses a comma-separated list of declarators with optional `=`
    /// initializers.
    ///
    /// Best-effort recovery on errors.
    /// The caller should invoke this when the next token starts a declarator.
    fn parse_variable_declaration_list(
        &mut self,
    ) -> crate::arena::ArenaVec<'arena, crate::ast::VariableDeclarator<'src, 'arena>> {
        let mut variables = self.arena.vec();
        while let Some((token, next_variable_name)) = self.peek_token_and_lexeme() {
            if token == Token::Identifier {
                self.advance(); // `Token::Identifier`
                let name = self.arena.string(next_variable_name);
                let initializer = if self.eat(Token::Assign) {
                    Some(self.parse_expression())
                } else {
                    None
                };
                variables.push(crate::ast::VariableDeclarator { name, initializer });
            } else {
                self.report_error_here(ParseErrorKind::DeclBadVariableIdentifier);
                // Try to recover to the next variable name
                self.recover_until(SyncLevel::ListSeparator);
            }

            // Can the list continue?
            // Loop cannot stall: each iteration consumes a token or breaks
            if !self.eat(Token::Comma) {
                break;
            }
        }
        // End-of-file branch
        variables
    }
}