lib/src/front_end/sequence_builder.dart

// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
import 'package:analyzer/dart/ast/ast.dart';
import 'package:analyzer/dart/ast/token.dart';

import '../ast_extensions.dart';
import '../back_end/code_writer.dart';
import '../piece/piece.dart';
import '../piece/sequence.dart';
import '../piece/text.dart';
import 'piece_factory.dart';

/// Incrementally builds a [SequencePiece], including handling comments and
/// newlines that may appear before, between, or after its contents.
///
/// Comments are handled specially here so that we can give them better
/// formatting than we would be able to if we treated all comments generally.
///
/// Most comments appear around statements in a block, members in a class, or
/// at the top level of a file. For those, we treat them essentially like
/// separate statements inside the sequence. This lets us gracefully handle
/// indenting them and supporting blank lines around them the same way we handle
/// other statements or members in a sequence.
final class SequenceBuilder {
  final PieceFactory _visitor;

  /// The opening bracket before the elements, if any.
  Piece? _leftBracket;

  /// The series of elements in the sequence.
  final List<SequenceElementPiece> _elements = [];

  /// The closing bracket after the elements, if any.
  Piece? _rightBracket;

  /// Whether a blank line should be allowed after the current element.
  bool _allowBlank = false;

  SequenceBuilder(this._visitor);

  Piece build({bool forceSplit = false}) {
    // If the sequence only contains a single piece, just return it directly
    // and discard the unnecessary wrapping.
    if (_leftBracket == null &&
        _elements.length == 1 &&
        _elements.single.hangingComments.isEmpty &&
        _rightBracket == null) {
      return _elements.single.piece;
    }

    // If there are no elements, don't bother making a SequencePiece or
    // BlockPiece.
    if (_elements.isEmpty) {
      return _visitor.pieces.build(() {
        if (_leftBracket case var bracket?) _visitor.pieces.add(bracket);

        if (forceSplit || _leftBracket == null) {
          _visitor.pieces.add(NewlinePiece());
        }

        if (_rightBracket case var bracket?) _visitor.pieces.add(bracket);
      });
    }

    // Discard any trailing blank line after the last element.
    _elements.last.blankAfter = false;

    var sequence = SequencePiece(_elements);
    if ((_leftBracket, _rightBracket) case (var left?, var right?)) {
      return BlockPiece(left, sequence, right);
    }

    return sequence;
  }

  /// Adds the opening [bracket] to the built sequence.
  void leftBracket(Token bracket) {
    _leftBracket = _visitor.tokenPiece(bracket);
  }

  /// Adds the closing [bracket] to the built sequence along with any comments
  /// that precede it.
  void rightBracket(Token bracket) {
    // Place any comments before the bracket inside the block.
    addCommentsBefore(bracket);
    _rightBracket = _visitor.tokenPiece(bracket);
  }

  /// Adds [piece] to this sequence.
  ///
  /// The caller should have already called [addCommentsBefore()] with the
  /// first token in [piece].
  void add(Piece piece, {Indent? indent, bool allowBlankAfter = true}) {
    _elements.add(SequenceElementPiece(indent ?? Indent.none, piece));

    _allowBlank = allowBlankAfter;
  }

  /// Visits [node] and adds the resulting [Piece] to this sequence, handling
  /// any comments or blank lines that appear before it.
  void visit(AstNode node, {Indent? indent, bool allowBlankAfter = true}) {
    addCommentsBefore(node.firstNonCommentToken, indent: indent);
    add(
      _visitor.nodePiece(node),
      indent: indent,
      allowBlankAfter: allowBlankAfter,
    );
  }

  /// Appends a blank line before the next piece in the sequence.
  void addBlank() {
    if (_elements.isEmpty) return;
    if (!_allowBlank) return;
    _elements.last.blankAfter = true;
  }

  /// Writes any comments appearing before [token] to the sequence.
  ///
  /// Also handles blank lines between preceding comments and elements and the
  /// subsequent element.
  ///
  /// Comments between sequence elements get special handling where comments
  /// on their own line become standalone sequence elements.
  void addCommentsBefore(Token token, {Indent? indent}) {
    indent ??= Indent.none;

    var comments = _visitor.comments.takeCommentsBefore(token);

    // Edge case: if we require a blank line, but there exists one between
    // some of the comments, or after the last one, then we don't need to
    // enforce one before the first comment. Example:
    //
    //     library foo;
    //     // comment
    //
    //     class Bar {}
    //
    // Normally, a blank line is required after `library`, but since there is
    // one after the comment, we don't need one before it. This is mainly so
    // that commented out directives stick with their preceding group.
    if (comments.containsBlank && _elements.isNotEmpty) {
      _elements.last.blankAfter = false;
    }

    for (var i = 0; i < comments.length; i++) {
      var comment = _visitor.pieces.commentPiece(comments[i]);

      if (_elements.isNotEmpty && comments.isHanging(i)) {
        // Attach the comment to the previous element.
        _elements.last.hangingComments.add(comment);
      } else {
        if (comments.linesBefore(i) > 1) {
          // Always preserve a blank line above sequence-level comments.
          _allowBlank = true;
          addBlank();
        }

        // Write the comment as its own sequence piece.
        _elements.add(SequenceElementPiece(indent, comment));
      }
    }

    // Write a blank before the token if there should be one.
    if (comments.linesBeforeNextToken > 1) {
      // If we just wrote a comment, then allow a blank line between it and the
      // element.
      if (comments.isNotEmpty) _allowBlank = true;

      addBlank();
    }
  }
}