swift-pterodactyl / Sources / PterodactylSyntax / SyntaxTreeBuilder.swift

// SPDX-FileCopyrightText: 2025 The Project Pterodactyl Developers
// SPDX-FileCopyrightText: 2026 The Project Pterodactyl Developers
//
// SPDX-License-Identifier: MPL-2.0

public class SyntaxTreeBuilder {
	private enum Event: Equatable {
		case open
		case close(kind: SyntaxTreeKind, metadata: SyntaxTreeMetadata?, cancelled: Bool)
		case advance(token: Token, metadata: TokenMetadata?)
	}

	private var events: [Event] = []

	public init() {}

	public struct SubtreeHandle: ~Copyable {
		let parent: SyntaxTreeBuilder
		var kind: SyntaxTreeKind
		var metadata: SyntaxTreeMetadata?
		private var closed = false
		var cancelled = false

		init(parent: SyntaxTreeBuilder, kind: SyntaxTreeKind? = nil, metadata: SyntaxTreeMetadata? = nil) {
			self.parent = parent
			self.kind = kind ?? .error
			self.metadata = metadata

			parent.events.append(.open)
		}

		mutating func cancel() {
			cancelled = true
		}

		mutating func close() {
			precondition(!closed)
			parent.events.append(.close(kind: kind, metadata: metadata, cancelled: cancelled))
			closed = true
		}

		deinit {
			precondition(closed)
		}
	}

	public func open(kind: SyntaxTreeKind? = nil, metadata: SyntaxTreeMetadata? = nil) -> SubtreeHandle {
		SubtreeHandle(parent: self, kind: kind, metadata: metadata)
	}

	public func advance(token: Token, metadata: TokenMetadata?) {
		events.append(.advance(token: token, metadata: metadata))
	}

	public var tree: SyntaxTree {
		var stack: [SyntaxTree.MutableTree] = []

		for event in events {
			switch event {
			case .open:
				stack.append(SyntaxTree.MutableTree(kind: .error, metadata: nil, children: []))

			case .close(let kind, let metadata, let cancelled):
				guard var node = stack.popLast() else { fatalError("Unbalanced tree") }
				node.kind = kind
				node.metadata = metadata
				guard let parentIndex = stack.indices.last else { return node.tree }
				if cancelled {
					stack[parentIndex].children.append(contentsOf: node.children)
				} else {
					stack[parentIndex].children.append(.tree(node.tree))
				}

			case .advance(let token, let metadata):
				guard let parentIndex = stack.indices.last else {
					fatalError("Attempted to insert token at root of parse tree")
				}
				stack[parentIndex]
					.children
					.append(.token(token, metadata: metadata))
			}
		}

		fatalError("Unbalanced syntax tree")
	}
}


extension Array {
	fileprivate mutating func modifyLast(_ modifier: (inout Element) -> Void) {
		if var last = popLast() {
			modifier(&last)
			append(last)
		}
	}
}