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

	private var events: [Event] = []

	public init() {}

	public struct Checkpoint {
		fileprivate let position: Int
	}

	public var checkpoint: Checkpoint {
		Checkpoint(position: events.count)
	}

	public mutating func openAtCheckpoint(_ checkpoint: Checkpoint) {
		events.insert(.open, at: checkpoint.position)
	}

	public mutating func open() {
		events.append(.open)
	}

	public mutating func close(kind: SyntaxTreeKind, metadata: SyntaxTreeMetadata? = nil) {
		events.append(.close(kind: kind, metadata: metadata))
	}

	public mutating 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):
				guard var node = stack.popLast() else { fatalError("More close events than open events") }
				node.kind = kind
				node.metadata = metadata
				guard let parentIndex = stack.indices.last else { return node.tree }
				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))
			}
		}

		if !stack.isEmpty {
			fatalError("More open events than close events")
		}

		return SyntaxTree(kind: .error, children: [])
	}
}