AutoCodable/Sources/AutoCodableMacro/AutoCodableMacro.swift

// The Swift Programming Language
// https://docs.swift.org/swift-book
import SwiftSyntax
import SwiftSyntaxBuilder
import SwiftSyntaxMacros
import SwiftDiagnostics
import Foundation

// 宏实现：自动生成 Codable 协议的实现
public struct AutoCodableMacro: MemberMacro {
    public static func expansion(
        of node: AttributeSyntax,
        providingMembersOf declaration: some DeclGroupSyntax,
        in context: some MacroExpansionContext
    ) throws -> [DeclSyntax] {
        // 仅支持类和结构体
        guard let typeDecl: DeclGroupSyntax = declaration.as(ClassDeclSyntax.self) ?? declaration.as(StructDeclSyntax.self) else {
            context.diagnose(
                Diagnostic(
                    node: node,
                    message: AutoCodableMacroError.onlyClassesAndStructs
                )
            )
            return []
        }
        
        // 提取所有存储属性
        let properties: [(name: String, type: String)] = typeDecl.memberBlock.members.compactMap { member -> (name: String, type: String)? in
            // 1. 检查是否是变量声明
            guard let variableDecl = member.decl.as(VariableDeclSyntax.self) else {
                return nil
            }
            
            // 2. 确保只处理单个属性的声明
            guard variableDecl.bindings.count == 1,
                  let binding = variableDecl.bindings.first else {
                return nil
            }
            
            // 3. 提取属性名称
            guard let identifierPattern = binding.pattern.as(IdentifierPatternSyntax.self) else {
                return nil
            }
            let name = identifierPattern.identifier.text
            
            // 4. 提取属性类型
            guard let typeAnnotation = binding.typeAnnotation else {
                return nil
            }
            let type = typeAnnotation.type.description.trimmingCharacters(in: .whitespacesAndNewlines)
            
            return (name: name, type: type)
        }
        
        if properties.isEmpty {
            context.diagnose(
                Diagnostic(
                    node: node,
                    message: AutoCodableMacroError.noStoredProperties
                )
            )
        }
        
        // 生成 CodingKeys 枚举
        let codingKeys = try EnumDeclSyntax("enum CodingKeys: String, CodingKey") {
            for property in properties {
                DeclSyntax("case \(raw: property.name)")
            }
        }
        
        // 生成 init(from:) 方法
        let bodyBuilder: SyntaxNodeString = declaration is ClassDeclSyntax ? "required init(from decoder: Decoder) throws" : "init(from decoder: Decoder) throws"
        let initFrom = try InitializerDeclSyntax(bodyBuilder) {
            CodeBlockItemListSyntax {
                "let container = try decoder.container(keyedBy: CodingKeys.self)"
                for property in properties {
                    """
                    if let value = try? container.decode(\(raw: property.type).self, forKey: .\(raw: property.name)) {
                        self.\(raw: property.name) = value
                    }
                    """
                }
            }
        }
        
        // 生成 encode(to:) 方法
        let encodeTo = try FunctionDeclSyntax("func encode(to encoder: Encoder) throws") {
            CodeBlockItemListSyntax {
                "var container = encoder.container(keyedBy: CodingKeys.self)"
                for property in properties {
                    "try container.encode(\(raw: property.name), forKey: .\(raw: property.name))"
                }
            }
        }
        
        return [
            DeclSyntax(codingKeys),
            DeclSyntax(initFrom),
            DeclSyntax(encodeTo)
        ]
    }
}

// 错误提示定义
enum AutoCodableMacroError: DiagnosticMessage {
    case onlyClassesAndStructs
    case noStoredProperties
    
    var message: String {
        switch self {
        case .onlyClassesAndStructs:
            "AutoCodable can only be applied to classes and structs"
        case .noStoredProperties:
            "Type has no stored properties; Codable implementation will be empty"
        }
    }
    
    var diagnosticID: MessageID { .init(domain: "Codable", id: "\(self)") }
    var severity: DiagnosticSeverity { .error }
}