Luminate/Sources/LuminateObservationMacros/ObservableMacro.swift

//
//  ObservableMacro.swift
//  LuminateObservationMacros
//
//  Created by Brendan Szymanski on 6/15/26.
//

import SwiftSyntax
import SwiftSyntaxMacros

/// The `@Observable` macro implementation.
///
/// Expands in three phases:
/// 1. **MemberMacro**: Adds `_$observationRegistrar` + one stored backing property
///    (`_$<name>`) per tracked `var`, preserving type and initial value.
/// 2. **MemberAttributeMacro**: Applies `@ObservationTracked` to each stored `var`.
/// 3. **AccessorMacro**: Wraps each tracked property with `get`/`set` that read/write
///    the `_$<name>` backing property and call `_$observationRegistrar.didChange()` on
///    every set. No `init` accessor needed — the backing stored property handles
///    the initial value naturally.
public enum ObservableMacro: MemberMacro, MemberAttributeMacro, AccessorMacro {

    // MARK: - MemberMacro

    /// Adds the observation registrar and a stored backing property for each
    /// tracked `var`, preserving the original type and initializer expression.
    public static func expansion(
        of node: AttributeSyntax,
        providingMembersOf declaration: some DeclGroupSyntax,
        conformingTo protocols: [TypeSyntax],
        in context: some MacroExpansionContext
    ) throws -> [DeclSyntax] {
        var members: [DeclSyntax] = [
            """
            public var _$observationRegistrar: ObservationRegistrar = .init()
            """
        ]

        for member in declaration.memberBlock.members {
            guard let variableDecl = member.decl.as(VariableDeclSyntax.self),
                variableDecl.isStoredVar,
                !variableDecl.isObservationSynthesized
            else {
                continue
            }

            // For each binding in the declaration, synthesize a backing property
            for binding in variableDecl.bindings {
                guard let name = binding.pattern.as(IdentifierPatternSyntax.self)?.identifier.text
                else {
                    continue
                }

                let backingName = "_$" + name
                let initializer = binding.initializer?.trimmedDescription ?? ""

                // Include type annotation if present, otherwise let Swift infer from initializer
                if let typeAnnotation = binding.typeAnnotation {
                    let type = typeAnnotation.type.trimmedDescription
                    if initializer.isEmpty {
                        members.append("internal var \(raw: backingName): \(raw: type)")
                    } else {
                        members.append(
                            "internal var \(raw: backingName): \(raw: type) \(raw: initializer)")
                    }
                } else if !initializer.isEmpty {
                    members.append("internal var \(raw: backingName) \(raw: initializer)")
                }
                // If neither type annotation nor initializer, skip (would be a compile error in the original code anyway)
            }
        }

        return members
    }

    // MARK: - MemberAttributeMacro

    /// Adds `@ObservationTracked` to each stored `var` property (excluding synthesized backing members).
    public static func expansion(
        of node: AttributeSyntax,
        attachedTo declaration: some DeclGroupSyntax,
        providingAttributesFor member: some DeclSyntaxProtocol,
        in context: some MacroExpansionContext
    ) throws -> [AttributeSyntax] {
        guard let variableDecl = member.as(VariableDeclSyntax.self),
            variableDecl.isStoredVar,
            !variableDecl.isObservationSynthesized,
            !variableDecl.isBackingProperty
        else {
            return []
        }
        return [
            AttributeSyntax(stringLiteral: "@ObservationTracked")
        ]
    }

    // MARK: - AccessorMacro

    /// Wraps the property with `get`/`set` that read from / write to `_$<name>` backing property
    /// and call `_$observationRegistrar.didChange()` on every set.
    public static func expansion(
        of node: AttributeSyntax,
        providingAccessorsOf declaration: some DeclSyntaxProtocol,
        in context: some MacroExpansionContext
    ) throws -> [AccessorDeclSyntax] {
        guard let varDecl = declaration.as(VariableDeclSyntax.self),
            let binding = varDecl.bindings.first,
            let name = binding.pattern.as(IdentifierPatternSyntax.self)?.identifier.text
        else {
            return []
        }

        let backingName = "_$" + name

        return [
            """
            get {
                \(raw: backingName)
            }
            """,
            """
            set {
                \(raw: backingName) = newValue
                _$observationRegistrar.didChange()
            }
            """,
        ]
    }
}

// MARK: - VariableDeclSyntax Helpers

extension VariableDeclSyntax {

    /// Whether this declaration is a stored `var` (not `let`, not computed).
    var isStoredVar: Bool {
        guard bindingSpecifier.tokenKind == .keyword(.var) else {
            return false
        }
        return bindings.allSatisfy { $0.accessorBlock == nil }
    }

    /// Whether this declaration was synthesized by `@Observable` (`_$observationRegistrar`).
    var isObservationSynthesized: Bool {
        bindings.contains { binding in
            guard let pattern = binding.pattern.as(IdentifierPatternSyntax.self) else {
                return false
            }
            return pattern.identifier.text == "_$observationRegistrar"
        }
    }

    /// Whether this declaration is a backing property (`_$<name>`) synthesized by `@Observable`.
    var isBackingProperty: Bool {
        bindings.allSatisfy { binding in
            guard let pattern = binding.pattern.as(IdentifierPatternSyntax.self) else {
                return false
            }
            return pattern.identifier.text.hasPrefix("_$")
        }
    }
}