todos/App/Sources/View Models/TaskGroupListViewModel.swift

import SwiftData
import SwiftUI

extension TaskGroupListView {

    /// View model for managing the logic for the task group list view.
    ///
    /// This observable class encapsulates the state and business logic for the task group list view.
    /// It follows the MVVM pattern to separate presentation logic from the view layer.
    ///
    /// ## Responsibilities
    /// - Providing access to all task groups for iteration
    /// - Efficiently counting tasks per group using SwiftData fetch descriptors
    /// - Managing date calculations for group categorization
    @Observable
    final class ViewModel {

        // MARK: Properties

        /// The calendar used for date calculations.
        ///
        /// This calendar is used to determine day boundaries for categorizing tasks into date groups: today, upcoming, overdue.
        /// Defaults to the current calendar.
        private let calendar: Calendar

        // MARK: Initialization

        /// Initializes a new view model instance.
        ///
        /// - Parameter calendar: The calendar to use for date calculations. Defaults to `.current`.
        init(calendar: Calendar = .current) {
            self.calendar = calendar
        }

        // MARK: Computed

        /// Returns all available task groups in an predetermined order.
        ///
        /// This computed property provides access to all task group cases for iteration in the view.
        ///
        /// - Returns: An array of all `TodoGroup` cases in a predetermined order.
        var groups: [TodoGroup] {
            TodoGroup.allCases
        }

        // MARK: Functions

        /// Calculates the number of tasks for a given task group using SwiftData.
        ///
        /// This method performs an efficient database-level count using SwiftData's `fetchCount` method with predicate-based filtering.
        /// Tasks are categorized based on their due date and completion status.
        ///
        /// - Parameters:
        ///   - group: The task group to count tasks for.
        ///   - modelContext: The SwiftData model context to query.
        /// - Returns: The number of tasks in the specified group, or 0 if an error occurs.
        func count(
            for group: TodoGroup,
            in modelContext: ModelContext
        ) -> Int {
            let today = calendar.startOfDay(for: .now)
            let tomorrow = calendar.date(byAdding: .day, value: 1, to: today) ?? .now

            let fetchDescriptor: FetchDescriptor<Todo> =
                switch group {
                    case .completed:
                        .init(predicate: #Predicate { $0.completed != nil })
                    case .overdue:
                        .init(predicate: #Predicate { $0.completed == nil && $0.due < today })
                    case .today:
                        .init(predicate: #Predicate { $0.completed == nil && $0.due >= today && $0.due < tomorrow })
                    case .upcoming:
                        .init(predicate: #Predicate { $0.completed == nil && $0.due >= tomorrow })
                }

            do {
                return try modelContext.fetchCount(fetchDescriptor)
            }
            catch {
                print("Error fetching count for group \(group): \(error)")
                return 0
            }
        }

    }
}