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 from an in-memory collection.
        ///
        /// This method filters the provided tasks array to count only those tasks that belong
        /// to the specified group based on their due date and completion status. Tasks are
        /// categorized as follows:
        ///
        /// - **Completed**: Tasks with a non-nil `completed` date
        /// - **Overdue**: Incomplete tasks due before today
        /// - **Today**: Incomplete tasks due today (from start of day to end of day)
        /// - **Upcoming**: Incomplete tasks due tomorrow or later
        ///
        /// - Parameters:
        ///   - tasks: The array of tasks to filter and count.
        ///   - group: The task group to count tasks for.
        /// - Returns: The number of tasks in the specified group.
        func count(
            _ tasks: [Todo],
            for group: TodoGroup
        ) -> Int {
            let today = calendar.startOfDay(for: .now)
            let tomorrow = calendar.date(byAdding: .day, value: 1, to: today) ?? .now

            return tasks
                .filter { task in
                    switch group {
                        case .completed:
                            task.completed != nil
                        case .overdue:
                            task.completed == nil && task.due < today
                        case .today:
                            task.completed == nil && task.due >= today && task.due < tomorrow
                        case .upcoming:
                            task.completed == nil && task.due >= tomorrow
                    }
                }
                .count
        }

    }
}