using System.Text;
using ClawdDotNet.Core.Organisation;

namespace ClawdDotNet.Core.Tasks;

/// <summary>
/// #714: Why a task exists — its parent, that one's parent, up to the goal of the unit.
/// Taken from Paperclip (Organisation-Konzept, 9a): an agent that only sees its own order
/// cannot weigh it. "Write a product text" is a different job when it serves "launch the
/// spring collection by Friday" than when it serves "clean up old listings".
///
/// <para><b>Purpose, not precondition.</b> <see cref="TaskItem.BlockedBy"/> says when a task
/// may run; the parent says what it is for. Both can point to the same task, and mean
/// different things.</para>
///
/// <para>Walked at run time, never stored as text: a parent that is renamed or finished
/// shows up as it is now. The walk stops at a cycle and after <see cref="MaxDepth"/> steps,
/// so a broken chain cannot hang a run or flood its prompt.</para>
/// </summary>
public static class GoalChain
{
    public const int MaxDepth = 8;

    /// <summary>The parents of a task, nearest first. Stops at a missing parent or a cycle.</summary>
    public static async Task<IReadOnlyList<TaskItem>> ParentsAsync(
        TaskItem task, Func<string, CancellationToken, Task<TaskItem?>> lookup, CancellationToken ct)
    {
        var parents = new List<TaskItem>();
        var seen = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { task.Id };
        var next = task.ParentId;

        while (!string.IsNullOrWhiteSpace(next) && parents.Count < MaxDepth && seen.Add(next))
        {
            if (await lookup(next, ct) is not { } parent)
                break;
            parents.Add(parent);
            next = parent.ParentId;
        }
        return parents;
    }

    /// <summary>
    /// Whether making <paramref name="parentId"/> the parent of <paramref name="taskId"/>
    /// would close a circle — the parent is the task itself or lies below it.
    /// </summary>
    public static async Task<bool> WouldCycleAsync(
        string taskId, string parentId, Func<string, CancellationToken, Task<TaskItem?>> lookup, CancellationToken ct)
    {
        if (string.Equals(taskId, parentId, StringComparison.OrdinalIgnoreCase))
            return true;

        var seen = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
        var next = parentId;
        while (!string.IsNullOrWhiteSpace(next) && seen.Add(next))
        {
            if (string.Equals(next, taskId, StringComparison.OrdinalIgnoreCase))
                return true;
            next = (await lookup(next, ct))?.ParentId;
        }
        return false;
    }

    /// <summary>
    /// The goal above the chain: that of the unit of the topmost task that belongs to one —
    /// a goal a human set on the top level belongs to no unit, so the next one down counts —
    /// or of the nearest unit above it that has a goal. <c>null</c> without units or without
    /// a goal.
    /// </summary>
    public static (string UnitName, string Goal)? UnitGoal(IReadOnlyList<TaskItem> fromTop, OrgChart? org)
    {
        if (org is not { HasUnits: true })
            return null;
        var unitId = fromTop.Select(t => TaskBoard.UnitOf(t, org)).FirstOrDefault(u => u is not null);
        if (org.Unit(unitId) is not { } unit)
            return null;

        foreach (var candidate in new[] { unit }.Concat(org.Ancestors(unit.Id)))
        {
            if (!string.IsNullOrWhiteSpace(candidate.Goal))
                return (candidate.Name, candidate.Goal.Trim());
        }
        return null;
    }

    /// <summary>
    /// The paragraph that goes into the run: "you do this because … because … because".
    /// <c>null</c> when there is nothing above the task — then the run looks as before.
    /// </summary>
    public static string? Describe(TaskItem task, IReadOnlyList<TaskItem> parents, OrgChart? org)
    {
        var goal = UnitGoal([.. parents.Reverse(), task], org);
        if (parents.Count == 0 && goal is null)
            return null;

        var sb = new StringBuilder();
        sb.AppendLine("Why this task exists — it serves, from nearest to farthest:");
        foreach (var parent in parents)
            sb.AppendLine($"- {parent.Id} \"{parent.Title}\" ({TaskText.Of(parent.Status)})");
        if (goal is { } g)
            sb.AppendLine($"- the goal of the unit \"{g.UnitName}\": {g.Goal}");
        sb.Append("Weigh your work against this. If the task no longer serves it — the parent is done or canceled, or the task contradicts the goal — say so in your result instead of working blindly.");
        return sb.ToString();
    }
}