using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;

namespace ZymonicServices;

/// <summary>Applies a unique sequence of object instances without resetting a bound collection.
/// Call on the thread which owns the collection.</summary>
public static class ZymonicCollectionProjection
{
    public static void Apply<T>(ObservableCollection<T> target, IReadOnlyList<T> desired) where T : class
    {
        var retained = new HashSet<T>(desired, ReferenceEqualityComparer.Instance);
        if (retained.Count != desired.Count)
            throw new ArgumentException("Projection must contain unique instances.", nameof(desired));

        for (int i = target.Count - 1; i >= 0; i--)
            if (!retained.Contains(target[i])) target.RemoveAt(i);
        for (int i = 0; i < desired.Count; i++)
        {
            if (i < target.Count && ReferenceEquals(target[i], desired[i])) continue;
            int existing = -1;
            for (int j = i + 1; j < target.Count; j++)
            {
                if (ReferenceEquals(target[j], desired[i])) { existing = j; break; }
            }
            if (existing >= 0) target.Move(existing, i);
            else target.Insert(i, desired[i]);
        }
        while (target.Count > desired.Count) target.RemoveAt(target.Count - 1);
    }
}