// Licensed to StellaOps under the AGPL-3.0-or-later license.

using System.Collections.Immutable;
using StellaOps.ReachGraph.Schema;

namespace StellaOps.ReachGraph.Deduplication;

/// <summary>
/// An edge that has been deduplicated from multiple source edges.
/// Preserves provenance by tracking all contributing sources.
/// </summary>
public sealed record DeduplicatedEdge
{
    /// <summary>
    /// Gets the semantic key for this edge.
    /// </summary>
    public required EdgeSemanticKey Key { get; init; }

    /// <summary>
    /// Gets the source node ID (from entry point).
    /// </summary>
    public required string From { get; init; }

    /// <summary>
    /// Gets the target node ID (to sink).
    /// </summary>
    public required string To { get; init; }

    /// <summary>
    /// Gets the aggregated explanation for this edge.
    /// </summary>
    public required EdgeExplanation Why { get; init; }

    /// <summary>
    /// Gets the set of source identifiers that contributed this edge.
    /// </summary>
    public required ImmutableHashSet<string> Sources { get; init; }

    /// <summary>
    /// Gets the maximum strength (weight) among all contributing sources.
    /// </summary>
    public required double Strength { get; init; }

    /// <summary>
    /// Gets the timestamp of the most recent observation of this edge.
    /// </summary>
    public required DateTimeOffset LastSeen { get; init; }

    /// <summary>
    /// Gets the number of contributing sources.
    /// </summary>
    public int SourceCount => Sources.Count;

    /// <summary>
    /// Gets whether this edge has multiple confirming sources.
    /// </summary>
    public bool IsCorroborated => Sources.Count > 1;
}

/// <summary>
/// Builder for creating <see cref="DeduplicatedEdge"/> instances by merging multiple source edges.
/// </summary>
public sealed class DeduplicatedEdgeBuilder
{
    private readonly EdgeSemanticKey _key;
    private readonly string _from;
    private readonly string _to;
    private readonly HashSet<string> _sources = new(StringComparer.Ordinal);
    private EdgeExplanation? _explanation;
    private double _maxStrength;
    private DateTimeOffset _lastSeen = DateTimeOffset.MinValue;

    /// <summary>
    /// Initializes a new builder for the given semantic key.
    /// </summary>
    public DeduplicatedEdgeBuilder(EdgeSemanticKey key, string from, string to)
    {
        _key = key;
        _from = from;
        _to = to;
    }

    /// <summary>
    /// Adds a source edge to this builder.
    /// </summary>
    /// <param name="sourceId">The source identifier (e.g., feed name, analyzer ID).</param>
    /// <param name="explanation">The edge explanation from this source.</param>
    /// <param name="strength">The strength/weight from this source.</param>
    /// <param name="observedAt">When this source observed the edge.</param>
    /// <returns>This builder for chaining.</returns>
    public DeduplicatedEdgeBuilder AddSource(
        string sourceId,
        EdgeExplanation explanation,
        double strength,
        DateTimeOffset observedAt)
    {
        _sources.Add(sourceId);

        // Keep the strongest explanation
        if (strength > _maxStrength || _explanation is null)
        {
            _maxStrength = strength;
            _explanation = explanation;
        }

        // Track most recent observation
        if (observedAt > _lastSeen)
        {
            _lastSeen = observedAt;
        }

        return this;
    }

    /// <summary>
    /// Builds the deduplicated edge.
    /// </summary>
    /// <returns>The deduplicated edge with merged provenance.</returns>
    /// <exception cref="InvalidOperationException">If no sources were added.</exception>
    public DeduplicatedEdge Build()
    {
        if (_sources.Count == 0 || _explanation is null)
        {
            throw new InvalidOperationException("At least one source must be added before building.");
        }

        return new DeduplicatedEdge
        {
            Key = _key,
            From = _from,
            To = _to,
            Why = _explanation,
            Sources = _sources.ToImmutableHashSet(StringComparer.Ordinal),
            Strength = _maxStrength,
            LastSeen = _lastSeen
        };
    }
}