/**
 * NodeId - Content-Addressed Node Identifier
 * Sprint: SPRINT_9100_0001_0001 (Core Resolver Package)
 * Task: RESOLVER-9100-002
 *
 * A content-addressed identifier for graph nodes.
 * NodeId = sha256(normalize(kind + ":" + key))
 * 
 * Guarantees:
 * - Same (kind, key) → same NodeId
 * - Different (kind, key) → different NodeId (collision resistant)
 * - Deterministic ordering via ordinal string comparison
 */

using System.Security.Cryptography;
using System.Text;

namespace StellaOps.Resolver;

/// <summary>
/// Content-addressed node identifier computed as SHA256 of normalized kind:key.
/// Immutable value type for deterministic graph operations.
/// </summary>
public readonly record struct NodeId : IComparable<NodeId>, IEquatable<NodeId>
{
    private readonly string _value;

    /// <summary>
    /// The SHA256 hex digest (lowercase, 64 characters).
    /// </summary>
    public string Value => _value ?? string.Empty;

    private NodeId(string value) => _value = value;

    /// <summary>
    /// Creates a NodeId from a pre-computed digest value.
    /// Use <see cref="From(string, string)"/> for computing from kind/key.
    /// </summary>
    /// <param name="digest">A valid SHA256 hex digest (64 lowercase hex chars).</param>
    public static NodeId FromDigest(string digest)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(digest);
        if (digest.Length != 64)
            throw new ArgumentException("NodeId digest must be 64 hex characters", nameof(digest));

        return new NodeId(digest.ToLowerInvariant());
    }

    /// <summary>
    /// Computes a NodeId from kind and key.
    /// Applies NFC normalization before hashing.
    /// </summary>
    /// <param name="kind">Node kind (e.g., "package", "file", "symbol").</param>
    /// <param name="key">Node key (e.g., PURL, file path, symbol name).</param>
    /// <returns>Content-addressed NodeId.</returns>
    public static NodeId From(string kind, string key)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(kind);
        ArgumentException.ThrowIfNullOrWhiteSpace(key);

        // NFC normalize inputs for Unicode consistency
        var normalizedKind = kind.Normalize(NormalizationForm.FormC);
        var normalizedKey = key.Normalize(NormalizationForm.FormC);

        var input = $"{normalizedKind}:{normalizedKey}";
        var hash = SHA256.HashData(Encoding.UTF8.GetBytes(input));
        var digest = Convert.ToHexString(hash).ToLowerInvariant();

        return new NodeId(digest);
    }

    /// <summary>
    /// Ordinal comparison for deterministic ordering.
    /// </summary>
    public int CompareTo(NodeId other)
        => string.Compare(Value, other.Value, StringComparison.Ordinal);

    /// <summary>
    /// Equality is based on digest value.
    /// </summary>
    public bool Equals(NodeId other)
        => string.Equals(Value, other.Value, StringComparison.Ordinal);

    public override int GetHashCode()
        => Value.GetHashCode(StringComparison.Ordinal);

    public override string ToString() => Value;

    public static bool operator <(NodeId left, NodeId right) => left.CompareTo(right) < 0;
    public static bool operator >(NodeId left, NodeId right) => left.CompareTo(right) > 0;
    public static bool operator <=(NodeId left, NodeId right) => left.CompareTo(right) <= 0;
    public static bool operator >=(NodeId left, NodeId right) => left.CompareTo(right) >= 0;
}