/**
 * Policy - Policy Model for Resolver
 * Sprint: SPRINT_9100_0001_0001 (Core Resolver Package)
 * Task: RESOLVER-9100-005
 *
 * Represents the policy used for verdict evaluation.
 * Policy digest is included in FinalDigest for reproducibility.
 */

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

namespace StellaOps.Resolver;

/// <summary>
/// Policy configuration for deterministic resolution.
/// </summary>
/// <param name="Version">Policy version string.</param>
/// <param name="Rules">Policy rules as JSON.</param>
/// <param name="ConstantsDigest">SHA256 digest of policy constants.</param>
public sealed record Policy(
    string Version,
    JsonElement Rules,
    string ConstantsDigest)
{
    private string? _digest;

    /// <summary>
    /// SHA256 digest of the policy (version + rules + constants).
    /// </summary>
    public string Digest => _digest ??= ComputeDigest();

    private string ComputeDigest()
    {
        var input = $"{Version}:{Rules.GetRawText()}:{ConstantsDigest}";
        var hash = SHA256.HashData(Encoding.UTF8.GetBytes(input));
        return Convert.ToHexString(hash).ToLowerInvariant();
    }

    /// <summary>
    /// Creates a policy from version and rules JSON.
    /// </summary>
    public static Policy Create(string version, JsonElement rules, string constantsDigest = "")
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(version);
        return new Policy(version, rules, constantsDigest);
    }

    /// <summary>
    /// Creates an empty policy for testing.
    /// </summary>
    public static Policy Empty => new("1.0.0", JsonDocument.Parse("{}").RootElement, "");
}