// Copyright (c) StellaOps. Licensed under the BUSL-1.1.


using StellaOps.HybridLogicalClock;
using System.Security.Cryptography;
using System.Text;

namespace StellaOps.Eventing.Internal;

/// <summary>
/// Generates deterministic event IDs from event content.
/// </summary>
internal static class EventIdGenerator
{
    /// <summary>
    /// Generates a deterministic event ID using SHA-256 hash of inputs.
    /// </summary>
    /// <param name="correlationId">The correlation ID.</param>
    /// <param name="tHlc">The HLC timestamp.</param>
    /// <param name="service">The service name.</param>
    /// <param name="kind">The event kind.</param>
    /// <returns>First 32 hex characters of SHA-256 hash (128 bits).</returns>
    public static string Generate(
        string correlationId,
        HlcTimestamp tHlc,
        string service,
        string kind)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(correlationId);
        ArgumentException.ThrowIfNullOrWhiteSpace(service);
        ArgumentException.ThrowIfNullOrWhiteSpace(kind);

        using var hasher = IncrementalHash.CreateHash(HashAlgorithmName.SHA256);

        // Append all inputs in deterministic order
        hasher.AppendData(Encoding.UTF8.GetBytes(correlationId));
        hasher.AppendData(Encoding.UTF8.GetBytes(tHlc.ToSortableString()));
        hasher.AppendData(Encoding.UTF8.GetBytes(service));
        hasher.AppendData(Encoding.UTF8.GetBytes(kind));

        var hash = hasher.GetHashAndReset();

        // Return first 16 bytes (128 bits) as lowercase hex (32 chars)
        return Convert.ToHexString(hash.AsSpan(0, 16)).ToLowerInvariant();
    }

    /// <summary>
    /// Computes SHA-256 digest of the payload.
    /// </summary>
    /// <param name="payload">The canonicalized JSON payload.</param>
    /// <returns>SHA-256 digest bytes.</returns>
    public static byte[] ComputePayloadDigest(string payload)
    {
        ArgumentNullException.ThrowIfNull(payload);
        return SHA256.HashData(Encoding.UTF8.GetBytes(payload));
    }
}