// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System;
using System.Text.Json;
using System.Text.Json.Serialization.Metadata;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Caching.Distributed;
using Microsoft.Shared.Diagnostics;
 
namespace Microsoft.Extensions.AI;
 
/// <summary>
/// Represents a delegating embedding generator that caches the results of embedding generation calls,
/// storing them as JSON in an <see cref="IDistributedCache"/>.
/// </summary>
/// <typeparam name="TInput">The type from which embeddings will be generated.</typeparam>
/// <typeparam name="TEmbedding">The type of embeddings to generate.</typeparam>
/// <remarks>
/// The provided implementation of <see cref="IEmbeddingGenerator{TInput, TEmbedding}"/> is thread-safe for concurrent
/// use so long as the employed <see cref="IDistributedCache"/> is similarly thread-safe for concurrent use.
/// </remarks>
public class DistributedCachingEmbeddingGenerator<TInput, TEmbedding> : CachingEmbeddingGenerator<TInput, TEmbedding>
    where TEmbedding : Embedding
{
    private readonly IDistributedCache _storage;
    private JsonSerializerOptions _jsonSerializerOptions;
 
    /// <summary>Initializes a new instance of the <see cref="DistributedCachingEmbeddingGenerator{TInput, TEmbedding}"/> class.</summary>
    /// <param name="innerGenerator">The underlying <see cref="IEmbeddingGenerator{TInput, TEmbedding}"/>.</param>
    /// <param name="storage">A <see cref="IDistributedCache"/> instance that will be used as the backing store for the cache.</param>
    /// <exception cref="ArgumentNullException"><paramref name="storage"/> is <see langword="null"/>.</exception>
    public DistributedCachingEmbeddingGenerator(IEmbeddingGenerator<TInput, TEmbedding> innerGenerator, IDistributedCache storage)
        : base(innerGenerator)
    {
        _ = Throw.IfNull(storage);
        _storage = storage;
        _jsonSerializerOptions = AIJsonUtilities.DefaultOptions;
    }
 
    /// <summary>Gets or sets JSON serialization options to use when serializing cache data.</summary>
    /// <exception cref="ArgumentNullException"><paramref name="value"/> is <see langword="null"/>.</exception>
    public JsonSerializerOptions JsonSerializerOptions
    {
        get => _jsonSerializerOptions;
        set
        {
            _ = Throw.IfNull(value);
            _jsonSerializerOptions = value;
        }
    }
 
    /// <inheritdoc />
    protected override async Task<TEmbedding?> ReadCacheAsync(string key, CancellationToken cancellationToken)
    {
        _ = Throw.IfNull(key);
        _jsonSerializerOptions.MakeReadOnly();
 
        if (await _storage.GetAsync(key, cancellationToken) is byte[] existingJson)
        {
            return JsonSerializer.Deserialize(existingJson, (JsonTypeInfo<TEmbedding>)_jsonSerializerOptions.GetTypeInfo(typeof(TEmbedding)));
        }
 
        return null;
    }
 
    /// <inheritdoc />
    protected override async Task WriteCacheAsync(string key, TEmbedding value, CancellationToken cancellationToken)
    {
        _ = Throw.IfNull(key);
        _ = Throw.IfNull(value);
        _jsonSerializerOptions.MakeReadOnly();
 
        var newJson = JsonSerializer.SerializeToUtf8Bytes(value, (JsonTypeInfo<TEmbedding>)_jsonSerializerOptions.GetTypeInfo(typeof(TEmbedding)));
        await _storage.SetAsync(key, newJson, cancellationToken);
    }
 
    /// <summary>Computes a cache key for the specified values.</summary>
    /// <param name="values">The values to inform the key.</param>
    /// <returns>The computed key.</returns>
    /// <remarks>
    /// <para>
    /// The <paramref name="values"/> are serialized to JSON using <see cref="JsonSerializerOptions"/> in order to compute the key.
    /// </para>
    /// <para>
    /// The generated cache key is not guaranteed to be stable across releases of the library.
    /// </para>
    /// </remarks>
    protected override string GetCacheKey(params ReadOnlySpan<object?> values) =>
        AIJsonUtilities.HashDataToString(values, _jsonSerializerOptions);
}