using System.Text;

namespace TarninaHub.Application.Matchmaking;

/// <summary>
/// Deterministic, offline embedding provider (feature hashing). Produces
/// L2-normalized vectors so cosine similarity reflects shared vocabulary.
/// Interchangeable with a real LLM provider behind <see cref="IEmbeddingProvider"/>
/// (see ADR-0005). Used as the default until an external provider is wired.
/// </summary>
public sealed class HashingEmbeddingProvider : IEmbeddingProvider
{
    public int Dimension => EmbeddingConstants.Dimension;

    public Task<float[]> EmbedAsync(string text, CancellationToken cancellationToken)
    {
        return Task.FromResult(Embed(text));
    }

    public Task<IReadOnlyList<float[]>> EmbedBatchAsync(
        IReadOnlyList<string> texts,
        CancellationToken cancellationToken)
    {
        IReadOnlyList<float[]> result = texts.Select(Embed).ToList();

        return Task.FromResult(result);
    }

    public float[] Embed(string text)
    {
        var vector = new float[Dimension];

        if (string.IsNullOrWhiteSpace(text))
        {
            return vector;
        }

        foreach (var token in Tokenize(text))
        {
            var index = (int)(Fnv1a(token) % Dimension);
            vector[index] += 1f;
        }

        Normalize(vector);

        return vector;
    }

    private static IEnumerable<string> Tokenize(string text)
    {
        var buffer = new StringBuilder();

        foreach (var character in text)
        {
            if (char.IsLetterOrDigit(character))
            {
                buffer.Append(char.ToLowerInvariant(character));
            }
            else if (buffer.Length > 0)
            {
                yield return buffer.ToString();
                buffer.Clear();
            }
        }

        if (buffer.Length > 0)
        {
            yield return buffer.ToString();
        }
    }

    private static void Normalize(float[] vector)
    {
        double sumOfSquares = vector.Sum(value => (double)value * value);
        var magnitude = Math.Sqrt(sumOfSquares);

        if (magnitude <= 0)
        {
            return;
        }

        for (var i = 0; i < vector.Length; i++)
        {
            vector[i] = (float)(vector[i] / magnitude);
        }
    }

    private static uint Fnv1a(string token)
    {
        const uint offset = 2166136261;
        const uint prime = 16777619;

        var hash = offset;

        foreach (var character in token)
        {
            hash ^= character;
            hash *= prime;
        }

        return hash;
    }
}