using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Text.Json;
using System.Threading;
using System.Threading.Tasks;

namespace StellaOps.Scanner.Surface.FS;

/// <summary>
/// Verifies determinism metadata on a Surface manifest by checking composition recipe,
/// layer fragment attestations, and DSSE payload integrity.
/// </summary>
public sealed class SurfaceManifestDeterminismVerifier
{
    private static readonly JsonSerializerOptions SerializerOptions = new(JsonSerializerDefaults.Web)
    {
        WriteIndented = false
    };

    public async Task<SurfaceDeterminismVerificationResult> VerifyAsync(
        SurfaceManifestDocument manifest,
        Func<SurfaceManifestArtifact, Task<ReadOnlyMemory<byte>>> artifactLoader,
        CancellationToken cancellationToken = default)
    {
        if (manifest is null)
        {
            throw new ArgumentNullException(nameof(manifest));
        }

        if (artifactLoader is null)
        {
            throw new ArgumentNullException(nameof(artifactLoader));
        }

        var errors = new List<string>();
        var merkleRoot = (manifest.DeterminismMerkleRoot ?? manifest.Determinism?.MerkleRoot)?.Trim().ToLowerInvariant();

        if (string.IsNullOrWhiteSpace(merkleRoot))
        {
            errors.Add("determinism.merkleRoot missing from manifest.");
        }

        var artifactsByDigest = manifest.Artifacts.ToDictionary(a => a.Digest, StringComparer.OrdinalIgnoreCase);
        var artifactsByUri = manifest.Artifacts.Where(a => !string.IsNullOrWhiteSpace(a.Uri))
            .ToDictionary(a => a.Uri, StringComparer.OrdinalIgnoreCase);

        // Validate composition recipe first; it anchors the Merkle root.
        var recipe = manifest.Artifacts.FirstOrDefault(a => string.Equals(a.Kind, "composition.recipe", StringComparison.Ordinal));
        if (recipe is null)
        {
            errors.Add("composition.recipe artifact missing.");
        }
        else
        {
            var recipeBytes = await LoadAndValidateDigestAsync(recipe, artifactLoader, errors, cancellationToken).ConfigureAwait(false);
            if (recipeBytes.Length > 0)
            {
                var computedRoot = ComputeSha256Hex(recipeBytes.Span);
                if (string.IsNullOrWhiteSpace(merkleRoot))
                {
                    merkleRoot = computedRoot;
                }
                else if (!string.Equals(merkleRoot, computedRoot, StringComparison.Ordinal))
                {
                    errors.Add($"determinism.merkleRoot mismatch: manifest={merkleRoot}, recipe={computedRoot}.");
                }

                await VerifyAttestationAsync(
                    recipe,
                    recipeBytes,
                    expectedPayloadType: recipe.MediaType,
                    artifactsByDigest,
                    artifactsByUri,
                    artifactLoader,
                    errors,
                    cancellationToken).ConfigureAwait(false);
            }
        }

        // Validate each layer fragment and its DSSE.
        foreach (var fragment in manifest.Artifacts.Where(a => string.Equals(a.Kind, "layer.fragments", StringComparison.Ordinal)))
        {
            var fragmentBytes = await LoadAndValidateDigestAsync(fragment, artifactLoader, errors, cancellationToken).ConfigureAwait(false);
            if (fragmentBytes.Length == 0)
            {
                continue;
            }

            await VerifyAttestationAsync(
                fragment,
                fragmentBytes,
                expectedPayloadType: fragment.MediaType,
                artifactsByDigest,
                artifactsByUri,
                artifactLoader,
                errors,
                cancellationToken).ConfigureAwait(false);
        }

        return new SurfaceDeterminismVerificationResult(errors.Count == 0, merkleRoot, errors);
    }

    private static async Task<ReadOnlyMemory<byte>> LoadAndValidateDigestAsync(
        SurfaceManifestArtifact artifact,
        Func<SurfaceManifestArtifact, Task<ReadOnlyMemory<byte>>> loader,
        List<string> errors,
        CancellationToken cancellationToken)
    {
        try
        {
            cancellationToken.ThrowIfCancellationRequested();
            var bytes = await loader(artifact).ConfigureAwait(false);
            if (bytes.Length == 0)
            {
                errors.Add($"artifact:{artifact.Kind} ({artifact.Digest}) content missing.");
                return ReadOnlyMemory<byte>.Empty;
            }

            var computedDigest = $"sha256:{ComputeSha256Hex(bytes.Span)}";
            if (!string.Equals(computedDigest, artifact.Digest, StringComparison.OrdinalIgnoreCase))
            {
                errors.Add($"artifact:{artifact.Kind} digest mismatch (manifest={artifact.Digest}, computed={computedDigest}).");
            }

            return bytes;
        }
        catch (Exception ex)
        {
            errors.Add($"artifact:{artifact.Kind} load failed: {ex.Message}");
            return ReadOnlyMemory<byte>.Empty;
        }
    }

    private static async Task VerifyAttestationAsync(
        SurfaceManifestArtifact target,
        ReadOnlyMemory<byte> targetContent,
        string expectedPayloadType,
        IReadOnlyDictionary<string, SurfaceManifestArtifact> artifactsByDigest,
        IReadOnlyDictionary<string, SurfaceManifestArtifact> artifactsByUri,
        Func<SurfaceManifestArtifact, Task<ReadOnlyMemory<byte>>> loader,
        List<string> errors,
        CancellationToken cancellationToken)
    {
        if (target.Attestations is null || target.Attestations.Count == 0)
        {
            errors.Add($"artifact:{target.Kind} missing dsse attestation.");
            return;
        }

        var attestation = target.Attestations.FirstOrDefault(a => string.Equals(a.Kind, "dsse", StringComparison.Ordinal));
        if (attestation is null)
        {
            errors.Add($"artifact:{target.Kind} missing dsse attestation.");
            return;
        }

        if (!artifactsByDigest.TryGetValue(attestation.Digest, out var dsseArtifact) &&
            (!string.IsNullOrWhiteSpace(attestation.Uri) && !artifactsByUri.TryGetValue(attestation.Uri, out dsseArtifact)))
        {
            errors.Add($"artifact:{target.Kind} attestation not found in manifest (digest={attestation.Digest}).");
            return;
        }

        if (dsseArtifact is null)
        {
            errors.Add($"artifact:{target.Kind} attestation lookup returned null instance.");
            return;
        }

        var dsseBytes = await LoadAndValidateDigestAsync(dsseArtifact, loader, errors, cancellationToken).ConfigureAwait(false);
        if (dsseBytes.Length == 0)
        {
            return;
        }

        try
        {
            using var doc = JsonDocument.Parse(dsseBytes.ToArray(), new JsonDocumentOptions { AllowTrailingCommas = false });
            var root = doc.RootElement;

            if (!root.TryGetProperty("payloadType", out var payloadTypeProp))
            {
                errors.Add($"artifact:{target.Kind} attestation payloadType missing.");
                return;
            }

            var payloadType = payloadTypeProp.GetString() ?? string.Empty;
            if (!string.Equals(payloadType, expectedPayloadType, StringComparison.Ordinal))
            {
                errors.Add($"artifact:{target.Kind} attestation payloadType mismatch (expected={expectedPayloadType}, actual={payloadType}).");
            }

            if (!root.TryGetProperty("payload", out var payloadProp))
            {
                errors.Add($"artifact:{target.Kind} attestation payload missing.");
                return;
            }

            var payload = DecodeBase64Url(payloadProp.GetString());
            if (!payload.Span.SequenceEqual(targetContent.Span))
            {
                errors.Add($"artifact:{target.Kind} attestation payload does not match artifact content.");
            }

            if (root.TryGetProperty("signatures", out var sigArray) &&
                sigArray.ValueKind == JsonValueKind.Array &&
                sigArray.GetArrayLength() > 0)
            {
                var sigNode = sigArray[0];
                if (sigNode.TryGetProperty("sig", out var sigValue))
                {
                    var sigBytes = DecodeBase64Url(sigValue.GetString());
                    var sigText = Encoding.UTF8.GetString(sigBytes.Span);
                    var expectedSig = ComputeSha256Hex(targetContent.Span);
                    if (!string.Equals(sigText, expectedSig, StringComparison.OrdinalIgnoreCase))
                    {
                        errors.Add($"artifact:{target.Kind} attestation signature mismatch.");
                    }
                }
            }
        }
        catch (Exception ex)
        {
            errors.Add($"artifact:{target.Kind} attestation parse failed: {ex.Message}");
        }
    }

    private static string ComputeSha256Hex(ReadOnlySpan<byte> bytes)
    {
        Span<byte> hash = stackalloc byte[32];
        SHA256.HashData(bytes, hash);
        return Convert.ToHexString(hash).ToLowerInvariant();
    }

    private static ReadOnlyMemory<byte> DecodeBase64Url(string? value)
    {
        if (string.IsNullOrEmpty(value))
        {
            return ReadOnlyMemory<byte>.Empty;
        }

        var padded = value.Replace('-', '+').Replace('_', '/');
        switch (padded.Length % 4)
        {
            case 2: padded += "=="; break;
            case 3: padded += "="; break;
        }

        return Convert.FromBase64String(padded);
    }
}

public sealed record SurfaceDeterminismVerificationResult(
    bool Success,
    string? MerkleRoot,
    IReadOnlyList<string> Errors)
{
    public bool IsDeterministic => Success;
}