using System.Buffers;
using System.Collections.Concurrent;
using System.Collections.Immutable;
using System.Linq;
using System.Text;

namespace StellaOps.Scanner.Analyzers.Lang.Rust.Internal;

internal static class RustBinaryClassifier
{
    private static readonly ReadOnlyMemory<byte> ElfMagic = new byte[] { 0x7F, (byte)'E', (byte)'L', (byte)'F' };
    private static readonly ReadOnlyMemory<byte> SymbolPrefix = new byte[] { (byte)'_', (byte)'Z', (byte)'N' };

    private const int ChunkSize = 64 * 1024;
    private const int OverlapSize = 48;
    private const long MaxBinarySize = 128L * 1024L * 1024L;

    private static readonly HashSet<string> StandardCrates = new(StringComparer.Ordinal)
    {
        "core",
        "alloc",
        "std",
        "panic_unwind",
        "panic_abort",
    };

    private static readonly EnumerationOptions Enumeration = new()
    {
        MatchCasing = MatchCasing.CaseSensitive,
        IgnoreInaccessible = true,
        RecurseSubdirectories = true,
        AttributesToSkip = FileAttributes.Device | FileAttributes.ReparsePoint,
    };

    private static readonly ConcurrentDictionary<RustFileCacheKey, ImmutableArray<string>> CandidateCache = new();

    public static IReadOnlyList<RustBinaryInfo> Scan(string rootPath, CancellationToken cancellationToken)
    {
        if (string.IsNullOrWhiteSpace(rootPath))
        {
            throw new ArgumentException("Root path is required", nameof(rootPath));
        }

        var binaries = new List<RustBinaryInfo>();
        foreach (var path in Directory.EnumerateFiles(rootPath, "*", Enumeration))
        {
            cancellationToken.ThrowIfCancellationRequested();

            if (!IsEligibleBinary(path))
            {
                continue;
            }

            if (!RustFileCacheKey.TryCreate(path, out var key))
            {
                continue;
            }

            var candidates = CandidateCache.GetOrAdd(
                key,
                static (_, state) => ExtractCrateNames(state.Path, state.CancellationToken),
                (Path: path, CancellationToken: cancellationToken));

            binaries.Add(new RustBinaryInfo(path, candidates));
        }

        return binaries;
    }

    private static bool IsEligibleBinary(string path)
    {
        try
        {
            var info = new FileInfo(path);
            if (!info.Exists || info.Length == 0 || info.Length > MaxBinarySize)
            {
                return false;
            }

            using var stream = info.OpenRead();
            Span<byte> buffer = stackalloc byte[4];
            var read = stream.Read(buffer);
            if (read != 4)
            {
                return false;
            }

            return buffer.SequenceEqual(ElfMagic.Span);
        }
        catch (IOException)
        {
            return false;
        }
        catch (UnauthorizedAccessException)
        {
            return false;
        }
    }

    private static ImmutableArray<string> ExtractCrateNames(string path, CancellationToken cancellationToken)
    {
        var names = new HashSet<string>(StringComparer.Ordinal);
        var buffer = ArrayPool<byte>.Shared.Rent(ChunkSize + OverlapSize);
        var overlap = new byte[OverlapSize];
        var overlapLength = 0;

        try
        {
            using var stream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read);
            while (true)
            {
                cancellationToken.ThrowIfCancellationRequested();

                // Copy previous overlap to buffer prefix.
                if (overlapLength > 0)
                {
                    Array.Copy(overlap, 0, buffer, 0, overlapLength);
                }

                var read = stream.Read(buffer, overlapLength, ChunkSize);
                if (read <= 0)
                {
                    break;
                }

                var span = new ReadOnlySpan<byte>(buffer, 0, overlapLength + read);
                ScanForSymbols(span, names);

                overlapLength = Math.Min(OverlapSize, span.Length);
                if (overlapLength > 0)
                {
                    span[^overlapLength..].CopyTo(overlap);
                }
            }
        }
        catch (IOException)
        {
            return ImmutableArray<string>.Empty;
        }
        catch (UnauthorizedAccessException)
        {
            return ImmutableArray<string>.Empty;
        }
        finally
        {
            ArrayPool<byte>.Shared.Return(buffer);
        }

        if (names.Count == 0)
        {
            return ImmutableArray<string>.Empty;
        }

        var ordered = names
            .Where(static name => !string.IsNullOrWhiteSpace(name))
            .Select(static name => name.Trim())
            .Where(static name => name.Length > 1)
            .Where(name => !StandardCrates.Contains(name))
            .Distinct(StringComparer.Ordinal)
            .OrderBy(static name => name, StringComparer.Ordinal)
            .ToImmutableArray();

        return ordered;
    }

    private static void ScanForSymbols(ReadOnlySpan<byte> span, HashSet<string> names)
    {
        var prefix = SymbolPrefix.Span;
        var index = 0;

        while (index < span.Length)
        {
            var slice = span[index..];
            var offset = slice.IndexOf(prefix);
            if (offset < 0)
            {
                break;
            }

            index += offset + prefix.Length;
            if (index >= span.Length)
            {
                break;
            }

            var remaining = span[index..];
            if (!TryParseCrate(remaining, out var crate, out var consumed))
            {
                index += 1;
                continue;
            }

            if (!string.IsNullOrWhiteSpace(crate))
            {
                names.Add(crate);
            }

            index += Math.Max(consumed, 1);
        }
    }

    private static bool TryParseCrate(ReadOnlySpan<byte> span, out string? crate, out int consumed)
    {
        crate = null;
        consumed = 0;

        var i = 0;
        var length = 0;

        while (i < span.Length && span[i] is >= (byte)'0' and <= (byte)'9')
        {
            length = (length * 10) + (span[i] - (byte)'0');
            i++;

            if (length > 256)
            {
                return false;
            }
        }

        if (i == 0 || length <= 0 || i + length > span.Length)
        {
            return false;
        }

        crate = Encoding.ASCII.GetString(span.Slice(i, length));
        consumed = i + length;
        return true;
    }
}

internal sealed record RustBinaryInfo(string AbsolutePath, ImmutableArray<string> CrateCandidates)
{
    public string ComputeSha256()
    {
        if (RustFileHashCache.TryGetSha256(AbsolutePath, out var sha256) && !string.IsNullOrEmpty(sha256))
        {
            return sha256;
        }

        return string.Empty;
    }
}