using System;
using System.Collections.Generic;
using System.Buffers;
using System.IO;
using System.Text;

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

internal static class GoBinaryScanner
{
    private static readonly ReadOnlyMemory<byte> BuildInfoMagic = new byte[]
    {
        0xFF, (byte)' ', (byte)'G', (byte)'o', (byte)' ', (byte)'b', (byte)'u', (byte)'i', (byte)'l', (byte)'d', (byte)'i', (byte)'n', (byte)'f', (byte)':'
    };

    private static readonly ReadOnlyMemory<byte> BuildIdMarker = Encoding.ASCII.GetBytes("Go build ID:");
    private static readonly ReadOnlyMemory<byte> GoPclnTabMarker = Encoding.ASCII.GetBytes(".gopclntab");
    private static readonly ReadOnlyMemory<byte> GoVersionPrefix = Encoding.ASCII.GetBytes("go1.");

    public static IEnumerable<string> EnumerateCandidateFiles(string rootPath)
    {
        var enumeration = new EnumerationOptions
        {
            RecurseSubdirectories = true,
            IgnoreInaccessible = true,
            AttributesToSkip = FileAttributes.Device | FileAttributes.ReparsePoint,
            MatchCasing = MatchCasing.CaseSensitive,
        };

        foreach (var path in Directory.EnumerateFiles(rootPath, "*", enumeration))
        {
            yield return path;
        }
    }

    public static bool TryReadBuildInfo(string filePath, out string? goVersion, out string? moduleData)
    {
        goVersion = null;
        moduleData = null;

        FileInfo info;
        try
        {
            info = new FileInfo(filePath);
            if (!info.Exists || info.Length < 64 || info.Length > 128 * 1024 * 1024)
            {
                return false;
            }
        }
        catch (IOException)
        {
            return false;
        }
        catch (UnauthorizedAccessException)
        {
            return false;
        }
        catch (System.Security.SecurityException)
        {
            return false;
        }

        var length = info.Length;
        if (length <= 0)
        {
            return false;
        }

        var inspectLength = (int)Math.Min(length, int.MaxValue);
        var buffer = ArrayPool<byte>.Shared.Rent(inspectLength);

        try
        {
            using var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read);
            var totalRead = 0;

            while (totalRead < inspectLength)
            {
                var read = stream.Read(buffer, totalRead, inspectLength - totalRead);
                if (read <= 0)
                {
                    break;
                }

                totalRead += read;
            }

            if (totalRead < 64)
            {
                return false;
            }

            var span = new ReadOnlySpan<byte>(buffer, 0, totalRead);
            var offset = span.IndexOf(BuildInfoMagic.Span);
            if (offset < 0)
            {
                return false;
            }

            var view = span[offset..];
            return GoBuildInfoDecoder.TryDecode(view, out goVersion, out moduleData);
        }
        catch (IOException)
        {
            return false;
        }
        catch (UnauthorizedAccessException)
        {
            return false;
        }
        finally
        {
            Array.Clear(buffer, 0, inspectLength);
            ArrayPool<byte>.Shared.Return(buffer);
        }
    }

    public static bool TryClassifyStrippedBinary(string filePath, out GoStrippedBinaryClassification classification)
    {
        classification = default;

        FileInfo fileInfo;
        try
        {
            fileInfo = new FileInfo(filePath);
            if (!fileInfo.Exists)
            {
                return false;
            }
        }
        catch (IOException)
        {
            return false;
        }
        catch (UnauthorizedAccessException)
        {
            return false;
        }
        catch (System.Security.SecurityException)
        {
            return false;
        }

        var length = fileInfo.Length;
        if (length < 128)
        {
            return false;
        }

        const int WindowSize = 128 * 1024;
        var readSize = (int)Math.Min(length, WindowSize);
        var buffer = ArrayPool<byte>.Shared.Rent(readSize);

        try
        {
            using var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read);

            var headRead = stream.Read(buffer, 0, readSize);
            if (headRead <= 0)
            {
                return false;
            }

            var headSpan = new ReadOnlySpan<byte>(buffer, 0, headRead);
            var hasBuildId = headSpan.IndexOf(BuildIdMarker.Span) >= 0;
            var hasPcln = headSpan.IndexOf(GoPclnTabMarker.Span) >= 0;
            var goVersion = ExtractGoVersion(headSpan);

            if (length > headRead)
            {
                var tailSize = Math.Min(readSize, (int)length);
                if (tailSize > 0)
                {
                    stream.Seek(-tailSize, SeekOrigin.End);
                    var tailRead = stream.Read(buffer, 0, tailSize);
                    if (tailRead > 0)
                    {
                        var tailSpan = new ReadOnlySpan<byte>(buffer, 0, tailRead);
                        hasBuildId |= tailSpan.IndexOf(BuildIdMarker.Span) >= 0;
                        hasPcln |= tailSpan.IndexOf(GoPclnTabMarker.Span) >= 0;
                        goVersion ??= ExtractGoVersion(tailSpan);
                    }
                }
            }

            if (hasBuildId)
            {
                classification = new GoStrippedBinaryClassification(
                    filePath,
                    GoStrippedBinaryIndicator.BuildId,
                    goVersion);
                return true;
            }

            if (hasPcln && !string.IsNullOrEmpty(goVersion))
            {
                classification = new GoStrippedBinaryClassification(
                    filePath,
                    GoStrippedBinaryIndicator.GoRuntimeMarkers,
                    goVersion);
                return true;
            }

            return false;
        }
        finally
        {
            Array.Clear(buffer, 0, readSize);
            ArrayPool<byte>.Shared.Return(buffer);
        }
    }

    private static string? ExtractGoVersion(ReadOnlySpan<byte> data)
    {
        var prefix = GoVersionPrefix.Span;
        var span = data;

        while (!span.IsEmpty)
        {
            var index = span.IndexOf(prefix);
            if (index < 0)
            {
                return null;
            }

            var absoluteIndex = data.Length - span.Length + index;

            if (absoluteIndex > 0)
            {
                var previous = (char)data[absoluteIndex - 1];
                if (char.IsLetterOrDigit(previous))
                {
                    span = span[(index + 1)..];
                    continue;
                }
            }

            var start = absoluteIndex;
            var end = start + prefix.Length;

            while (end < data.Length && IsVersionCharacter((char)data[end]))
            {
                end++;
            }

            if (end - start <= prefix.Length)
            {
                span = span[(index + 1)..];
                continue;
            }

            var candidate = data[start..end];
            return Encoding.ASCII.GetString(candidate);
        }

        return null;
    }

    private static bool IsVersionCharacter(char value)
        => (value >= '0' && value <= '9')
           || (value >= 'a' && value <= 'z')
           || (value >= 'A' && value <= 'Z')
           || value is '.' or '-' or '+' or '_';
}