FileAnalyzer.cs

/*
 * Copyright 2022 faddenSoft
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
using System;
using System.Diagnostics;
using System.Reflection;
using System.Runtime.ExceptionServices;

using CommonUtil;
using DiskArc.Arc;
using DiskArc.Disk;
using static DiskArc.Defs;

namespace DiskArc {
    /// <summary>
    /// Disk image / file archive analysis functions.
    /// </summary>
    public static class FileAnalyzer {
        /// <summary>
        /// List of supported filesystems and multi-partition formats.
        /// </summary>
        /// <remarks>
        /// List the filesystems in the order they should be tested when a disk is scanned.
        /// </remarks>
        private static DiskLayoutEntry[] sDiskLayouts = {
            new DiskLayoutEntry("APM", FileSystemType.APM, typeof(Multi.APM)),
            new DiskLayoutEntry("MacTS", FileSystemType.MacTS, typeof(Multi.MacTS)),
            new DiskLayoutEntry("MicroDrive", FileSystemType.MicroDrive, typeof(Multi.MicroDrive)),
            new DiskLayoutEntry("FocusDrive", FileSystemType.FocusDrive, typeof(Multi.FocusDrive)),
            new DiskLayoutEntry("CFFA", FileSystemType.CFFA, typeof(Multi.CFFA)),
            // MSDOS/FAT (mostly to rule it out)

            // We want to check for DOS before ProDOS/Pascal/CPM, so that we can identify hybrids.
            new DiskLayoutEntry("DOS 3.x", FileSystemType.DOS33, typeof(FS.DOS)),
            new DiskLayoutEntry("ProDOS", FileSystemType.ProDOS, typeof(FS.ProDOS)),
            new DiskLayoutEntry("Pascal", FileSystemType.Pascal, typeof(FS.Pascal)),
            new DiskLayoutEntry("HFS", FileSystemType.HFS, typeof(FS.HFS)),
            new DiskLayoutEntry("MFS", FileSystemType.MFS, typeof(FS.MFS)),
            new DiskLayoutEntry("RDOS", FileSystemType.RDOS, typeof(FS.RDOS)),
            new DiskLayoutEntry("Gutenberg", FileSystemType.Gutenberg, typeof(FS.Gutenberg)),
            new DiskLayoutEntry("CPM", FileSystemType.CPM, typeof(FS.CPM)),

            new DiskLayoutEntry("Am/UniDOS", FileSystemType.AmUniDOS, typeof(Multi.AmUniDOS)),
            new DiskLayoutEntry("OzDOS", FileSystemType.OzDOS, typeof(Multi.OzDOS)),
            // ISO-9660? (rule out HFS first)
        };


        /// <summary>
        /// List of supported filesystems and multi-partition formats.
        /// </summary>
        public static DiskLayoutEntry[] DiskLayouts {
            get {
                // Entries are immutable, so a shallow copy is fine.
                DiskLayoutEntry[] arr = new DiskLayoutEntry[sDiskLayouts.Length];
                Array.Copy(sDiskLayouts, arr, arr.Length);
                return arr;
            }
        }

        /// <summary>
        /// One entry in the disk layout set.  Immutable.
        /// </summary>
        public class DiskLayoutEntry {
            /// <summary>
            /// Human-readable name.
            /// </summary>
            public string Name { get; }

            /// <summary>
            /// FileSystemType enumeration value.
            /// </summary>
            public FileSystemType FSType;

            /// <summary>
            /// Reference to filesystem class.
            /// </summary>
            private Type mImplClass;


            /// <summary>
            /// Constructs the object, using reflection to dig out static methods and properties.
            /// This is more efficient than using reflection every time, and also helps ensure
            /// that the implementations are complete.
            /// </summary>
            /// <param name="name">Short, human-readable name of class.</param>
            /// <param name="implClass">Implementation class.</param>
            /// <exception cref="Exception"></exception>
            public DiskLayoutEntry(string name, FileSystemType fsType, Type implClass) {
                // Make sure we're getting the right thing.
                Debug.Assert(implClass.GetInterface(nameof(IFileSystem)) != null ||
                             implClass.GetInterface(nameof(IMultiPart)) != null);

                Name = name;
                FSType = fsType;
                mImplClass = implClass;

                // Cache a reference to the constructor.
                ConstructorInfo? ctor = implClass.GetConstructor(
                    BindingFlags.Public | BindingFlags.Instance,
                    new Type[] { typeof(IChunkAccess), typeof(AppHook) });
                if (ctor == null) {
                    throw new Exception("Unable to find ctor in " + implClass.FullName);
                }
                mCtorInfo = ctor;

                // Cache reference to the TestImage function, which is a public static method
                // whose signature matches the TestImageFunc delegate.
                MethodInfo? method = implClass.GetMethod("TestImage",
                    BindingFlags.Public | BindingFlags.Static,
                    new Type[] { typeof(IChunkAccess), typeof(AppHook) });
                if (method == null) {
                    throw new Exception("Unable to find TestImage() in " + implClass.FullName);
                }
                // This will throw TypeInitializationException if the method signature is wrong.
                mTestImageFunc =
                        (TestImageFunc)Delegate.CreateDelegate(typeof(TestImageFunc), method);

                // Cache reference to the size test function.
                method = implClass.GetMethod("IsSizeAllowed",
                    BindingFlags.Public | BindingFlags.Static,
                    new Type[] { typeof(long) });
                if (method == null) {
                    throw new Exception("Unable to find IsSizeAllowed() in " + implClass.FullName);
                }
                mIsSizeAllowedFunc =
                    (IsSizeAllowedFunc)Delegate.CreateDelegate(typeof(IsSizeAllowedFunc), method);
            }

            /// <summary>
            /// Cached reflection reference to constructor.
            /// </summary>
            private ConstructorInfo mCtorInfo;

            /// <summary>
            /// Creates an instance of the filesystem.  This should only be called after a
            /// successful test result.
            /// </summary>
            /// <param name="source">Chunk source.</param>
            /// <param name="appHook">Application hook reference.</param>
            /// <param name="contents">Result: IFileSystem or IMultiPart object.</param>
            public void CreateInstance(IChunkAccess source, AppHook appHook,
                    out IDiskContents? contents) {
                Debug.Assert(source != null);
                object instance;
                try {
                    instance = mCtorInfo.Invoke(new object[] { source, appHook });
                } catch (TargetInvocationException ex) {
                    if (ex.InnerException != null) {
                        // Re-throwing an inner exception loses the stack trace.  Do this instead.
                        // https://stackoverflow.com/a/17091351/294248
                        ExceptionDispatchInfo.Capture(ex.InnerException).Throw();
                        // This is not reached, but the compiler doesn't know that.
                        throw ex.InnerException;
                    } else {
                        throw;
                    }
                }
                if (instance is IDiskContents) {
                    contents = (IDiskContents)instance;
                } else {
                    throw new NotImplementedException("Not expecting instance of " +
                        instance.GetType().ToString());
                }
            }

            /// <summary>
            /// Cached reflection reference to filesystem test function.
            /// </summary>
            private TestImageFunc mTestImageFunc;
            private delegate TestResult TestImageFunc(IChunkAccess chunkSource, AppHook appHook);

            /// <summary>
            /// Result from filesystem test.  Higher values are better.
            /// </summary>
            public enum TestResult {
                Undefined = 0,
                No = 1,
                Barely = 2,
                Maybe = 3,
                Good = 4,
                Yes = 5
            }

            /// <summary>
            /// Invokes the filesystem test function.
            /// </summary>
            /// <param name="chunkSource">Data chunk source.</param>
            /// <param name="appHook">Application hook reference.</param>
            /// <returns>True if the filesystem code recognizes this as its own.</returns>
            public TestResult TestImage(IChunkAccess chunkSource, AppHook appHook) {
                try {
                    return mTestImageFunc(chunkSource, appHook);
                } catch (Exception ex) {
                    appHook.LogW("Exception thrown from " + Name + " TestImage: " + ex.Message);
                    return TestResult.No;
                }
            }

            /// <summary>
            /// Cached reflection reference to filesystem creation size test function.
            /// </summary>
            private IsSizeAllowedFunc mIsSizeAllowedFunc;
            private delegate bool IsSizeAllowedFunc(long size);

            /// <summary>
            /// Invokes the filesystem creation size test function.
            /// </summary>
            /// <param name="chunkSource">Data chunk source.</param>
            /// <returns>True if a filesystem image of the specified size is allowed.</returns>
            public bool IsSizeAllowed(long size) {
                return mIsSizeAllowedFunc(size);
            }

            public override string ToString() {
                return "[FileSystemEntry: " + Name + "]";
            }
        }

        /// <summary>
        /// Invokes the filesystem-specific IsSizeAllowed() method.
        /// </summary>
        /// <param name="fsType">Filesystem type.</param>
        /// <param name="size">Size to test.</param>
        /// <returns>True if the filesystem can be placed on a volume of the specified
        ///   size.</returns>
        public static bool IsSizeAllowed(FileSystemType fsType, long size) {
            foreach (DiskLayoutEntry dle in sDiskLayouts) {
                if (dle.FSType == fsType) {
                    return dle.IsSizeAllowed(size);
                }
            }
            throw new ArgumentException("Unsupported file system type: " + fsType);
        }

        /// <summary>
        /// Invokes the filesystem-specific CreateInstance() method.
        /// </summary>
        /// <param name="fsType">Filesystem type.</param>
        /// <param name="source">Chunk source.  The IFileSystem object takes ownership.</param>
        /// <param name="appHook">Application hook reference.</param>
        /// <param name="contents">Result: IFileSystem or IMultiPart object.</param>
        public static void CreateInstance(FileSystemType fsType, IChunkAccess source,
                AppHook appHook, out IDiskContents? contents) {
            foreach (DiskLayoutEntry dle in sDiskLayouts) {
                if (dle.FSType == fsType) {
                    dle.CreateInstance(source, appHook, out contents);
                    return;
                }
            }
            throw new ArgumentException("Unsupported file system type: " + fsType);
        }

        /// <summary>
        /// Finds the file kind(s) associated with a filename extension.
        /// </summary>
        /// <param name="fileNameExt">Filename extension, with the leading '.'.</param>
        /// <param name="fileKind1">Result: what kind of file this is.</param>
        /// <param name="orderHint1">Result: the sector order hinted at by the filename (only
        ///   useful for 5.25" disk images).</param>
        /// <param name="fileKind2">Result: alternate possibility for file kind.</param>
        /// <returns>True if the extension was recognized, false if not.</returns>
        public static bool ExtensionToKind(string fileNameExt, out FileKind fileKind1,
                out SectorOrder orderHint1, out FileKind fileKind2, out bool okayForCreate) {
            okayForCreate = true;
            fileNameExt = fileNameExt.ToLowerInvariant();

            fileKind1 = fileKind2 = FileKind.Unknown;
            orderHint1 = SectorOrder.Unknown;

            switch (fileNameExt) {
                case ".co":     // "unofficial" CP/M-ordered
                case ".do":
                case ".po":
                case ".iso":
                case ".hdv":
                    fileKind1 = FileKind.UnadornedSector;
                    if (fileNameExt == ".co") {
                        orderHint1 = SectorOrder.CPM_KBlock;
                    } else if (fileNameExt == ".do") {
                        orderHint1 = SectorOrder.DOS_Sector;
                    } else {
                        orderHint1 = SectorOrder.ProDOS_Block;
                    }
                    break;
                case ".img":
                    fileKind1 = FileKind.UnadornedSector;
                    orderHint1 = SectorOrder.Physical;
                    fileKind2 = FileKind.DiskCopy;
                    break;
                case ".d13":
                    fileKind1 = FileKind.UnadornedSector;
                    orderHint1 = SectorOrder.DOS_Sector;
                    break;
                case ".dsk":
                    okayForCreate = false;      // ambiguous
                    fileKind1 = FileKind.UnadornedSector;
                    orderHint1 = SectorOrder.DOS_Sector;
                    fileKind2 = FileKind.DiskCopy;
                    break;
                case ".nib":
                    fileKind1 = FileKind.UnadornedNibble525;
                    orderHint1 = SectorOrder.Physical;
                    break;
                case ".nb2":
                    okayForCreate = false;      // not supported
                    fileKind1 = FileKind.UnadornedNibble525;
                    orderHint1 = SectorOrder.Physical;
                    break;
                case ".raw":     // could be unadorned sector or unadorned nibble
                    okayForCreate = false;      // ambiguous
                    fileKind1 = FileKind.UnadornedSector;
                    orderHint1 = SectorOrder.DOS_Sector;
                    fileKind2 = FileKind.UnadornedNibble525;
                    break;
                case ".woz":
                    fileKind1 = FileKind.Woz;
                    orderHint1 = SectorOrder.Physical;
                    break;
                case ".2mg":
                case ".2img":
                    fileKind1 = FileKind.TwoIMG;
                    break;
                case ".dc":
                case ".dc6":
                case ".image":
                    fileKind1 = FileKind.DiskCopy;
                    fileKind2 = FileKind.UnadornedSector;
                    orderHint1 = SectorOrder.ProDOS_Block;
                    break;
                case ".app":
                    fileKind1 = FileKind.Trackstar;
                    orderHint1 = SectorOrder.Physical;
                    break;
                case ".zip":
                    fileKind1 = FileKind.Zip;
                    break;
                case ".shk":
                case ".sdk":
                    fileKind1 = FileKind.NuFX;
                    break;
                case ".bxy":
                case ".sea":
                case ".bse":
                    okayForCreate = false;      // don't allow wrapper creation
                    fileKind1 = FileKind.NuFX;
                    break;
                case ".bny":
                case ".bqy":
                    fileKind1 = FileKind.Binary2;
                    break;
                case ".acu":
                    fileKind1 = FileKind.AppleLink;
                    break;
                case ".gz":
                    fileKind1 = FileKind.GZip;
                    break;
                case ".as":
                    fileKind1 = FileKind.AppleSingle;
                    break;
                //case ".bin":      // too generic; let the unknown-extension handler deal with it
                case ".macbin":
                    fileKind1 = FileKind.MacBinary;
                    break;
                case ".ddd":
                    fileKind1 = FileKind.DDD;
                    orderHint1 = SectorOrder.Physical;
                    break;
                default:
                    Debug.WriteLine("Unrecognized filename extension: " + fileNameExt);
                    return false;
            }
            return true;
        }

        /// <summary>
        /// Outcome of file analysis.
        /// </summary>
        public enum AnalysisResult {
            Undefined = 0,

            Success,                    // analysis successfully identified the file
            DubiousSuccess,             // analysis succeeded but some evidence of damage
            FileDamaged,                // file recognized but severely damaged (e.g. truncated)
            UnknownExtension,           // filename extension was not recognized
            ExtensionMismatch,          // file extension does not match file contents
            NotImplemented,             // part of the analyzer is not implemented
        }

        /// <summary>
        /// Analyzes the filename extension and contents of a file to determine what kind of
        /// file it is.
        /// </summary>
        /// <remarks>
        /// <para>This only analyzes the file structure.  It will determine if something appears
        /// to be a disk image, but will not try to determine the sector skew or filesystem
        /// type.</para>
        /// <para>If the result is <see cref="AnalysisResult.ExtensionMismatch"/>, and the
        /// caller wants to retry the analysis considering only the file contents, it may call
        /// this function again with an empty <paramref name="fileNameExt"/>.  This could be
        /// valuable if the file has other attributes, e.g. an HFS file type of 'dCpy'.</para>
        /// </remarks>
        /// <param name="stream">File data stream; must be seekable.</param>
        /// <param name="fileNameExt">Filename extension, with or without the leading '.';
        ///   may be empty.</param>
        /// <param name="kind">Result: what kind of file this is.</param>
        /// <param name="orderHint">Result: for disk image files, the sector order hinted at
        ///   by the filename.</param>
        /// <returns>Result indicating success or cause of failure.</returns>
        public static AnalysisResult Analyze(Stream stream, string fileNameExt, AppHook appHook,
                out FileKind kind, out SectorOrder orderHint) {
            if (!stream.CanSeek) {
                throw new ArgumentException("Stream must be seekable");
            }
            //if (stream.Length == 0) {
            //    throw new ArgumentException("Zero-length stream");
            //}

            AnalysisResult result = AnalysisResult.Success;

            kind = FileKind.Unknown;
            if (ExtensionToKind(fileNameExt, out FileKind kind1, out orderHint,
                    out FileKind kind2, out bool unused)) {
                // File extension was recognized.  See if the contents match.
                if (TestKind(stream, kind1, appHook)) {
                    kind = kind1;
                } else if (TestKind(stream, kind2, appHook)) {
                    kind = kind2;
                } else {
                    result = AnalysisResult.ExtensionMismatch;
                }
            } else {
                result = AnalysisResult.UnknownExtension;
            }

            // Handle special case.
            if (result == AnalysisResult.Success &&
                    fileNameExt == ".d13" && stream.Length != SECTOR_SIZE * 13 * 35) {
                result = AnalysisResult.ExtensionMismatch;
            }

            if (result == AnalysisResult.UnknownExtension) {
                // See if we can make sense of this.  We shouldn't find files without extensions
                // on the host filesystem, but files pulled out of an archive might not use a
                // filename extension for identification.
                foreach (FileKind testKind in sProbeKinds) {
                    if (TestKind(stream, testKind, appHook)) {
                        appHook.LogI("Extension '" + fileNameExt + "' was unknown, detected " +
                            testKind);
                        kind = testKind;
                        result = AnalysisResult.Success;
                        break;
                    }
                }
            }

            return result;
        }

        /// <summary>
        /// List of things to try, in order, when we don't recognize the file extension.
        /// </summary>
        private static FileKind[] sProbeKinds = new FileKind[] {
            // These can be detected reliably.
            FileKind.TwoIMG,
            FileKind.Woz,
            FileKind.Zip,
            FileKind.NuFX,
            FileKind.GZip,
            FileKind.AppleSingle,
            FileKind.DiskCopy,
            FileKind.AppleLink,
            FileKind.Binary2,           // test after NuFX (.BXY > .BNY)
            FileKind.MacBinary,         // very weak format detection
            // These are less definite, depending primarily on the size of the file.
            FileKind.Trackstar,
            FileKind.UnadornedNibble525,
            FileKind.UnadornedSector,   // this accepts almost any multiple of 512 bytes
            // Skip: DDD
        };

        /// <summary>
        /// Invokes the appropriate file kind tester.
        /// </summary>
        /// <param name="stream">File stream.</param>
        /// <param name="kind">File kind to test against.</param>
        /// <param name="appHook">Application hook reference.</param>
        /// <returns>Result of TestKind() function.</returns>
        public static bool TestKind(Stream stream, FileKind kind, AppHook appHook) {
            switch (kind) {
                case FileKind.AppleSingle:
                    return AppleSingle.TestKind(stream, appHook) ||
                        AppleSingle.TestDouble(stream, appHook);
                case FileKind.AppleLink:
                    return AppleLink.TestKind(stream, appHook);
                case FileKind.Binary2:
                    if (stream.Length == 0) {
                        // Empty Binary II archives are totally empty files.  The test code
                        // doesn't accept them, but we want to do so here.
                        return true;
                    }
                    return Binary2.TestKind(stream, appHook);
                case FileKind.DDD:
                    return false;       // TODO
                case FileKind.DiskCopy:
                    return DiskCopy.TestKind(stream, appHook);
                case FileKind.GZip:
                    return GZip.TestKind(stream, appHook);
                case FileKind.MacBinary:
                    return MacBinary.TestKind(stream, appHook);
                case FileKind.NuFX:
                    return NuFX.TestKind(stream, appHook);
                case FileKind.Trackstar:
                    return Trackstar.TestKind(stream, appHook);
                case FileKind.TwoIMG:
                    return TwoIMG.TestKind(stream, appHook);
                case FileKind.UnadornedSector:
                    return UnadornedSector.TestKind(stream, appHook);
                case FileKind.UnadornedNibble525:
                    return UnadornedNibble525.TestKind(stream, appHook);
                case FileKind.Woz:
                    return Woz.TestKind(stream, appHook);
                case FileKind.Zip:
                    return Zip.TestKind(stream, appHook);

                case FileKind.Unknown:
                    return false;
                default:
                    Debug.WriteLine("Unhandled file kind: " + kind);
                    Debug.Assert(false);
                    return false;
            }
        }

        /// <summary>
        /// Prepares a disk image file for use, based on the detected file kind.
        /// </summary>
        /// <remarks>
        /// <para>This just opens and validates the disk image file.  It does not analyze the
        /// disk format or filesystem.</para>
        /// </remarks>
        /// <param name="stream">Disk image file.</param>
        /// <param name="kind">Result from file analyzer; indicates block/sector vs. nibble.</param>
        /// <param name="appHook">Application hook reference.</param>
        /// <returns>A disk image object of the appropriate type, or null if the file could not
        ///   be opened as directed.</returns>
        /// <exception cref="DAException">Called with a non-disk-image kind.</exception>
        public static IDiskImage? PrepareDiskImage(Stream stream, FileKind kind, AppHook appHook) {
            if (!IsDiskImageFile(kind)) {
                throw new DAException("This method is only for disk images");
            }

            // Instantiate the specified disk file kind.  If we're being called after a successful
            // file test, this should not fail.
            IDiskImage? diskImage = null;
            try {
                switch (kind) {
                    case FileKind.DiskCopy:
                        diskImage = DiskCopy.OpenDisk(stream, appHook);
                        break;
                    case FileKind.Trackstar:
                        diskImage = Trackstar.OpenDisk(stream, appHook);
                        break;
                    case FileKind.TwoIMG:
                        diskImage = TwoIMG.OpenDisk(stream, appHook);
                        break;
                    case FileKind.UnadornedSector:
                        diskImage = UnadornedSector.OpenDisk(stream, appHook);
                        break;
                    case FileKind.UnadornedNibble525:
                        diskImage = UnadornedNibble525.OpenDisk(stream, appHook);
                        break;
                    case FileKind.Woz:
                        diskImage = Woz.OpenDisk(stream, appHook);
                        break;
                    default:
                        Debug.Assert(false, "Unexpected kind " + kind);
                        break;
                }
            } catch (NotSupportedException) {
                // The constructor rejected our file.
                appHook.LogE("Failed to open disk file with kind=" + kind);
            } catch (Exception ex) {
                appHook.LogE("Failed to open disk file with kind=" + kind + " (" + ex + ")");
                Debug.Assert(false, "unexpected exception");
            }

            return diskImage;
        }

        /// <summary>
        /// Analyzes nibble data to determine the low-level sector format.  Used to get a chunk
        /// access object for a nibble disk image.
        /// </summary>
        /// <param name="nibbles">Nibble data source.</param>
        /// <param name="codec">Nibble codec to use.  If null, various standard codecs will
        ///   be tried.</param>
        /// <param name="appHook">Application hook reference.</param>
        /// <returns>Chunk access object.</returns>
        public static IChunkAccess? AnalyzeDiskFormat(INibbleDataAccess nibbles,
                SectorCodec? codec, AppHook appHook) {
            if (codec != null) {
                // Currently just accepting what we're given.  This feels like the correct
                // approach.
                return new NibbleChunkAccess(nibbles, codec);
            } else {
                if (nibbles.DiskKind == MediaKind.GCR_525) {
                    return Analyze525(nibbles, appHook);
                } else {
                    return Analyze35(nibbles, appHook);
                }
            }
        }

        /// <summary>
        /// Tracks to examine when trying to determine the disk format of a 5.25" disk.
        /// </summary>
        /// <remarks>
        /// <para>Track 0 can be a little funky because it needs to be at least partly
        /// standard or the disk won't boot, so we start with track 1, which will usually
        /// have a DOS image or file data.  Track 17 is a natural for DOS, but sometimes we
        /// get a false positive if the copy protector decided to have a visible catalog
        /// track just for fun.  16 will be the first track with DOS data.  22 is mostly
        /// arbitrary; it's an even-numbered track past the catalog track (some protection
        /// schemes used different values for odd/even tracks).</para>
        /// </remarks>
        private static readonly uint[] sTracksToCheck = new uint[] { 1, 16, 17, 22 };

        /// <summary>
        /// Analyzes a 5.25" floppy disk image.
        /// </summary>
        private static IChunkAccess? Analyze525(INibbleDataAccess nibbles, AppHook appHook) {
            const int DMG_SCORE = 1;
            const int PARTIAL_SCORE = 2;
            const int FULL_SCORE = 4;
            int bestScore = 0;
            SectorCodec? bestCodec = null;

            foreach (StdSectorCodec.CodecIndex525 ci in
                    Enum.GetValues(typeof(StdSectorCodec.CodecIndex525))) {
                SectorCodec codec = StdSectorCodec.GetCodec(ci);

                // If we find at least 13 sectors on 3 tracks (or 10 sectors on 3 tracks for 5&3),
                // declare victory immediately.
                int expectedSectors =
                    codec.DataEncoding == SectorCodec.NibbleEncoding.GCR53 ? 13 : 16;
                int winThreshold = sTracksToCheck.Length * (expectedSectors - 3) * FULL_SCORE;

                int score = 0;
                foreach (uint track in sTracksToCheck) {
                    if (!nibbles.GetTrackBits(track, 0, out CircularBitBuffer? trackData)) {
                        // Track wasn't even stored.  Give up.
                        Debug.WriteLine("No data stored for track " + track);
                        return null;
                    }

                    // Find all the sectors.  If we come up completely empty, we either have the
                    // wrong codec, or it's a track that has a weird format because it's used for
                    // nibble counting or other tricks.
                    List<SectorPtr> sectorPtrs = codec.FindSectors(track, 0, trackData);

                    // Score the sectors based on contents.
                    foreach (SectorPtr secPtr in sectorPtrs) {
                        if (secPtr.Sector >= expectedSectors) {
                            score += DMG_SCORE;
                        } else if (secPtr.IsAddrDamaged || secPtr.IsDataDamaged) {
                            score += DMG_SCORE;
                        } else if (secPtr.DataFieldBitOffset == -1) {
                            // Undamaged but no data field, so this is probably newly-formatted
                            // DOS 3.2.
                            score += PARTIAL_SCORE;
                        } else {
                            score += FULL_SCORE;
                        }
                    }
                }

                if (score >= winThreshold) {
                    // Confidence level is high enough to just accept this.
                    appHook.LogD("Confidently accepting codec (score=" + score + "): " +
                        codec.Name);
                    return new NibbleChunkAccess(nibbles, codec);
                }
                if (score > bestScore) {
                    bestScore = score;
                    bestCodec = codec;
                    appHook.LogD("New best codec (score=" + score + "): " + codec.Name);
                }
            }

            if (bestScore < 12 * FULL_SCORE) {
                return null;
            }
            Debug.Assert(bestCodec != null);
            appHook.LogD("Choosing best codec (score=" + bestScore + "): " + bestCodec.Name);
            return new NibbleChunkAccess(nibbles, bestCodec);
        }

        /// <summary>
        /// Analyzes a 3.5" floppy disk image.
        /// </summary>
        private static IChunkAccess? Analyze35(INibbleDataAccess nibbles, AppHook appHook) {
            // These disks are usually ProDOS or HFS, which have key filesystem structures
            // on track zero.  I'm not sure what sort of see-through copy protection is used
            // on 3.5" disks, so for now we just do a quick check on track zero to confirm
            // that the disk is recognizable.

            StdSectorCodec.CodecIndex35 ci = StdSectorCodec.CodecIndex35.Std_35;
            SectorCodec codec = StdSectorCodec.GetCodec(ci);

            // Grab track 0 / side 0.
            uint track = 0;
            uint side = 0;
            if (!nibbles.GetTrackBits(track, side, out CircularBitBuffer? trackData)) {
                // Track wasn't even stored.  Give up.
                Debug.WriteLine("No data stored for track " + track + "/" + side);
                return null;
            }

            // Find all the sectors.  If we come up completely empty, we either have the
            // wrong codec, or it's a track that has a weird format because it's used for
            // nibble counting or other tricks.
            List<SectorPtr> sectorPtrs = codec.FindSectors(track, side, trackData);
            if (sectorPtrs.Count < 10) {
                appHook.LogD("3.5 sector format not recognized");
                return null;
            }

            appHook.LogD("Selecting 3.5 codec " + codec.Name);
            return new NibbleChunkAccess(nibbles, codec);
        }

        // List of file sector orders to test.
        private static SectorOrder[] sOrders = new SectorOrder[] {
            SectorOrder.DOS_Sector,
            SectorOrder.ProDOS_Block,
            SectorOrder.Physical,
            SectorOrder.CPM_KBlock      // only useful for disk images created by CP/M software
        };

        /// <summary>
        /// Analyzes a chunk of blocks or sectors to determine which filesystem is present.
        /// This is called by the disk image code, after it identifies the part of the stream
        /// where chunks are stored.
        /// </summary>
        /// <remarks>
        /// <para>For 5.25" disk images, determining the file order and filesystem must be
        /// done at the same time.  For all other images, the file order will either be
        /// "physical" (if it's a nibble image) or ProDOS block.</para>
        /// <para>If a bad block is encountered during analysis, this will return false.</para>
        ///
        /// <para>For a multi-partition format, the filesystems with the partitions will not
        /// be prepared.  Each partition will have an accessible IChunkAccess.</para>
        /// </remarks>
        /// <param name="chunkAccess">Chunk data source.</param>
        /// <param name="doProbeOrders">If set, probe for the proper file order.  Set to false
        ///   if the file order is already known.</param>
        /// <param name="appHook">Application callback hook.</param>
        /// <param name="contents">Result: if disk layout was a single filesystem, this
        ///   receives an IFileSystem object instance; if layout was a multi-partition format,
        ///   this receives an IMultiPart instance.  Otherwise null.</param>
        /// <returns>True if the disk layout was recognized; <paramref name="contents"/> will
        ///   be non-null.  If false, <paramref name="contents"/> will be null.</returns>
        /// <exception cref="DAException">Called with a disk image kind.</exception>
        public static bool AnalyzeFileSystem(IChunkAccess chunkAccess, bool doProbeOrders,
                AppHook appHook, out IDiskContents? contents) {
            SectorOrder initialOrder = chunkAccess.FileOrder;
            SectorOrder bestOrder = initialOrder;
            DiskLayoutEntry? bestEntry = null;
            DiskLayoutEntry.TestResult bestResult = DiskLayoutEntry.TestResult.No;

            Debug.Assert(chunkAccess is not GatedChunkAccess);
            try {
                // Try each filesystem in turn, stopping if we get a positive match.  If we get
                // something between "yes" and "no", remember the value, keeping the best one we find
                // (the search list should be in priority order).
                foreach (DiskLayoutEntry fse in sDiskLayouts) {
                    chunkAccess.FileOrder = initialOrder;

                    DiskLayoutEntry.TestResult result = fse.TestImage(chunkAccess, appHook);
                    if (result == DiskLayoutEntry.TestResult.Yes) {
                        appHook.LogD("FS match on " + fse.Name + ", fileOrder=" +
                            chunkAccess.FileOrder);
                        bestEntry = fse;
                        bestOrder = initialOrder;
                        break;
                    } else if (result > bestResult) {
                        // Remember this one, but keep going.
                        bestEntry = fse;
                        bestOrder = initialOrder;
                        bestResult = result;
                    }

                    // Didn't get a "yes", try different orderings if this is a sector image.
                    if (chunkAccess.HasSectors && doProbeOrders) {
                        foreach (SectorOrder order in sOrders) {
                            if (order == initialOrder) {
                                continue;       // don't do this one again
                            }
                            chunkAccess.FileOrder = order;
                            Debug.Assert(chunkAccess.FileOrder == order);
                            result = fse.TestImage(chunkAccess, appHook);
                            if (result == DiskLayoutEntry.TestResult.Yes) {
                                appHook.LogD("FS probe match on " + fse.Name + ", fileOrder=" +
                                    chunkAccess.FileOrder);
                                bestEntry = fse;
                                bestOrder = order;
                                bestResult = result;
                                break;
                            } else if (result > bestResult) {
                                // Best we've seen so far, remember it.  If we hit the same score a
                                // second time, we want to keep the first one, so that we prioritize
                                // hinted orders.
                                bestEntry = fse;
                                bestOrder = order;
                                bestResult = result;
                            }
                        }

                        // We'll take "good" or "yes".  If we insist on "yes" we'll potentially
                        // miss out on some DOS+ProDOS hybrids that have short catalog tracks.
                        // TODO: we may want to rework this to always take the highest-scoring
                        //   item, but keep track of the next-best.  If we have yes+yes or yes+good
                        //   then we can evaluate for hybrid.  This removes the need to check DOS
                        //   before ProDOS.
                        if (bestResult >= DiskLayoutEntry.TestResult.Good) {
                            break;
                        }
                    }
                }
            } catch (BadBlockException ex) {
                Debug.WriteLine("Filesystem analysis failed: " + ex.Message);
            }

            if (bestEntry != null) {
                chunkAccess.FileOrder = bestOrder;
                bestEntry.CreateInstance(chunkAccess, appHook, out contents);
                return true;
            } else {
                chunkAccess.FileOrder = initialOrder;
                contents = null;
                return false;
            }
        }

        /// <summary>
        /// Creates an instance of a filesystem, by type.
        /// </summary>
        /// <param name="type">Filesystem type.</param>
        /// <param name="chunkAccess">Chunk access object.</param>
        /// <param name="appHook">Application hook reference.</param>
        /// <returns>Filesystem object, or null if unable to create it.</returns>
        public static IFileSystem? CreateFileSystem(FileSystemType type, IChunkAccess chunkAccess,
                AppHook appHook) {
            Debug.Assert(chunkAccess is not GatedChunkAccess);
            foreach (DiskLayoutEntry fse in sDiskLayouts) {
                if (fse.FSType == type) {
                    DiskLayoutEntry.TestResult result = fse.TestImage(chunkAccess, appHook);
                    if (result <= DiskLayoutEntry.TestResult.No) {
                        Debug.WriteLine("Requested filesystem not found");
                        return null;
                    }
                    fse.CreateInstance(chunkAccess, appHook, out IDiskContents? contents);
                    return contents as IFileSystem;
                }
            }
            Debug.Assert(false, "Unable to find FS type " + type);
            return null;
        }

        /// <summary>
        /// Prepares a file archive for use, based on the detected file kind.
        /// </summary>
        /// <param name="stream">Stream with archive data.  Must be seekable, may be
        ///   read-only.</param>
        /// <param name="kind">How to treat the data stream.</param>
        /// <returns>Archive object reference, or null if the stream could not be opened as
        ///   the specified kind.</returns>
        /// <exception cref="DAException">Called with a disk image kind.</exception>
        ///
        /// <remarks>
        /// <para>Normally the caller will pass in the file kind determined by the analyzer.
        /// In some ambiguous cases the analyzer result can be overridden, e.g. a BXY file could
        /// be opened as Binary II rather than NuFX to edit the header.</para>
        /// </remarks>
        public static IArchive? PrepareArchive(Stream stream, FileKind kind, AppHook appHook) {
            if (IsDiskImageFile(kind)) {
                throw new DAException("This method is not for disk images");
            }

            IArchive? archive = null;
            try {
                switch (kind) {
                    case FileKind.AppleLink:
                        archive = AppleLink.OpenArchive(stream, appHook);
                        break;
                    case FileKind.AppleSingle:
                        archive = AppleSingle.OpenArchive(stream, appHook);
                        break;
                    case FileKind.Binary2:
                        archive = Binary2.OpenArchive(stream, appHook);
                        break;
                    case FileKind.GZip:
                        archive = GZip.OpenArchive(stream, appHook);
                        break;
                    case FileKind.MacBinary:
                        archive = MacBinary.OpenArchive(stream, appHook);
                        break;
                    case FileKind.NuFX:
                        archive = NuFX.OpenArchive(stream, appHook);
                        break;
                    case FileKind.Zip:
                        archive = Zip.OpenArchive(stream, appHook);
                        break;
                    default:
                        throw new NotImplementedException("Not implemented: " + kind);
                }
            } catch (NotSupportedException) {
                // Rejection.
                appHook.LogE("Failed to open archive file with kind=" + kind);
            }

            return archive;
        }
    }
}