servuo-plugins/overlay/Scripts/Custom/Bridge/BridgeCatalog.cs

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Drawing.Imaging;
using System.Globalization;
using System.IO;
using System.Runtime.InteropServices;
using System.Security.Cryptography;
using System.Text;

using Ultima;

namespace Server.Custom.Bridge
{
    /// <summary>
    ///     **The body catalogue** (docs/link/v8.md §4.8, §5, §6, §11 — protocol 8, phase 3).
    ///
    ///     One thumbnail per creature body: the working set that makes a bestiary, a marketplace
    ///     listing and a character sheet render. Everything deeper — every action, every frame —
    ///     is the same addressing scheme at a deeper key, fetched on demand in a later phase; this
    ///     is the set that is worth importing before anything asks for it, because on this
    ///     machine's client it is **787 sprites at about a kilobyte each**.
    ///
    ///     Two request kinds, which are §6's two stages for assets rather than for sources:
    ///
    ///     <c>assets.manifest</c> — `[{ key, sha256, bytes, width, height }]`, no pixels. The
    ///     website diffs it against what it already holds and asks only for what changed. That is
    ///     the whole difference between an Update and a re-download.
    ///
    ///     <c>assets.fetch</c> — the pixels, for an explicit list of keys.
    ///
    ///     ── **Why the manifest builds the pictures it refuses to send** ──
    ///
    ///     A manifest row carries a hash of the bytes, and the only way to hash bytes is to have
    ///     them. So the scan decodes, encodes to PNG and hashes, then sends the row and **keeps
    ///     the bytes** — a megabyte for the whole catalogue, against re-decoding all 787 sprites a
    ///     second time when the fetch arrives moments later.
    ///
    ///     ── **Why the manifest pages on TIME rather than on bytes** ──
    ///
    ///     Every other family on this plane pages because its rows are large. This one's rows are
    ///     ninety bytes and the whole catalogue is one page by the byte budget — but producing
    ///     that page means decoding 787 animations, and the sidecar gives a reply ten seconds
    ///     (§3.3). So the scan carries a **wall-clock budget** as well
    ///     (<see cref="BridgeConfig.AssetScanMs"/>) and cuts the page `limit` when it is spent,
    ///     resuming from its cursor on the next call. The byte budget is still enforced, because
    ///     the day a family's rows grow is not the day to discover only one of the two bounds was
    ///     real.
    ///
    ///     ── **Why `catalog` is derived from the sources and not minted per build** ──
    ///
    ///     A manifest walk and the fetch that follows it must be talking about the same client
    ///     files, or the website stitches one catalogue out of two. The obvious answer is a fresh
    ///     id per build, and it is wrong: this cache is released when it goes idle, so a rebuild
    ///     halfway through a slow import would change the id and force a restart although nothing
    ///     about the client moved. So the id is a hash of what actually decides the bytes — every
    ///     anim file's size and mtime, both direction settings and
    ///     <see cref="BridgeAssets.EXTRACTOR_VERSION"/>. It is stable across a rebuild and it
    ///     changes exactly when an operator patches their client.
    ///
    ///     ── **The never-sweep rule, and the 357** ──
    ///
    ///     Nothing here asks a file type for an index it does not own, and nothing here trusts the
    ///     library's own success. <see cref="BridgeAssetValidator.ResolveAnimation"/> takes
    ///     `BodyConverter.Convert`'s answer and reports nothing if it leads nowhere (sweeping
    ///     instead puts a giant spider on the gargoyle page, decoding cleanly); and every body is
    ///     put through <see cref="BridgeAssetValidator.CheckEntry"/> and
    ///     <c>RecordReader.AnimationSane</c> **before** it is decoded, because a body whose index
    ///     entry reads `length 0` gets a bitmap back anyway — the previously-decoded creature's,
    ///     from the library's reused stream buffer. That is 357 of the 1,144 bodies the library
    ///     claims on a stock client, and importing them would have written 357 duplicate
    ///     portraits whose subject depended on the order this walk happened to run in.
    /// </summary>
    public static class BridgeCatalog
    {
        /// <summary>The only family this phase serves. §5's key scheme covers the rest.</summary>
        private const string Family = "body";

        /// <summary>Bodies are addressable to 2047; the sweep behind §4.8 covered exactly this.</summary>
        private const int MaxBody = 2047;

        /// <summary>The catalogue is first frames only. Deep keys are phase 6.</summary>
        private const int CatalogAction = 0;

        public static void Initialize()
        {
            if (!BridgeConfig.Enabled)
                return;

            BridgeBoot.RegisterHandler("assets.manifest", OnManifest);
            BridgeBoot.RegisterHandler("assets.fetch", OnFetch);
        }

        // ── the cache ────────────────────────────────────────────────────────────────────────

        private sealed class Sprite
        {
            public string Key;
            public int Body;
            public int Direction;
            public int FileType;
            public string Sha256;
            public byte[] Png;
            public int Width;
            public int Height;
        }

        private sealed class Catalog
        {
            public string Id;
            public readonly Dictionary<string, Sprite> ByKey =
                new Dictionary<string, Sprite>(StringComparer.Ordinal);
            public readonly List<Sprite> Order = new List<Sprite>();

            /// <summary>The next body the scan has yet to look at.</summary>
            public int Next = 1;

            public bool Complete;
            public DateTime LastUsed;
        }

        private static readonly object _sync = new object();
        private static Catalog _catalog;

        private static readonly TimeSpan IdleFor = TimeSpan.FromMinutes(5);

        // ── assets.manifest ──────────────────────────────────────────────────────────────────

        private static void OnManifest(Dictionary<string, object> o)
        {
            string reqId;

            if (!Admit(o, "assets.manifest", out reqId))
                return;

            var family = BridgeJson.GetString(o, "family") ?? Family;

            if (!String.Equals(family, Family, StringComparison.Ordinal))
            {
                // Named rather than ignored: `family` exists so §5's statics and land can join
                // this envelope in phase 5 without a second request kind, and a website that
                // asked for one of those against a phase-3 overlay must be told it asked too
                // early rather than handed a body catalogue it did not request.
                BridgeAssets.Fail(reqId, "BAD_REQUEST",
                    "this shard serves the '" + Family + "' asset family only (asked for '"
                    + family + "')");
                return;
            }

            var cursor = BridgeJson.GetString(o, "cursor");

            BridgeAssets.Accept(reqId, "assets.manifest", () => ReplyManifest(reqId, cursor));
        }

        /// <summary>
        ///     Worker thread. Scans forward from the cursor until the byte budget or the time
        ///     budget is spent, hashing what it decodes and keeping the bytes for the fetch.
        /// </summary>
        private static void ReplyManifest(string reqId, string cursor)
        {
            string imagingReason;

            if (!BridgeAssets.ImagingOk(out imagingReason))
            {
                // Never a stack trace and never a 500: on a Linux host without libgdiplus this is
                // the expected outcome, and it is actionable in one line (§4.4).
                BridgeAssets.Fail(reqId, "UNAVAILABLE",
                    "this shard host cannot render images - Mono's System.Drawing needs "
                    + "libgdiplus. Install it (apt-get install libgdiplus) and re-run the import. "
                    + "Cliloc and atlas import are unaffected. (" + imagingReason + ")");
                return;
            }

            string id = SourceId();

            Catalog catalog;

            lock (_sync)
            {
                if (_catalog == null || _catalog.Id != id)
                    _catalog = new Catalog { Id = id };

                catalog = _catalog;
                catalog.LastUsed = DateTime.UtcNow;
            }

            int from = ParseBodyCursor(cursor);

            var sb = BridgeJson.Begin("assets.manifest.ok");

            sb.Str("reqId", reqId)
              .Str("family", Family)
              // What the website compares across pages, and across the fetch that follows. A
              // change means the operator patched their client mid-import and the half already
              // read describes files that no longer exist.
              .Str("catalog", catalog.Id)
              .Num("extractorVersion", BridgeAssets.EXTRACTOR_VERSION)
              .Num("maxBody", MaxBody)
              .Num("from", from);

            WritePlayerBodies(sb);

            var page = new BridgeAssets.PageBuilder(sb, "rows", BridgeConfig.AssetBatchBytes);

            int scanned = 0;
            int last = from - 1;
            bool timedOut = false;
            bool budgetCut = false;

            var deadline = DateTime.UtcNow.AddMilliseconds(BridgeConfig.AssetScanMs);

            using (var readers = new Readers())
            {
                int body = from;

                for (; body <= MaxBody; body++)
                {
                    // Checked before the body rather than after it, so the budget bounds the reply
                    // rather than the reply plus one more decode. One sprite is milliseconds; the
                    // ceiling this lives under is ten seconds and the cost of overshooting it is
                    // the whole page, retried.
                    if (body > from && DateTime.UtcNow >= deadline)
                    {
                        timedOut = true;
                        break;
                    }

                    scanned++;
                    last = body;

                    Sprite sprite = Resolve(catalog, readers, body);

                    if (sprite == null)
                        continue;

                    var item = new StringBuilder(128);

                    item.Append("{\"key\":");
                    BridgeJson.Text(item, sprite.Key);
                    item.Append(",\"sha256\":\"").Append(sprite.Sha256).Append('"');
                    item.Append(",\"bytes\":").Append(sprite.Png.Length.ToString(CultureInfo.InvariantCulture));
                    item.Append(",\"width\":").Append(sprite.Width.ToString(CultureInfo.InvariantCulture));
                    item.Append(",\"height\":").Append(sprite.Height.ToString(CultureInfo.InvariantCulture));
                    item.Append(",\"body\":").Append(sprite.Body.ToString(CultureInfo.InvariantCulture));
                    item.Append(",\"direction\":").Append(sprite.Direction.ToString(CultureInfo.InvariantCulture));
                    item.Append('}');

                    if (!page.TryAdd(item.ToString(), "b:" + body.ToString(CultureInfo.InvariantCulture)))
                    {
                        // The budget stopped this page BEFORE this body's row went on it, so the
                        // next page must resume AT this body rather than after it. Getting this
                        // one line wrong drops exactly one creature from the catalogue per page,
                        // which nothing downstream could ever notice.
                        budgetCut = true;
                        last = body - 1;
                        scanned--;
                        break;
                    }
                }
            }

            if (timedOut)
                page.Cut("limit");

            // The walk reached the end of the addressable range without either budget stopping it.
            // Derived from the two flags rather than from the row count, because a page that ends
            // exactly on a boundary is indistinguishable from a finished one by count alone —
            // §3.4's whole argument for `cut` existing.
            bool finished = !timedOut && !budgetCut && last >= MaxBody;

            int held;

            lock (_sync)
            {
                if (_catalog == catalog)
                {
                    catalog.Next = Math.Max(catalog.Next, last + 1);
                    catalog.LastUsed = DateTime.UtcNow;

                    if (finished)
                        catalog.Complete = true;
                }

                held = catalog.Order.Count;
            }

            page.Close();

            // Past Close(), which is normally the mistake BridgeCliloc's `from` comment warns
            // about — but these three are not knowable until the scan has run, and they cost
            // about fifty bytes against PageBuilder's 256-byte reserve, of which Close() itself
            // spends around forty. Anything larger than this belongs before the page opens.
            sb.Num("scanned", scanned)
              .Num("held", held)
              .Bool("complete", finished);

            BridgeLink.Emit(sb.End());

            Sweep();
        }

        // ── assets.fetch ─────────────────────────────────────────────────────────────────────

        private static void OnFetch(Dictionary<string, object> o)
        {
            string reqId;

            if (!Admit(o, "assets.fetch", out reqId))
                return;

            var keys = BridgeJson.GetStringList(o, "keys");

            if (keys.Count == 0)
            {
                BridgeAssets.Fail(reqId, "BAD_REQUEST",
                    "assets.fetch requires a non-empty `keys` array");
                return;
            }

            if (keys.Count > BridgeConfig.AssetFetchKeys)
            {
                BridgeAssets.Fail(reqId, "BAD_REQUEST",
                    "assets.fetch takes at most " + BridgeConfig.AssetFetchKeys
                    + " keys per request (asked for " + keys.Count + ")");
                return;
            }

            var catalog = BridgeJson.GetString(o, "catalog");
            var cursor = BridgeJson.GetString(o, "cursor");

            BridgeAssets.Accept(reqId, "assets.fetch", () => ReplyFetch(reqId, keys, catalog, cursor));
        }

        private static void ReplyFetch(string reqId, List<string> keys, string expected, string cursor)
        {
            string imagingReason;

            if (!BridgeAssets.ImagingOk(out imagingReason))
            {
                BridgeAssets.Fail(reqId, "UNAVAILABLE",
                    "this shard host cannot render images - Mono's System.Drawing needs "
                    + "libgdiplus. (" + imagingReason + ")");
                return;
            }

            string id = SourceId();

            if (expected != null && expected != id)
            {
                // The client files moved between the manifest and this fetch. Refusing is the only
                // honest answer: the keys were chosen against a catalogue that no longer describes
                // what is on disk, and serving them would mix two clients in one import with no
                // error anywhere.
                BridgeAssets.Fail(reqId, "UNREADABLE",
                    "the shard's client files changed since that manifest was read (catalog "
                    + expected + " is now " + id + "); start the import again");
                return;
            }

            Catalog catalog;

            lock (_sync)
            {
                if (_catalog == null || _catalog.Id != id)
                    _catalog = new Catalog { Id = id };

                catalog = _catalog;
                catalog.LastUsed = DateTime.UtcNow;
            }

            int from = ParseKeyCursor(cursor);

            var sb = BridgeJson.Begin("assets.fetch.ok");

            sb.Str("reqId", reqId)
              .Str("family", Family)
              .Str("catalog", catalog.Id)
              .Num("extractorVersion", BridgeAssets.EXTRACTOR_VERSION)
              .Num("asked", keys.Count)
              .Num("from", from);

            var page = new BridgeAssets.PageBuilder(sb, "rows", BridgeConfig.AssetBatchBytes);

            int i = from;

            using (var readers = new Readers())
            {
                for (; i < keys.Count; i++)
                {
                    var item = Render(catalog, readers, keys[i]);

                    if (!page.TryAdd(item, "k:" + (i + 1).ToString(CultureInfo.InvariantCulture)))
                        break;
                }
            }

            page.Close();

            sb.Num("sent", page.Count);

            BridgeLink.Emit(sb.End());

            Sweep();
        }

        /// <summary>
        ///     One key to one row, with the bytes.
        ///
        ///     A key this shard cannot serve is a **row**, not a failed request: the website asked
        ///     for a list, and one key naming a body whose art this client does not carry must not
        ///     cost the other four hundred. `status` distinguishes the two ways that happens —
        ///     `absent` (this client has no art at that key, the expected answer for two thirds of
        ///     the player bodies) and `unsupported` (a key shape this phase does not serve, which
        ///     is a website bug rather than a client gap).
        /// </summary>
        private static string Render(Catalog catalog, Readers readers, string key)
        {
            int body;

            if (!TryParseKey(key, out body))
            {
                var bad = new StringBuilder(96);
                bad.Append("{\"key\":");
                BridgeJson.Text(bad, key);
                bad.Append(",\"status\":\"unsupported\"}");
                return bad.ToString();
            }

            Sprite sprite = Resolve(catalog, readers, body);

            var item = new StringBuilder(2048);

            item.Append("{\"key\":");
            BridgeJson.Text(item, key);

            if (sprite == null)
            {
                item.Append(",\"status\":\"absent\"}");
                return item.ToString();
            }

            item.Append(",\"status\":\"ok\"");
            item.Append(",\"sha256\":\"").Append(sprite.Sha256).Append('"');
            item.Append(",\"bytes\":").Append(sprite.Png.Length.ToString(CultureInfo.InvariantCulture));
            item.Append(",\"width\":").Append(sprite.Width.ToString(CultureInfo.InvariantCulture));
            item.Append(",\"height\":").Append(sprite.Height.ToString(CultureInfo.InvariantCulture));
            item.Append(",\"body\":").Append(sprite.Body.ToString(CultureInfo.InvariantCulture));
            item.Append(",\"direction\":").Append(sprite.Direction.ToString(CultureInfo.InvariantCulture));
            item.Append(",\"png\":\"").Append(Convert.ToBase64String(sprite.Png)).Append("\"}");

            return item.ToString();
        }

        // ── decode ───────────────────────────────────────────────────────────────────────────

        /// <summary>
        ///     The catalogue entry for one body, decoded and hashed on first sight and cached
        ///     after. Returns null when this client has no art for it — which is an ordinary
        ///     answer for well over half of the addressable range, not a failure.
        /// </summary>
        private static Sprite Resolve(Catalog catalog, Readers readers, int body)
        {
            string key = Key(body);

            lock (_sync)
            {
                Sprite cached;

                if (catalog.ByKey.TryGetValue(key, out cached))
                    return cached;
            }

            int direction = IsPlayerBody(body)
                ? BridgeConfig.AssetPlayerDirection
                : BridgeConfig.AssetCreatureDirection;

            int fileType, at;
            string reason;

            if (!BridgeAssetValidator.ResolveAnimation(body, CatalogAction, direction,
                    out fileType, out at, out reason))
                return null;

            FileIndex index = readers.Index(fileType);

            if (index == null)
                return null;

            if (BridgeAssetValidator.CheckEntry(index, at, readers.MulLength(fileType),
                    readers.VerdataLength, out reason) != BridgeAssetValidator.Verdict.Ok)
            {
                // The `length 0` case lands here, and it is the 357. The library would hand back
                // the previously-decoded body's bitmap for every one of them.
                return null;
            }

            var reader = readers.Reader(fileType);

            if (reader == null)
                return null;

            // `maxFrames: 1` because that is what `firstFrame: true` decodes. Checking frames
            // nobody reads would invent refusals, and a checker that refuses real art is worse
            // than no checker at all.
            if (!reader.AnimationSane(index, at, 1, out reason))
                return null;

            Sprite sprite;

            try
            {
                sprite = Decode(key, body, direction, fileType);
            }
            catch (Exception e)
            {
                Console.WriteLine("[Bridge] catalogue: body {0}: {1}: {2}",
                    body, e.GetType().Name, e.Message);
                return null;
            }

            if (sprite == null)
                return null;

            lock (_sync)
            {
                if (!catalog.ByKey.ContainsKey(key))
                {
                    catalog.ByKey[key] = sprite;
                    catalog.Order.Add(sprite);
                }

                return catalog.ByKey[key];
            }
        }

        private static Sprite Decode(string key, int body, int direction, int fileType)
        {
            int hue = 0;

            // `preserveHue: false` — the catalogue is the creature's own art, and a body-level hue
            // from Body.def belongs to a specific mob rather than to the species. §5's key scheme
            // is where a hued variant is expressed (`static/3922/h33`), not here.
            Frame[] frames = Animations.GetAnimation(body, CatalogAction, direction, ref hue, false, true);

            if (frames == null || frames.Length == 0 || frames[0] == null)
                return null;

            Bitmap bitmap = frames[0].Bitmap;

            if (bitmap == null || bitmap.Width <= 0 || bitmap.Height <= 0)
                return null;

            byte[] png = ToPng(bitmap);

            if (png == null)
                return null;

            return new Sprite
            {
                Key = key,
                Body = body,
                Direction = direction,
                FileType = fileType,
                Png = png,
                Width = bitmap.Width,
                Height = bitmap.Height,
                Sha256 = Hash(png)
            };
        }

        /// <summary>
        ///     ARGB1555 to a PNG with a transparent background.
        ///
        ///     <c>Frame</c> writes 16-bit ARGB1555: a pixel the sprite does not cover is left as
        ///     zero and a pixel it does cover carries the top bit set. Saving that format straight
        ///     to PNG asks GDI+ to make the conversion, and what it does with a one-bit alpha
        ///     channel varies by platform — on Mono it is a different implementation entirely. A
        ///     sprite that came back with a black rectangle behind it would look fine in a test
        ///     that only checked the bytes decoded, and wrong on every page that showed it.
        ///
        ///     So the expansion is done here, explicitly: alpha bit clear becomes fully
        ///     transparent, and each 5-bit channel is widened to 8 bits by repeating its high bits
        ///     (<c>(c &lt;&lt; 3) | (c &gt;&gt; 2)</c>) rather than by shifting alone, which would
        ///     cap white at 248 and tint the whole catalogue.
        /// </summary>
        private static byte[] ToPng(Bitmap source)
        {
            var rect = new Rectangle(0, 0, source.Width, source.Height);

            if (source.PixelFormat != PixelFormat.Format16bppArgb1555)
            {
                // Not what this library has ever produced. Save it rather than reinterpret it:
                // guessing at an unknown layout is how a catalogue fills with confident nonsense.
                using (var ms = new MemoryStream())
                {
                    source.Save(ms, ImageFormat.Png);
                    return ms.ToArray();
                }
            }

            using (var target = new Bitmap(source.Width, source.Height, PixelFormat.Format32bppArgb))
            {
                BitmapData src = source.LockBits(rect, ImageLockMode.ReadOnly, PixelFormat.Format16bppArgb1555);
                BitmapData dst = null;

                try
                {
                    dst = target.LockBits(rect, ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb);

                    var line = new short[source.Width];
                    var outLine = new int[source.Width];

                    for (int y = 0; y < source.Height; y++)
                    {
                        Marshal.Copy(new IntPtr(src.Scan0.ToInt64() + ((long)y * src.Stride)),
                            line, 0, source.Width);

                        for (int x = 0; x < source.Width; x++)
                        {
                            int p = line[x] & 0xFFFF;

                            if ((p & 0x8000) == 0)
                            {
                                outLine[x] = 0;
                                continue;
                            }

                            int r = (p >> 10) & 0x1F;
                            int g = (p >> 5) & 0x1F;
                            int b = p & 0x1F;

                            outLine[x] = unchecked((int)0xFF000000)
                                         | (((r << 3) | (r >> 2)) << 16)
                                         | (((g << 3) | (g >> 2)) << 8)
                                         | ((b << 3) | (b >> 2));
                        }

                        Marshal.Copy(outLine, 0, new IntPtr(dst.Scan0.ToInt64() + ((long)y * dst.Stride)),
                            source.Width);
                    }
                }
                finally
                {
                    if (dst != null)
                        target.UnlockBits(dst);

                    source.UnlockBits(src);
                }

                using (var ms = new MemoryStream())
                {
                    target.Save(ms, ImageFormat.Png);
                    return ms.ToArray();
                }
            }
        }

        private static string Hash(byte[] bytes)
        {
            using (var sha = SHA256.Create())
            {
                byte[] digest = sha.ComputeHash(bytes);
                var sb = new StringBuilder(digest.Length * 2);

                foreach (byte b in digest)
                    sb.Append(b.ToString("x2", CultureInfo.InvariantCulture));

                return sb.ToString();
            }
        }

        // ── player bodies (§5.2) ─────────────────────────────────────────────────────────────

        /// <summary>
        ///     Asked of the shard, never hardcoded.
        ///
        ///     Every registered race carries four body ids, and a shard that calls `RegisterRace`
        ///     adds ids no table of ours could contain. Even on stock ServUO a hardcoded list
        ///     would already be wrong in a way that is easy to miss: `RaceDefinitions.cs` passes
        ///     the gargoyle's ghost bodies in the OPPOSITE order to the other two races.
        ///
        ///     This set is the whole of what §5.1 gives direction 0 — head-on, facing the viewer,
        ///     because a character is a portrait and should look at you. Everything else takes
        ///     direction 1, the front three-quarter, because head-on is the least legible view of
        ///     a four-legged creature: a wolf seen from the front is a dark blob.
        /// </summary>
        /// <summary>
        ///     Cached for the life of the process: `RegisterRace` runs at Configure time, before
        ///     anything on this plane can be asked a question, so the set cannot change under a
        ///     running shard. Rebuilding it per body would enumerate every race 2,047 times per
        ///     scan to answer a question whose answer is twelve integers.
        /// </summary>
        private static HashSet<int> _playerBodies;

        private static HashSet<int> PlayerBodies()
        {
            var cached = _playerBodies;

            if (cached != null)
                return cached;

            var set = new HashSet<int>();

            try
            {
                foreach (var race in Race.AllRaces)
                {
                    if (race == null)
                        continue;

                    set.Add(race.MaleBody);
                    set.Add(race.FemaleBody);
                    set.Add(race.MaleGhostBody);
                    set.Add(race.FemaleGhostBody);
                }
            }
            catch (Exception e)
            {
                Console.WriteLine("[Bridge] catalogue: cannot enumerate races: {0}", e.Message);
            }

            set.Remove(0);

            _playerBodies = set;

            return set;
        }

        private static bool IsPlayerBody(int body)
        {
            return PlayerBodies().Contains(body);
        }

        private static void WritePlayerBodies(StringBuilder sb)
        {
            var bodies = new List<int>(PlayerBodies());
            bodies.Sort();

            sb.Append(",\"playerBodies\":[");

            for (int i = 0; i < bodies.Count; i++)
            {
                if (i > 0)
                    sb.Append(',');

                sb.Append(bodies[i].ToString(CultureInfo.InvariantCulture));
            }

            sb.Append(']');
        }

        // ── keys, cursors and the source id ──────────────────────────────────────────────────

        private static string Key(int body)
        {
            return "body/" + body.ToString(CultureInfo.InvariantCulture)
                   + "/a" + CatalogAction.ToString(CultureInfo.InvariantCulture);
        }

        /// <summary>
        ///     `body/&lt;id&gt;/a0`, and nothing else in this phase. A deeper key
        ///     (`body/400/a2/f3`) is well-formed under §5 and simply not served yet, so it comes
        ///     back `unsupported` rather than being silently read as its own first frame.
        /// </summary>
        private static bool TryParseKey(string key, out int body)
        {
            body = 0;

            if (key == null)
                return false;

            string[] parts = key.Split('/');

            if (parts.Length != 3 || parts[0] != "body")
                return false;

            if (!Int32.TryParse(parts[1], NumberStyles.None, CultureInfo.InvariantCulture, out body))
                return false;

            if (body < 1 || body > MaxBody)
                return false;

            return parts[2] == "a" + CatalogAction.ToString(CultureInfo.InvariantCulture);
        }

        private static int ParseBodyCursor(string cursor)
        {
            if (cursor == null)
                return 1;

            int value;

            if (cursor.StartsWith("b:", StringComparison.Ordinal)
                && Int32.TryParse(cursor.Substring(2), NumberStyles.None, CultureInfo.InvariantCulture, out value))
                return Math.Max(1, value + 1);

            return 1;
        }

        private static int ParseKeyCursor(string cursor)
        {
            if (cursor == null)
                return 0;

            int value;

            if (cursor.StartsWith("k:", StringComparison.Ordinal)
                && Int32.TryParse(cursor.Substring(2), NumberStyles.None, CultureInfo.InvariantCulture, out value))
                return Math.Max(0, value);

            return 0;
        }

        /// <summary>
        ///     Everything that decides the bytes, hashed into one short id.
        ///
        ///     Deliberately (size, mtime) rather than content: §6 makes exactly the same choice
        ///     for the source gate, and for the same reason — the anim files are 195 MB and
        ///     hashing them on every page of a walk would turn a manifest into a minute.
        ///     `assets.sources` is where an operator gets content hashes, computed off the request
        ///     path; this is a "did it move while I was reading" check, which (size, mtime)
        ///     answers.
        /// </summary>
        private static string SourceId()
        {
            var sb = new StringBuilder(256);

            sb.Append(BridgeAssets.EXTRACTOR_VERSION)
              .Append(':').Append(BridgeConfig.AssetPlayerDirection)
              .Append(':').Append(BridgeConfig.AssetCreatureDirection);

            for (int fileType = 1; fileType <= 5; fileType++)
            {
                sb.Append('|');

                string path = BridgeAssetValidator.AnimDataPath(fileType);

                if (path == null)
                    continue;

                try
                {
                    var info = new FileInfo(path);

                    if (!info.Exists)
                        continue;

                    sb.Append(info.Length).Append(',').Append(info.LastWriteTimeUtc.Ticks);
                }
                catch
                {
                    // An unreadable file is itself a state, and one that must not change from page
                    // to page without being noticed. Leaving the slot empty does that.
                }
            }

            return Hash(Encoding.UTF8.GetBytes(sb.ToString())).Substring(0, 16);
        }

        // ── shared plumbing ──────────────────────────────────────────────────────────────────

        /// <summary>
        ///     The two gates every request on this plane passes: a correlation id, and the
        ///     operator's consent. Both refuse rather than answer.
        /// </summary>
        private static bool Admit(Dictionary<string, object> o, string kind, out string reqId)
        {
            reqId = BridgeJson.GetString(o, "reqId");

            if (reqId == null)
            {
                BridgeAssets.Fail(null, "BAD_REQUEST", kind + " requires a reqId");
                return false;
            }

            if (!BridgeConfig.AssetsEnabled)
            {
                BridgeAssets.Fail(reqId, "DISABLED", "asset extraction is disabled on this shard");
                return false;
            }

            return true;
        }

        /// <summary>
        ///     The five anim files' index and record readers, opened for one reply and closed with
        ///     it. Holding them across replies would keep handles on the operator's client files
        ///     for as long as the cache lives, for no gain: opening five is microseconds and a
        ///     page decodes hundreds of sprites through them.
        /// </summary>
        private sealed class Readers : IDisposable
        {
            private readonly FileIndex[] _index = new FileIndex[6];
            private readonly BridgeAssetValidator.RecordReader[] _reader =
                new BridgeAssetValidator.RecordReader[6];
            private readonly long[] _length = new long[6];
            private readonly bool[] _open = new bool[6];

            public readonly long VerdataLength;

            public Readers()
            {
                VerdataLength = BridgeAssetValidator.MulLength(VerdataPath());
            }

            private static string VerdataPath()
            {
                try
                {
                    return Files.GetFilePath("verdata.mul");
                }
                catch
                {
                    return null;
                }
            }

            private void Ensure(int fileType)
            {
                if (fileType < 1 || fileType > 5 || _open[fileType])
                    return;

                _open[fileType] = true;

                string path = BridgeAssetValidator.AnimDataPath(fileType);

                if (path == null)
                    return;

                try
                {
                    _index[fileType] = BridgeAssetValidator.OpenAnimIndex(fileType);
                    _length[fileType] = BridgeAssetValidator.MulLength(path);
                    _reader[fileType] = new BridgeAssetValidator.RecordReader(path, VerdataPath());
                }
                catch (Exception e)
                {
                    Console.WriteLine("[Bridge] catalogue: anim file type {0}: {1}",
                        fileType, e.Message);
                }
            }

            public FileIndex Index(int fileType)
            {
                Ensure(fileType);
                return fileType >= 1 && fileType <= 5 ? _index[fileType] : null;
            }

            public long MulLength(int fileType)
            {
                Ensure(fileType);
                return fileType >= 1 && fileType <= 5 ? _length[fileType] : 0;
            }

            public BridgeAssetValidator.RecordReader Reader(int fileType)
            {
                Ensure(fileType);
                return fileType >= 1 && fileType <= 5 ? _reader[fileType] : null;
            }

            public void Dispose()
            {
                for (int i = 1; i <= 5; i++)
                {
                    if (_reader[i] == null)
                        continue;

                    try
                    {
                        _reader[i].Dispose();
                    }
                    catch
                    {
                        // Closing a read-only handle. Nothing useful is left to do.
                    }
                }
            }
        }

        /// <summary>
        ///     Lets a megabyte of the operator's client art go once nothing has asked for it in
        ///     five minutes. A rebuild costs one scan and, because <see cref="SourceId"/> is
        ///     derived from the files rather than minted per build, it produces the same catalogue
        ///     id — so an import that spans the drop does not have to start over.
        /// </summary>
        private static void Sweep()
        {
            lock (_sync)
            {
                if (_catalog == null)
                    return;

                if (DateTime.UtcNow - _catalog.LastUsed > IdleFor)
                    _catalog = null;
            }
        }

        public static string Status()
        {
            lock (_sync)
            {
                if (_catalog == null)
                    return "catalog(empty)";

                return String.Format("catalog(id={0} held={1} next={2} complete={3})",
                    _catalog.Id, _catalog.Order.Count, _catalog.Next, _catalog.Complete);
            }
        }
    }
}