Back UnitCastingInfo / UnitChannelInfo for remote units

Vanilla stores no cast record for other units, so UnitCastingInfo(other)
returned nil and remote channels had no timing. Co-hook the SMSG_SPELL_START
handler (0x006E7640, gated on opcode 0x131) -- the only packet carrying
another unit's cast with a server-authoritative cast time -- and cache it
per caster GUID (cast vs. channel split by AttributesEx channel bits).

- UnitCastingInfo(unit): full timing for any unit within its cast window;
  enemy/target/focus/nameplate cast bars now work, not just the player.
- UnitChannelInfo(unit): remote channels get real start/end times when the
  channel's begin was observed, validated against the live +0x228 field so a
  stale entry never sticks; falls back to the old no-times path otherwise.
- Player path unchanged (self-tracked); the cache skips caster==PlayerGuid.

Factor the CDataStore packet reader out of Aura::Source into a shared
Net::PacketReader header (vtable-correct) used by both packet hooks.

Best-effort: only casts begun while the unit was in range are seen, and an
interrupted remote cast lingers until its computed end (1.12 sends no
per-unit interrupt packet to hook).

docs/API.md

@@ -7951,15 +7951,19 @@ C_Spell.UnitCastingInfo("player")
 `startTimeMs`/`endTimeMs` share `GetTime()`'s epoch (`GetTime()*1000`),
 so progress is `(GetTime()*1000 - startTimeMs) / (endTimeMs - startTimeMs)`.
 
-> **Local player only.** Vanilla 1.12 stores no cast times anywhere
-> readable (the cast bar is Lua-driven off `SPELLCAST_START`), and it
-> tracks regular casts for nobody but you — other units' casts exist
-> only as transient animation visuals. So `C_Spell.UnitCastingInfo(otherUnit)`
-> returns `nil`. We self-track the player's cast on a per-frame tick
-> (`VAR_CURRENT_CAST_SPELL`), stamping `startTimeMs = now` and
-> `endTimeMs = now + base cast time`. Instant casts return `nil` (no
-> cast bar). See [`C_Spell.UnitChannelInfo`](#c_spellunitchannelinfounit--c_spellchannelinfo)
-> for channels, which *are* visible on other units.
+> **Works for any unit.** Vanilla 1.12 stores no cast times on the
+> CGUnit (the cast bar is Lua-driven off `SPELLCAST_START`), so we
+> source them two ways: the **local player** is self-tracked on a
+> per-frame tick (`VAR_CURRENT_CAST_SPELL`, stamping `startTimeMs = now`,
+> `endTimeMs = now + effective cast time`); **other units** come from a
+> co-hook on `SMSG_SPELL_START` (`Spell::Cast`), the one packet carrying
+> another unit's cast with a server-authoritative cast time, cached per
+> caster GUID. So `C_Spell.UnitCastingInfo("target")` now backs
+> enemy/target/focus/nameplate cast bars. Instant casts return `nil` (no
+> cast bar). Best-effort for remote units: only casts that began while the
+> unit was in range are seen, and — since 1.12 has no per-unit interrupt
+> packet to hook — an *interrupted* remote cast lingers until its computed
+> `endTimeMs` rather than clearing instantly.
 >
 > `isTradeskill` is real — the spell's `SPELL_ATTR_TRADESPELL` flag, so
 > profession recipe casts (smelting, cooking/enchanting recipes, …)
@@ -7991,13 +7995,15 @@ C_Spell.ChannelInfo()
 --  907894314, 907902314, false, false, 10187
 ```
 
-Unlike casts, the channeled spellID **is** broadcast for every unit
-(`UNIT_FIELD_CHANNEL_SPELL`), so `C_Spell.UnitChannelInfo("target")`
-works on other units — returning `name`/`displayName`/`textureID`/`spellID`
-with **`nil` start/end times** (we only track the local player's channel
-start). For the player, the full timing is present
-(`endTimeMs - startTimeMs` = channel duration). Same placeholder fields
-as `C_Spell.UnitCastingInfo`.
+The channeled spellID **is** broadcast for every unit
+(`UNIT_FIELD_CHANNEL_SPELL`), so `C_Spell.UnitChannelInfo("target")` works
+on other units. Timing now comes from the same `SMSG_SPELL_START` co-hook
+as casts: when we observed the channel begin, the remote unit returns full
+**start/end times** (validated against the live `UNIT_FIELD_CHANNEL_SPELL`
+so a stale cache entry never applies to a different/ended channel);
+otherwise it falls back to `name`/`displayName`/`textureID`/`spellID` with
+**`nil` times**. The player path is unchanged (full timing). Same
+placeholder fields as `C_Spell.UnitCastingInfo`.
 
 ## SpellBook
 

src/Offsets.h

@@ -695,6 +695,17 @@ enum Offsets {
     // same Octo/Turtle 1.12.1 client). Per-cast frequency, not per-frame.
     FUN_SPELL_GO = 0x006E7A70,
 
+    // SMSG packet handler that processes `SMSG_SPELL_START` (opcode
+    // `0x131`) among others — `int __fastcall(uint32_t unk, uint32_t
+    // opCode, uint32_t unk2, CDataStore *packet)`. For `0x131` the body is
+    // `itemGuid(packed), casterGuid(packed), spellId(u32), castFlags(u16),
+    // castTime(u32), targetMask, …`. This is the only place the client sees
+    // *another* unit's cast with a server-authoritative cast time, so
+    // `Spell::Cast` co-hooks it (gated on `opCode == 0x131`) to back
+    // remote-unit `UnitCastingInfo` / `UnitChannelInfo`. Mirrored from
+    // nampower's SpellStartHandlerHook.
+    FUN_SPELL_START_HANDLER = 0x006E7640,
+
     // `CGUnit_C::OnAuraAdded` — `__thiscall void(CGUnit *unit, uint32_t
     // slot, uint32_t spellId)` (i.e. `__fastcall(unit /*ecx*/, edx_unused,
     // slot, spellId)`). Fires for EVERY aura that lands on a tracked unit,

src/aura/Source.cpp

@@ -18,6 +18,7 @@
 #include "Data.h"
 #include "Game.h"
 #include "Offsets.h"
+#include "net/PacketReader.h"
 #include "spell/Lookup.h"
 #include "tick/WorldTick.h"
 #include "unit/Identity.h"
@@ -28,39 +29,7 @@ namespace Aura::Source {
 
 namespace {
 
-// Minimal view of the engine's CDataStore packet reader. Only the fields
-// the read path touches; layout matches nampower's `CDataStore`. Incoming
-// SMSG packets are a single contiguous buffer (`m_base == 0`,
-// `m_alloc == size`), so the simple `m_buffer - m_base + m_read` read with
-// an `m_size` bound is sufficient — same critical guard as the engine's
-// `CDataStore::Get`.
-//
-// CDataStore is POLYMORPHIC in the engine (virtual Internal*/Reset/etc.),
-// so a vtable pointer occupies offset 0 and the data members start at
-// +0x04. Omitting `vtable` here would shift every field by 4 and make
-// `m_buffer` read the vptr — yielding a garbage read address (the cause of
-// an early ACCESS_VIOLATION in the SpellGo hook).
-struct CDataStore {
-    void *vtable;       // +0x00
-    uint8_t *m_buffer;  // +0x04
-    uint32_t m_base;    // +0x08
-    uint32_t m_alloc;   // +0x0C
-    uint32_t m_size;    // +0x10
-    uint32_t m_read;    // +0x14
-};
-
-template <typename T>
-T ReadField(CDataStore *p) {
-    T val{};
-    const uint32_t end = p->m_read + sizeof(T);
-    if (end > p->m_size) {
-        p->m_read = p->m_size + 1; // poison: further reads short-circuit
-        return val;
-    }
-    val = *reinterpret_cast<const T *>(p->m_buffer - p->m_base + p->m_read);
-    p->m_read += sizeof(T);
-    return val;
-}
+using Net::CDataStore;
 
 // A cast applies an aura iff any of its three effects names an aura. The
 // EffectApplyAuraName[3] int32 array sits at `+0x16C` (already used by the
@@ -181,10 +150,10 @@ void __fastcall SpellGo_h(uint64_t *itemGUID, uint64_t *casterGUID,
 
     if (packet != nullptr) {
         const uint32_t savedRead = packet->m_read;
-        ReadField<int16_t>(packet); // castFlags (unused)
-        const uint8_t numHit = ReadField<uint8_t>(packet);
+        Net::Read<int16_t>(packet); // castFlags (unused)
+        const uint8_t numHit = Net::Read<uint8_t>(packet);
         for (int i = 0; i < numHit; ++i) {
-            const uint64_t guid = ReadField<uint64_t>(packet);
+            const uint64_t guid = Net::Read<uint64_t>(packet);
             if (numTargets < kMaxTargets)
                 targets[numTargets++] = guid;
         }

src/net/PacketReader.h

@@ -0,0 +1,68 @@
+// This file is part of ClassicAPI.
+//
+// ClassicAPI is free software: you can redistribute it and/or modify it under the terms
+// of the GNU Lesser General Public License as published by the Free Software Foundation, either
+// version 3 of the License, or (at your option) any later version.
+//
+// ClassicAPI is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+// PURPOSE. See the GNU Lesser General Public License for more details.
+
+#pragma once
+
+#include <cstdint>
+
+// Minimal reader for the engine's `CDataStore` packet buffer, shared by the
+// packet co-hooks (`Aura::Source`'s SpellGo, `Spell::Cast`'s SpellStart).
+//
+// `CDataStore` is POLYMORPHIC in the engine (virtual Internal*/Reset/etc.),
+// so a vtable pointer sits at offset 0 and the data members start at +0x04.
+// Omitting `vtable` shifts every field by 4 and makes `m_buffer` read the
+// vptr — a garbage read address (this caused an early ACCESS_VIOLATION in
+// the SpellGo hook before the slot was added).
+//
+// Incoming SMSG packets are a single contiguous buffer (`m_base == 0`,
+// `m_alloc == size`), so the simple `m_buffer - m_base + m_read` read with
+// an `m_size` bound is sufficient — the same critical guard as the engine's
+// own `CDataStore::Get`. Callers should save `m_read` before reading and
+// restore it before handing the packet to the original handler.
+
+namespace Net {
+
+struct CDataStore {
+    void *vtable;       // +0x00
+    uint8_t *m_buffer;  // +0x04
+    uint32_t m_base;    // +0x08
+    uint32_t m_alloc;   // +0x0C
+    uint32_t m_size;    // +0x10
+    uint32_t m_read;    // +0x14
+};
+
+// Reads a fixed-width field and advances the cursor. On overrun it poisons
+// `m_read` (so subsequent reads short-circuit) and returns a zero value.
+template <typename T>
+T Read(CDataStore *p) {
+    T val{};
+    const uint32_t end = p->m_read + sizeof(T);
+    if (end > p->m_size) {
+        p->m_read = p->m_size + 1;
+        return val;
+    }
+    val = *reinterpret_cast<const T *>(p->m_buffer - p->m_base + p->m_read);
+    p->m_read += sizeof(T);
+    return val;
+}
+
+// Reads a WoW packed GUID: a 1-byte mask followed by one byte per set bit,
+// each contributing the corresponding byte of the 64-bit value.
+inline uint64_t ReadPackedGuid(CDataStore *p) {
+    uint64_t guid = 0;
+    const uint8_t mask = Read<uint8_t>(p);
+    for (int i = 0; i < 8; ++i) {
+        if (mask & (1u << i))
+            guid |= static_cast<uint64_t>(Read<uint8_t>(p)) << (i * 8);
+    }
+    return guid;
+}
+
+} // namespace Net

src/spell/Cast.cpp

@@ -30,13 +30,17 @@
 // Lua's `GetTime()` scales by 0.001 — so startMs/endMs are directly
 // comparable to `GetTime()*1000` in addon progress math.
 //
-// Scope (vanilla ceiling, confirmed against the binary — see TODO #68):
-//   - Player casts/channels: full timing.
-//   - Other units' CHANNELS: spellID/name/texture from the broadcast
-//     +0x228 field, but no times (we don't track per-remote-unit start).
-//   - Other units' regular CASTS: nil. Vanilla doesn't store them — the
-//     casting animation is driven by transient spell-visual objects, not
-//     a queryable cast record.
+// Other units have no engine-stored cast record, so we capture their casts
+// from the `SMSG_SPELL_START` packet (the only place the client sees
+// another unit's cast + a server-authoritative cast time) into a per-caster
+// cache — see the SpellStart co-hook below. Scope:
+//   - Player casts/channels: full timing (self-tracked, exact engine math).
+//   - Other units' regular CASTS: full timing while within the cast window,
+//     from the SpellStart cache. No clean per-unit interrupt signal exists
+//     in 1.12, so an interrupted remote cast lingers until its computed end.
+//   - Other units' CHANNELS: the broadcast +0x228 field gates "channeling
+//     now"; the SpellStart cache adds real start/end times when we observed
+//     the channel begin (else spellID/name/texture with nil times).
 //
 // Modern-signature fields vanilla can't fill are structurally-correct
 // placeholders: castID / castBarID = nil, notInterruptible = false,
@@ -45,8 +49,10 @@
 #include "Game.h"
 #include "Offsets.h"
 #include "dbc/Lookup.h"
+#include "net/PacketReader.h"
 #include "spell/Lookup.h"
 #include "tick/WorldTick.h"
+#include "unit/Identity.h"
 
 #include <cstdint>
 
@@ -204,6 +210,118 @@ bool ArgIsPlayer(void *L) {
     return u != nullptr && u == Resolve("player");
 }
 
+// ---- Remote-unit cast tracking (SMSG_SPELL_START) ------------------------
+
+// Vanilla stores no cast record for other units, so we capture their casts
+// from SMSG_SPELL_START — the only packet carrying another unit's cast with
+// a server-authoritative cast time. Keyed by caster GUID (a unit casts one
+// thing at a time). Regular casts back `UnitCastingInfo`; channels add real
+// times to `UnitChannelInfo` (validated against the live +0x228 field).
+
+constexpr int OFF_ATTRIBUTES_EX = 0x1C; // Spell.dbc AttributesEx
+constexpr uint32_t SPELL_ATTR_EX_CHANNELED = 0x4 | 0x40; // IS_CHANNELED | SELF
+
+bool IsChannelSpell(const uint8_t *rec) {
+    return (*reinterpret_cast<const uint32_t *>(rec + OFF_ATTRIBUTES_EX) &
+            SPELL_ATTR_EX_CHANNELED) != 0;
+}
+
+// Channel duration for a non-player caster — base (skipMod=1), since we
+// only have the local player's spell-mod tables.
+int RemoteChannelDurationMs(const uint8_t *rec) {
+    const int ms = reinterpret_cast<GetDuration_t>(
+        Offsets::FUN_GET_SPELL_DURATION)(rec, 0, /*skipMod*/ 1);
+    return ms > 0 ? ms : 0;
+}
+
+struct RemoteCast {
+    uint64_t casterGuid;
+    int spellID;
+    int startMs;
+    int endMs;
+    bool isChannel;
+    bool used;
+};
+
+constexpr int kRemoteCastSlots = 64;
+RemoteCast g_remoteCasts[kRemoteCastSlots];
+int g_remoteCursor = 0;
+
+void StoreRemoteCast(uint64_t caster, int spellID, int startMs, int endMs,
+                     bool isChannel) {
+    const RemoteCast entry{caster, spellID, startMs, endMs, isChannel, true};
+    // One active cast per unit — replace any existing entry for this caster.
+    for (auto &e : g_remoteCasts) {
+        if (e.used && e.casterGuid == caster) {
+            e = entry;
+            return;
+        }
+    }
+    const int now = NowMs();
+    for (auto &e : g_remoteCasts) {
+        if (!e.used || now >= e.endMs) {
+            e = entry;
+            return;
+        }
+    }
+    g_remoteCasts[g_remoteCursor] = entry;
+    g_remoteCursor = (g_remoteCursor + 1) % kRemoteCastSlots;
+}
+
+const RemoteCast *FindRemoteCast(uint64_t caster) {
+    if (caster == 0)
+        return nullptr;
+    for (const auto &e : g_remoteCasts) {
+        if (e.used && e.casterGuid == caster)
+            return &e;
+    }
+    return nullptr;
+}
+
+void RecordRemoteStart(uint64_t caster, int spellID, uint32_t castTime) {
+    if (caster == 0 || spellID == 0)
+        return;
+    // The local player uses the self-tracked g_cast / g_channel path.
+    if (caster == Unit::Identity::PlayerGuid())
+        return;
+    const uint8_t *rec = Spell::Lookup::RecordForID(spellID);
+    if (rec == nullptr)
+        return;
+    const int now = NowMs();
+    const bool channel = IsChannelSpell(rec);
+    const int endMs = channel ? now + RemoteChannelDurationMs(rec)
+                              : now + static_cast<int>(castTime);
+    StoreRemoteCast(caster, spellID, now, endMs, channel);
+}
+
+// Co-hook on the SMSG_SPELL_START handler. Gated on opCode 0x131 so every
+// other packet this handler processes is a cheap early-out. Packet layout
+// mirrors nampower's SpellStartHandlerHook.
+using SpellStartHandler_t = int(__fastcall *)(uint32_t unk, uint32_t opCode,
+                                              uint32_t unk2,
+                                              Net::CDataStore *packet);
+SpellStartHandler_t g_origSpellStart = nullptr;
+
+int __fastcall SpellStartHandler_h(uint32_t unk, uint32_t opCode,
+                                   uint32_t unk2, Net::CDataStore *packet) {
+    if (opCode == 0x131 && packet != nullptr) {
+        const uint32_t saved = packet->m_read;
+        Net::ReadPackedGuid(packet); // itemGuid (unused)
+        const uint64_t caster = Net::ReadPackedGuid(packet);
+        const int spellID = static_cast<int>(Net::Read<uint32_t>(packet));
+        Net::Read<uint16_t>(packet); // castFlags (unused)
+        const uint32_t castTime = Net::Read<uint32_t>(packet);
+        packet->m_read = saved; // restore before the engine re-parses
+        RecordRemoteStart(caster, spellID, castTime);
+    }
+    return g_origSpellStart(unk, opCode, unk2, packet);
+}
+
+const Game::HookAutoRegister _spellStartHook{
+    Offsets::FUN_SPELL_START_HANDLER,
+    reinterpret_cast<void *>(&SpellStartHandler_h),
+    reinterpret_cast<void **>(&g_origSpellStart)};
+
 } // namespace
 
 // Per-frame: detect the player's cast/channel start, stamp times.
@@ -237,15 +355,25 @@ static const Tick::WorldTick::AutoSubscribe _tickSub{&OnWorldTick};
 // `CastingInfo()` — local player's cast, no token lookup.
 static int __fastcall Script_CastingInfo(void *L) { return PushCastInfo(L, g_cast); }
 
-// `UnitCastingInfo(unit)` — only the local player has regular-cast data
-// in vanilla; any other unit returns nil.
+// `UnitCastingInfo(unit)` — local player from self-tracking; other units
+// from the SMSG_SPELL_START cache (regular casts still within their cast
+// window). An interrupted remote cast lingers until its computed end time
+// (no clean per-unit interrupt signal in 1.12) — acceptable best-effort.
 static int __fastcall Script_UnitCastingInfo(void *L) {
     if (!Game::Lua::IsString(L, 1)) {
         Game::Lua::Error(L, "Usage: C_Spell.UnitCastingInfo(\"unit\")");
         return 0;
     }
-    if (ArgIsPlayer(L))
+    const char *token = Game::Lua::ToString(L, 1);
+    void *u = (token != nullptr) ? Resolve(token) : nullptr;
+    if (u == nullptr)
+        return 0;
+    if (u == Resolve("player"))
         return PushCastInfo(L, g_cast);
+
+    const RemoteCast *rc = FindRemoteCast(Unit::Identity::GuidForObject(u));
+    if (rc != nullptr && !rc->isChannel && NowMs() < rc->endMs)
+        return PushCastInfo(L, TrackedSpell{rc->spellID, rc->startMs, rc->endMs});
     return 0;
 }
 
@@ -272,7 +400,15 @@ static int __fastcall Script_UnitChannelInfo(void *L) {
         static_cast<const uint8_t *>(u) + Offsets::OFF_UNIT_DESCRIPTOR);
     if (desc == nullptr)
         return 0;
+    // The live +0x228 field is authoritative for "is this unit channeling
+    // right now"; the SMSG_SPELL_START cache adds real start/end times when
+    // we observed the channel begin (and still matches the current spell).
     const int spellID = *reinterpret_cast<const int *>(desc + Offsets::OFF_UNIT_FIELD_CHANNEL_SPELL);
+    if (spellID == 0)
+        return 0;
+    const RemoteCast *rc = FindRemoteCast(Unit::Identity::GuidForObject(u));
+    if (rc != nullptr && rc->isChannel && rc->spellID == spellID && NowMs() < rc->endMs)
+        return PushChannelInfo(L, spellID, rc->startMs, rc->endMs, /*haveTimes*/ true);
     return PushChannelInfo(L, spellID, 0, 0, /*haveTimes*/ false);
 }