♻️ Clean up `cel_header.hpp`

1. Rename `CelGetFrameStart` to `CelGetFrame`, in line with the other 2
   functions with the same name and load the `uint32_t` safely.
2. Remove redundant `FrameHeader`, simply use `LoadLE16`.
3. Document all the functions.

Source/engine/cel_header.hpp

@@ -10,46 +10,48 @@
 
 namespace devilution {
 
-inline byte *CelGetFrameStart(byte *pCelBuff, int nCel)
+/**
+ * Returns the pointer to the start of the frame data (often a header).
+ */
+inline byte *CelGetFrame(byte *data, int frame)
 {
-	const auto *pFrameTable = reinterpret_cast<const std::uint32_t *>(pCelBuff);
-
-	return &pCelBuff[SDL_SwapLE32(pFrameTable[nCel])];
+	const std::uint32_t begin = LoadLE32(&data[frame * sizeof(std::uint32_t)]);
+	return &data[begin];
 }
 
-inline byte *CelGetFrame(byte *pCelBuff, int nCel, int *nDataSize)
+/**
+ * Returns the pointer to the start of the frame data (often a header) and sets `frameSize` to the size of the data in bytes.
+ */
+inline byte *CelGetFrame(byte *data, int frame, int *frameSize)
 {
-	const std::uint32_t nCellStart = LoadLE32(&pCelBuff[nCel * sizeof(std::uint32_t)]);
-	*nDataSize = static_cast<int>(LoadLE32(&pCelBuff[(nCel + 1) * sizeof(std::uint32_t)]) - nCellStart);
-	return &pCelBuff[nCellStart];
+	const std::uint32_t begin = LoadLE32(&data[frame * sizeof(std::uint32_t)]);
+	*frameSize = static_cast<int>(LoadLE32(&data[(frame + 1) * sizeof(std::uint32_t)]) - begin);
+	return &data[begin];
 }
 
-inline const byte *CelGetFrame(const byte *pCelBuff, int nCel, int *nDataSize)
+/**
+ * Returns the pointer to the start of the frame data (often a header) and sets `frameSize` to the size of the data in bytes.
+ */
+inline const byte *CelGetFrame(const byte *data, int frame, int *frameSize)
 {
-	const std::uint32_t nCellStart = LoadLE32(&pCelBuff[nCel * sizeof(std::uint32_t)]);
-	*nDataSize = static_cast<int>(LoadLE32(&pCelBuff[(nCel + 1) * sizeof(std::uint32_t)]) - nCellStart);
-	return &pCelBuff[nCellStart];
+	const std::uint32_t begin = LoadLE32(&data[frame * sizeof(std::uint32_t)]);
+	*frameSize = static_cast<int>(LoadLE32(&data[(frame + 1) * sizeof(std::uint32_t)]) - begin);
+	return &data[begin];
 }
 
-struct FrameHeader {
-	uint16_t row0;
-	uint16_t row32;
-	uint16_t row64;
-	uint16_t row96;
-	uint16_t row128;
-};
-
-inline const byte *CelGetFrameClipped(const byte *pCelBuff, int nCel, int *nDataSize)
+/**
+ * Returns the pointer to the start of the frame's pixel data and sets `frameSize` to the size of the data in bytes.
+ */
+inline const byte *CelGetFrameClipped(const byte *data, int frame, int *frameSize)
 {
-	const byte *pRLEBytes = CelGetFrame(pCelBuff, nCel, nDataSize);
-
-	FrameHeader frameHeader;
-	std::memcpy(&frameHeader, pRLEBytes, sizeof(FrameHeader));
+	const byte *frameData = CelGetFrame(data, frame, frameSize);
 
-	std::uint16_t nDataStart = SDL_SwapLE16(frameHeader.row0);
-	*nDataSize -= nDataStart;
+	// The frame begins with a header that consists of 5 little-endian 16-bit integers
+	// pointing to the start of the pixel data for rows 0, 32, 64, 96, and 128.
+	const std::uint16_t begin = LoadLE16(frameData);
 
-	return &pRLEBytes[nDataStart];
+	*frameSize -= begin;
+	return &frameData[begin];
 }
 
 } // namespace devilution

Source/missiles.cpp

@@ -1127,7 +1127,7 @@ void LoadMissileGFX(BYTE mi)
 		sprintf(pszName, "Missiles\\%s.CL2", mfd->mName);
 		byte *file = LoadFileInMem(pszName).release();
 		for (unsigned i = 0; i < mfd->mAnimFAmt; i++)
-			mfd->mAnimData[i] = CelGetFrameStart(file, i);
+			mfd->mAnimData[i] = CelGetFrame(file, i);
 	} else if (mfd->mAnimFAmt == 1) {
 		sprintf(pszName, "Missiles\\%s.CL2", mfd->mName);
 		mfd->mAnimData[0] = LoadFileInMem(pszName).release();

Source/monster.cpp

@@ -159,7 +159,7 @@ void InitMonsterTRN(CMonster &monst)
 
 		for (int j = 0; j < 8; j++) {
 			Cl2ApplyTrans(
-			    CelGetFrameStart(monst.Anims[i].CMem.get(), j),
+			    CelGetFrame(monst.Anims[i].CMem.get(), j),
 			    colorTranslations,
 			    monst.Anims[i].Frames);
 		}
@@ -359,7 +359,7 @@ void InitMonsterGFX(int monst)
 			if (Monsters[monst].mtype != MT_GOLEM || (animletter[anim] != 's' && animletter[anim] != 'd')) {
 
 				for (i = 0; i < 8; i++) {
-					byte *pCelStart = CelGetFrameStart(celBuf, i);
+					byte *pCelStart = CelGetFrame(celBuf, i);
 					Monsters[monst].Anims[anim].CelSpritesForDirections[i].emplace(pCelStart, width);
 				}
 			} else {

Source/player.cpp

@@ -581,7 +581,7 @@ void SetPlayerGPtrs(const char *path, std::unique_ptr<byte[]> &data, std::array<
 	data = LoadFileInMem(path);
 
 	for (int i = 0; i < 8; i++) {
-		byte *pCelStart = CelGetFrameStart(data.get(), i);
+		byte *pCelStart = CelGetFrame(data.get(), i);
 		anim[i].emplace(pCelStart, width);
 	}
 }

Source/towners.cpp

@@ -238,7 +238,7 @@ void InitCows(TownerStruct &towner, const TownerInit &initData)
 	towner.animOrder = nullptr;
 	towner.animOrderSize = 0;
 	for (int i = 0; i < 8; i++) {
-		towner._tNAnim[i] = CelGetFrameStart(CowCels.get(), i);
+		towner._tNAnim[i] = CelGetFrame(CowCels.get(), i);
 	}
 	NewTownerAnim(towner, towner._tNAnim[initData.dir], 12, 3);
 	towner._tAnimFrame = GenerateRnd(11) + 1;

Source/utils/endian.hpp

@@ -4,6 +4,14 @@
 
 namespace devilution {
 
+template <typename T>
+constexpr std::uint16_t LoadLE16(const T *b)
+{
+	static_assert(sizeof(T) == 1, "invalid argument");
+	// NOLINTNEXTLINE(readability-magic-numbers)
+	return (static_cast<std::uint16_t>(b[1]) << 8) | static_cast<std::uint16_t>(b[0]);
+}
+
 template <typename T>
 constexpr std::uint32_t LoadLE32(const T *b)
 {