pcm/PcmPack: simplify unpack_sample()

src/pcm/PcmPack.cxx

@@ -45,22 +45,45 @@ pcm_pack_24(uint8_t *dest, const int32_t *src, const int32_t *src_end)
 	}
 }
 
-static void
-unpack_sample(int32_t *dest0, const uint8_t *src)
+/**
+ * Construct a signed 24 bit integer from three bytes into a int32_t.
+ */
+static constexpr int32_t
+ConstructS24(uint8_t low, uint8_t mid, uint8_t high)
 {
-	uint8_t *dest = (uint8_t *)dest0;
+	return int32_t(low) | (int32_t(mid) << 8) | (int32_t(high) << 16) |
+		/* extend the sign bit */
+		(high & 0x80 ? ~int32_t(0xffffff) : 0);
+}
 
-	if (IsBigEndian())
-		/* extend the sign bit to the most fourth byte */
-		*dest++ = *src & 0x80 ? 0xff : 0x00;
+/**
+ * Read a packed signed little-endian 24 bit integer.
+ */
+gcc_pure
+static int32_t
+ReadS24LE(const uint8_t *src)
+{
+	return ConstructS24(src[0], src[1], src[2]);
+}
 
-	*dest++ = *src++;
-	*dest++ = *src++;
-	*dest++ = *src;
+/**
+ * Read a packed signed big-endian 24 bit integer.
+ */
+gcc_pure
+static int32_t
+ReadS24BE(const uint8_t *src)
+{
+	return ConstructS24(src[2], src[1], src[0]);
+}
 
-	if (IsLittleEndian())
-		/* extend the sign bit to the most fourth byte */
-		*dest++ = *src & 0x80 ? 0xff : 0x00;
+/**
+ * Read a packed signed native-endian 24 bit integer.
+ */
+gcc_pure
+static int32_t
+ReadS24(const uint8_t *src)
+{
+	return IsBigEndian() ? ReadS24BE(src) : ReadS24LE(src);
 }
 
 void
@@ -70,7 +93,7 @@ pcm_unpack_24(int32_t *dest, const uint8_t *src, const uint8_t *src_end)
 	   parameter to the unpack_sample() inline function) */
 
 	while (src < src_end) {
-		unpack_sample(dest++, src);
+		*dest++ = ReadS24(src);
 		src += 3;
 	}
 }