LibVideo/VP9: Specify which spec section defines certain behaviors

Author: https://github.com/FalseHonesty
Commit: https://github.com/SerenityOS/serenity/commit/91572a49c4a
Pull-request: https://github.com/SerenityOS/serenity/pull/8349
Reviewed-by: https://github.com/Lubrsi

Userland/Libraries/LibVideo/VP9/BitStream.cpp

@@ -37,13 +37,6 @@ u8 BitStream::read_f(size_t n)
     return result;
 }
 
-i8 BitStream::read_s(size_t n)
-{
-    auto value = read_f(n);
-    auto sign = read_bit();
-    return sign ? -value : value;
-}
-
 u8 BitStream::read_f8()
 {
     if (!m_current_byte.has_value())
@@ -63,30 +56,6 @@ u16 BitStream::read_f16()
     return (read_f8() << 8u) | read_f8();
 }
 
-u8 BitStream::read_literal(size_t n)
-{
-    u8 return_value = 0;
-    for (size_t i = 0; i < n; i++) {
-        return_value = (2 * return_value) + read_bool(128);
-    }
-    return return_value;
-}
-
-u64 BitStream::get_position()
-{
-    return (m_bytes_read * 8) + (7 - m_current_bit_position);
-}
-
-size_t BitStream::bytes_remaining()
-{
-    return m_bytes_remaining;
-}
-
-size_t BitStream::bits_remaining()
-{
-    return (bytes_remaining() * 8) + m_current_bit_position + 1;
-}
-
 /* 9.2.1 */
 bool BitStream::init_bool(size_t bytes)
 {
@@ -139,4 +108,35 @@ bool BitStream::exit_bool()
     return padding_element == 0;
 }
 
+u8 BitStream::read_literal(size_t n)
+{
+    u8 return_value = 0;
+    for (size_t i = 0; i < n; i++) {
+        return_value = (2 * return_value) + read_bool(128);
+    }
+    return return_value;
+}
+
+i8 BitStream::read_s(size_t n)
+{
+    auto value = read_f(n);
+    auto sign = read_bit();
+    return sign ? -value : value;
+}
+
+u64 BitStream::get_position()
+{
+    return (m_bytes_read * 8) + (7 - m_current_bit_position);
+}
+
+size_t BitStream::bytes_remaining()
+{
+    return m_bytes_remaining;
+}
+
+size_t BitStream::bits_remaining()
+{
+    return (bytes_remaining() * 8) + m_current_bit_position + 1;
+}
+
 }

Userland/Libraries/LibVideo/VP9/BitStream.h

@@ -21,20 +21,25 @@ public:
 
     u8 read_byte();
     bool read_bit();
+
+    /* (9.1) */
     u8 read_f(size_t n);
-    i8 read_s(size_t n);
     u8 read_f8();
     u16 read_f16();
+
+    /* (9.2) */
+    bool init_bool(size_t bytes);
+    bool read_bool(u8 probability);
+    bool exit_bool();
     u8 read_literal(size_t n);
 
+    /* (4.9.2) */
+    i8 read_s(size_t n);
+
     u64 get_position();
     size_t bytes_remaining();
     size_t bits_remaining();
 
-    bool init_bool(size_t bytes);
-    bool read_bool(u8 probability);
-    bool exit_bool();
-
 private:
     u8 const* m_data_ptr { nullptr };
     size_t m_bytes_remaining { 0 };

Userland/Libraries/LibVideo/VP9/Decoder.cpp

@@ -22,6 +22,7 @@ Decoder::Decoder()
 {
 }
 
+/* (6.1) */
 bool Decoder::parse_frame(ByteBuffer const& frame_data)
 {
     m_bit_stream = make<BitStream>(frame_data.data(), frame_data.size());
@@ -49,6 +50,7 @@ bool Decoder::parse_frame(ByteBuffer const& frame_data)
     return true;
 }
 
+/* (6.2) */
 bool Decoder::uncompressed_header()
 {
     auto frame_marker = m_bit_stream->read_f(2);

Userland/Libraries/LibVideo/VP9/Decoder.h

@@ -40,6 +40,7 @@ private:
         return StudioSwing;
     }
 
+    /* (6.2) Uncompressed Header Syntax */
     bool uncompressed_header();
     bool frame_sync_code();
     bool color_config();
@@ -59,6 +60,7 @@ private:
     bool setup_past_independence();
     bool trailing_bits();
 
+    /* (6.3) Compressed Header Syntax */
     bool compressed_header();
     bool read_tx_mode();
     bool tx_mode_probs();
@@ -79,6 +81,7 @@ private:
     u8 update_mv_prob(u8 prob);
     bool setup_compound_reference_mode();
 
+    /* (6.4) Decode Tiles Syntax */
     bool decode_tiles();
     bool clear_above_context();
     u32 get_tile_offset(u32 tile_num, u32 mis, u32 tile_size_log2);

Userland/Libraries/LibVideo/VP9/SyntaxElementCounter.h

@@ -44,6 +44,7 @@ enum class SyntaxElementType {
 
 class SyntaxElementCounter final {
 public:
+    /* (8.3) Clear Counts Process */
     void clear_counts();
 
     u8 m_counts_intra_mode[BLOCK_SIZE_GROUPS][INTRA_MODES];

Userland/Libraries/LibVideo/VP9/TreeParser.h

@@ -41,10 +41,14 @@ public:
         TreeSelectionValue m_value;
     };
 
+    /* (9.3.3) */
     template<typename T = int>
     T parse_tree(SyntaxElementType type);
+    /* (9.3.1) */
     TreeSelection select_tree(SyntaxElementType type);
+    /* (9.3.2) */
     u8 select_tree_probability(SyntaxElementType type, u8 node);
+    /* (9.3.4) */
     void count_syntax_element(SyntaxElementType type, int value);
 
     void set_default_intra_mode_variables(u8 idx, u8 idy)