LibVideo: Cache 64 bits at a time for reading in BitStream

Reads will now be done in larger chunks at a time.

The public read_byte() function was removed in favor of a private
fill_reservoir() function which will be used to fill the 64-bit
reservoir field which will then be bit-shifted and masked as necessary
for subsequent arbitrary bit-sized reads.

read_f(n) was renamed to read_bits to be clearer about its use.

Userland/Libraries/LibVideo/VP9/BitStream.cpp

@@ -5,60 +5,67 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <AK/Error.h>
-
 #include "BitStream.h"
 
 namespace Video::VP9 {
 
-ErrorOr<u8> BitStream::read_byte()
+ErrorOr<void> BitStream::fill_reservoir()
 {
-    if (m_bytes_remaining < 1)
-        return Error::from_string_literal("read_byte: Out of data.");
-    VERIFY(m_bytes_remaining >= 1);
-    m_bytes_remaining--;
-    return *(m_data_ptr++);
+    VERIFY(m_reservoir_bits_remaining == 0);
+    if (m_data_ptr == m_end_ptr)
+        return Error::from_string_literal("Stream is out of data");
+    VERIFY(m_data_ptr < m_end_ptr);
+    m_reservoir = 0;
+
+    size_t byte_index;
+    for (byte_index = 0; m_data_ptr < m_end_ptr && byte_index < sizeof(m_reservoir); byte_index++) {
+        m_reservoir = (m_reservoir << 8) | *m_data_ptr;
+        m_data_ptr++;
+    }
+
+    m_reservoir_bits_remaining = byte_index * 8;
+    m_reservoir <<= (sizeof(m_reservoir) - byte_index) * 8;
+    return {};
+}
+
+ErrorOr<u64> BitStream::read_bits(u8 bit_count)
+{
+    if (bit_count > sizeof(u64) * 8)
+        return Error::from_string_literal("Requested read is too large");
+    u64 result = 0;
+
+    while (bit_count > 0) {
+        if (m_reservoir_bits_remaining == 0)
+            TRY(fill_reservoir());
+
+        u64 batch_bits = min(bit_count, m_reservoir_bits_remaining);
+        u64 bit_shift = (sizeof(m_reservoir) * 8u) - batch_bits;
+
+        result = (result << batch_bits) | m_reservoir >> bit_shift;
+        m_reservoir <<= batch_bits;
+        bit_count -= batch_bits;
+        m_reservoir_bits_remaining -= batch_bits;
+        m_bits_read += batch_bits;
+    }
+
+    return result;
 }
 
 ErrorOr<bool> BitStream::read_bit()
 {
-    if (!m_current_byte.has_value()) {
-        m_current_byte = TRY(read_byte());
-        m_current_bit_position = 7;
-    }
-
-    bool bit_value = m_current_byte.value() & (1u << m_current_bit_position);
-    if (--m_current_bit_position < 0)
-        m_current_byte.clear();
-    return bit_value;
-}
-
-ErrorOr<u8> BitStream::read_f(size_t n)
-{
-    u8 result = 0;
-    for (size_t i = 0; i < n; i++) {
-        result = (2 * result) + TRY(read_bit());
-    }
-    return result;
+    auto value = TRY(read_bits(1));
+    VERIFY(value <= 2);
+    return value != 0;
 }
 
 ErrorOr<u8> BitStream::read_f8()
 {
-    if (!m_current_byte.has_value())
-        return read_byte();
-
-    auto high_bits = m_current_byte.value() & ((1u << m_current_bit_position) - 1);
-    u8 remaining_bits = 7 - m_current_bit_position;
-    m_current_byte = TRY(read_byte());
-    m_current_bit_position = 7;
-    auto low_bits = (m_current_byte.value() >> (8u - remaining_bits)) & ((1u << remaining_bits) - 1);
-    m_current_bit_position -= remaining_bits;
-    return (high_bits << remaining_bits) | low_bits;
+    return TRY(read_bits(8));
 }
 
 ErrorOr<u16> BitStream::read_f16()
 {
-    return (TRY(read_f8()) << 8u) | TRY(read_f8());
+    return TRY(read_bits(16));
 }
 
 /* 9.2.1 */
@@ -108,7 +115,7 @@ ErrorOr<bool> BitStream::read_bool(u8 probability)
 ErrorOr<void> BitStream::exit_bool()
 {
     // FIXME: I'm not sure if this call to min is spec compliant, or if there is an issue elsewhere earlier in the parser.
-    auto padding_element = TRY(read_f(min(m_bool_max_bits, (u64)bits_remaining())));
+    auto padding_element = TRY(read_bits(min(m_bool_max_bits, (u64)bits_remaining())));
 
     // FIXME: It is a requirement of bitstream conformance that enough padding bits are inserted to ensure that the final coded byte of a frame is not equal to a superframe marker.
     //  A byte b is equal to a superframe marker if and only if (b & 0xe0)is equal to 0xc0, i.e. if the most significant 3 bits are equal to 0b110.
@@ -128,24 +135,24 @@ ErrorOr<u8> BitStream::read_literal(size_t n)
 
 ErrorOr<i8> BitStream::read_s(size_t n)
 {
-    auto value = TRY(read_f(n));
+    auto value = TRY(read_bits(n));
     auto sign = TRY(read_bit());
     return sign ? -value : value;
 }
 
 u64 BitStream::get_position()
 {
-    return (m_bytes_read * 8) + (7 - m_current_bit_position);
+    return m_bits_read;
 }
 
 size_t BitStream::bytes_remaining()
 {
-    return m_bytes_remaining;
+    return (m_end_ptr - m_data_ptr) + (m_reservoir_bits_remaining / 8);
 }
 
 size_t BitStream::bits_remaining()
 {
-    return (bytes_remaining() * 8) + m_current_bit_position + 1;
+    return ((m_end_ptr - m_data_ptr) * sizeof(m_data_ptr) * 8) + m_reservoir_bits_remaining;
 }
 
 }

Userland/Libraries/LibVideo/VP9/BitStream.h

@@ -7,6 +7,7 @@
 
 #pragma once
 
+#include <AK/Error.h>
 #include <AK/Optional.h>
 #include <AK/Types.h>
 
@@ -16,15 +17,14 @@ class BitStream {
 public:
     BitStream(u8 const* data, size_t size)
         : m_data_ptr(data)
-        , m_bytes_remaining(size)
+        , m_end_ptr(data + size)
     {
     }
 
-    ErrorOr<u8> read_byte();
     ErrorOr<bool> read_bit();
 
+    ErrorOr<u64> read_bits(u8 bit_count);
     /* (9.1) */
-    ErrorOr<u8> read_f(size_t n);
     ErrorOr<u8> read_f8();
     ErrorOr<u16> read_f16();
 
@@ -42,11 +42,13 @@ public:
     size_t bits_remaining();
 
 private:
+    ErrorOr<void> fill_reservoir();
+
     u8 const* m_data_ptr { nullptr };
-    size_t m_bytes_remaining { 0 };
-    Optional<u8> m_current_byte;
-    i8 m_current_bit_position { 0 };
-    u64 m_bytes_read { 0 };
+    u8 const* m_end_ptr { nullptr };
+    u64 m_reservoir;
+    u8 m_reservoir_bits_remaining { 0 };
+    size_t m_bits_read { 0 };
 
     u8 m_bool_value { 0 };
     u8 m_bool_range { 0 };

Userland/Libraries/LibVideo/VP9/Parser.cpp

@@ -95,7 +95,7 @@ ErrorOr<void> Parser::refresh_probs()
 /* (6.2) */
 ErrorOr<void> Parser::uncompressed_header()
 {
-    auto frame_marker = TRY(m_bit_stream->read_f(2));
+    auto frame_marker = TRY(m_bit_stream->read_bits(2));
     if (frame_marker != 2)
         return Error::from_string_literal("uncompressed_header: Frame marker must be 2");
     auto profile_low_bit = TRY(m_bit_stream->read_bit());
@@ -105,7 +105,7 @@ ErrorOr<void> Parser::uncompressed_header()
         return Error::from_string_literal("uncompressed_header: Profile 3 reserved bit was non-zero");
     auto show_existing_frame = TRY(m_bit_stream->read_bit());
     if (show_existing_frame) {
-        m_frame_to_show_map_index = TRY(m_bit_stream->read_f(3));
+        m_frame_to_show_map_index = TRY(m_bit_stream->read_bits(3));
         m_header_size_in_bytes = 0;
         m_refresh_frame_flags = 0;
         m_loop_filter_level = 0;
@@ -128,7 +128,7 @@ ErrorOr<void> Parser::uncompressed_header()
         m_frame_is_intra = !m_show_frame && TRY(m_bit_stream->read_bit());
 
         if (!m_error_resilient_mode) {
-            m_reset_frame_context = TRY(m_bit_stream->read_f(2));
+            m_reset_frame_context = TRY(m_bit_stream->read_bits(2));
         } else {
             m_reset_frame_context = 0;
         }
@@ -150,7 +150,7 @@ ErrorOr<void> Parser::uncompressed_header()
         } else {
             m_refresh_frame_flags = TRY(m_bit_stream->read_f8());
             for (auto i = 0; i < 3; i++) {
-                m_ref_frame_idx[i] = TRY(m_bit_stream->read_f(3));
+                m_ref_frame_idx[i] = TRY(m_bit_stream->read_bits(3));
                 m_ref_frame_sign_bias[LastFrame + i] = TRY(m_bit_stream->read_bit());
             }
             TRY(frame_size_with_refs());
@@ -167,7 +167,7 @@ ErrorOr<void> Parser::uncompressed_header()
         m_frame_parallel_decoding_mode = true;
     }
 
-    m_frame_context_idx = TRY(m_bit_stream->read_f(2));
+    m_frame_context_idx = TRY(m_bit_stream->read_bits(2));
     if (m_frame_is_intra || m_error_resilient_mode) {
         setup_past_independence();
         if (m_frame_type == KeyFrame || m_error_resilient_mode || m_reset_frame_context == 3) {
@@ -209,7 +209,7 @@ ErrorOr<void> Parser::color_config()
         m_bit_depth = 8;
     }
 
-    auto color_space = TRY(m_bit_stream->read_f(3));
+    auto color_space = TRY(m_bit_stream->read_bits(3));
     VERIFY(color_space <= RGB);
     m_color_space = static_cast<ColorSpace>(color_space);
 
@@ -291,15 +291,15 @@ ErrorOr<void> Parser::read_interpolation_filter()
     if (TRY(m_bit_stream->read_bit())) {
         m_interpolation_filter = Switchable;
     } else {
-        m_interpolation_filter = literal_to_type[TRY(m_bit_stream->read_f(2))];
+        m_interpolation_filter = literal_to_type[TRY(m_bit_stream->read_bits(2))];
     }
     return {};
 }
 
 ErrorOr<void> Parser::loop_filter_params()
 {
-    m_loop_filter_level = TRY(m_bit_stream->read_f(6));
-    m_loop_filter_sharpness = TRY(m_bit_stream->read_f(3));
+    m_loop_filter_level = TRY(m_bit_stream->read_bits(6));
+    m_loop_filter_sharpness = TRY(m_bit_stream->read_bits(3));
     m_loop_filter_delta_enabled = TRY(m_bit_stream->read_bit());
     if (m_loop_filter_delta_enabled) {
         if (TRY(m_bit_stream->read_bit())) {
@@ -362,7 +362,7 @@ ErrorOr<void> Parser::segmentation_params()
             m_feature_enabled[i][j] = feature_enabled;
             if (feature_enabled) {
                 auto bits_to_read = segmentation_feature_bits[j];
-                feature_value = TRY(m_bit_stream->read_f(bits_to_read));
+                feature_value = TRY(m_bit_stream->read_bits(bits_to_read));
                 if (segmentation_feature_signed[j]) {
                     if (TRY(m_bit_stream->read_bit()))
                         feature_value = -feature_value;
@@ -764,7 +764,7 @@ ErrorOr<void> Parser::decode_tiles()
             if (last_tile)
                 tile_size = m_bit_stream->bytes_remaining();
             else
-                tile_size = TRY(m_bit_stream->read_f(32));
+                tile_size = TRY(m_bit_stream->read_bits(32));
 
             m_mi_row_start = get_tile_offset(tile_row, m_mi_rows, m_tile_rows_log2);
             m_mi_row_end = get_tile_offset(tile_row + 1, m_mi_rows, m_tile_rows_log2);