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LibGfx/JPEG: Introduce peek_bits and use it in read_bits

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While already providing a performance improvement by removing the loop
in `read_bits`, this method was introduced to optimize `next_symbol`.
This commit is contained in:
Lucas CHOLLET 2023-04-29 15:37:28 -04:00 • committed by Andreas Kling
parent 86ce9cc30f
commit 5ec2aa4dcc
1 changed files with 30 additions and 18 deletions
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48
Userland/Libraries/LibGfx/ImageFormats/JPEGLoader.cpp
View file

@ -12,6 +12,7 @@
#include <AK/HashMap.h>
#include <AK/Math.h>
#include <AK/MemoryStream.h>
#include <AK/NumericLimits.h>
#include <AK/String.h>
#include <AK/Try.h>
#include <AK/Vector.h>
@ -253,27 +254,38 @@ public:
return Error::from_string_literal("Reading too much huffman bits at once");
}
u16 value = 0;
while (count--) {
if (m_byte_offset >= m_stream.size()) {
dbgln_if(JPEG_DEBUG, "Huffman stream exhausted. This could be an error!");
return Error::from_string_literal("Huffman stream exhausted.");
}
u8 const current_byte = m_stream[m_byte_offset];
u8 const current_bit = 1u & (current_byte >> (7 - m_bit_offset)); // MSB first.
m_bit_offset++;
value = (value << 1) | current_bit;
if (m_bit_offset == 8) {
m_byte_offset++;
m_bit_offset = 0;
}
}
u16 const value = peek_bits(count);
discard_bits(count);
return value;
}
u16 peek_bits(u8 count) const
{
using BufferType = u32;
constexpr static auto max = NumericLimits<BufferType>::max();
auto const mask = max >> (8 + m_bit_offset);
BufferType msb_buffer {};
if (m_byte_offset + 0 < m_stream.size())
msb_buffer |= (static_cast<BufferType>(m_stream[m_byte_offset + 0]) << (2 * 8));
if (m_byte_offset + 1 < m_stream.size())
msb_buffer |= (static_cast<BufferType>(m_stream[m_byte_offset + 1]) << (1 * 8));
if (m_byte_offset + 2 < m_stream.size())
msb_buffer |= (static_cast<BufferType>(m_stream[m_byte_offset + 2]) << (0 * 8));
return (mask & msb_buffer) >> (3 * 8 - m_bit_offset - count);
}
void discard_bits(u8 count)
{
m_bit_offset += count;
auto const carry = m_bit_offset / 8;
m_bit_offset -= 8 * carry;
m_byte_offset += carry;
}
void advance_to_byte_boundary()
{
if (m_bit_offset > 0) {