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b8f80501ec
The context can vary for every bit we read. This does not affect the one use in the test which reuses the same context for all bits, but it is necessary for future changes.
96 lines
2.5 KiB
C++
96 lines
2.5 KiB
C++
/*
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* Copyright (c) 2024, Nico Weber <thakis@chromium.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/MemoryStream.h>
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#include <AK/OwnPtr.h>
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#include <LibGfx/ImageFormats/ImageDecoder.h>
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namespace Gfx {
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struct JBIG2LoadingContext;
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namespace JBIG2 {
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// E.3 Arithmetic decoding procedure, but with the changes described in
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// Annex G Arithmetic decoding procedure (software conventions).
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// Exposed for testing.
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class ArithmeticDecoder {
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public:
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struct Context {
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u16 I; // Index I stored for context CX (E.2.4)
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u8 is_mps; // "More probable symbol" (E.1.1). 0 or 1.
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};
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static ErrorOr<ArithmeticDecoder> initialize(ReadonlyBytes data);
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bool get_next_bit(Context& context);
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private:
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ArithmeticDecoder(ReadonlyBytes data)
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: m_data(data)
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{
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}
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ReadonlyBytes m_data;
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// The code below uses names from the spec, so that the algorithms look exactly like the flowcharts in the spec.
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// Abbreviations:
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// "CX": "Context" (E.1)
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// "D": "Decision" (as in "encoder input" / "decoder output") (E.1)
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// "I(CX)": "Index I stored for context CX" (E.2.4)
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// "MPS": "More probable symbol" (E.1.1)
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// "LPS": "Less probable symbol" (E.1.1)
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void INITDEC();
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u8 DECODE(); // Returns a single decoded bit.
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u8 MPS_EXCHANGE();
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u8 LPS_EXCHANGE();
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void RENORMD();
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void BYTEIN();
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u8 B(size_t offset = 0) const; // Byte pointed to by BP.
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size_t BP; // Pointer into compressed data.
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// E.3.1 Decoder code register conventions
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u32 C; // Consists of u16 C_high, C_low.
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u16 A; // Current value of the fraction. Fixed precision; 0x8000 is equivalent to 0.75.
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u8 CT; // Count of the number of bits in C.
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Context* CX;
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static u16& I(Context* cx) { return cx->I; }
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static u8& MPS(Context* cx) { return cx->is_mps; }
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static u16 Qe(u16);
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static u8 NMPS(u16);
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static u8 NLPS(u16);
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static u8 SWITCH(u16);
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};
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}
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class JBIG2ImageDecoderPlugin : public ImageDecoderPlugin {
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public:
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static bool sniff(ReadonlyBytes);
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static ErrorOr<NonnullOwnPtr<ImageDecoderPlugin>> create(ReadonlyBytes);
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virtual ~JBIG2ImageDecoderPlugin() override = default;
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virtual IntSize size() override;
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virtual ErrorOr<ImageFrameDescriptor> frame(size_t index, Optional<IntSize> ideal_size = {}) override;
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static ErrorOr<ByteBuffer> decode_embedded(Vector<ReadonlyBytes>);
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private:
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JBIG2ImageDecoderPlugin();
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OwnPtr<JBIG2LoadingContext> m_context;
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};
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}
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