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- /*
- * Copyright (c) 2024, Nico Weber <thakis@chromium.org>
- *
- * SPDX-License-Identifier: BSD-2-Clause
- */
- #pragma once
- #include <AK/MemoryStream.h>
- #include <AK/OwnPtr.h>
- #include <LibGfx/ImageFormats/ImageDecoder.h>
- namespace Gfx {
- struct JBIG2LoadingContext;
- namespace JBIG2 {
- // E.3 Arithmetic decoding procedure, but with the changes described in
- // Annex G Arithmetic decoding procedure (software conventions).
- // Exposed for testing.
- class ArithmeticDecoder {
- public:
- struct Context {
- u8 I { 0 }; // Index I stored for context CX (E.2.4)
- u8 is_mps { 0 }; // "More probable symbol" (E.1.1). 0 or 1.
- };
- static ErrorOr<ArithmeticDecoder> initialize(ReadonlyBytes data);
- bool get_next_bit(Context& context);
- private:
- ArithmeticDecoder(ReadonlyBytes data)
- : m_data(data)
- {
- }
- ReadonlyBytes m_data;
- // The code below uses names from the spec, so that the algorithms look exactly like the flowcharts in the spec.
- // Abbreviations:
- // "CX": "Context" (E.1)
- // "D": "Decision" (as in "encoder input" / "decoder output") (E.1)
- // "I(CX)": "Index I stored for context CX" (E.2.4)
- // "MPS": "More probable symbol" (E.1.1)
- // "LPS": "Less probable symbol" (E.1.1)
- void INITDEC();
- u8 DECODE(); // Returns a single decoded bit.
- u8 MPS_EXCHANGE();
- u8 LPS_EXCHANGE();
- void RENORMD();
- void BYTEIN();
- u8 B(size_t offset = 0) const; // Byte pointed to by BP.
- size_t BP { 0 }; // Pointer into compressed data.
- // E.3.1 Decoder code register conventions
- u32 C { 0 }; // Consists of u16 C_high, C_low.
- u16 A { 0 }; // Current value of the fraction. Fixed precision; 0x8000 is equivalent to 0.75.
- u8 CT { 0 }; // Count of the number of bits in C.
- Context* CX { nullptr };
- static u8& I(Context* cx) { return cx->I; }
- static u8& MPS(Context* cx) { return cx->is_mps; }
- static u16 Qe(u16);
- static u8 NMPS(u16);
- static u8 NLPS(u16);
- static u8 SWITCH(u16);
- };
- }
- class JBIG2ImageDecoderPlugin : public ImageDecoderPlugin {
- public:
- static bool sniff(ReadonlyBytes);
- static ErrorOr<NonnullOwnPtr<ImageDecoderPlugin>> create(ReadonlyBytes);
- virtual ~JBIG2ImageDecoderPlugin() override = default;
- virtual IntSize size() override;
- virtual ErrorOr<ImageFrameDescriptor> frame(size_t index, Optional<IntSize> ideal_size = {}) override;
- static ErrorOr<ByteBuffer> decode_embedded(Vector<ReadonlyBytes>);
- private:
- JBIG2ImageDecoderPlugin();
- OwnPtr<JBIG2LoadingContext> m_context;
- };
- }
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