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75 lines
2.1 KiB
C++
75 lines
2.1 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/Span.h>
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namespace Gfx {
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// This is the arithmetic decoder described in Annex E of the JBIG2 spec.
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// See JBIG2Loader.cpp for the JBIG2 spec link.
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//
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// It's also used in JPEG2000 and also described in Annex C of the JPEG2000 spec.
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// See JPEG2000Loader.cpp for the JPEG2000 spec link.
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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 QMArithmeticDecoder {
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public:
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struct Context {
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u8 I { 0 }; // Index I stored for context CX (E.2.4)
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u8 is_mps { 0 }; // "More probable symbol" (E.1.1). 0 or 1.
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};
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static ErrorOr<QMArithmeticDecoder> initialize(ReadonlyBytes data);
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bool get_next_bit(Context& context);
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private:
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QMArithmeticDecoder(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 { 0 }; // Pointer into compressed data.
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// E.3.1 Decoder code register conventions
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u32 C { 0 }; // Consists of u16 C_high, C_low.
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u16 A { 0 }; // Current value of the fraction. Fixed precision; 0x8000 is equivalent to 0.75.
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u8 CT { 0 }; // Count of the number of bits in C.
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Context* CX { nullptr };
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static u8& 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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