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111 lines
3.2 KiB
C++
111 lines
3.2 KiB
C++
//===--- Varint.h - Variable length integer encoding ------------*- C++ -*-===//
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//
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// This source file is part of the Swift.org open source project
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//
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// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
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// Licensed under Apache License v2.0 with Runtime Library Exception
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//
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// See https://swift.org/LICENSE.txt for license information
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// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
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//
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//===----------------------------------------------------------------------===//
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///
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/// \file Varint.h
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/// \brief Defines transformations of integral types to/from variable length
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/// 7-bit encoding.
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//===----------------------------------------------------------------------===//
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#include <numeric>
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#include <type_traits>
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#include "llvm/ADT/SmallVector.h"
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#ifndef SWIFT_BASIC_VARINT_H
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#define SWIFT_BASIC_VARINT_H
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namespace swift {
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namespace Varint {
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/// Encode an unsigned integral type to a variable length 7-bit-encoded sequence
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/// of bytes.
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template <typename T>
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llvm::SmallVector<uint8_t, 10> encode(
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typename std::enable_if<
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std::is_integral<T>::value && std::is_unsigned<T>::value, T
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>::type i
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) {
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llvm::SmallVector<uint8_t, 10> bytes;
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do {
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uint8_t b = i & 0x7F;
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i >>= 7;
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if (i)
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b |= 0x80;
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bytes.push_back(b);
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} while (i);
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return bytes;
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}
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/// Encode a signed integral type to a variable length 7-bit-encoded sequence of
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/// bytes.
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///
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/// This transforms the signed value into an unsigned value and delegates
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/// to the unsigned version of `encode`.
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template <typename T>
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llvm::SmallVector<uint8_t, 10> encode(
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typename std::enable_if<
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std::is_integral<T>::value && std::is_signed<T>::value, T
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>::type i
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) {
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// Zig-zag encode the signed integer into the unsigned integer type.
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// Negative numbers are encoded as unsigned odd numbers in the
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// unsigned type, positive numbers are even. This prioritizes the
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// smaller numbers around zero, while making it compatible with
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// 7-bit encoding:
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// -3 -> 5
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// -2 -> 3
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// -1 -> 1
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// 0 -> 0
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// 1 -> 2
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// 2 -> 4
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// 3 -> 6
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typename std::make_unsigned<T>::type z = i < 0 ? ~(i << 1) : (i << 1);
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return encode<decltype(z)>(z);
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}
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/// Decode a variable length 7-bit encoded sequence of bytes to an unsigned
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/// integer type.
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template <typename T>
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typename std::enable_if<
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std::is_integral<T>::value && std::is_unsigned<T>::value, T
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>::type
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decode(const uint8_t *bytes) {
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size_t i = 0;
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size_t shift = 0;
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T decoded = 0;
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do {
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decoded |= T(bytes[i] & 0x7F) << shift;
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shift += 7;
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} while (bytes[i++] & 0x80);
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return decoded;
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}
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/// Decode a variable length 7-bit-encoded sequence of bytes to a signed integer
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/// type.
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///
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/// This delegates to the unsigned version of `decode` and transforms the
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/// value back into its signed version.
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template <typename T>
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typename std::enable_if<
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std::is_integral<T>::value && std::is_signed<T>::value, T
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>::type
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decode(const uint8_t *bytes) {
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auto decoded = decode<typename std::make_unsigned<T>::type>(bytes);
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// Zig-zag decode back into the signed integer type.
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return decoded & 1 ? ~(decoded >> 1) : (decoded >> 1);
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}
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} // end namespace Varint
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} // end namespace swift
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#endif // SWIFT_BASIC_VARINT_H
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