107 lines
2.9 KiB
C++
107 lines
2.9 KiB
C++
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#ifndef STAR_VLQ_ENCODING_HPP
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#define STAR_VLQ_ENCODING_HPP
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#include "StarMemory.hpp"
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namespace Star {
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// Write an unsigned integer as a VLQ (Variable Length Quantity). Writes the
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// integer in 7 byte chunks, with the 8th bit of each octet indicates whether
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// another chunk follows. Endianness independent, as the chunks are always
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// written most significant first. Returns number of octet written (writes a
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// maximum of a 64 bit integer, so a maximum of 10)
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template <typename OutputIterator>
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size_t writeVlqU(uint64_t x, OutputIterator out) {
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size_t i;
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for (i = 9; i > 0; --i) {
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if (x & ((uint64_t)(127) << (i * 7)))
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break;
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}
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for (size_t j = 0; j < i; ++j)
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*out++ = (uint8_t)((x >> ((i - j) * 7)) & 127) | 128;
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*out++ = (uint8_t)(x & 127);
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return i + 1;
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}
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inline size_t vlqUSize(uint64_t x) {
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size_t i;
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for (i = 9; i > 0; --i) {
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if (x & ((uint64_t)(127) << (i * 7)))
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break;
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}
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return i + 1;
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}
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// Read a VLQ (Variable Length Quantity) encoded unsigned integer. Returns
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// number of bytes read. Reads a *maximum of 10 bytes*, cannot read a larger
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// than 64 bit integer! If no end marker is found within 'maxBytes' or 10
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// bytes, whichever is smaller, then will return NPos to signal error.
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template <typename InputIterator>
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size_t readVlqU(uint64_t& x, InputIterator in, size_t maxBytes = 10) {
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x = 0;
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for (size_t i = 0; i < min<size_t>(10, maxBytes); ++i) {
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uint8_t oct = *in++;
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x = (x << 7) | (uint64_t)(oct & 127);
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if (!(oct & 128))
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return i + 1;
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}
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return NPos;
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}
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// Write a VLQ (Variable Length Quantity) encoded signed integer. Encoded by
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// making the sign bit the least significant bit in the integer. Returns
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// number of bytes written.
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template <typename OutputIterator>
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size_t writeVlqI(int64_t v, OutputIterator out) {
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uint64_t target;
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// If negative, then add 1 to properly encode -2^63
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if (v < 0)
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target = ((-(v + 1)) << 1) | 1;
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else
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target = v << 1;
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return writeVlqU(target, out);
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}
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inline size_t vlqISize(int64_t v) {
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uint64_t target;
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// If negative, then add 1 to properly encode -2^63
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if (v < 0)
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target = ((-(v + 1)) << 1) | 1;
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else
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target = v << 1;
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return vlqUSize(target);
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}
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// Read a VLQ (Variable Length Quantity) encoded signed integer. Returns
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// number of bytes read. Reads a *maximum of 10 bytes*, cannot read a larger
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// than 64 bit integer! If no end marker is found within 'maxBytes' or 10
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// bytes, whichever is smaller, then will return NPos to signal error.
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template <typename InputIterator>
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size_t readVlqI(int64_t& v, InputIterator in, size_t maxBytes = 10) {
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uint64_t source;
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size_t bytes = readVlqU(source, in, maxBytes);
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if (bytes == NPos)
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return NPos;
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bool negative = (source & 1);
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// If negative, then need to undo the +1 transformation to encode -2^63
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if (negative)
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v = -(int64_t)(source >> 1) - 1;
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else
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v = (int64_t)(source >> 1);
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return bytes;
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}
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}
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#endif
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