Varint Index in C++

Varint is a method of encoding integer values using a variable number of bytes. In C++, a varint index is an index into a data structure that uses varint encoding to store integer values.

Code Example

#include <iostream>
#include <vector>

class VarintIndex {
public:
    explicit VarintIndex(size_t size)
        : data_(size) {}

    void Set(size_t index, uint64_t value) {
        size_t offset = index * sizeof(uint64_t);
        size_t bytes_written = 0;
        do {
            uint8_t byte = value & 0x7f;
            value >>= 7;
            if (value != 0) {
                byte |= 0x80;
            }
            data_[offset + bytes_written] = byte;
            ++bytes_written;
        } while (value != 0);
    }

    uint64_t Get(size_t index) const {
        size_t offset = index * sizeof(uint64_t);
        uint64_t value = 0;
        uint64_t shift = 0;
        do {
            uint8_t byte = data_[offset];
            value |= (uint64_t)(byte & 0x7f) << shift;
            shift += 7;
        } while (byte & 0x80);
        return value;
    }

private:
    std::vector<uint8_t> data_;
};

Explanation

The above code implements a varint index using a std::vector<uint8_t> to store varint-encoded integers. The Set and Get methods take an index and a uint64_t value as input. The Set method encodes the value as a varint and stores the encoded bytes at the specified index. The Get method decodes the varint-encoded value and returns it.

The encoding scheme works by writing the least significant 7 bits of the integer to a byte, and then shifting the integer right by 7 bits. This process is repeated until the entire integer has been encoded. The most significant bit of each byte is set to 1 if there are more bytes in the encoding, and 0 otherwise.

Conclusion

Varint encoding is a useful technique for efficiently storing integer values in a compact format. In C++, a varint index can be implemented using a vector of bytes to store the varint-encoded integers. The code snippet presented demonstrates the basic operations of setting and getting values from a varint index.