首先,請注意varints作爲實際字節發送,而不是字符1
和0
的字符串。
對於一個無符號的varint,我相信下面的代碼會爲你解碼,假設你已經在data
指向的緩衝區中獲得了varint數據。該示例函數返回參考參數int decoded_bytes
中解碼的字節數。
uint64_t decode_unsigned_varint(const uint8_t *const data, int &decoded_bytes)
{
int i = 0;
uint64_t decoded_value = 0;
int shift_amount = 0;
do
{
decoded_value |= (uint64_t)(data[i] & 0x7F) << shift_amount;
shift_amount += 7;
} while ((data[i++] & 0x80) != 0);
decoded_bytes = i;
return decoded_value;
}
要解碼簽署varint,您可以使用調用第一第二個功能:
int64_t decode_signed_varint(const uint8_t *const data, int &decoded_bytes)
{
uint64_t unsigned_value = decode_unsigned_varint(data, decoded_bytes);
return (int64_t)(unsigned_value & 1 ? ~(unsigned_value >> 1)
: (unsigned_value >> 1));
}
我相信這兩個功能是正確的。我使用下面的代碼進行了一些基本測試,以驗證Google頁面中的幾個數據點。輸出是正確的。
#include <stdint.h>
#include <iostream>
uint64_t decode_unsigned_varint(const uint8_t *const data, int &decoded_bytes)
{
int i = 0;
uint64_t decoded_value = 0;
int shift_amount = 0;
do
{
decoded_value |= (uint64_t)(data[i] & 0x7F) << shift_amount;
shift_amount += 7;
} while ((data[i++] & 0x80) != 0);
decoded_bytes = i;
return decoded_value;
}
int64_t decode_signed_varint(const uint8_t *const data, int &decoded_bytes)
{
uint64_t unsigned_value = decode_unsigned_varint(data, decoded_bytes);
return (int64_t)(unsigned_value & 1 ? ~(unsigned_value >> 1)
: (unsigned_value >> 1));
}
uint8_t ex_p300[] = { 0xAC, 0x02 };
uint8_t ex_n1 [] = { 0x01 };
using namespace std;
int main()
{
int decoded_bytes_p300;
uint64_t p300;
p300 = decode_unsigned_varint(ex_p300, decoded_bytes_p300);
int decoded_bytes_n1;
int64_t n1;
n1 = decode_signed_varint(ex_n1, decoded_bytes_n1);
cout << "p300 = " << p300
<< " decoded_bytes_p300 = " << decoded_bytes_p300 << endl;
cout << "n1 = " << n1
<< " decoded_bytes_n1 = " << decoded_bytes_n1 << endl;
return 0;
}
要編碼varints,可以使用以下函數。請注意,緩衝區uint8_t *const data
應該至少有10個字節的空間,因爲最大的varint長度爲10個字節。
的#include
// Encode an unsigned 64-bit varint. Returns number of encoded bytes.
// 'buffer' must have room for up to 10 bytes.
int encode_unsigned_varint(uint8_t *const buffer, uint64_t value)
{
int encoded = 0;
do
{
uint8_t next_byte = value & 0x7F;
value >>= 7;
if (value)
next_byte |= 0x80;
buffer[encoded++] = next_byte;
} while (value);
return encoded;
}
// Encode a signed 64-bit varint. Works by first zig-zag transforming
// signed value into an unsigned value, and then reusing the unsigned
// encoder. 'buffer' must have room for up to 10 bytes.
int encode_signed_varint(uint8_t *const buffer, int64_t value)
{
uint64_t uvalue;
uvalue = uint64_t(value < 0 ? ~(value << 1) : (value << 1));
return encode_unsigned_varint(buffer, uvalue);
}
我以前從來沒見過varints,但我想谷歌的「varint C庫」,並得到了一些打擊。 –