3 Int Coding
輕鬆一刻，前面約定中講過Leveldb使用了很多VarInt型編碼，典型的如後面將涉及到的各種key。其中的編碼、解碼函數分爲VarInt和FixedInt兩種。int32和int64操作都是類似的。
3.1 Decode
首先是FixedInt編碼，直接上代碼，很簡單明瞭。
void EncodeFixed32(char* buf, uint32_t value)
{
if (port::kLittleEndian) {
memcpy(buf, &value,sizeof(value));
} else {
buf[0] = value & 0xff;
buf[1] = (value >> 8)& 0xff;
buf[2] = (value >> 16)& 0xff;
buf[3] = (value >> 24)& 0xff;
}
}
下面是VarInt編碼，int32和int64格式，代碼如下，有效位是7bit的，因此把uint32按7bit分割，對unsigned char賦值時，超出0xFF會自動截斷，因此直接*(ptr++) = v|B即可，不需要再把(v|B)與0xFF作&操作。
char* EncodeVarint32(char* dst, uint32_t v)
{
unsigned char* ptr =reinterpret_cast<unsigned char*>(dst);
static const int B = 128;
if (v < (1<<7)) {
*(ptr++) = v;
} else if (v < (1<<14)){
*(ptr++) = v | B;
*(ptr++) = v>>7;
} else if (v < (1<<21)){
*(ptr++) = v | B;
*(ptr++) = (v>>7) | B;
*(ptr++) = v>>14;
} else if (v < (1<<28)){
*(ptr++) = v | B;
*(ptr++) = (v>>7) | B;
*(ptr++) = (v>>14) | B;
*(ptr++) = v>>21;
} else {
*(ptr++) = v | B;
*(ptr++) = (v>>7) | B;
*(ptr++) = (v>>14) | B;
*(ptr++) = (v>>21) | B;
*(ptr++) = v>>28;
}
return reinterpret_cast<char*>(ptr);
}
char* EncodeVarint64(char* dst, uint64_t v) {// 對於uint64，直接循環
static const int B = 128;
unsigned char* ptr =reinterpret_cast<unsigned char*>(dst);
while (v >= B) {
*(ptr++) = (v & (B-1)) |B;
v >>= 7;
}
*(ptr++) =static_cast<unsigned char>(v);
returnreinterpret_cast<char*>(ptr);
}
3.2 Decode
Fixed Int的Decode，操作，代碼：
inline uint32_t DecodeFixed32(const char* ptr)
{
if (port::kLittleEndian) {
uint32_t result;
memcpy(&result, ptr,sizeof(result)); // gcc optimizes this to a plain load
return result;
} else {
return((static_cast<uint32_t>(static_cast<unsigned char>(ptr[0])))
|(static_cast<uint32_t>(static_cast<unsigned char>(ptr[1])) <<8)
| (static_cast<uint32_t>(static_cast<unsignedchar>(ptr[2])) << 16)
|(static_cast<uint32_t>(static_cast<unsigned char>(ptr[3])) <<24));
}
}
再來看看VarInt的解碼，很簡單，依次讀取1byte，直到最高位爲0的byte結束，取低7bit，作(<<7)移位操作組合成Int。看代碼：
const char* GetVarint32Ptr(const char* p, const char* limit, uint32_t* value)
{
if (p < limit) {
uint32_t result =*(reinterpret_cast<const unsigned char*>(p));
if ((result & 128) == 0) {
*value = result;
return p + 1;
}
}
return GetVarint32PtrFallback(p,limit, value);
}
const char* GetVarint32PtrFallback(const char* p, const char* limit,uint32_t* value)
{
uint32_t result = 0;
for (uint32_t shift = 0; shift<= 28 && p < limit; shift += 7) {
uint32_t byte =*(reinterpret_cast<const unsigned char*>(p));
p++;
if (byte & 128) { // More bytes are present
result |= ((byte & 127)<< shift);
} else {
result |= (byte <<shift);
*value = result;
returnreinterpret_cast<const char*>(p);
}
}
return NULL;
}
