How does NSData's implementation of the hash method work? - nsdata

Question

When calling hash on the built in NSData class in the foundation framework -- what implementation is used to return the hash value? (CRC32, something else?)

Answer 1

Something else. Actually it's an implementation detail, which does not need to use a fixed algorithm in different versions.
You can check the implementation in the open source version of Core Foundation. Note that NSData is toll-free bridged to CFDataRef. From http://opensource.apple.com/source/CF/CF-635.21/CFData.c:
static CFHashCode __CFDataHash(CFTypeRef cf) {
CFDataRef data = (CFDataRef)cf;
return CFHashBytes((uint8_t *)CFDataGetBytePtr(data), __CFMin(__CFDataLength(data), 80));
}
we see that the first 80 bytes are used to compute the hash. The function CFHashBytes is implemented as using the ELF hash algorithm:
#define ELF_STEP(B) T1 = (H << 4) + B; T2 = T1 & 0xF0000000; if (T2) T1 ^= (T2 >> 24); T1 &= (~T2); H = T1;
CFHashCode CFHashBytes(uint8_t *bytes, CFIndex length) {
/* The ELF hash algorithm, used in the ELF object file format */
UInt32 H = 0, T1, T2;
SInt32 rem = length;
while (3 < rem) {
ELF_STEP(bytes[length - rem]);
ELF_STEP(bytes[length - rem + 1]);
ELF_STEP(bytes[length - rem + 2]);
ELF_STEP(bytes[length - rem + 3]);
rem -= 4;
}
switch (rem) {
case 3: ELF_STEP(bytes[length - 3]);
case 2: ELF_STEP(bytes[length - 2]);
case 1: ELF_STEP(bytes[length - 1]);
case 0: ;
}
return H;
}
#undef ELF_STEP