feather

FEATure HashER

Build & Install

• CPP
  ```bash cd PATH/TO/FEATHER mkdir build && cd build

Build with unit-test, without python-binding

cmake ../ -DFEATHERBUILDTESTS=ON -DFEATHERBUILDPY_BINDER=OFF

Build without unit-test and python-binding

cmake ../ -DFEATHERBUILDTESTS=OFF -DFEATHERBUILDPY_BINDER=OFF

Build python binding

cmake ../ -DFEATHERBUILDTESTS=OFF -DFEATHERBUILDPYBINDER=ON -DPYTHONEXECUTABLE=/usr/bin//python3.7

make -j12 ```

• Python with pip
  bash python -m pip install git+https://github.com/innerNULL/feather.git -vvv # or python -m pip install https://github.com/innerNULL/feather/archive/refs/heads/main.zip -vvv # or python -m pip install pyfeather here is what you may see:
  Processing /Path/To/feather Building wheels for collected packages: pyfeather Building wheel for pyfeather (setup.py) ... done Created wheel for pyfeather: filename=pyfeather-0.0.1-cp37-cp37m-macosx_10_15_x86_64.whl size=1284474 sha256=e3f9d0be1e7578274f3fcecb854c1e66336a24985b8e6ff4213375d76463299e Stored in directory: /private/var/folders/4q/50_2647d1yb47jt9j6plwx2r0000gq/T/pip-ephem-wheel-cache-996awbes/wheels/0f/bd/93/b6936ec0c1169201de264147e21ae7e2bb894720b34bcdce79 Successfully built pyfeather Installing collected packages: pyfeather Successfully installed pyfeather-0.0.1

How to Use

Feature-Hash

Here is an simple example: ```python import pyfeather from typing import List

Loading the pre-defined feature schema.

feahash = pyfeather.FeaHash("./conf/feather.conf")

Getting hash of value 2 and '2' of 'fea1', which is an discrete

feature, and both hash results should be same.

fea1hashstr2: List[int] = feahash.GetFeaHash("fea1", "2") fea1hashint2: List[int] = feahash.GetFeaHash("fea1", 2)

[10100070] [10100070]

print(fea1hashstr2, fea1hashint2)

Getting hash of value 3.14 and '3.14' and 5.12 of "fea10", which

is an continuous feature, and the hash-bucket of any value of

this feature should always be 0, so all value has same feature

hash result.

fea10hashfloat3p14: List[int] = feahash.GetFeaHash("fea10", 3.14) fea10hashstr3p14: List[int] = feahash.GetFeaHash("fea10", '3.14') fea10hashfloat5p12: List[int] = feahash.GetFeaHash("fea10", 5.12)

[11000000] [11000000] [11000000]

print(fea10hashfloat3p14, fea10hashstr3p14, fea10hashfloat5p12)

Getting hash of value of [4.0, 3.0, 2.0, 1.0] and [1.0, 2.0, 3.0, 4.0]

of "fea11", which is an vector feature with dimension as 4, so all

4-dim vectors' feature-hash-bucket of this feature should always

be [0, 1, 2, 3], and like continusous-feature, all feature-hash of

any value of this feature should be same.

fea11hash4to1: List[int] = feahash.GetFeaHash("fea11", [4.0, 3.0, 2.0, 1.0]) fea11hash1to4: List[int] = feahash.GetFeaHash("fea11", [1.0, 2.0, 3.0, 4.0])

[11100000, 11100001, 11100002, 11100003]

[11100000, 11100001, 11100002, 11100003]

print(fea11hash4to1, fea11hash1to4) ```

Feature Hashing

Notions

• Feature Value:
  Defined by FeaValue, which is a unified wrapping of feature data. Feature-data could be classified into three types: discrete-feature, continuous-feature, vector-feature.
  
  • Discrete Feature:
    The input could be std::string, int32_t, float, double, no matter what input type it is, the input will be casted into std::string and saved as FeaValue::discrete_val.
    
  • Continuous Feature:
    The input could be std::string, int32_t, float, double, no matter what input type it is, the input will be casted into float and saved as FeaValue::continuous_val.
    
  • Vector Feature:
    The input could be std::vector<std::string>, std::vector<int>, std::vector<float>, no matter what input type it is, the input will be casted into std::vector<float> and saved as FeaValue::vec_val.
    

FeaValue instance will also record some feature meta-data, such as feature-type, 0 for discrete-feature, 1 for continuous-feature, 2 for vector-feature.

Besides, FeaValue support transform feature-value to feature-value's hash id by FeaValue::GetHash, according feature-type:
* Discrete Feature Hash:
Just the result of calling std::hash on FeaValue::discrete_val.
* Continuous Feature Hash:
Always returns 1 as feature-hash. This is because continuous-feature actually don't needs feature-hashing, so its Feature-Slot's bucket-size should always be 1, so by assign its feather hash-id to 1, when using mod function to hash-id on slot-bucket-size, we can always assign continuous-feature value on 0-bucket of its slot.
* Vector Feature Hash:
Similiar to continuous-feature case, vector-feature also doesn't need feature-hashing operation. So we can assign each element in vector to slot-bucket which id corresponding to element's index.
To do this, we can let each element's hash-id as element-index + slot-bucket-size, when using mod function to hash-id on slot-bucket-size, we can always assign each element's slot-bucket to element-index.
For example, a 3-dim vector [3.14, 5.21, 6.79], its slot-bucket-size must be 3 (same with its dimension), we assign each element's hash-id as [3 + 0, 3 + 1, 3 + 2], and so each element's slot-bucket-id is [3 % 3, 4 % 3, 5 % 3].

• Feature Slot
  Defined in FeaSlot. Each feature corresponing to a "slot", the feature-slot takes responsibility to map each feature value's hash to a bucket by executing mod operation on feature value's hash with slot's bucket-size.

• Feature Hash
  Define in FeaHash. There is a config which defines schema of target features, the 1st column is feature-name, 2nd column is feature-slot id, 3rd column is feature-slot hash-bucket number, 4th column is feature-type (0 for discrete-feathe, 1 for continuous-feature, 2 for vector-feature). The FeaHash will record/register all slots at construction function.

• Feature Indexer(TODO)

• Feature Extractor
  The base class is FeaExtractor, but the mainly using case is mapping feature's hash-id/index and value (in continuous and vector feature case) to libsvm format, which can be done by LibSVMExtractor.

• Bucket-ID and Bucket-Code
  Briefly, Bucket-ID is int32_t, Bucket-Code is std::string which digits/length should be fixed. Here are some example about mapping bucket-id to bucket-code, which all bucket-code has 5 digits:

  • bucket-id: 5 -> bucket-code: '00005'
    
  • bucket-id: 54234 -> bucket-code: '54234'
    
  • bucket-id: 568 -> bucket-code: '00568'
    

Algorithm

Each feature has a slot and hash-bucket size, the finally hash of this feature is a int64 in the format as ${SLOT}${HASH-BUCKET-CODE}, since the top digit of this int64 is controled by slot, so each feature's finally hash value will far away with each other, the second part is 'part of hash value of feature value according feature hash-bucket size' because, hash value of feature value is a int64, finally-hash is also a int64, if we just concat slot and hash value of feature value, then the finally result number will have posibillity to overflow from int64 range.

by the way, in case we want adjust each feature-slot's hash-bucket size, we can maintain a hash-ring/consistant hash for each slot.

TODO

• FeaHash::Hash2IndexDictBuild should supports rebuild mode.