QuSpin-Core (work in progress)

Low-level C++ and Cython API for QuSpin.

TODO:

1. generate code coverage results for both Cython and C++ tests.

2. create unit tests for C++ API.

   a. This is done inside the header files. Here is an example from src/quspin_core/includes/quspin/basis/bitbasis/bits.h

    ```
    #ifdef QUSPIN_UNIT_TESTS
   
    TEST_CASE("get_bit_substring") {
   
        using namespace quspin::basis;
   
        bit_set<uint8_t> state(0b1010111);
   
        CHECK(get_sub_bitstring(state,0) == 1);
        CHECK(get_sub_bitstring(state,3) == 0);
        CHECK(get_sub_bitstring(state,7) == 0);
        CHECK(get_sub_bitstring(state,9) == 0);
   
        int l1[2] = {0,1};
        int l2[3] = {0,3,6};
   
        CHECK(get_sub_bitstring(state,l1,2) == 0b11);
        CHECK(get_sub_bitstring(state,l2,3) == 0b101);
   
    }
   
    #endif
    ```
   

   All one has to do is define the macro boundary #ifdef QUSPIN_UNIT_TESTS ... #endif and write tests inside the body of the if statement. Following the doctest instructions to set up tests. To run the tests you need CMAKE and you can run the following commands in the terminal from QuSpin-Core root directory:

   i. cmake -B build .

   ii. cmake --build build/

   iii cd build/

   iv. ctest

   it will output something like:

   Test project .../QuSpin-Core/build
   Start 1: test_cpp
   1/1 Test #1: test_cpp .........................   Passed    0.07 sec
   
   100% tests passed, 0 tests failed out of 1
   
   Total Test time (real) =   0.11 sec
   

   These tests will automatically get run every time the code is updated on github so make these test as light as possible. The idea is to test the most basic functionality with simple cases that can be worked out by hand.

3. design C++ ABI (Application backend interface)

   a. preliminary design concept and implementation.

   b. revise during development of Cython API

4. Cython API:

   a. design

   b. test

5. fermion core ABI/API:

   a. design

   b. test

6. Start to plan integration for QuSpin-Core into QuSpin

   a. move relevant C++ code from QuSpin to QuSpin-Core

     i. redesign
   
    ii. test
   

   b. replace old backend withthe new backend

      i. removing old cython code for 1D basis sets
        
      ii.redesign thee  backend interface 
        
      iii. potentially remove old arguments from functions