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Fix default constructor of QubitRegister #94

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Fix default constructor of QubitRegister #94

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7 changes: 4 additions & 3 deletions src/qureg_init.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -35,8 +35,9 @@ QubitRegister<Type>::QubitRegister()

fusion = false;

Resize(1UL);
state_storage[0] = {1., 0.};
Resize(2UL);
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QubitRegister::Resize() will be deprecated in the next release, but for the moment this is ok.

Always keep in mind that the argument of Resize corresponds to the new number of global amplitudes.
IQS requires at least 2 amplitudes per MPI rank so calling Resize(2) works only for 1 MPI rank.

This points to a consistent use: the argument-less constructor for QubitRegister should be used only to create a 1-qubit state in state |1> and when a single MPI rank is used.

state[0] = {1., 0.};
state[1] = {0., 0.};

if (nprocs > 1) {
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The assert will trigger if the argument-less constructor is called with more than 1 MPI ranks.

The current unit tests are not capturing this failure since they are located in one_qubit_register_test.hpp and all its tests are skipped when there is more than 1 MPI rank (see lines 26-27 there).
If you add an identical test "IntializeWithDefault" to state_initialization_test.hpp you will see it fails when 2 or more MPI ranks are used.

fprintf(stderr,
Expand All @@ -57,7 +58,7 @@ void QubitRegister<Type>::Resize(std::size_t new_num_amplitudes)
log2_nprocs = iqs::ilog2(nprocs);

// FIXME GG: I believe this limits the use of "resize" to adding a single qubit
if(GlobalSize()) assert(GlobalSize() * 2UL == new_num_amplitudes);
// if(GlobalSize()) assert(GlobalSize() * 2UL == new_num_amplitudes);
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Resize() will be deprecated in the next release in favor of a more descriptive method that clarifies how the state is expanded or reduced in size.

num_qubits = iqs::ilog2(new_num_amplitudes);

local_size_ = UL(1L << UL(num_qubits - log2_nprocs));
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14 changes: 14 additions & 0 deletions unit_test/include/one_qubit_register_test.hpp
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Expand Up @@ -37,6 +37,20 @@ class OneQubitRegisterTest : public ::testing::Test
//////////////////////////////////////////////////////////////////////////////
// Test macros:

TEST_F(OneQubitRegisterTest, InitializeWithDefault)
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As mentioned in another comment, this unit test is run only when there are at most 1 MPI ranks.

{
ComplexDP amplitude;

iqs::QubitRegister<ComplexDP> psi_0;
// |psi_0> = |0>
amplitude = psi_0.GetGlobalAmplitude(0);
ASSERT_DOUBLE_EQ(amplitude.real(), 1.);
ASSERT_DOUBLE_EQ(amplitude.imag(), 0.);
amplitude = psi_0.GetGlobalAmplitude(1);
ASSERT_DOUBLE_EQ(amplitude.real(), 0.);
ASSERT_DOUBLE_EQ(amplitude.imag(), 0.);
}

TEST_F(OneQubitRegisterTest, InitializeInComputationalBasis)
{
ComplexDP amplitude;
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