Function validate_quregCanBeReduced

quest/src/core/validation.cpp:4289–4313

 */

void validate_quregCanBeReduced(Qureg qureg, int numTraceQubits, const char* caller) {

    // 0 < numTraceQubits <= numQubits is assured by validate_targets(), but
    // numTraceQubits == numQubtis is permitted there though forbidden here
    assertThat(numTraceQubits < qureg.numQubits, report::NUM_TRACE_QUBITS_EQUALS_QUREG_SIZE, caller);

    // when not distributed, there are no further restrictions
    if (!qureg.isDistributed)
        return;

    // if the reduced qureg were hypothetically permitted to be non-distributed 
    // despite qureg being distributed, then maxTr <= numQb - ceil(logNumNodes/2).
    // we presently do not support this however, and insist the reduced qureg
    // matches the input qureg's distribution, such that the maximum traced qubits
    // is simply that which still retains at least one column per node
    int maxNumTraceQubits = qureg.numQubits - qureg.logNumNodes;

    tokenSubs vars = {
        {"${NUM_TRACE_QUBITS}", numTraceQubits},
        {"${MAX_TRACE_QUBITS}", maxNumTraceQubits},
        {"${NUM_QUREG_QUBITS}", qureg.numQubits},
        {"${NUM_NODES}",        qureg.numNodes}};

    assertThat(numTraceQubits <= maxNumTraceQubits, report::NUM_TRACE_QUBITS_EXCEEDS_DISTRIBUTED_MAX, vars, caller);
}

void validate_quregCanBeSetToReducedDensMatr(Qureg out, Qureg in, int numTraceQubits, const char* caller) {