from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
from qiskit import Aer, execute
from qiskit.tools.visualization import plot_histogram

# Half adder
# input qubits: a, b
# output qubits: s (sum) and cout (carry-out)
def halfadd(circ, a, b, s, cout) :
    circ.cx(a, s)
    circ.cx(b, s)
    circ.ccx(a, b, cout)

# Create registers and circuit
a = QuantumRegister(1, 'a')  # input
b = QuantumRegister(1, 'b')
s = QuantumRegister(2, 'out')  # output
qc = QuantumCircuit(a,b,s)

# Build a circuit that initializes the input bits to a full superposition
# Calls the adder function and then measures the output register

qc.h(a)
qc.h(b)
qc.barrier()
halfadd(qc, a, b, s[0], s[1])
qc.barrier()
m = ClassicalRegister(2, 'sum')
qc.add_register(m)
qc.measure(s,m)

# Simulate and show results
backend = Aer.get_backend('qasm_simulator')
job = execute(qc, backend, shots=4096)  # shots default = 1024
result = job.result()
print(result.get_counts())

qc.draw(output='mpl')
plot_histogram(result.get_counts())