# ClassicalFunction compiler (qiskit.circuit.classicalfunction)#

## Overview#

The classical function compiler provides the necessary tools to map a classical potentially irreversible functions into quantum circuits. Below is a simple example of how to synthesize a simple boolean function defined using Python into a QuantumCircuit:

from qiskit.circuit.classicalfunction import classical_function
from qiskit.circuit.classicalfunction.types import Int1

@classical_function
def grover_oracle(a: Int1, b: Int1, c: Int1, d: Int1) -> Int1:
return (not a and b and not c and d)

quantum_circuit = grover_oracle.synth(registerless=False)
quantum_circuit.draw('text')

a: ββoββ
β
b: βββ ββ
β
c: ββoββ
β
d: βββ ββ
βββ΄ββ
return: β€ X β
βββββ

Following Qiskitβs little-endian bit ordering convention, the left-most bit (a) is the most significant bit and the right-most bit (d) is the least significant bit.

## Supplementary Information#

### Tweedledum#

Tweedledum is a C++-17 header-only library that implements a large set of reversible (and quantum) synthesis, optimization, and mapping algorithms. The classical function compiler relies on it and its dependencies to both represent logic networks and synthesize them into quantum circuits.

### ClassicalFunction data types#

At the moment, the only type supported by the classical_function compilers is qiskit.circuit.classicalfunction.types.Int1. The classical function function to parse must include type hints (just Int1 for now). The resulting gate will be a gate in the size of the sum of all the parameters and the return.

The type Int1 means the classical function will only operate at bit level.

## ClassicalFunction compiler API#

### classical_function#

Decorator for a classical function that returns a ClassicalFunction object.

### ClassicalFunction#

 ClassicalFunction(source[, name]) Represent a classical function and its logic network. BooleanExpression(expression[, name, var_order]) The Boolean Expression gate.

### Exceptions#

 ClassicalFunctionCompilerTypeError(*message) ClassicalFunction compiler type error. ClassicalFunctionParseError(*message) ClassicalFunction compiler parse error. ClassicalFunctionCompilerTypeError(*message) ClassicalFunction compiler type error.