# Source code for qutip.topology

    __all__ = ['berry_curvature', 'plot_berry_curvature']

    from qutip import (Qobj, tensor, basis, qeye, isherm, sigmax, sigmay, sigmaz)
    import numpy as np

    try:
        import matplotlib.pyplot as plt
    except:
        pass

    def berry_curvature(eigfs):
        """Computes the discretized Berry curvature on the two dimensional grid
        of parameters. The function works well for cases with no band mixing.

        Parameters
        ==========
        eigfs : numpy ndarray
            4 dimensional numpy ndarray where the first two indices are for the two
            discrete values of the two parameters and the third is the index of the
            occupied bands. The fourth dimension holds the eigenfunctions.

        Returns
        -------
        b_curv : numpy ndarray
            A two dimensional array of the discretized Berry curvature defined for
            the values of the two parameters defined in the eigfs.
        """
        nparam0 = eigfs.shape[0]
        nparam1 = eigfs.shape[1]
        nocc = eigfs.shape[2]
        b_curv = np.zeros((nparam0-1, nparam1-1), dtype=float)

        for i in range(nparam0-1):
            for j in range(nparam1-1):
                rect_prd = np.identity(nocc, dtype=complex)
                innP0 = np.zeros([nocc, nocc], dtype=complex)
                innP1 = np.zeros([nocc, nocc], dtype=complex)
                innP2 = np.zeros([nocc, nocc], dtype=complex)
                innP3 = np.zeros([nocc, nocc], dtype=complex)

                for k in range(nocc):
                    for l in range(nocc):
                        wf0 = eigfs[i, j, k, :]
                        wf1 = eigfs[i+1, j, l, :]
                        innP0[k, l] = np.dot(wf0.conjugate(), wf1)

                        wf1 = eigfs[i+1, j, k, :]
                        wf2 = eigfs[i+1, j+1, l, :]
                        innP1[k, l] = np.dot(wf1.conjugate(), wf2)

                        wf2 = eigfs[i+1, j+1, k, :]
                        wf3 = eigfs[i, j+1, l, :]
                        innP2[k, l] = np.dot(wf2.conjugate(), wf3)

                        wf3 = eigfs[i, j+1, k, :]
                        wf0 = eigfs[i, j, l, :]
                        innP3[k, l] = np.dot(wf3.conjugate(), wf0)

                rect_prd = np.dot(rect_prd, innP0)
                rect_prd = np.dot(rect_prd, innP1)
                rect_prd = np.dot(rect_prd, innP2)
                rect_prd = np.dot(rect_prd, innP3)

                dett = np.linalg.det(rect_prd)
                curl_z = np.angle(dett)
                b_curv[i, j] = curl_z

        return b_curv

    def plot_berry_curvature(eigfs):
        """Plots the discretized Berry curvature on the two dimensional grid
        of parameters. The function works well for cases with no band mixing."""
        b_curv = berry_curvature(eigfs)
        fig, ax = plt.subplots()
        ax.imshow(b_curv, origin="lower")
        ax.set_title("Berry curvature")
        ax.set_xlabel(r"$Parameter0$")
        ax.set_ylabel(r"$Parameter1$")
        fig.tight_layout()
        fig.savefig("berry_curvature.pdf")