def get_adjacency_matrix(df_connected):
    '''
    Return matrix where $a_{i,j} = 1$ indicates polygon $i$ is connected to
    polygon $j$.

    Also, return mapping (and reverse mapping) from original keys in
    `df_connected` to zero-based integer index used for matrix rows and
    columns.
    '''
    sorted_path_keys = np.sort(np.unique(df_connected[['source', 'target']]
                                         .values.ravel()))
    indexed_paths = pd.Series(sorted_path_keys)
    path_indexes = pd.Series(indexed_paths.index, index=sorted_path_keys)

    adjacency_matrix = np.zeros((path_indexes.shape[0], ) * 2, dtype=int)
    for i_key, j_key in df_connected[['source', 'target']].values:
        i, j = path_indexes.loc[[i_key, j_key]]
        adjacency_matrix[i, j] = 1
        adjacency_matrix[j, i] = 1
    return adjacency_matrix, indexed_paths, path_indexes