"""
Implementation of optimized einsum.

"""
from __future__ import division, absolute_import, print_function

from numpy.core.multiarray import c_einsum
from numpy.core.numeric import asarray, asanyarray, result_type

__all__ = ['einsum', 'einsum_path']

einsum_symbols = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
einsum_symbols_set = set(einsum_symbols)


def _compute_size_by_dict(indices, idx_dict):
    """
    Computes the product of the elements in indices based on the dictionary
    idx_dict.

    Parameters
    ----------
    indices : iterable
        Indices to base the product on.
    idx_dict : dictionary
        Dictionary of index sizes

    Returns
    -------
    ret : int
        The resulting product.

    Examples
    --------
    >>> _compute_size_by_dict('abbc', {'a': 2, 'b':3, 'c':5})
    90

    """
    ret = 1
    for i in indices:
        ret *= idx_dict[i]
    return ret


def _find_contraction(positions, input_sets, output_set):
    """
    Finds the contraction for a given set of input and output sets.

    Parameters
    ----------
    positions : iterable
        Integer positions of terms used in the contraction.
    input_sets : list
        List of sets that represent the lhs side of the einsum subscript
    output_set : set
        Set that represents the rhs side of the overall einsum subscript

    Returns
    -------
    new_result : set
        The indices of the resulting contraction
    remaining : list
        List of sets that have not been contracted, the new set is appended to
        the end of this list
    idx_removed : set
        Indices removed from the entire contraction
    idx_contraction : set
        The indices used in the current contraction

    Examples
    --------

    # A simple dot product test case
    >>> pos = (0, 1)
    >>> isets = [set('ab'), set('bc')]
    >>> oset = set('ac')
    >>> _find_contraction(pos, isets, oset)
    ({'a', 'c'}, [{'a', 'c'}], {'b'}, {'a', 'b', 'c'})

    # A more complex case with additional terms in the contraction
    >>> pos = (0, 2)
    >>> isets = [set('abd'), set('ac'), set('bdc')]
    >>> oset = set('ac')
    >>> _find_contraction(pos, isets, oset)
    ({'a', 'c'}, [{'a', 'c'}, {'a', 'c'}], {'b', 'd'}, {'a', 'b', 'c', 'd'})
    """

    idx_contract = set()
    idx_remain = output_set.copy()
    remaining = []
    for ind, value in enumerate(input_sets):
        if ind in positions:
            idx_contract |= value
        else:
            remaining.append(value)
            idx_remain |= value

    new_result = idx_remain & idx_contract
    idx_removed = (idx_contract - new_result)
    remaining.append(new_result)

    return (new_result, remaining, idx_removed, idx_contract)


def _optimal_path(input_sets, output_set, idx_dict, memory_limit):
    """
    Computes all possible pair contractions, sieves the results based
    on ``memory_limit`` and returns the lowest cost path. This algorithm
    scales factorial with respect to the elements in the list ``input_sets``.

    Parameters
    ----------
    input_sets : list
        List of sets that represent the lhs side of the einsum subscript
    output_set : set
        Set that represents the rhs side of the overall einsum subscript
    idx_dict : dictionary
        Dictionary of index sizes
    memory_limit : int
        The maximum number of elements in a temporary array

    Returns
    -------
    path : list
        The optimal contraction order within the memory limit constraint.

    Examples
    --------
    >>> isets = [set('abd'), set('ac'), set('bdc')]
    >>> oset = set('')
    >>> idx_sizes = {'a': 1, 'b':2, 'c':3, 'd':4}
    >>> _path__optimal_path(isets, oset, idx_sizes, 5000)
    [(0, 2), (0, 1)]
    """

    full_results = [(0, [], input_sets)]
    for iteration in range(len(input_sets) - 1):
        iter_results = []

        # Compute all unique pairs
        comb_iter = []
        for x in range(len(input_sets) - iteration):
            for y in range(x + 1, len(input_sets) - iteration):
                comb_iter.append((x, y))

        for curr in full_results:
            cost, positions, remaining = curr
            for con in comb_iter:

                # Find the contraction
                cont = _find_contraction(con, remaining, output_set)
                new_result, new_input_sets, idx_removed, idx_contract = cont

                # Sieve the results based on memory_limit
                new_size = _compute_size_by_dict(new_result, idx_dict)
                if new_size > memory_limit:
                    continue

                # Find cost
                new_cost = _compute_size_by_dict(idx_contract, idx_dict)
                if idx_removed:
                    new_cost *= 2

                # Build (total_cost, positions, indices_remaining)
                new_cost += cost
                new_pos = positions + [con]
                iter_results.append((new_cost, new_pos, new_input_sets))

        # Update list to iterate over
        full_results = iter_results

    # If we have not found anything return single einsum contraction
    if len(full_results) == 0:
        return [tuple(range(len(input_sets)))]

    path = min(full_results, key=lambda x: x[0])[1]
    return path


def _greedy_path(input_sets, output_set, idx_dict, memory_limit):
    """
    Finds the path by contracting the best pair until the input list is
    exhausted. The best pair is found by minimizing the tuple
    ``(-prod(indices_removed), cost)``.  What this amounts to is prioritizing
    matrix multiplication or inner product operations, then Hadamard like
    operations, and finally outer operations. Outer products are limited by
    ``memory_limit``. This algorithm scales cubically with respect to the
    number of elements in the list ``input_sets``.

    Parameters
    ----------
    input_sets : list
        List of sets that represent the lhs side of the einsum subscript
    output_set : set
        Set that represents the rhs side of the overall einsum subscript
    idx_dict : dictionary
        Dictionary of index sizes
    memory_limit_limit : int
        The maximum number of elements in a temporary array

    Returns
    -------
    path : list
        The greedy contraction order within the memory limit constraint.

    Examples
    --------
    >>> isets = [set('abd'), set('ac'), set('bdc')]
    >>> oset = set('')
    >>> idx_sizes = {'a': 1, 'b':2, 'c':3, 'd':4}
    >>> _path__greedy_path(isets, oset, idx_sizes, 5000)
    [(0, 2), (0, 1)]
    """

    if len(input_sets) == 1:
        return [(0,)]

    path = []
    for iteration in range(len(input_sets) - 1):
        iteration_results = []
        comb_iter = []

        # Compute all unique pairs
        for x in range(len(input_sets)):
            for y in range(x + 1, len(input_sets)):
                comb_iter.append((x, y))

        for positions in comb_iter:

            # Find the contraction
            contract = _find_contraction(positions, input_sets, output_set)
            idx_result, new_input_sets, idx_removed, idx_contract = contract

            # Sieve the results based on memory_limit
            if _compute_size_by_dict(idx_result, idx_dict) > memory_limit:
                continue

            # Build sort tuple
            removed_size = _compute_size_by_dict(idx_removed, idx_dict)
            cost = _compute_size_by_dict(idx_contract, idx_dict)
            sort = (-removed_size, cost)

            # Add contraction to possible choices
            iteration_results.append([sort, positions, new_input_sets])

        # If we did not find a new contraction contract remaining
        if len(iteration_results) == 0:
            path.append(tuple(range(len(input_sets))))
            break

        # Sort based on first index
        best = min(iteration_results, key=lambda x: x[0])
        path.append(best[1])
        input_sets = best[2]

    return path


def _parse_einsum_input(operands):
    """
    A reproduction of einsum c side einsum parsing in python.

    Returns
    -------
    input_strings : str
        Parsed input strings
    output_string : str
        Parsed output string
    operands : list of array_like
        The operands to use in the numpy contraction

    Examples
    --------
    The operand list is simplified to reduce printing:

    >>> a = np.random.rand(4, 4)
    >>> b = np.random.rand(4, 4, 4)
    >>> __parse_einsum_input(('...a,...a->...', a, b))
    ('za,xza', 'xz', [a, b])

    >>> __parse_einsum_input((a, [Ellipsis, 0], b, [Ellipsis, 0]))
    ('za,xza', 'xz', [a, b])
    """

    if len(operands) == 0:
        raise ValueError("No input operands")

    if isinstance(operands[0], str):
        subscripts = operands[0].replace(" ", "")
        operands = [asanyarray(v) for v in operands[1:]]

        # Ensure all characters are valid
        for s in subscripts:
            if s in '.,->':
                continue
            if s not in einsum_symbols:
                raise ValueError("Character %s is not a valid symbol." % s)

    else:
        tmp_operands = list(operands)
        operand_list = []
        subscript_list = []
        for p in range(len(operands) // 2):
            operand_list.append(tmp_operands.pop(0))
            subscript_list.append(tmp_operands.pop(0))

        output_list = tmp_operands[-1] if len(tmp_operands) else None
        operands = [asanyarray(v) for v in operand_list]
        subscripts = ""
        last = len(subscript_list) - 1
        for num, sub in enumerate(subscript_list):
            for s in sub:
                if s is Ellipsis:
                    subscripts += "..."
                elif isinstance(s, int):
                    subscripts += einsum_symbols[s]
                else:
                    raise TypeError("For this input type lists must contain "
                                    "either int or Ellipsis")
            if num != last:
                subscripts += ","

        if output_list is not None:
            subscripts += "->"
            for s in output_list:
                if s is Ellipsis:
                    subscripts += "..."
                elif isinstance(s, int):
                    subscripts += einsum_symbols[s]
                else:
                    raise TypeError("For this input type lists must contain "
                                    "either int or Ellipsis")
    # Check for proper "->"
    if ("-" in subscripts) or (">" in subscripts):
        invalid = (subscripts.count("-") > 1) or (subscripts.count(">") > 1)
        if invalid or (subscripts.count("->") != 1):
            raise ValueError("Subscripts can only contain one '->'.")

    # Parse ellipses
    if "." in subscripts:
        used = subscripts.replace(".", "").replace(",", "").replace("->", "")
        unused = list(einsum_symbols_set - set(used))
        ellipse_inds = "".join(unused)
        longest = 0

        if "->" in subscripts:
            input_tmp, output_sub = subscripts.split("->")
            split_subscripts = input_tmp.split(",")
            out_sub = True
        else:
            split_subscripts = subscripts.split(',')
            out_sub = False

        for num, sub in enumerate(split_subscripts):
            if "." in sub:
                if (sub.count(".") != 3) or (sub.count("...") != 1):
                    raise ValueError("Invalid Ellipses.")

                # Take into account numerical values
                if operands[num].shape == ():
                    ellipse_count = 0
                else:
                    ellipse_count = max(operands[num].ndim, 1)
                    ellipse_count -= (len(sub) - 3)

                if ellipse_count > longest:
                    longest = ellipse_count

                if ellipse_count < 0:
                    raise ValueError("Ellipses lengths do not match.")
                elif ellipse_count == 0:
                    split_subscripts[num] = sub.replace('...', '')
                else:
                    rep_inds = ellipse_inds[-ellipse_count:]
                    split_subscripts[num] = sub.replace('...', rep_inds)

        subscripts = ",".join(split_subscripts)
        if longest == 0:
            out_ellipse = ""
        else:
            out_ellipse = ellipse_inds[-longest:]

        if out_sub:
            subscripts += "->" + output_sub.replace("...", out_ellipse)
        else:
            # Special care for outputless ellipses
            output_subscript = ""
            tmp_subscripts = subscripts.replace(",", "")
            for s in sorted(set(tmp_subscripts)):
                if s not in (einsum_symbols):
                    raise ValueError("Character %s is not a valid symbol." % s)
                if tmp_subscripts.count(s) == 1:
                    output_subscript += s
            normal_inds = ''.join(sorted(set(output_subscript) -
                                         set(out_ellipse)))

            subscripts += "->" + out_ellipse + normal_inds

    # Build output string if does not exist
    if "->" in subscripts:
        input_subscripts, output_subscript = subscripts.split("->")
    else:
        input_subscripts = subscripts
        # Build output subscripts
        tmp_subscripts = subscripts.replace(",", "")
        output_subscript = ""
        for s in sorted(set(tmp_subscripts)):
            if s not in einsum_symbols:
                raise ValueError("Character %s is not a valid symbol." % s)
            if tmp_subscripts.count(s) == 1:
                output_subscript += s

    # Make sure output subscripts are in the input
    for char in output_subscript:
        if char not in input_subscripts:
            raise ValueError("Output character %s did not appear in the input"
                             % char)

    # Make sure number operands is equivalent to the number of terms
    if len(input_subscripts.split(',')) != len(operands):
        raise ValueError("Number of einsum subscripts must be equal to the "
                         "number of operands.")

    return (input_subscripts, output_subscript, operands)


def einsum_path(*operands, **kwargs):
    """
    einsum_path(subscripts, *operands, optimize='greedy')

    Evaluates the lowest cost contraction order for an einsum expression by
    considering the creation of intermediate arrays.

    Parameters
    ----------
    subscripts : str
        Specifies the subscripts for summation.
    *operands : list of array_like
        These are the arrays for the operation.
    optimize : {bool, list, tuple, 'greedy', 'optimal'}
        Choose the type of path. If a tuple is provided, the second argument is
        assumed to be the maximum intermediate size created. If only a single
        argument is provided the largest input or output array size is used
        as a maximum intermediate size.

        * if a list is given that starts with ``einsum_path``, uses this as the
          contraction path
        * if False no optimization is taken
        * if True defaults to the 'greedy' algorithm
        * 'optimal' An algorithm that combinatorially explores all possible
          ways of contracting the listed tensors and choosest the least costly
          path. Scales exponentially with the number of terms in the
          contraction.
        * 'greedy' An algorithm that chooses the best pair contraction
          at each step. Effectively, this algorithm searches the largest inner,
          Hadamard, and then outer products at each step. Scales cubically with
          the number of terms in the contraction. Equivalent to the 'optimal'
          path for most contractions.

        Default is 'greedy'.

    Returns
    -------
    path : list of tuples
        A list representation of the einsum path.
    string_repr : str
        A printable representation of the einsum path.

    Notes
    -----
    The resulting path indicates which terms of the input contraction should be
    contracted first, the result of this contraction is then appended to the
    end of the contraction list. This list can then be iterated over until all
    intermediate contractions are complete.

    See Also
    --------
    einsum, linalg.multi_dot

    Examples
    --------

    We can begin with a chain dot example. In this case, it is optimal to
    contract the ``b`` and ``c`` tensors first as reprsented by the first
    element of the path ``(1, 2)``. The resulting tensor is added to the end
    of the contraction and the remaining contraction ``(0, 1)`` is then
    completed.

    >>> a = np.random.rand(2, 2)
    >>> b = np.random.rand(2, 5)
    >>> c = np.random.rand(5, 2)
    >>> path_info = np.einsum_path('ij,jk,kl->il', a, b, c, optimize='greedy')
    >>> print(path_info[0])
    ['einsum_path', (1, 2), (0, 1)]
    >>> print(path_info[1])
      Complete contraction:  ij,jk,kl->il
             Naive scaling:  4
         Optimized scaling:  3
          Naive FLOP count:  1.600e+02
      Optimized FLOP count:  5.600e+01
       Theoretical speedup:  2.857
      Largest intermediate:  4.000e+00 elements
    -------------------------------------------------------------------------
    scaling                  current                                remaining
    -------------------------------------------------------------------------
       3                   kl,jk->jl                                ij,jl->il
       3                   jl,ij->il                                   il->il


    A more complex index transformation example.

    >>> I = np.random.rand(10, 10, 10, 10)
    >>> C = np.random.rand(10, 10)
    >>> path_info = np.einsum_path('ea,fb,abcd,gc,hd->efgh', C, C, I, C, C,
                                   optimize='greedy')

    >>> print(path_info[0])
    ['einsum_path', (0, 2), (0, 3), (0, 2), (0, 1)]
    >>> print(path_info[1])
      Complete contraction:  ea,fb,abcd,gc,hd->efgh
             Naive scaling:  8
         Optimized scaling:  5
          Naive FLOP count:  8.000e+08
      Optimized FLOP count:  8.000e+05
       Theoretical speedup:  1000.000
      Largest intermediate:  1.000e+04 elements
    --------------------------------------------------------------------------
    scaling                  current                                remaining
    --------------------------------------------------------------------------
       5               abcd,ea->bcde                      fb,gc,hd,bcde->efgh
       5               bcde,fb->cdef                         gc,hd,cdef->efgh
       5               cdef,gc->defg                            hd,defg->efgh
       5               defg,hd->efgh                               efgh->efgh
    """

    # Make sure all keywords are valid
    valid_contract_kwargs = ['optimize', 'einsum_call']
    unknown_kwargs = [k for (k, v) in kwargs.items() if k
                      not in valid_contract_kwargs]
    if len(unknown_kwargs):
        raise TypeError("Did not understand the following kwargs:"
                        " %s" % unknown_kwargs)

    # Figure out what the path really is
    path_type = kwargs.pop('optimize', False)
    if path_type is True:
        path_type = 'greedy'
    if path_type is None:
        path_type = False

    memory_limit = None

    # No optimization or a named path algorithm
    if (path_type is False) or isinstance(path_type, str):
        pass

    # Given an explicit path
    elif len(path_type) and (path_type[0] == 'einsum_path'):
        pass

    # Path tuple with memory limit
    elif ((len(path_type) == 2) and isinstance(path_type[0], str) and
            isinstance(path_type[1], (int, float))):
        memory_limit = int(path_type[1])
        path_type = path_type[0]

    else:
        raise TypeError("Did not understand the path: %s" % str(path_type))

    # Hidden option, only einsum should call this
    einsum_call_arg = kwargs.pop("einsum_call", False)

    # Python side parsing
    input_subscripts, output_subscript, operands = _parse_einsum_input(operands)
    subscripts = input_subscripts + '->' + output_subscript

    # Build a few useful list and sets
    input_list = input_subscripts.split(',')
    input_sets = [set(x) for x in input_list]
    output_set = set(output_subscript)
    indices = set(input_subscripts.replace(',', ''))

    # Get length of each unique dimension and ensure all dimensions are correct
    dimension_dict = {}
    for tnum, term in enumerate(input_list):
        sh = operands[tnum].shape
        if len(sh) != len(term):
            raise ValueError("Einstein sum subscript %s does not contain the "
                             "correct number of indices for operand %d.",
                             input_subscripts[tnum], tnum)
        for cnum, char in enumerate(term):
            dim = sh[cnum]
            if char in dimension_dict.keys():
                if dimension_dict[char] != dim:
                    raise ValueError("Size of label '%s' for operand %d does "
                                     "not match previous terms.", char, tnum)
            else:
                dimension_dict[char] = dim

    # Compute size of each input array plus the output array
    size_list = []
    for term in input_list + [output_subscript]:
        size_list.append(_compute_size_by_dict(term, dimension_dict))
    max_size = max(size_list)

    if memory_limit is None:
        memory_arg = max_size
    else:
        memory_arg = memory_limit

    # Compute naive cost
    # This isnt quite right, need to look into exactly how einsum does this
    naive_cost = _compute_size_by_dict(indices, dimension_dict)
    indices_in_input = input_subscripts.replace(',', '')
    mult = max(len(input_list) - 1, 1)
    if (len(indices_in_input) - len(set(indices_in_input))):
        mult *= 2
    naive_cost *= mult

    # Compute the path
    if (path_type is False) or (len(input_list) in [1, 2]) or (indices == output_set):
        # Nothing to be optimized, leave it to einsum
        path = [tuple(range(len(input_list)))]
    elif path_type == "greedy":
        # Maximum memory should be at most out_size for this algorithm
        memory_arg = min(memory_arg, max_size)
        path = _greedy_path(input_sets, output_set, dimension_dict, memory_arg)
    elif path_type == "optimal":
        path = _optimal_path(input_sets, output_set, dimension_dict, memory_arg)
    elif path_type[0] == 'einsum_path':
        path = path_type[1:]
    else:
        raise KeyError("Path name %s not found", path_type)

    cost_list, scale_list, size_list, contraction_list = [], [], [], []

    # Build contraction tuple (positions, gemm, einsum_str, remaining)
    for cnum, contract_inds in enumerate(path):
        # Make sure we remove inds from right to left
        contract_inds = tuple(sorted(list(contract_inds), reverse=True))

        contract = _find_contraction(contract_inds, input_sets, output_set)
        out_inds, input_sets, idx_removed, idx_contract = contract

        cost = _compute_size_by_dict(idx_contract, dimension_dict)
        if idx_removed:
            cost *= 2
        cost_list.append(cost)
        scale_list.append(len(idx_contract))
        size_list.append(_compute_size_by_dict(out_inds, dimension_dict))

        tmp_inputs = []
        for x in contract_inds:
            tmp_inputs.append(input_list.pop(x))

        # Last contraction
        if (cnum - len(path)) == -1:
            idx_result = output_subscript
        else:
            sort_result = [(dimension_dict[ind], ind) for ind in out_inds]
            idx_result = "".join([x[1] for x in sorted(sort_result)])

        input_list.append(idx_result)
        einsum_str = ",".join(tmp_inputs) + "->" + idx_result

        contraction = (contract_inds, idx_removed, einsum_str, input_list[:])
        contraction_list.append(contraction)

    opt_cost = sum(cost_list) + 1

    if einsum_call_arg:
        return (operands, contraction_list)

    # Return the path along with a nice string representation
    overall_contraction = input_subscripts + "->" + output_subscript
    header = ("scaling", "current", "remaining")

    speedup = naive_cost / opt_cost
    max_i = max(size_list)

    path_print  = "  Complete contraction:  %s\n" % overall_contraction
    path_print += "         Naive scaling:  %d\n" % len(indices)
    path_print += "     Optimized scaling:  %d\n" % max(scale_list)
    path_print += "      Naive FLOP count:  %.3e\n" % naive_cost
    path_print += "  Optimized FLOP count:  %.3e\n" % opt_cost
    path_print += "   Theoretical speedup:  %3.3f\n" % speedup
    path_print += "  Largest intermediate:  %.3e elements\n" % max_i
    path_print += "-" * 74 + "\n"
    path_print += "%6s %24s %40s\n" % header
    path_print += "-" * 74

    for n, contraction in enumerate(contraction_list):
        inds, idx_rm, einsum_str, remaining = contraction
        remaining_str = ",".join(remaining) + "->" + output_subscript
        path_run = (scale_list[n], einsum_str, remaining_str)
        path_print += "\n%4d    %24s %40s" % path_run

    path = ['einsum_path'] + path
    return (path, path_print)


# Rewrite einsum to handle different cases
def einsum(*operands, **kwargs):
    """
    einsum(subscripts, *operands, out=None, dtype=None, order='K',
           casting='safe', optimize=False)

    Evaluates the Einstein summation convention on the operands.

    Using the Einstein summation convention, many common multi-dimensional
    array operations can be represented in a simple fashion.  This function
    provides a way to compute such summations. The best way to understand this
    function is to try the examples below, which show how many common NumPy
    functions can be implemented as calls to `einsum`.

    Parameters
    ----------
    subscripts : str
        Specifies the subscripts for summation.
    operands : list of array_like
        These are the arrays for the operation.
    out : {ndarray, None}, optional
        If provided, the calculation is done into this array.
    dtype : {data-type, None}, optional
        If provided, forces the calculation to use the data type specified.
        Note that you may have to also give a more liberal `casting`
        parameter to allow the conversions. Default is None.
    order : {'C', 'F', 'A', 'K'}, optional
        Controls the memory layout of the output. 'C' means it should
        be C contiguous. 'F' means it should be Fortran contiguous,
        'A' means it should be 'F' if the inputs are all 'F', 'C' otherwise.
        'K' means it should be as close to the layout as the inputs as
        is possible, including arbitrarily permuted axes.
        Default is 'K'.
    casting : {'no', 'equiv', 'safe', 'same_kind', 'unsafe'}, optional
        Controls what kind of data casting may occur.  Setting this to
        'unsafe' is not recommended, as it can adversely affect accumulations.

          * 'no' means the data types should not be cast at all.
          * 'equiv' means only byte-order changes are allowed.
          * 'safe' means only casts which can preserve values are allowed.
          * 'same_kind' means only safe casts or casts within a kind,
            like float64 to float32, are allowed.
          * 'unsafe' means any data conversions may be done.

        Default is 'safe'.
    optimize : {False, True, 'greedy', 'optimal'}, optional
        Controls if intermediate optimization should occur. No optimization
        will occur if False and True will default to the 'greedy' algorithm.
        Also accepts an explicit contraction list from the ``np.einsum_path``
        function. See ``np.einsum_path`` for more details. Default is False.

    Returns
    -------
    output : ndarray
        The calculation based on the Einstein summation convention.

    See Also
    --------
    einsum_path, dot, inner, outer, tensordot, linalg.multi_dot

    Notes
    -----
    .. versionadded:: 1.6.0

    The subscripts string is a comma-separated list of subscript labels,
    where each label refers to a dimension of the corresponding operand.
    Repeated subscripts labels in one operand take the diagonal.  For example,
    ``np.einsum('ii', a)`` is equivalent to ``np.trace(a)``.

    Whenever a label is repeated, it is summed, so ``np.einsum('i,i', a, b)``
    is equivalent to ``np.inner(a,b)``.  If a label appears only once,
    it is not summed, so ``np.einsum('i', a)`` produces a view of ``a``
    with no changes.

    The order of labels in the output is by default alphabetical.  This
    means that ``np.einsum('ij', a)`` doesn't affect a 2D array, while
    ``np.einsum('ji', a)`` takes its transpose.

    The output can be controlled by specifying output subscript labels
    as well.  This specifies the label order, and allows summing to
    be disallowed or forced when desired.  The call ``np.einsum('i->', a)``
    is like ``np.sum(a, axis=-1)``, and ``np.einsum('ii->i', a)``
    is like ``np.diag(a)``.  The difference is that `einsum` does not
    allow broadcasting by default.

    To enable and control broadcasting, use an ellipsis.  Default
    NumPy-style broadcasting is done by adding an ellipsis
    to the left of each term, like ``np.einsum('...ii->...i', a)``.
    To take the trace along the first and last axes,
    you can do ``np.einsum('i...i', a)``, or to do a matrix-matrix
    product with the left-most indices instead of rightmost, you can do
    ``np.einsum('ij...,jk...->ik...', a, b)``.

    When there is only one operand, no axes are summed, and no output
    parameter is provided, a view into the operand is returned instead
    of a new array.  Thus, taking the diagonal as ``np.einsum('ii->i', a)``
    produces a view.

    An alternative way to provide the subscripts and operands is as
    ``einsum(op0, sublist0, op1, sublist1, ..., [sublistout])``. The examples
    below have corresponding `einsum` calls with the two parameter methods.

    .. versionadded:: 1.10.0

    Views returned from einsum are now writeable whenever the input array
    is writeable. For example, ``np.einsum('ijk...->kji...', a)`` will now
    have the same effect as ``np.swapaxes(a, 0, 2)`` and
    ``np.einsum('ii->i', a)`` will return a writeable view of the diagonal
    of a 2D array.

    .. versionadded:: 1.12.0

    Added the ``optimize`` argument which will optimize the contraction order
    of an einsum expression. For a contraction with three or more operands this
    can greatly increase the computational efficiency at the cost of a larger
    memory footprint during computation.

    See ``np.einsum_path`` for more details.

    Examples
    --------
    >>> a = np.arange(25).reshape(5,5)
    >>> b = np.arange(5)
    >>> c = np.arange(6).reshape(2,3)

    >>> np.einsum('ii', a)
    60
    >>> np.einsum(a, [0,0])
    60
    >>> np.trace(a)
    60

    >>> np.einsum('ii->i', a)
    array([ 0,  6, 12, 18, 24])
    >>> np.einsum(a, [0,0], [0])
    array([ 0,  6, 12, 18, 24])
    >>> np.diag(a)
    array([ 0,  6, 12, 18, 24])

    >>> np.einsum('ij,j', a, b)
    array([ 30,  80, 130, 180, 230])
    >>> np.einsum(a, [0,1], b, [1])
    array([ 30,  80, 130, 180, 230])
    >>> np.dot(a, b)
    array([ 30,  80, 130, 180, 230])
    >>> np.einsum('...j,j', a, b)
    array([ 30,  80, 130, 180, 230])

    >>> np.einsum('ji', c)
    array([[0, 3],
           [1, 4],
           [2, 5]])
    >>> np.einsum(c, [1,0])
    array([[0, 3],
           [1, 4],
           [2, 5]])
    >>> c.T
    array([[0, 3],
           [1, 4],
           [2, 5]])

    >>> np.einsum('..., ...', 3, c)
    array([[ 0,  3,  6],
           [ 9, 12, 15]])
    >>> np.einsum(',ij', 3, C)
    array([[ 0,  3,  6],
           [ 9, 12, 15]])
    >>> np.einsum(3, [Ellipsis], c, [Ellipsis])
    array([[ 0,  3,  6],
           [ 9, 12, 15]])
    >>> np.multiply(3, c)
    array([[ 0,  3,  6],
           [ 9, 12, 15]])

    >>> np.einsum('i,i', b, b)
    30
    >>> np.einsum(b, [0], b, [0])
    30
    >>> np.inner(b,b)
    30

    >>> np.einsum('i,j', np.arange(2)+1, b)
    array([[0, 1, 2, 3, 4],
           [0, 2, 4, 6, 8]])
    >>> np.einsum(np.arange(2)+1, [0], b, [1])
    array([[0, 1, 2, 3, 4],
           [0, 2, 4, 6, 8]])
    >>> np.outer(np.arange(2)+1, b)
    array([[0, 1, 2, 3, 4],
           [0, 2, 4, 6, 8]])

    >>> np.einsum('i...->...', a)
    array([50, 55, 60, 65, 70])
    >>> np.einsum(a, [0,Ellipsis], [Ellipsis])
    array([50, 55, 60, 65, 70])
    >>> np.sum(a, axis=0)
    array([50, 55, 60, 65, 70])

    >>> a = np.arange(60.).reshape(3,4,5)
    >>> b = np.arange(24.).reshape(4,3,2)
    >>> np.einsum('ijk,jil->kl', a, b)
    array([[ 4400.,  4730.],
           [ 4532.,  4874.],
           [ 4664.,  5018.],
           [ 4796.,  5162.],
           [ 4928.,  5306.]])
    >>> np.einsum(a, [0,1,2], b, [1,0,3], [2,3])
    array([[ 4400.,  4730.],
           [ 4532.,  4874.],
           [ 4664.,  5018.],
           [ 4796.,  5162.],
           [ 4928.,  5306.]])
    >>> np.tensordot(a,b, axes=([1,0],[0,1]))
    array([[ 4400.,  4730.],
           [ 4532.,  4874.],
           [ 4664.,  5018.],
           [ 4796.,  5162.],
           [ 4928.,  5306.]])

    >>> a = np.arange(6).reshape((3,2))
    >>> b = np.arange(12).reshape((4,3))
    >>> np.einsum('ki,jk->ij', a, b)
    array([[10, 28, 46, 64],
           [13, 40, 67, 94]])
    >>> np.einsum('ki,...k->i...', a, b)
    array([[10, 28, 46, 64],
           [13, 40, 67, 94]])
    >>> np.einsum('k...,jk', a, b)
    array([[10, 28, 46, 64],
           [13, 40, 67, 94]])

    >>> # since version 1.10.0
    >>> a = np.zeros((3, 3))
    >>> np.einsum('ii->i', a)[:] = 1
    >>> a
    array([[ 1.,  0.,  0.],
           [ 0.,  1.,  0.],
           [ 0.,  0.,  1.]])

    """

    # Grab non-einsum kwargs
    optimize_arg = kwargs.pop('optimize', False)

    # If no optimization, run pure einsum
    if optimize_arg is False:
        return c_einsum(*operands, **kwargs)

    valid_einsum_kwargs = ['out', 'dtype', 'order', 'casting']
    einsum_kwargs = {k: v for (k, v) in kwargs.items() if
                     k in valid_einsum_kwargs}

    # Make sure all keywords are valid
    valid_contract_kwargs = ['optimize'] + valid_einsum_kwargs
    unknown_kwargs = [k for (k, v) in kwargs.items() if
                      k not in valid_contract_kwargs]

    if len(unknown_kwargs):
        raise TypeError("Did not understand the following kwargs: %s"
                        % unknown_kwargs)

    # Special handeling if out is specified
    specified_out = False
    out_array = einsum_kwargs.pop('out', None)
    if out_array is not None:
        specified_out = True

    # Build the contraction list and operand
    operands, contraction_list = einsum_path(*operands, optimize=optimize_arg,
                                             einsum_call=True)
    # Start contraction loop
    for num, contraction in enumerate(contraction_list):
        inds, idx_rm, einsum_str, remaining = contraction
        tmp_operands = []
        for x in inds:
            tmp_operands.append(operands.pop(x))

        # If out was specified
        if specified_out and ((num + 1) == len(contraction_list)):
            einsum_kwargs["out"] = out_array

        # Do the contraction
        new_view = c_einsum(einsum_str, *tmp_operands, **einsum_kwargs)

        # Append new items and derefernce what we can
        operands.append(new_view)
        del tmp_operands, new_view

    if specified_out:
        return out_array
    else:
        return operands[0]
