"""Neighbor sampling APIs"""

from .._ffi.function import _init_api
from .. import backend as F
from ..base import DGLError, EID
from ..heterograph import DGLHeteroGraph
from .. import ndarray as nd
from .. import utils

__all__ = [
    'sample_neighbors',
    'select_topk']

def sample_neighbors(g, nodes, fanout, edge_dir='in', prob=None, replace=False):
    """Sample neighboring edges of the given nodes and return the induced subgraph.

    For each node, a number of inbound (or outbound when ``edge_dir == 'out'``) edges
    will be randomly chosen.  The graph returned will then contain all the nodes in the
    original graph, but only the sampled edges.

    Node/edge features are not preserved. The original IDs of
    the sampled edges are stored as the `dgl.EID` feature in the returned graph.

    Parameters
    ----------
    g : DGLGraph
        The graph.  Must be on CPU.
    nodes : tensor or dict
        Node IDs to sample neighbors from.

        This argument can take a single ID tensor or a dictionary of node types and ID tensors.
        If a single tensor is given, the graph must only have one type of nodes.
    fanout : int or dict[etype, int]
        The number of edges to be sampled for each node on each edge type.

        This argument can take a single int or a dictionary of edge types and ints.
        If a single int is given, DGL will sample this number of edges for each node for
        every edge type.

        If -1 is given for a single edge type, all the neighboring edges with that edge
        type will be selected.
    edge_dir : str, optional
        Determines whether to sample inbound or outbound edges.

        Can take either ``in`` for inbound edges or ``out`` for outbound edges.
    prob : str, optional
        Feature name used as the (unnormalized) probabilities associated with each
        neighboring edge of a node.  The feature must have only one element for each
        edge.

        The features must be non-negative floats, and the sum of the features of
        inbound/outbound edges for every node must be positive (though they don't have
        to sum up to one).  Otherwise, the result will be undefined.
    replace : bool, optional
        If True, sample with replacement.

    Returns
    -------
    DGLGraph
        A sampled subgraph containing only the sampled neighboring edges.  It is on CPU.

    Examples
    --------
    Assume that you have the following graph

    >>> g = dgl.graph(([0, 0, 1, 1, 2, 2], [1, 2, 0, 1, 2, 0]))

    And the weights

    >>> g.edata['prob'] = torch.FloatTensor([0., 1., 0., 1., 0., 1.])

    To sample one inbound edge for node 0 and node 1:

    >>> sg = dgl.sampling.sample_neighbors(g, [0, 1], 1)
    >>> sg.edges(order='eid')
    (tensor([1, 0]), tensor([0, 1]))
    >>> sg.edata[dgl.EID]
    tensor([2, 0])

    To sample one inbound edge for node 0 and node 1 with probability in edge feature
    ``prob``:

    >>> sg = dgl.sampling.sample_neighbors(g, [0, 1], 1, prob='prob')
    >>> sg.edges(order='eid')
    (tensor([2, 1]), tensor([0, 1]))

    With ``fanout`` greater than the number of actual neighbors and without replacement,
    DGL will take all neighbors instead:

    >>> sg = dgl.sampling.sample_neighbors(g, [0, 1], 3)
    >>> sg.edges(order='eid')
    (tensor([1, 2, 0, 1]), tensor([0, 0, 1, 1]))
    """
    if not isinstance(nodes, dict):
        if len(g.ntypes) > 1:
            raise DGLError("Must specify node type when the graph is not homogeneous.")
        nodes = {g.ntypes[0] : nodes}
    assert g.device == F.cpu(), "Graph must be on CPU."

    nodes = utils.prepare_tensor_dict(g, nodes, 'nodes')
    nodes_all_types = []
    for ntype in g.ntypes:
        if ntype in nodes:
            nodes_all_types.append(F.to_dgl_nd(nodes[ntype]))
        else:
            nodes_all_types.append(nd.array([], ctx=nd.cpu()))

    if not isinstance(fanout, dict):
        fanout_array = [int(fanout)] * len(g.etypes)
    else:
        if len(fanout) != len(g.etypes):
            raise DGLError('Fan-out must be specified for each edge type '
                           'if a dict is provided.')
        fanout_array = [None] * len(g.etypes)
        for etype, value in fanout.items():
            fanout_array[g.get_etype_id(etype)] = value
    fanout_array = F.to_dgl_nd(F.tensor(fanout_array, dtype=F.int64))

    if prob is None:
        prob_arrays = [nd.array([], ctx=nd.cpu())] * len(g.etypes)
    else:
        prob_arrays = []
        for etype in g.canonical_etypes:
            if prob in g.edges[etype].data:
                prob_arrays.append(F.to_dgl_nd(g.edges[etype].data[prob]))
            else:
                prob_arrays.append(nd.array([], ctx=nd.cpu()))

    subgidx = _CAPI_DGLSampleNeighbors(g._graph, nodes_all_types, fanout_array,
                                       edge_dir, prob_arrays, replace)
    induced_edges = subgidx.induced_edges
    ret = DGLHeteroGraph(subgidx.graph, g.ntypes, g.etypes)
    for i, etype in enumerate(ret.canonical_etypes):
        ret.edges[etype].data[EID] = induced_edges[i]
    return ret

def select_topk(g, k, weight, nodes=None, edge_dir='in', ascending=False):
    """Select the neighboring edges with k-largest (or k-smallest) weights of the given
    nodes and return the induced subgraph.

    For each node, a number of inbound (or outbound when ``edge_dir == 'out'``) edges
    with the largest (or smallest when ``ascending == True``) weights will be chosen.
    The graph returned will then contain all the nodes in the original graph, but only
    the sampled edges.

    Node/edge features are not preserved. The original IDs of
    the sampled edges are stored as the `dgl.EID` feature in the returned graph.

    Parameters
    ----------
    g : DGLGraph
        The graph.  Must be on CPU.
    k : int or dict[etype, int]
        The number of edges to be selected for each node on each edge type.

        This argument can take a single int or a dictionary of edge types and ints.
        If a single int is given, DGL will select this number of edges for each node for
        every edge type.

        If -1 is given for a single edge type, all the neighboring edges with that edge
        type will be selected.
    weight : str
        Feature name of the weights associated with each edge.  The feature should have only
        one element for each edge.  The feature can be either int32/64 or float32/64.
    nodes : tensor or dict, optional
        Node IDs to sample neighbors from.

        This argument can take a single ID tensor or a dictionary of node types and ID tensors.
        If a single tensor is given, the graph must only have one type of nodes.

        If None, DGL will select the edges for all nodes.
    edge_dir : str, optional
        Determines whether to sample inbound or outbound edges.

        Can take either ``in`` for inbound edges or ``out`` for outbound edges.
    ascending : bool, optional
        If True, DGL will return edges with k-smallest weights instead of
        k-largest weights.

    Returns
    -------
    DGLGraph
        A sampled subgraph containing only the sampled neighboring edges.  It is on CPU.

    Examples
    --------
    >>> g = dgl.graph(([0, 0, 1, 1, 2, 2], [1, 2, 0, 1, 2, 0]))
    >>> g.edata['weight'] = torch.FloatTensor([0, 1, 0, 1, 0, 1])
    >>> sg = dgl.sampling.select_topk(g, 1, 'weight')
    >>> sg.edges(order='eid')
    (tensor([2, 1, 0]), tensor([0, 1, 2]))
    """
    # Rectify nodes to a dictionary
    if nodes is None:
        nodes = {ntype: F.arange(0, g.number_of_nodes(ntype)) for ntype in g.ntypes}
    elif not isinstance(nodes, dict):
        if len(g.ntypes) > 1:
            raise DGLError("Must specify node type when the graph is not homogeneous.")
        nodes = {g.ntypes[0] : nodes}
    assert g.device == F.cpu(), "Graph must be on CPU."

    # Parse nodes into a list of NDArrays.
    nodes = utils.prepare_tensor_dict(g, nodes, 'nodes')
    nodes_all_types = []
    for ntype in g.ntypes:
        if ntype in nodes:
            nodes_all_types.append(F.to_dgl_nd(nodes[ntype]))
        else:
            nodes_all_types.append(nd.array([], ctx=nd.cpu()))

    if not isinstance(k, dict):
        k_array = [int(k)] * len(g.etypes)
    else:
        if len(k) != len(g.etypes):
            raise DGLError('K value must be specified for each edge type '
                           'if a dict is provided.')
        k_array = [None] * len(g.etypes)
        for etype, value in k.items():
            k_array[g.get_etype_id(etype)] = value
    k_array = F.to_dgl_nd(F.tensor(k_array, dtype=F.int64))

    weight_arrays = []
    for etype in g.canonical_etypes:
        if weight in g.edges[etype].data:
            weight_arrays.append(F.to_dgl_nd(g.edges[etype].data[weight]))
        else:
            raise DGLError('Edge weights "{}" do not exist for relation graph "{}".'.format(
                weight, etype))

    subgidx = _CAPI_DGLSampleNeighborsTopk(
        g._graph, nodes_all_types, k_array, edge_dir, weight_arrays, bool(ascending))
    induced_edges = subgidx.induced_edges
    ret = DGLHeteroGraph(subgidx.graph, g.ntypes, g.etypes)
    for i, etype in enumerate(ret.canonical_etypes):
        ret.edges[etype].data[EID] = induced_edges[i]
    return ret

_init_api('dgl.sampling.neighbor', __name__)
