"""
Generators for interval graph.
"""
from collections.abc import Sequence
import networkx as nx

__all__ = ["interval_graph"]


def interval_graph(intervals):
    """ Generates an interval graph for a list of intervals given.

    In graph theory, an interval graph is an undirected graph formed from a set
    of closed intervals on the real line, with a vertex for each interval
    and an edge between vertices whose intervals intersect.
    It is the intersection graph of the intervals.

    More information can be found at:
    https://en.wikipedia.org/wiki/Interval_graph

    Parameters
    ----------
    intervals : a sequence of intervals, say (l, r) where l is the left end,
    and r is the right end of the closed interval.

    Returns
    -------
    G : networkx graph

    Examples
    --------
    >>> intervals = [(-2, 3), [1, 4], (2, 3), (4, 6)]
    >>> G = nx.interval_graph(intervals)
    >>> sorted(G.edges)
    [((-2, 3), (1, 4)), ((-2, 3), (2, 3)), ((1, 4), (2, 3)), ((1, 4), (4, 6))]

    Raises
    --------
    :exc:`TypeError`
        if `intervals` contains None or an element which is not
        collections.abc.Sequence or not a length of 2.
    :exc:`ValueError`
        if `intervals` contains an interval such that min1 > max1
        where min1,max1 = interval
    """
    intervals = list(intervals)
    for interval in intervals:
        if not (isinstance(interval, Sequence) and len(interval) == 2):
            raise TypeError(
                "Each interval must have length 2, and be a "
                "collections.abc.Sequence such as tuple or list."
            )
        if interval[0] > interval[1]:
            raise ValueError(
                f"Interval must have lower value first. " f"Got {interval}"
            )

    graph = nx.Graph()

    tupled_intervals = [tuple(interval) for interval in intervals]
    graph.add_nodes_from(tupled_intervals)

    while tupled_intervals:
        min1, max1 = interval1 = tupled_intervals.pop()
        for interval2 in tupled_intervals:
            min2, max2 = interval2
            if max1 >= min2 and max2 >= min1:
                graph.add_edge(interval1, interval2)
    return graph