diff env/lib/python3.9/site-packages/networkx/algorithms/coloring/equitable_coloring.py @ 0:4f3585e2f14b draft default tip

"planemo upload commit 60cee0fc7c0cda8592644e1aad72851dec82c959"

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/env/lib/python3.9/site-packages/networkx/algorithms/coloring/equitable_coloring.py	Mon Mar 22 18:12:50 2021 +0000
@@ -0,0 +1,513 @@
+"""
+Equitable coloring of graphs with bounded degree.
+"""
+
+import networkx as nx
+from collections import defaultdict
+
+__all__ = ["equitable_color"]
+
+
+def is_coloring(G, coloring):
+    """Determine if the coloring is a valid coloring for the graph G."""
+    # Verify that the coloring is valid.
+    for (s, d) in G.edges:
+        if coloring[s] == coloring[d]:
+            return False
+    return True
+
+
+def is_equitable(G, coloring, num_colors=None):
+    """Determines if the coloring is valid and equitable for the graph G."""
+
+    if not is_coloring(G, coloring):
+        return False
+
+    # Verify whether it is equitable.
+    color_set_size = defaultdict(int)
+    for color in coloring.values():
+        color_set_size[color] += 1
+
+    if num_colors is not None:
+        for color in range(num_colors):
+            if color not in color_set_size:
+                # These colors do not have any vertices attached to them.
+                color_set_size[color] = 0
+
+    # If there are more than 2 distinct values, the coloring cannot be equitable
+    all_set_sizes = set(color_set_size.values())
+    if len(all_set_sizes) == 0 and num_colors is None:  # Was an empty graph
+        return True
+    elif len(all_set_sizes) == 1:
+        return True
+    elif len(all_set_sizes) == 2:
+        a, b = list(all_set_sizes)
+        return abs(a - b) <= 1
+    else:  # len(all_set_sizes) > 2:
+        return False
+
+
+def make_C_from_F(F):
+    C = defaultdict(lambda: [])
+    for node, color in F.items():
+        C[color].append(node)
+
+    return C
+
+
+def make_N_from_L_C(L, C):
+    nodes = L.keys()
+    colors = C.keys()
+    return {
+        (node, color): sum(1 for v in L[node] if v in C[color])
+        for node in nodes
+        for color in colors
+    }
+
+
+def make_H_from_C_N(C, N):
+    return {
+        (c1, c2): sum(1 for node in C[c1] if N[(node, c2)] == 0)
+        for c1 in C.keys()
+        for c2 in C.keys()
+    }
+
+
+def change_color(u, X, Y, N, H, F, C, L):
+    """Change the color of 'u' from X to Y and update N, H, F, C."""
+    assert F[u] == X and X != Y
+
+    # Change the class of 'u' from X to Y
+    F[u] = Y
+
+    for k in C.keys():
+        # 'u' witnesses an edge from k -> Y instead of from k -> X now.
+        if N[u, k] == 0:
+            H[(X, k)] -= 1
+            H[(Y, k)] += 1
+
+    for v in L[u]:
+        # 'v' has lost a neighbor in X and gained one in Y
+        N[(v, X)] -= 1
+        N[(v, Y)] += 1
+
+        if N[(v, X)] == 0:
+            # 'v' witnesses F[v] -> X
+            H[(F[v], X)] += 1
+
+        if N[(v, Y)] == 1:
+            # 'v' no longer witnesses F[v] -> Y
+            H[(F[v], Y)] -= 1
+
+    C[X].remove(u)
+    C[Y].append(u)
+
+
+def move_witnesses(src_color, dst_color, N, H, F, C, T_cal, L):
+    """Move witness along a path from src_color to dst_color."""
+    X = src_color
+    while X != dst_color:
+        Y = T_cal[X]
+        # Move _any_ witness from X to Y = T_cal[X]
+        w = [x for x in C[X] if N[(x, Y)] == 0][0]
+        change_color(w, X, Y, N=N, H=H, F=F, C=C, L=L)
+        X = Y
+
+
+def pad_graph(G, num_colors):
+    """Add a disconnected complete clique K_p such that the number of nodes in
+    the graph becomes a multiple of `num_colors`.
+
+    Assumes that the graph's nodes are labelled using integers.
+
+    Returns the number of nodes with each color.
+    """
+
+    n_ = len(G)
+    r = num_colors - 1
+
+    # Ensure that the number of nodes in G is a multiple of (r + 1)
+    s = n_ // (r + 1)
+    if n_ != s * (r + 1):
+        p = (r + 1) - n_ % (r + 1)
+        s += 1
+
+        # Complete graph K_p between (imaginary) nodes [n_, ... , n_ + p]
+        K = nx.relabel_nodes(nx.complete_graph(p), {idx: idx + n_ for idx in range(p)})
+        G.add_edges_from(K.edges)
+
+    return s
+
+
+def procedure_P(V_minus, V_plus, N, H, F, C, L, excluded_colors=None):
+    """Procedure P as described in the paper."""
+
+    if excluded_colors is None:
+        excluded_colors = set()
+
+    A_cal = set()
+    T_cal = {}
+    R_cal = []
+
+    # BFS to determine A_cal, i.e. colors reachable from V-
+    reachable = [V_minus]
+    marked = set(reachable)
+    idx = 0
+
+    while idx < len(reachable):
+        pop = reachable[idx]
+        idx += 1
+
+        A_cal.add(pop)
+        R_cal.append(pop)
+
+        # TODO: Checking whether a color has been visited can be made faster by
+        # using a look-up table instead of testing for membership in a set by a
+        # logarithmic factor.
+        next_layer = []
+        for k in C.keys():
+            if (
+                H[(k, pop)] > 0
+                and k not in A_cal
+                and k not in excluded_colors
+                and k not in marked
+            ):
+                next_layer.append(k)
+
+        for dst in next_layer:
+            # Record that `dst` can reach `pop`
+            T_cal[dst] = pop
+
+        marked.update(next_layer)
+        reachable.extend(next_layer)
+
+    # Variables for the algorithm
+    b = len(C) - len(A_cal)
+
+    if V_plus in A_cal:
+        # Easy case: V+ is in A_cal
+        # Move one node from V+ to V- using T_cal to find the parents.
+        move_witnesses(V_plus, V_minus, N=N, H=H, F=F, C=C, T_cal=T_cal, L=L)
+    else:
+        # If there is a solo edge, we can resolve the situation by
+        # moving witnesses from B to A, making G[A] equitable and then
+        # recursively balancing G[B - w] with a different V_minus and
+        # but the same V_plus.
+
+        A_0 = set()
+        A_cal_0 = set()
+        num_terminal_sets_found = 0
+        made_equitable = False
+
+        for W_1 in R_cal[::-1]:
+
+            for v in C[W_1]:
+                X = None
+
+                for U in C.keys():
+                    if N[(v, U)] == 0 and U in A_cal and U != W_1:
+                        X = U
+
+                # v does not witness an edge in H[A_cal]
+                if X is None:
+                    continue
+
+                for U in C.keys():
+                    # Note: Departing from the paper here.
+                    if N[(v, U)] >= 1 and U not in A_cal:
+                        X_prime = U
+                        w = v
+
+                        # Finding the solo neighbor of w in X_prime
+                        y_candidates = [
+                            node
+                            for node in L[w]
+                            if F[node] == X_prime and N[(node, W_1)] == 1
+                        ]
+
+                        if len(y_candidates) > 0:
+                            y = y_candidates[0]
+                            W = W_1
+
+                            # Move w from W to X, now X has one extra node.
+                            change_color(w, W, X, N=N, H=H, F=F, C=C, L=L)
+
+                            # Move witness from X to V_minus, making the coloring
+                            # equitable.
+                            move_witnesses(
+                                src_color=X,
+                                dst_color=V_minus,
+                                N=N,
+                                H=H,
+                                F=F,
+                                C=C,
+                                T_cal=T_cal,
+                                L=L,
+                            )
+
+                            # Move y from X_prime to W, making W the correct size.
+                            change_color(y, X_prime, W, N=N, H=H, F=F, C=C, L=L)
+
+                            # Then call the procedure on G[B - y]
+                            procedure_P(
+                                V_minus=X_prime,
+                                V_plus=V_plus,
+                                N=N,
+                                H=H,
+                                C=C,
+                                F=F,
+                                L=L,
+                                excluded_colors=excluded_colors.union(A_cal),
+                            )
+                            made_equitable = True
+                            break
+
+                if made_equitable:
+                    break
+            else:
+                # No node in W_1 was found such that
+                # it had a solo-neighbor.
+                A_cal_0.add(W_1)
+                A_0.update(C[W_1])
+                num_terminal_sets_found += 1
+
+            if num_terminal_sets_found == b:
+                # Otherwise, construct the maximal independent set and find
+                # a pair of z_1, z_2 as in Case II.
+
+                # BFS to determine B_cal': the set of colors reachable from V+
+                B_cal_prime = set()
+                T_cal_prime = {}
+
+                reachable = [V_plus]
+                marked = set(reachable)
+                idx = 0
+                while idx < len(reachable):
+                    pop = reachable[idx]
+                    idx += 1
+
+                    B_cal_prime.add(pop)
+
+                    # No need to check for excluded_colors here because
+                    # they only exclude colors from A_cal
+                    next_layer = [
+                        k
+                        for k in C.keys()
+                        if H[(pop, k)] > 0 and k not in B_cal_prime and k not in marked
+                    ]
+
+                    for dst in next_layer:
+                        T_cal_prime[pop] = dst
+
+                    marked.update(next_layer)
+                    reachable.extend(next_layer)
+
+                # Construct the independent set of G[B']
+                I_set = set()
+                I_covered = set()
+                W_covering = {}
+
+                B_prime = [node for k in B_cal_prime for node in C[k]]
+
+                # Add the nodes in V_plus to I first.
+                for z in C[V_plus] + B_prime:
+                    if z in I_covered or F[z] not in B_cal_prime:
+                        continue
+
+                    I_set.add(z)
+                    I_covered.add(z)
+                    I_covered.update([nbr for nbr in L[z]])
+
+                    for w in L[z]:
+                        if F[w] in A_cal_0 and N[(z, F[w])] == 1:
+                            if w not in W_covering:
+                                W_covering[w] = z
+                            else:
+                                # Found z1, z2 which have the same solo
+                                # neighbor in some W
+                                z_1 = W_covering[w]
+                                # z_2 = z
+
+                                Z = F[z_1]
+                                W = F[w]
+
+                                # shift nodes along W, V-
+                                move_witnesses(
+                                    W, V_minus, N=N, H=H, F=F, C=C, T_cal=T_cal, L=L
+                                )
+
+                                # shift nodes along V+ to Z
+                                move_witnesses(
+                                    V_plus,
+                                    Z,
+                                    N=N,
+                                    H=H,
+                                    F=F,
+                                    C=C,
+                                    T_cal=T_cal_prime,
+                                    L=L,
+                                )
+
+                                # change color of z_1 to W
+                                change_color(z_1, Z, W, N=N, H=H, F=F, C=C, L=L)
+
+                                # change color of w to some color in B_cal
+                                W_plus = [
+                                    k
+                                    for k in C.keys()
+                                    if N[(w, k)] == 0 and k not in A_cal
+                                ][0]
+                                change_color(w, W, W_plus, N=N, H=H, F=F, C=C, L=L)
+
+                                # recurse with G[B \cup W*]
+                                excluded_colors.update(
+                                    [
+                                        k
+                                        for k in C.keys()
+                                        if k != W and k not in B_cal_prime
+                                    ]
+                                )
+                                procedure_P(
+                                    V_minus=W,
+                                    V_plus=W_plus,
+                                    N=N,
+                                    H=H,
+                                    C=C,
+                                    F=F,
+                                    L=L,
+                                    excluded_colors=excluded_colors,
+                                )
+
+                                made_equitable = True
+                                break
+
+                    if made_equitable:
+                        break
+                else:
+                    assert False, (
+                        "Must find a w which is the solo neighbor "
+                        "of two vertices in B_cal_prime."
+                    )
+
+            if made_equitable:
+                break
+
+
+def equitable_color(G, num_colors):
+    """Provides equitable (r + 1)-coloring for nodes of G in O(r * n^2) time
+     if deg(G) <= r. The algorithm is described in [1]_.
+
+     Attempts to color a graph using r colors, where no neighbors of a node
+     can have same color as the node itself and the number of nodes with each
+     color differ by at most 1.
+
+     Parameters
+     ----------
+     G : networkX graph
+        The nodes of this graph will be colored.
+
+     num_colors : number of colors to use
+        This number must be at least one more than the maximum degree of nodes
+        in the graph.
+
+     Returns
+     -------
+     A dictionary with keys representing nodes and values representing
+     corresponding coloring.
+
+     Examples
+     --------
+     >>> G = nx.cycle_graph(4)
+     >>> d = nx.coloring.equitable_color(G, num_colors=3)
+     >>> nx.algorithms.coloring.equitable_coloring.is_equitable(G, d)
+     True
+
+     Raises
+     ------
+     NetworkXAlgorithmError
+         If the maximum degree of the graph ``G`` is greater than
+         ``num_colors``.
+
+     References
+     ----------
+     .. [1] Kierstead, H. A., Kostochka, A. V., Mydlarz, M., & Szemerédi, E.
+         (2010). A fast algorithm for equitable coloring. Combinatorica, 30(2),
+         217-224.
+    """
+
+    # Map nodes to integers for simplicity later.
+    nodes_to_int = {}
+    int_to_nodes = {}
+
+    for idx, node in enumerate(G.nodes):
+        nodes_to_int[node] = idx
+        int_to_nodes[idx] = node
+
+    G = nx.relabel_nodes(G, nodes_to_int, copy=True)
+
+    # Basic graph statistics and sanity check.
+    if len(G.nodes) > 0:
+        r_ = max([G.degree(node) for node in G.nodes])
+    else:
+        r_ = 0
+
+    if r_ >= num_colors:
+        raise nx.NetworkXAlgorithmError(
+            f"Graph has maximum degree {r_}, needs "
+            f"{r_ + 1} (> {num_colors}) colors for guaranteed coloring."
+        )
+
+    # Ensure that the number of nodes in G is a multiple of (r + 1)
+    pad_graph(G, num_colors)
+
+    # Starting the algorithm.
+    # L = {node: list(G.neighbors(node)) for node in G.nodes}
+    L_ = {node: [] for node in G.nodes}
+
+    # Arbitrary equitable allocation of colors to nodes.
+    F = {node: idx % num_colors for idx, node in enumerate(G.nodes)}
+
+    C = make_C_from_F(F)
+
+    # The neighborhood is empty initially.
+    N = make_N_from_L_C(L_, C)
+
+    # Currently all nodes witness all edges.
+    H = make_H_from_C_N(C, N)
+
+    # Start of algorithm.
+    edges_seen = set()
+
+    for u in sorted(G.nodes):
+        for v in sorted(G.neighbors(u)):
+
+            # Do not double count edges if (v, u) has already been seen.
+            if (v, u) in edges_seen:
+                continue
+
+            edges_seen.add((u, v))
+
+            L_[u].append(v)
+            L_[v].append(u)
+
+            N[(u, F[v])] += 1
+            N[(v, F[u])] += 1
+
+            if F[u] != F[v]:
+                # Were 'u' and 'v' witnesses for F[u] -> F[v] or F[v] -> F[u]?
+                if N[(u, F[v])] == 1:
+                    H[F[u], F[v]] -= 1  # u cannot witness an edge between F[u], F[v]
+
+                if N[(v, F[u])] == 1:
+                    H[F[v], F[u]] -= 1  # v cannot witness an edge between F[v], F[u]
+
+        if N[(u, F[u])] != 0:
+            # Find the first color where 'u' does not have any neighbors.
+            Y = [k for k in C.keys() if N[(u, k)] == 0][0]
+            X = F[u]
+            change_color(u, X, Y, N=N, H=H, F=F, C=C, L=L_)
+
+            # Procedure P
+            procedure_P(V_minus=X, V_plus=Y, N=N, H=H, F=F, C=C, L=L_)
+
+    return {int_to_nodes[x]: F[x] for x in int_to_nodes}