90 lines
2.2 KiB
Java
90 lines
2.2 KiB
Java
"use strict";
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var _ = require("../lodash");
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var Graph = require("../graphlib").Graph;
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var slack = require("./util").slack;
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module.exports = feasibleTree;
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/*
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* Constructs a spanning tree with tight edges and adjusted the input node's
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* ranks to achieve this. A tight edge is one that is has a length that matches
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* its "minlen" attribute.
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*
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* The basic structure for this function is derived from Gansner, et al., "A
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* Technique for Drawing Directed Graphs."
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*
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* Pre-conditions:
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*
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* 1. Graph must be a DAG.
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* 2. Graph must be connected.
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* 3. Graph must have at least one node.
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* 5. Graph nodes must have been previously assigned a "rank" property that
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* respects the "minlen" property of incident edges.
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* 6. Graph edges must have a "minlen" property.
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*
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* Post-conditions:
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*
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* - Graph nodes will have their rank adjusted to ensure that all edges are
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* tight.
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*
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* Returns a tree (undirected graph) that is constructed using only "tight"
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* edges.
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*/
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function feasibleTree(g) {
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var t = new Graph({ directed: false });
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// Choose arbitrary node from which to start our tree
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var start = g.nodes()[0];
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var size = g.nodeCount();
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t.setNode(start, {});
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var edge, delta;
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while (tightTree(t, g) < size) {
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edge = findMinSlackEdge(t, g);
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delta = t.hasNode(edge.v) ? slack(g, edge) : -slack(g, edge);
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shiftRanks(t, g, delta);
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}
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return t;
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}
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/*
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* Finds a maximal tree of tight edges and returns the number of nodes in the
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* tree.
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*/
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function tightTree(t, g) {
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function dfs(v) {
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_.forEach(g.nodeEdges(v), function(e) {
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var edgeV = e.v,
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w = (v === edgeV) ? e.w : edgeV;
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if (!t.hasNode(w) && !slack(g, e)) {
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t.setNode(w, {});
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t.setEdge(v, w, {});
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dfs(w);
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}
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});
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}
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_.forEach(t.nodes(), dfs);
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return t.nodeCount();
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}
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/*
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* Finds the edge with the smallest slack that is incident on tree and returns
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* it.
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*/
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function findMinSlackEdge(t, g) {
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return _.minBy(g.edges(), function(e) {
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if (t.hasNode(e.v) !== t.hasNode(e.w)) {
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return slack(g, e);
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}
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});
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}
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function shiftRanks(t, g, delta) {
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_.forEach(t.nodes(), function(v) {
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g.node(v).rank += delta;
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});
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}
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