Rename articles prefixed with js-

snippets/js/s/data-structures-graph.md

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+---
+title: JavaScript Data Structures - Graph
+shortTitle: Graph
+type: story
+language: javascript
+tags: [class]
+author: chalarangelo
+cover: purple-flower-macro-1
+excerpt: A graph is a data structure consisting of a set of vertices connected by a set of edges.
+dateModified: 2021-08-17T05:00:00-04:00
+---
+
+### Definition
+
+A graph is a data structure consisting of a set of nodes or vertices and a set of edges that represent connections between those nodes. Graphs can be directed or undirected, while their edges can be assigned numeric weights.
+
+![JavaScript Graph visualization](./illustrations/ds-graph.png)
+
+Each node in a graph data structure must have the following properties:
+
+- `key`: The key of the node
+- `value`: The value of the node
+
+Each edge in a graph data structure must have the following properties:
+
+- `a`: The starting node of the edge
+- `b`: The target node of the edge
+- `weight`: An optional numeric weight value for the edge
+
+The main operations of a graph data structure are:
+
+- `addNode`: Inserts a new node with the specific key and value
+- `addEdge`: Inserts a new edge between two given nodes, optionally setting its weight
+- `removeNode`: Removes the node with the specified key
+- `removeEdge`: Removes the edge between two given nodes
+- `findNode`: Retrieves the node with the given key
+- `hasEdge`: Checks if the graph has an edge between two given nodes
+- `setEdgeWeight`: Sets the weight of a given edge
+- `getEdgeWeight`: Gets the weight of a given edge
+- `adjacent`: Finds all nodes for which an edge exists from a given node
+- `indegree`: Calculates the total number of edges to a given node
+- `outdegree`: Calculates the total number of edges from a given node
+
+### Implementation
+
+```js
+class Graph {
+  constructor(directed = true) {
+    this.directed = directed;
+    this.nodes = [];
+    this.edges = new Map();
+  }
+
+  addNode(key, value = key) {
+    this.nodes.push({ key, value });
+  }
+
+  addEdge(a, b, weight) {
+    this.edges.set(JSON.stringify([a, b]), { a, b, weight });
+    if (!this.directed)
+      this.edges.set(JSON.stringify([b, a]), { a: b, b: a, weight });
+  }
+
+  removeNode(key) {
+    this.nodes = this.nodes.filter(n => n.key !== key);
+    [...this.edges.values()].forEach(({ a, b }) => {
+      if (a === key || b === key) this.edges.delete(JSON.stringify([a, b]));
+    });
+  }
+
+  removeEdge(a, b) {
+    this.edges.delete(JSON.stringify([a, b]));
+    if (!this.directed) this.edges.delete(JSON.stringify([b, a]));
+  }
+
+  findNode(key) {
+    return this.nodes.find(x => x.key === key);
+  }
+
+  hasEdge(a, b) {
+    return this.edges.has(JSON.stringify([a, b]));
+  }
+
+  setEdgeWeight(a, b, weight) {
+    this.edges.set(JSON.stringify([a, b]), { a, b, weight });
+    if (!this.directed)
+      this.edges.set(JSON.stringify([b, a]), { a: b, b: a, weight });
+  }
+
+  getEdgeWeight(a, b) {
+    return this.edges.get(JSON.stringify([a, b])).weight;
+  }
+
+  adjacent(key) {
+    return [...this.edges.values()].reduce((acc, { a, b }) => {
+      if (a === key) acc.push(b);
+      return acc;
+    }, []);
+  }
+
+  indegree(key) {
+    return [...this.edges.values()].reduce((acc, { a, b }) => {
+      if (b === key) acc++;
+      return acc;
+    }, 0);
+  }
+
+  outdegree(key) {
+    return [...this.edges.values()].reduce((acc, { a, b }) => {
+      if (a === key) acc++;
+      return acc;
+    }, 0);
+  }
+}
+```
+
+- Create a `class` with a `constructor` that initializes an empty array, `nodes`, and a `Map`, `edges`, for each instance. The optional argument, `directed`, specifies if the graph is directed or not.
+
+- Define an `addNode()` method, which uses `Array.prototype.push()` to add a new node in the `nodes` array.
+- Define an `addEdge()` method, which uses `Map.prototype.set()` to add a new edge to the `edges` Map, using `JSON.stringify()` to produce a unique key.
+- Define a `removeNode()` method, which uses `Array.prototype.filter()` and `Map.prototype.delete()` to remove the given node and any edges connected to it.
+- Define a `removeEdge()` method, which uses `Map.prototype.delete()` to remove the given edge.
+- Define a `findNode()` method, which uses `Array.prototype.find()` to return the given node, if any.
+- Define a `hasEdge()` method, which uses `Map.prototype.has()` and `JSON.stringify()` to check if the given edge exists in the `edges` Map.
+- Define a `setEdgeWeight()` method, which uses `Map.prototype.set()` to set the weight of the appropriate edge, whose key is produced by `JSON.stringify()`.
+- Define a `getEdgeWeight()` method, which uses `Map.prototype.get()` to get the eight of the appropriate edge, whose key is produced by `JSON.stringify()`.
+- Define an `adjacent()` method, which uses `Map.prototype.values()`, `Array.prototype.reduce()` and `Array.prototype.push()` to find all nodes connected to the given node.
+- Define an `indegree()` method, which uses `Map.prototype.values()` and `Array.prototype.reduce()` to count the number of edges to the given node.
+- Define an `outdegree()` method, which uses `Map.prototype.values()` and `Array.prototype.reduce()` to count the number of edges from the given node.
+
+```js
+const g = new Graph();
+
+g.addNode('a');
+g.addNode('b');
+g.addNode('c');
+g.addNode('d');
+
+g.addEdge('a', 'c');
+g.addEdge('b', 'c');
+g.addEdge('c', 'b');
+g.addEdge('d', 'a');
+
+g.nodes.map(x => x.value);  // ['a', 'b', 'c', 'd']
+[...g.edges.values()].map(({ a, b }) => `${a} => ${b}`);
+// ['a => c', 'b => c', 'c => b', 'd => a']
+
+g.adjacent('c');            // ['b']
+
+g.indegree('c');            // 2
+g.outdegree('c');           // 1
+
+g.hasEdge('d', 'a');        // true
+g.hasEdge('a', 'd');        // false
+
+g.removeEdge('c', 'b');
+
+[...g.edges.values()].map(({ a, b }) => `${a} => ${b}`);
+// ['a => c', 'b => c', 'd => a']
+
+g.removeNode('c');
+
+g.nodes.map(x => x.value);  // ['a', 'b', 'd']
+[...g.edges.values()].map(({ a, b }) => `${a} => ${b}`);
+// ['d => a']
+
+g.setEdgeWeight('d', 'a', 5);
+g.getEdgeWeight('d', 'a');  // 5
+```