Add JS example for Huffman Encoding

Author: zsparal

chapters/data_compression/huffman/code/javascript/huffman.js

@@ -0,0 +1,84 @@
+function encode(str) {
+ const tree = createTree(str);
+ const codebook = createCodebook(tree);
+ return {
+ string: [...str].map(c => codebook[c]).join(""),
+ tree,
+ codebook
+ };
+
+ function createTree(str) {
+ const chars = [...str];
+ const charCounts = chars.reduce((counts, char) => {
+ counts[char] = (counts[char] || 0) + 1;
+ return counts;
+ }, {});
+
+ const nodes = Object.entries(charCounts).map(([key, weight]) => ({ key, weight }));
+ const priorityQueue = makeQueue(nodes);
+ while (priorityQueue.data.length > 1) {
+ const left = priorityQueue.dequeue();
+ const right = priorityQueue.dequeue();
+ priorityQueue.enqueue({ weight: left.weight + right.weight, left, right });
+ }
+ return priorityQueue.dequeue();
+ }
+
+ function createCodebook(tree) {
+ return function recurse(node, bitstring, dict) {
+ if (!node.left && !node.right) {
+ dict[node.key] = bitstring;
+ } else {
+ if (node.left) {
+ recurse(node.left, bitstring + "0", dict);
+ }
+
+ if (node.right) {
+ recurse(node.right, bitstring + "1", dict);
+ }
+ }
+ return dict;
+ }(tree, "", {});
+ }
+}
+
+function decode(bitstring, tree) {
+ const result = [];
+ let node = tree;
+
+ for (const bit of [...bitstring]) {
+ node = bit === "0" ? node.left : node.right;
+ if (!node.left && !node.right) {
+ result.push(node.key);
+ node = tree;
+ }
+ }
+
+ return result.join("");
+}
+
+// This queue implementation is horribly inefficient, but a proper, heap-based implementation would
+// be longer that the algorithm itself
+function makeQueue(iterable) {
+ return {
+ data: [...iterable].sort((a, b) => a.weight - b.weight),
+ enqueue(value) {
+ const target = this.data.findIndex(x => x.weight > value.weight);
+ if (target === -1) {
+ this.data.push(value);
+ } else {
+ this.data = [...this.data.slice(0, target), value, ...this.data.slice(target)];
+ }
+ },
+ dequeue() {
+ const [result] = this.data.splice(0, 1);
+ return result;
+ }
+ };
+}
+
+const encoded = encode("Hello, World!");
+const decoded = decode(encoded.string, encoded.tree);
+console.log(encoded.string);
+console.log(decoded);
+

chapters/data_compression/huffman/huffman.md

@@ -81,6 +81,9 @@ Program.cs
 {% sample lang="py" %}
 ### Python
 [import, lang:"python"](code/python/huffman.py)
+{% sample lang="js" %}
+### JavaScript
+[import, lang:"javascript"](code/javascript/huffman.js)
 {% endmethod %}