Add starter file sniffer for detecting file types

file/sniffer.js

@@ -0,0 +1,63 @@
+/**
+ * File Sniffer.
+ *
+ * Licensed under the MIT License
+ *
+ * Copyright(c) 2020 Google Inc.
+ */
+
+const fileSignatures = {
+  'image/png': [0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A],
+};
+
+// TODO: Build a tree of signature bytes that end in a MIME type.
+class Node {
+  /**
+   * @param {number} value 
+   */
+  constructor(value) {
+    this.value = value;
+    this.children = {};
+    this.mimeType = undefined;
+  }
+}
+
+// Top-level node in the tree.
+const root = new Node();
+let maxDepth = 0;
+
+// Construct the tree.
+(function() {
+  for (const mimeType in fileSignatures) {
+    let curNode = root;
+    let depth = 0;
+    for (const byte of fileSignatures[mimeType]) {
+      if (curNode.children[byte] === undefined) {
+        curNode.children[byte] = new Node(byte);
+      }
+      depth++;
+      curNode = curNode.children[byte];
+    }
+    if (maxDepth < depth) {
+      maxDepth = depth;
+    }
+    curNode.mimeType = mimeType;
+  }
+})();
+
+/**
+ * @param {ArrayBuffer} ab
+ * @return {string} The MIME type of the buffer, or undefined.
+ */
+export function findMimeType(ab) {
+  const depth = ab.byteLength < maxDepth ? ab.byteLength : maxDepth;
+  const arr = new Uint8Array(ab).subarray(0, depth);
+  let curNode = root;
+  for (const byte of arr) {
+    if (curNode.children[byte] === undefined) return undefined;
+    curNode = curNode.children[byte];
+    if (curNode.mimeType) return curNode.mimeType;
+  }
+}
+
+debugger;