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Tweak the file signature sniffer so byte tree initialization happens lazily.

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This commit is contained in:
Jeff Schiller 2022-01-14 12:17:03 -08:00
parent 96ba25aef7
commit 7746b8c03f
3 changed files with 57 additions and 32 deletions
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75
file/sniffer.js
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@ -38,6 +38,10 @@ const fileSignatures = {
// * an OGG container can be resolved to OGG Audio, OGG Video
// * an HEIF container can be resolved to AVIF, HEIC
/**
* Represents a single byte in the tree. If this node terminates a known MIME type (see magic
* numbers above), then the mimeType field will be set.
*/
class Node {
/** @param {number} value */
constructor(value) {
@ -47,49 +51,64 @@ class Node {
}
}
// Top-level node in the tree.
const root = new Node();
/** Top-level node in the byte tree. */
let root = null;
/** The maximum depth of the byte tree. */
let maxDepth = 0;
// Construct the tree, erroring if overlapping mime types are possible.
for (const mimeType in fileSignatures) {
for (const signature of fileSignatures[mimeType]) {
let curNode = root;
let depth = 0;
for (const byte of signature) {
if (curNode.children[byte] === undefined) {
if (byte === '??' && !curNode.children['??'] && Object.keys(curNode.children).length > 0) {
throw 'Cannot add a placeholder child to a node that has non-placeholder children';
} else if (byte !== '??' && curNode.children['??']) {
throw 'Cannot add a non-placeholder child to a node that has a placeholder child';
/**
* This function initializes the byte tree. It is lazily called upon findMimeType(), but if you care
* about when the tree initializes (like in startup, etc), you can call it yourself here.
*/
export function initialize() {
root = new Node();
// Construct the tree, erroring if overlapping mime types are possible.
for (const mimeType in fileSignatures) {
for (const signature of fileSignatures[mimeType]) {
let curNode = root;
let depth = 0;
for (const byte of signature) {
if (curNode.children[byte] === undefined) {
if (byte === '??' && !curNode.children['??'] && Object.keys(curNode.children).length > 0) {
throw 'Cannot add a placeholder child to a node that has non-placeholder children';
} else if (byte !== '??' && curNode.children['??']) {
throw 'Cannot add a non-placeholder child to a node that has a placeholder child';
}
curNode.children[byte] = new Node(byte);
}
curNode.children[byte] = new Node(byte);
depth++;
curNode = curNode.children[byte];
} // for each byte
if (maxDepth < depth) {
maxDepth = depth;
}
depth++;
curNode = curNode.children[byte];
} // for each byte
if (maxDepth < depth) {
maxDepth = depth;
}
if (curNode.mimeType) {
throw `File signature collision: ${curNode.mimeType} overlaps with ${mimeType}`;
} else if (Object.keys(curNode.children).length > 0) {
throw `${mimeType} signature is not unique, it collides with other mime types`;
}
curNode.mimeType = mimeType;
} // for each signature
if (curNode.mimeType) {
throw `File signature collision: ${curNode.mimeType} overlaps with ${mimeType}`;
} else if (Object.keys(curNode.children).length > 0) {
throw `${mimeType} signature is not unique, it collides with other mime types`;
}
curNode.mimeType = mimeType;
} // for each signature
}
}
/**
* Finds the likely MIME type represented by the ArrayBuffer.
* @param {ArrayBuffer} ab
* @return {string} The MIME type of the buffer, or undefined.
*/
export function findMimeType(ab) {
if (!root) {
initializeTree();
}
const depth = ab.byteLength < maxDepth ? ab.byteLength : maxDepth;
const arr = new Uint8Array(ab).subarray(0, depth);
let curNode = root;
// Step through bytes, updating curNode as it walks down the byte tree.
for (const byte of arr) {
// If this node has a placeholder child, just step into it.
if (curNode.children['??']) {