upstream-tldraw/apps/dotcom/sync-worker/src/utils/base64.ts

// Chunked base64 helpers. String.fromCharCode(...bytes) overflows the engine's argument limit on
// large buffers (a PNG screenshot is megabytes), so encoding walks the bytes in 32k chunks.

export function arrayBufferToBase64(buffer: ArrayBuffer): string {
	const bytes = new Uint8Array(buffer)
	// workerd (V8 14+) has the native encoder, which skips the intermediate binary string that
	// matters when the .tldr download inlines multi-MB assets inside a memory-capped DO. Node < 25
	// under vitest does not have it.
	if (typeof bytes.toBase64 === 'function') return bytes.toBase64()
	return btoa(bytesToBinaryString(bytes))
}

export function base64ToArrayBuffer(base64: string): ArrayBuffer {
	return binaryStringToBytes(atob(base64)).buffer
}

// Base64url (RFC 4648 §5): the URL-safe alphabet with padding stripped, used for the signed
// render tokens that ride in the render page's query string.
export function base64UrlEncode(bytes: Uint8Array): string {
	return btoa(bytesToBinaryString(bytes)).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '')
}

export function base64UrlDecode(value: string): Uint8Array<ArrayBuffer> {
	return binaryStringToBytes(atob(value.replace(/-/g, '+').replace(/_/g, '/')))
}

function bytesToBinaryString(bytes: Uint8Array): string {
	let binary = ''
	for (let i = 0; i < bytes.length; i += 0x8000) {
		binary += String.fromCharCode(...bytes.subarray(i, i + 0x8000))
	}
	return binary
}

function binaryStringToBytes(binary: string): Uint8Array<ArrayBuffer> {
	const bytes = new Uint8Array(new ArrayBuffer(binary.length))
	for (let i = 0; i < binary.length; i++) {
		bytes[i] = binary.charCodeAt(i)
	}
	return bytes
}