Receipt Parser

import { MAX_FILE_BYTES, MIME_EXTENSIONS } from "../constants/defaults";import type { SupportedMimeType, UploadResult } from "../types/receipt";const MULTIPART_OVERHEAD_BYTES = 64 * 1024;function startsWith(bytes: Uint8Array, signature: number[], offset = 0): boolean {  return signature.every((byte, i) => bytes[offset + i] === byte);}/** Detects the file type from magic bytes so a wrong or missing Content-Type doesn't matter. */export function detectMimeType(data: ArrayBuffer): SupportedMimeType | null {  const bytes = new Uint8Array(data, 0, Math.min(data.byteLength, 12));  if (startsWith(bytes, [0x25, 0x50, 0x44, 0x46])) return "application/pdf";  if (startsWith(bytes, [0xff, 0xd8, 0xff])) return "image/jpeg";  if (startsWith(bytes, [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])) return "image/png";  if (    startsWith(bytes, [0x52, 0x49, 0x46, 0x46]) &&    startsWith(bytes, [0x57, 0x45, 0x42, 0x50], 8)  ) {    return "image/webp";  }  return null;}function sanitizeFileName(name: string | null | undefined): string | null {  const cleaned = (name ?? "")    .replace(/[^\w.\- ]+/g, "")    .trim()    .slice(0, 100);  return cleaned || null;}/** Reads a receipt from a raw request body or a multipart `file` field. */export async function readUpload(request: Request, nameParam?: string): Promise<UploadResult> {  const tooLarge = {    ok: false,    code: "PAYLOAD_TOO_LARGE",    message: `File exceeds the ${MAX_FILE_BYTES / (1024 * 1024)}MB limit`,    status: 413,  } as const;  const missing = {    ok: false,    code: "MISSING_FILE",    message: "Send the receipt as the raw request body or as multipart field 'file'",    status: 400,  } as const;  const declaredLength = Number(request.headers.get("content-length") ?? 0);  if (declaredLength > MAX_FILE_BYTES + MULTIPART_OVERHEAD_BYTES) return tooLarge;  const contentType = (request.headers.get("content-type") ?? "").toLowerCase();  let data: ArrayBuffer;  let name: string | null;  if (contentType.startsWith("multipart/form-data")) {    let form: FormData;    try {      form = await request.formData();    } catch {      return missing;    }    // workers-types declares FormData.get() as string | null, but file fields are File at runtime.    const field: unknown = form.get("file");    if (!(field instanceof File)) return missing;    data = await field.arrayBuffer();    name = sanitizeFileName(field.name);  } else {    data = await request.arrayBuffer();    name = sanitizeFileName(nameParam);  }  if (data.byteLength === 0) return missing;  if (data.byteLength > MAX_FILE_BYTES) return tooLarge;  const mimeType = detectMimeType(data);  if (!mimeType) {    return {      ok: false,      code: "UNSUPPORTED_MEDIA_TYPE",      message: "Unsupported file type. Use PDF, JPEG, PNG, or WebP",      status: 415,    };  }  return {    ok: true,    file: { name: name ?? `receipt.${MIME_EXTENSIONS[mimeType]}`, mimeType, data },  };}