# All files / helpers generate-fid.ts

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50% Branches 2/4
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     /**
     * @license
     * Copyright 2019 Google LLC
     *
     * Licensed under the Apache License, Version 2.0 (the "License");
     * you may not use this file except in compliance with the License.
     * You may obtain a copy of the License at
     *
     *   http://www.apache.org/licenses/LICENSE-2.0
     *
     * Unless required by applicable law or agreed to in writing, software
     * distributed under the License is distributed on an "AS IS" BASIS,
     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     * See the License for the specific language governing permissions and
     * limitations under the License.
     */

    import { bufferToBase64UrlSafe } from './buffer-to-base64-url-safe';

    export const VALID_FID_PATTERN = /^[cdef][\w-]{21}$/;
    export const INVALID_FID = '';

    /**
     * Generates a new FID using random values from Web Crypto API.
     * Returns an empty string if FID generation fails for any reason.
     */
    export function generateFid(): string {
      try {
        // A valid FID has exactly 22 base64 characters, which is 132 bits, or 16.5
        // bytes. our implementation generates a 17 byte array instead.
        const fidByteArray = new Uint8Array(17);
        const crypto =
          self.crypto || ((self as unknown) as { msCrypto: Crypto }).msCrypto;
        crypto.getRandomValues(fidByteArray);

        // Replace the first 4 random bits with the constant FID header of 0b0111.
        fidByteArray[0] = 0b01110000 + (fidByteArray[0] % 0b00010000);

        const fid = encode(fidByteArray);

        return VALID_FID_PATTERN.test(fid) ? fid : INVALID_FID;
      } catch {
        // FID generation errored
        return INVALID_FID;
      }
    }

    /** Converts a FID Uint8Array to a base64 string representation. */
    function encode(fidByteArray: Uint8Array): string {
      const b64String = bufferToBase64UrlSafe(fidByteArray);

      // Remove the 23rd character that was added because of the extra 4 bits at the
      // end of our 17 byte array, and the '=' padding.
      return b64String.substr(0, 22);
    }