# All files / helpers generate-fid.ts

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     /**
     * @license
     * Copyright 2019 Google Inc.
     *
     * 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';

    /** Generates a new FID using random values from Web Crypto API. */
    export function generateFid(): string {
      // 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);
      crypto.getRandomValues(fidByteArray);

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

      return encode(fidByteArray);
    }

    /** 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);
    }