# All files / local encoded_resource_path.ts

96.72% Statements 59/61
89.47% Branches 17/19
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     /**
     * Copyright 2017 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 { ResourcePath } from '../model/path';
    import { assert, fail } from '../util/assert';

    /**
     * Helpers for dealing with resource paths stored in IndexedDB.
     *
     * Resource paths in their canonical string form do not sort as the server
     * sorts them. Specifically the server splits paths into segments first and then
     * sorts, putting end-of-segment before any character. In a UTF-8 string
     * encoding the slash ('/') that denotes the end-of-segment naturally comes
     * after other characters so the intent here is to encode the path delimiters in
     * such a way that the resulting strings sort naturally.
     *
     * Resource paths are also used for prefix scans so it's important to
     * distinguish whole segments from any longer segments of which they might be a
     * prefix. For example, it's important to make it possible to scan documents in
     * a collection "foo" without encountering documents in a collection "foobar".
     *
     * Separate from the concerns about resource path ordering and separation,
     * On Android, SQLite imposes additional restrictions since it does not handle
     * keys with embedded NUL bytes particularly well. Rather than change the
     * implementation we keep the encoding identical to keep the ports similar.
     *
     * Taken together this means resource paths when encoded for storage in
     * IndexedDB have the following characteristics:
     *
     *   * Segment separators ("/") sort before everything else.
     *   * All paths have a trailing separator.
     *   * NUL bytes do not exist in the output, since IndexedDB doesn't treat them
     * well.
     *
     * Therefore resource paths are encoded into string form using the following
     * rules:
     *
     *   * '\x01' is used as an escape character.
     *   * Path separators are encoded as "\x01\x01"
     *   * NUL bytes are encoded as "\x01\x10"
     *   * '\x01' is encoded as "\x01\x11"
     *
     * This encoding leaves some room between path separators and the NUL byte
     * just in case we decide to support integer document ids after all.
     *
     * Note that characters treated specially by the backend ('.', '/', and '~')
     * are not treated specially here. This class assumes that any unescaping of
     * resource path strings into actual ResourcePath objects will handle these
     * characters there.
     */
    export type EncodedResourcePath = string;

    const escapeChar = '\u0001';
    const encodedSeparatorChar = '\u0001';
    const encodedNul = '\u0010';
    const encodedEscape = '\u0011';

    /**
     * Encodes a resource path into a IndexedDb-compatible string form.
     */
    export function encode(path: ResourcePath): EncodedResourcePath {
      let result = '';
      for (let i = 0; i < path.length; i++) {
        if (result.length > 0) {
          result = encodeSeparator(result);
        }
        result = encodeSegment(path.get(i), result);
      }
      return encodeSeparator(result);
    }

    /** Encodes a single segment of a resource path into the given result */
    function encodeSegment(segment: string, resultBuf: string) {
      let result = resultBuf;
      const length = segment.length;
      for (let i = 0; i < length; i++) {
        const c = segment.charAt(i);
        switch (c) {
          case '\0':
            result += escapeChar + encodedNul;
            break;
          case escapeChar:
            result += escapeChar + encodedEscape;
            break;
          default:
            result += c;
        }
      }
      return result;
    }

    /** Encodes a path separator into the given result */
    function encodeSeparator(result: string): string {
      return result + escapeChar + encodedSeparatorChar;
    }

    /**
     * Decodes the given IndexedDb-compatible string form of a resource path into
     * a ResourcePath instance. Note that this method is not suitable for use with
     * decoding resource names from the server; those are One Platform format
     * strings.
     */
    export function decode(path: EncodedResourcePath): ResourcePath {
      // Event the empty path must encode as a path of at least length 2. A path
      // with exactly 2 must be the empty path.
      const length = path.length;
      assert(length >= 2, 'Invalid path ' + path);
      if (length === 2) {
        assert(
          path.charAt(0) === escapeChar && path.charAt(1) === encodedSeparatorChar,
          'Non-empty path ' + path + ' had length 2'
        );
        return ResourcePath.EMPTY_PATH;
      }

      // Escape characters cannot exist past the second-to-last position in the
      // source value.
      const lastReasonableEscapeIndex = length - 2;

      const segments: string[] = [];
      let segmentBuilder = '';

      for (let start = 0; start < length; ) {
        // The last two characters of a valid encoded path must be a separator, so
        // there must be an end to this segment.
        const end = path.indexOf(escapeChar, start);
        Iif (end < 0 || end > lastReasonableEscapeIndex) {
          fail('Invalid encoded resource path: "' + path + '"');
        }

        const next = path.charAt(end + 1);
        switch (next) {
          case encodedSeparatorChar:
            const currentPiece = path.substring(start, end);
            let segment;
            if (segmentBuilder.length === 0) {
              // Avoid copying for the common case of a segment that excludes \0
              // and \001
              segment = currentPiece;
            } else {
              segmentBuilder += currentPiece;
              segment = segmentBuilder;
              segmentBuilder = '';
            }
            segments.push(segment);
            break;
          case encodedNul:
            segmentBuilder += path.substring(start, end);
            segmentBuilder += '\0';
            break;
          case encodedEscape:
            // The escape character can be used in the output to encode itself.
            segmentBuilder += path.substring(start, end + 1);
            break;
          default:
            fail('Invalid encoded resource path: "' + path + '"');
        }

        start = end + 2;
      }

      return new ResourcePath(segments);
    }

    /**
     * Computes the prefix successor of the given path, computed by encode above.
     * A prefix successor is the first key that cannot be prefixed by the given
     * path. It's useful for defining the end of a prefix scan such that all keys
     * in the scan have the same prefix.
     *
     * Note that this is not a general prefix successor implementation, which is
     * tricky to get right with Strings, given that they encode down to UTF-8.
     * Instead this relies on the fact that all paths encoded by this class are
     * always terminated with a separator, and so a successor can always be
     * cheaply computed by incrementing the last character of the path.
     */
    export function prefixSuccessor(path: EncodedResourcePath) {
      const c = path.charCodeAt(path.length - 1);
      // TODO(mcg): this really should be a general thing, but not worth it right
      // now
      assert(c === 1, 'successor may only operate on paths generated by encode');
      return path.substring(0, path.length - 1) + String.fromCharCode(c + 1);
    }