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• [DIR] ..
• [FILE] CachedKeyDecoder.ts
• [FILE] Decoder.ts
• [FILE] Encoder.ts
• [FILE] ExtData.ts
• [FILE] ExtensionCodec.ts
• [FILE] context.ts
• [FILE] decode.ts
• [FILE] decodeAsync.ts
• [FILE] encode.ts
• [FILE] index.ts
• [FILE] timestamp.ts
• [DIR] utils

Просмотр файла: CachedKeyDecoder.ts

import { utf8DecodeJs } from "./utils/utf8";

interface KeyCacheRecord {
  readonly bytes: Uint8Array;
  readonly value: string;
}

const DEFAULT_MAX_KEY_LENGTH = 16;
const DEFAULT_MAX_LENGTH_PER_KEY = 16;

export interface KeyDecoder {
  canBeCached(byteLength: number): boolean;
  decode(bytes: Uint8Array, inputOffset: number, byteLength: number): string;
}

export class CachedKeyDecoder implements KeyDecoder {
  hit = 0;
  miss = 0;
  private readonly caches: Array<Array<KeyCacheRecord>>;

  constructor(readonly maxKeyLength = DEFAULT_MAX_KEY_LENGTH, readonly maxLengthPerKey = DEFAULT_MAX_LENGTH_PER_KEY) {
    // avoid `new Array(N)` to create a non-sparse array for performance.
    this.caches = [];
    for (let i = 0; i < this.maxKeyLength; i++) {
      this.caches.push([]);
    }
  }

  public canBeCached(byteLength: number) {
    return byteLength > 0 && byteLength <= this.maxKeyLength;
  }

  private get(bytes: Uint8Array, inputOffset: number, byteLength: number): string | null {
    const records = this.caches[byteLength - 1];
    const recordsLength = records.length;

    FIND_CHUNK: for (let i = 0; i < recordsLength; i++) {
      const record = records[i];
      const recordBytes = record.bytes;

      for (let j = 0; j < byteLength; j++) {
        if (recordBytes[j] !== bytes[inputOffset + j]) {
          continue FIND_CHUNK;
        }
      }
      return record.value;
    }
    return null;
  }

  private store(bytes: Uint8Array, value: string) {
    const records = this.caches[bytes.length - 1];
    const record: KeyCacheRecord = { bytes, value };

    if (records.length >= this.maxLengthPerKey) {
      // `records` are full!
      // Set `record` to a randomized position.
      records[(Math.random() * records.length) | 0] = record;
    } else {
      records.push(record);
    }
  }

  public decode(bytes: Uint8Array, inputOffset: number, byteLength: number): string {
    const cachedValue = this.get(bytes, inputOffset, byteLength);
    if (cachedValue != null) {
      this.hit++;
      return cachedValue;
    }
    this.miss++;

    const value = utf8DecodeJs(bytes, inputOffset, byteLength);
    // Ensure to copy a slice of bytes because the byte may be NodeJS Buffer and Buffer#slice() returns a reference to its internal ArrayBuffer.
    const slicedCopyOfBytes = Uint8Array.prototype.slice.call(bytes, inputOffset, inputOffset + byteLength);
    this.store(slicedCopyOfBytes, value);
    return value;
  }
}

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