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CachedKeyDecoder.ts

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

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;

}

interface KeyCacheRecord {

readonly bytes: Uint8Array;

readonly str: string;

}

export class CachedKeyDecoder implements KeyDecoder {

hit = 0;

miss = 0;

private readonly caches: Array<Array<KeyCacheRecord>>;

readonly maxKeyLength: number;

readonly maxLengthPerKey: number;

constructor(maxKeyLength = DEFAULT_MAX_KEY_LENGTH, maxLengthPerKey = DEFAULT_MAX_LENGTH_PER_KEY) {

this.maxKeyLength = maxKeyLength;

this.maxLengthPerKey = maxLengthPerKey;

// avoid `new Array(N)`, which makes a sparse array,

// because a sparse array is typically slower than a non-sparse array.

this.caches = [];

for (let i = 0; i < this.maxKeyLength; i++) {

this.caches.push([]);

}

public canBeCached(byteLength: number): boolean {

return byteLength > 0 && byteLength <= this.maxKeyLength;

}

private find(bytes: Uint8Array, inputOffset: number, byteLength: number): string | null {

const records = this.caches[byteLength - 1]!;

FIND_CHUNK: for (const record of records) {

const recordBytes = record.bytes;

for (let j = 0; j < byteLength; j++) {

if (recordBytes[j] !== bytes[inputOffset + j]) {

continue FIND_CHUNK;

}

return record.str;

}

return null;

}

private store(bytes: Uint8Array, value: string) {

const records = this.caches[bytes.length - 1]!;

const record: KeyCacheRecord = { bytes, str: value };

if (records.length >= this.maxLengthPerKey) {

// `records` are full!

// Set `record` to an arbitrary position.

records[(Math.random() * records.length) | 0] = record;

} else {

records.push(record);

}

public decode(bytes: Uint8Array, inputOffset: number, byteLength: number): string {

const cachedValue = this.find(bytes, inputOffset, byteLength);

if (cachedValue != null) {

this.hit++;

return cachedValue;

}

this.miss++;

const str = utf8DecodeJs(bytes, inputOffset, byteLength);

// Ensure to copy a slice of bytes because the bytes may be a 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, str);

return str;

}