After compressing data using zip.deflate, how can I obtain the length of the encoded data?

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Question

After compressing data with zip.deflate and calling zStream.totalOut, the totalOut value of zStream remains unchanged at 0 even after encoding. How can I obtain the length of the encoded data?

import { zlib } from '@kit.BasicServicesKit';

async function getTotalOutOfzStream() {
  let str = 'hello world!';
  let arrayBufferIn = new ArrayBuffer(str.length);
  let byteArray = new Uint8Array(arrayBufferIn);
  for (let i = 0, j = str.length; i < j; i++) {
    byteArray[i] = str.charCodeAt(i)
  }
  let arrayBufferOut = new ArrayBuffer(100);
  let zStream: zlib.ZStream = {
    nextIn: arrayBufferIn,
    availableIn: arrayBufferIn.byteLength,
    nextOut: arrayBufferOut,
    availableOut: arrayBufferOut.byteLength,
    totalOut: 0
  };
  try {
    let zip = zlib.createZipSync();
    await zip.deflateInit(zStream, zlib.CompressLevel.COMPRESS_LEVEL_BEST_SPEED)
    await zip.deflate(zStream, zlib.CompressFlushMode.FINISH)

    // Use zStream to obtain totalOut
    let totalOut = zStream.totalOut
    console.info('The total out of zStream is ' + totalOut)
  } catch (e) {
    console.error(e)
  }
}

Short Answer

After each operation completes, you must call getZStream() again to obtain the latest status. For reference: Compression and Decompression.

 import { zlib } from '@kit.BasicServicesKit';

async function getTotalOutOfzStream() {
  let str = 'hello world!';
  let arrayBufferIn = new ArrayBuffer(str.length);
  let byteArray = new Uint8Array(arrayBufferIn);
  for (let i = 0, j = str.length; i < j; i++) {
    byteArray[i] = str.charCodeAt(i)
  }
  let arrayBufferOut = new ArrayBuffer(100);
  let zStream: zlib.ZStream = {
    nextIn: arrayBufferIn,
    availableIn: arrayBufferIn.byteLength,
    nextOut: arrayBufferOut,
    availableOut: arrayBufferOut.byteLength,
    totalOut: 0
  };
  try {
    let zip = zlib.createZipSync();
    await zip.deflateInit(zStream, zlib.CompressLevel.COMPRESS_LEVEL_BEST_SPEED)
    await zip.deflate(zStream, zlib.CompressFlushMode.FINISH)

    // Re-execute getZStream() to obtain the latest status, then get totalOut.
    let totalOut = (await zip.getZStream()).totalOut
    console.info('The total out of zStream is ' + totalOut)
  } catch (e) {
    console.error(e)
  }
}

Applicable Scenarios

Here are the applicable scenarios for obtaining the encoded data length using totalOut:

Data Transmission Optimization

When sending compressed data over networks, knowing the exact compressed size allows for proper buffer allocation and efficient bandwidth utilization
Useful for implementing progress tracking during file uploads/downloads of compressed data

Memory Management and Buffer Allocation

Determining the precise size of compressed data before writing to storage or transmission
Avoiding overallocation of buffers by using the actual compressed size rather than estimated maximums

Data Storage Efficiency

When storing compressed data in databases or filesystems, using totalOut ensures only the actual compressed bytes are stored
Calculating compression ratios for monitoring storage optimization effectiveness

Streaming Data Processing

In real-time data compression pipelines, tracking compressed output size for proper chunking and processing
Monitoring compression performance across different data types and sizes

Resource Monitoring and Logging