TS/JS 使用pako.js 压缩字符串和二进制

因为项目需要压缩字符串和二进制，找到了pako这个库：
https://github.com/nodeca/pako
https://gitee.com/renew_old_romance/pako/tree/master
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/pako/index.d.ts

一、简单介绍

参考Javascript 简单实现Gzip 压缩字符串 基于pako.js

<script>
    function input() {
        const input = document.getElementById('input').value;
        const ys = zip(input);
        const jy = unzip(ys);
        document.getElementById('yw').innerText = input;
        document.getElementById('ys').innerText = ys;
        document.getElementById('jy').innerText = jy;
        document.getElementById('ysl').innerText = 
        (Math.round(length(ys) / length(input) * 10000) / 100.00 + "%");

    }

    // 解压
    function unzip(b64Data) {
        let strData = atob(b64Data);
        const charData = strData.split('').map(function (x) {
            return x.charCodeAt(0);
        });
        const binData = new Uint8Array(charData);
        const data = pako.inflate(binData);
        strData = String.fromCharCode.apply(null, new Uint16Array(data));
        return decodeURIComponent(strData);
    }

    // 压缩
    function zip(str) {
        const binaryString = pako.gzip(encodeURIComponent(str), {to: 'string'})
        return btoa(binaryString);
    }

    // 占用字节数计算(UTF-8)
    function length(str) {
        let total = 0, charCode, i, len;
        for (i = 0, len = str.length; i < len; i++) {
            charCode = str.charCodeAt(i);
            if (charCode <= 0x007f) {
                total += 1;
            } else if (charCode <= 0x07ff) {
                total += 2;
            } else if (charCode <= 0xffff) {
                total += 3;
            } else {
                total += 4;
            }
        }
        return total;
    }
</script>

二、二进制压缩/解压缩

    //压缩
    public static deflatePlan(b: model_a.Byte): Uint8Array {
        return pako.deflate(new Uint8Array(b.buffer));
    }

    //解压
    public static inflatePlan(b: Uint8Array): Uint8Array {
        return pako.inflate(b);
    }

1.压缩后存储成本地文件

let e = PlanTool.encodePlan(data);
console.log("编码数据:", e);

let d = PlanTool.decodePlan(e);
console.log("画墙方案编码然后解码:", d);
//压缩后存储
let plan = PlanTool.deflatePlan(e);
PlanTool.saveForWebBrowser(new Blob([plan.buffer]), "plan");

//---------------------------------------------
// let str = JSON.stringify(obj);
// let b = new Blob([str]);
public static saveForWebBrowser(b: Blob, fileName: string) {
    let downloadLink = document.createElement('a');
    downloadLink.download = fileName;
    downloadLink.innerHTML = 'Download File';
    if (window.URL != null) {
        downloadLink.href = window.URL.createObjectURL(b);
    } else {
        downloadLink.href = window.URL.createObjectURL(b);
        downloadLink.onclick = () => {
            document.body.removeChild(downloadLink);
        };
        downloadLink.style.display = 'none';
        document.body.appendChild(downloadLink);
    }
    downloadLink.click();
}

2.加载本地文件后，解压缩

//https://developer.mozilla.org/zh-CN/docs/Web/API/XMLHttpRequest/Sending_and_Receiving_Binary_Data
let oReq = new XMLHttpRequest();
oReq.open("GET", "resources/plan.txt", true);
oReq.responseType = "arraybuffer";
oReq.onload = () => {
    let arrayBuffer = oReq.response as ArrayBuffer; // 注意:不是oReq.responseText
    if (arrayBuffer) {
        //解压
        let byteArray = new Uint8Array(arrayBuffer);
        let d = PlanTool.inflatePlan(byteArray);
        //解码
        let b = new model_a.Byte(d.buffer);
        let decodeData = PlanTool.decodePlan(b);
        console.log("画墙方案编码然后解码:", decodeData);

        let scene = Eventtower.instance.getAtt(TGetGloba.GET_STAGE_MANAGER).scene;
        let idToContent = Eventtower.instance.getAtt(TGetGloba.GET_STAGE_MANAGER).idToContent;
        scene.decodeToObject(decodeData, idToContent);
        //性能测试
        PlanTool.benchmarkDecodePlan(b);
        PlanTool.benchmarkInflatePlan(byteArray);
    }
};
oReq.send(null);

三、字符串压缩/解压缩

private unzip(b64Data) {
  var strData = atob(b64Data);
  // Convert binary string to character-number array
  var charData = strData.split('').map(function (x) { return x.charCodeAt(0); });
  // Turn number array into byte-array
  var binData = new Uint8Array(charData);
  // unzip
  var data = pako.inflate(binData);
  // Convert gunzipped byteArray back to ascii string:
  return this.ab2str(data);
}

private zip(str) {
  var binaryString = pako.gzip(str, { to: 'string' });
  let v = this.ab2str(binaryString as any);
  return btoa(v);
}

// ArrayBuffer转为字符串，参数为ArrayBuffer对象
private ab2str(buf: ArrayBuffer): string {
  var binaryString = '';
  let bytes = new Uint16Array(buf);
  let length = bytes.length;
  for (var i = 0; i < length; i++) {
    binaryString += String.fromCharCode(bytes[i]);
  }
  return binaryString;
}

因为字符串需要与后端通讯，所以使用了bota/atob进行base64编码。

1.ArrayBuffer转为字符串的问题

关于字符串与二进制处理，可以参考jsmpeg系列一 基础知识 字符处理 ArrayBuffer TypedArray，其中提到了ArrayBuffer与字符串的互相转换。

ArrayBuffer转为字符串，或者字符串转为ArrayBuffer，有一个前提，即字符串的编码方法是确定的。假定字符串采用UTF-16编码（JavaScript的内部编码方式），可以自己编写转换函数。

// ArrayBuffer转为字符串，参数为ArrayBuffer对象
function ab2str(buf) {
   return String.fromCharCode.apply(null, new Uint16Array(buf));
}

// 字符串转为ArrayBuffer对象，参数为字符串
function str2ab(str) {
    var buf = new ArrayBuffer(str.length*2); // 每个字符占用2个字节
    var bufView = new Uint16Array(buf);
    for (var i=0, strLen=str.length; i<strLen; i++) {
         bufView[i] = str.charCodeAt(i);
    }
    return buf;
}

但是，ab2str这种写法，在实际使用中，如果buf过大，会有Maximum call stack size exceeded堆栈溢出。

可以参考javascript - js数组转字符串 - 在字符串和ArrayBuffers之间转换，改为for的写法：

var
  binaryString = '',
  bytes = new Uint8Array(arrayBuffer),
  length = bytes.length;
for (var i = 0; i < length; i++) {
  binaryString += String.fromCharCode(bytes[i]);
}