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Buffer

buffer 基本使用

// Buffer 类在全局作用域中，因此无需使用 require('buffer').Buffer。
// buffer 实例类似于整数数组, 但对应于 v8 堆外部的固定大小的原始内存分配
const char = '我';

const buffer = Buffer.from(char);
console.log(buffer);        // <Buffer e6 88 91>

const hex = buffer.toString('hex');
console.log(hex);           // e68891

const utf8 = buffer.toString('utf8');
console.log(utf8);          // 我

const ascii = buffer.toString('ascii');
console.log(ascii);


const binary = char.charCodeAt(0).toString(2);
console.log(binary);         // 110001000010001

const eight = char.charCodeAt(0).toString(8);
console.log(eight);         // 61021

const ten = char.charCodeAt(0);
console.log(ten);           // 25105

const sixteen = char.charCodeAt(0).toString(16);
console.log(sixteen);       // 6211
// 我的 Unicode 值 uni6211, uni6211, &#x6211;
// 使用 buffer
const buffer = Buffer.allocUnsafe(26);
for (let i = 0; i < 26; i++) {
    // 存入十进制的 ASCII 值, 97 为 十进制 a 的 ASCII 值
    buffer[i] = i+ 97;
}
console.log(buffer);                             // <Buffer 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 78 79 7a>
console.log(buffer.toString('ascii'));  // abcdefghijklmnopqrstuvwxyz
console.log(buffer.toString('hex'));    // 6162636465666768696a6b6c6d6e6f707172737475767778797a
console.log(buffer.toString('utf8'));   // abcdefghijklmnopqrstuvwxyz
console.log(buffer.toString('base64')); // YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXo=

读取二进制流

const fs = require('fs');
const read = fs.createReadStream('arrow.png');
let pngBuffer = null;
read.on('data', data => {
    if (!pngBuffer) pngBuffer = data;
    else pngBuffer = pngBuffer.concat(data);
});

read.on('close', () => {
    console.log(pngBuffer);
    const hex = pngBuffer.toString('hex');

    const len = 8;
    const header = hex.substr(0,len);
    console.log(header);    // 89504e47, png 文件头
    const end = hex.substr(hex.length-len, len);
    console.log(end);       // ae426082, png 文件尾
});

buffer 与 TypeArray

// Buffer 与 TypeArray
const arr = new Uint16Array(2);
// const arr = new Uint8Array(2);
arr[0] = 7000;
arr[1] = 8000;
console.log(arr);   // Uint16Array [ 7000, 8000 ]

const buf1 = Buffer.from(arr);          // 拷贝 buffer
const buf2 = Buffer.from(arr.buffer);   // 共享 buffer


console.log(buf1);      // <Buffer 58 40>
console.log(buf2);      // <Buffer 58 1b 40 1f>

arr[0] = 63;
arr[1] = 64;

console.log(buf1);      // <Buffer 58 40>
console.log(buf2);      // <Buffer 3f 00 40 00>

buffer 与 StringDecode

// buffer 与 StringDecode

const { StringDecoder } = require('string_decoder');

// Decode to utf8
const decode = new StringDecoder('utf8');
const cent = Buffer.from([0xC2, 0xA2]);
console.log(cent);                      // <Buffer c2 a2>
console.log(`${decode.write(cent)}`);   // ¢

const euro = Buffer.from([0xE2, 0x82, 0xAC]);
console.log(euro);                      // <Buffer e2 82 ac>
console.log(`${decode.write(euro)}`);   // €

// 分段 Decode
let str = decode.write(Buffer.from('e2', 'hex'));
console.log(`${str}`);  //
str = decode.write(Buffer.from('82', 'hex'));
console.log(`${str}`);  //
str = decode.end(Buffer.from('ac', 'hex'));
console.log(`${str}`);  // €


// Decode to hex
const decodeHex = new StringDecoder('hex');
const centStr = '¢';
console.log(`${decodeHex.write(Buffer.from(centStr, 'utf8'))}`);   // c2a2
const euroStr = '€';
console.log(`${decodeHex.write(Buffer.from(euroStr, 'utf8'))}`);   // e282ac