我试图读取一个大文件一行在一次。我在Quora上找到了一个关于这个主题的问题，但我错过了一些联系，把整个事情联系在一起。
var Lazy=require("lazy");
new Lazy(process.stdin)
.lines
.forEach(
function(line) {
console.log(line.toString());
}
);
process.stdin.resume();
我想要弄清楚的是如何一次从文件中读取一行，而不是像本例中那样读取STDIN。
我试着:
fs.open('./VeryBigFile.csv', 'r', '0666', Process);
function Process(err, fd) {
if (err) throw err;
// DO lazy read
}
但这并不奏效。我知道在必要时我可以使用PHP之类的东西，但我想弄清楚这个问题。
我不认为其他答案会起作用，因为文件比我运行它的服务器的内存大得多。
虽然您可能应该像上面的答案所建议的那样使用readline模块，但readline似乎面向命令行接口，而不是行读取。关于缓冲，它也有点不透明。(任何需要流式行阅读器的人可能都想调整缓冲区大小)。readline模块大约有1000行，而这个，加上统计和测试，是34行。
const EventEmitter = require('events').EventEmitter;
class LineReader extends EventEmitter{
constructor(f, delim='\n'){
super();
this.totalChars = 0;
this.totalLines = 0;
this.leftover = '';
f.on('data', (chunk)=>{
this.totalChars += chunk.length;
let lines = chunk.split(delim);
if (lines.length === 1){
this.leftover += chunk;
return;
}
lines[0] = this.leftover + lines[0];
this.leftover = lines[lines.length-1];
if (this.leftover) lines.pop();
this.totalLines += lines.length;
for (let l of lines) this.onLine(l);
});
// f.on('error', ()=>{});
f.on('end', ()=>{console.log('chars', this.totalChars, 'lines', this.totalLines)});
}
onLine(l){
this.emit('line', l);
}
}
//Command line test
const f = require('fs').createReadStream(process.argv[2], 'utf8');
const delim = process.argv[3];
const lineReader = new LineReader(f, delim);
lineReader.on('line', (line)=> console.log(line));
下面是一个更短的版本，没有统计数据，只有19行:
class LineReader extends require('events').EventEmitter{
constructor(f, delim='\n'){
super();
this.leftover = '';
f.on('data', (chunk)=>{
let lines = chunk.split(delim);
if (lines.length === 1){
this.leftover += chunk;
return;
}
lines[0] = this.leftover + lines[0];
this.leftover = lines[lines.length-1];
if (this.leftover)
lines.pop();
for (let l of lines)
this.emit('line', l);
});
}
}
我有一个小模块，它做得很好，被相当多的其他项目使用npm readline注意，在节点v10中有一个本地readline模块，所以我重新发布了我的模块linebyline https://www.npmjs.com/package/linebyline
如果你不想使用这个模块，函数很简单:
var fs = require('fs'),
EventEmitter = require('events').EventEmitter,
util = require('util'),
newlines = [
13, // \r
10 // \n
];
var readLine = module.exports = function(file, opts) {
if (!(this instanceof readLine)) return new readLine(file);
EventEmitter.call(this);
opts = opts || {};
var self = this,
line = [],
lineCount = 0,
emit = function(line, count) {
self.emit('line', new Buffer(line).toString(), count);
};
this.input = fs.createReadStream(file);
this.input.on('open', function(fd) {
self.emit('open', fd);
})
.on('data', function(data) {
for (var i = 0; i < data.length; i++) {
if (0 <= newlines.indexOf(data[i])) { // Newline char was found.
lineCount++;
if (line.length) emit(line, lineCount);
line = []; // Empty buffer.
} else {
line.push(data[i]); // Buffer new line data.
}
}
}).on('error', function(err) {
self.emit('error', err);
}).on('end', function() {
// Emit last line if anything left over since EOF won't trigger it.
if (line.length){
lineCount++;
emit(line, lineCount);
}
self.emit('end');
}).on('close', function() {
self.emit('close');
});
};
util.inherits(readLine, EventEmitter);
function createLineReader(fileName){
var EM = require("events").EventEmitter
var ev = new EM()
var stream = require("fs").createReadStream(fileName)
var remainder = null;
stream.on("data",function(data){
if(remainder != null){//append newly received data chunk
var tmp = new Buffer(remainder.length+data.length)
remainder.copy(tmp)
data.copy(tmp,remainder.length)
data = tmp;
}
var start = 0;
for(var i=0; i<data.length; i++){
if(data[i] == 10){ //\n new line
var line = data.slice(start,i)
ev.emit("line", line)
start = i+1;
}
}
if(start<data.length){
remainder = data.slice(start);
}else{
remainder = null;
}
})
stream.on("end",function(){
if(null!=remainder) ev.emit("line",remainder)
})
return ev
}
//---------main---------------
fileName = process.argv[2]
lineReader = createLineReader(fileName)
lineReader.on("line",function(line){
console.log(line.toString())
//console.log("++++++++++++++++++++")
})
