Javascript 如何压缩字符串？

How to compress a string?

提问by FurtiveFelon

I would like to have a reversible compression for a type of string so that i can include it in URLs without keeping track of what it refers to. The string i would like to compress is SVG path string, here is a short primer: http://apike.ca/prog_svg_paths.html

我想对一种类型的字符串进行可逆压缩，以便我可以将它包含在 URL 中，而无需跟踪它所指的内容。我想压缩的字符串是 SVG 路径字符串，这是一个简短的入门：http: //apike.ca/prog_svg_paths.html

Basically, the string contains a character, followed by arbitrary number of integers, then another character followed by arbitrary number of integers and so on.

基本上，字符串包含一个字符，后跟任意数量的整数，然后是另一个字符，后跟任意数量的整数，依此类推。

If anyone knows of a good resource for this, it would be much appreciated!

如果有人知道这方面的好资源，将不胜感激！

Jason

杰森

回答by olliej

Many compression algorithms are well documented, a couple even have js implementations:

许多压缩算法都有详细记录，有几个甚至有 js 实现：

• GZipA common (reasonably) good compression algorithm, I know there's a JS impl, i'm just hunting the URL

• LZWAnother question points to an LZW implementation in JS

• Arithmetic coding(i did this, but the model it uses is stupid so doesn't achieve the best compression rates it could)

• GZip一种常见的（合理的）好的压缩算法，我知道有一个 JS impl，我只是在寻找 URL

• LZW另一个问题指向 JS 中的 LZW 实现

• 算术编码（我这样做了，但它使用的模型很愚蠢，因此无法实现最佳压缩率）

回答by pp19dd

Sounds like you might benefit from single and double RLE compression.

听起来您可能会从单 RLE 和双 RLE 压缩中受益。

A primer on this can be seen here:

可以在此处查看有关此内容的入门：

http://pp19dd.com/2011/10/query-string-limits-encoding-hundreds-of-checkboxes-with-rle/#demo

http://pp19dd.com/2011/10/query-string-limits-encoding-hundreds-of-checkboxes-with-rle/#demo

The library should be flexible enough to modify your compression pattern to something more preferable. The writeup explains how this works; might be a good start to optimize your SVG case.

该库应该足够灵活，可以将您的压缩模式修改为更可取的。这篇文章解释了这是如何工作的；可能是优化 SVG 案例的良好开端。

回答by pepkin88

You could try Huffman compression. Number of different chars is 20-30, and if the string is long, compression should be effective.

你可以试试霍夫曼压缩。不同字符数为20-30，如果字符串很长，压缩应该有效。

回答by Simon Hutchison

The following solution returns a compressed Base64 encoded string.

以下解决方案返回压缩的 Base64 编码字符串。

Create a file called zip.js with the code below and then see usage below that.

使用下面的代码创建一个名为 zip.js 的文件，然后查看下面的用法。

// Apply LZW-compression to a string and return base64 compressed string.
export function zip (s) {
  try {
    var dict = {}
    var data = (s + '').split('')
    var out = []
    var currChar
    var phrase = data[0]
    var code = 256
    for (var i = 1; i < data.length; i++) {
      currChar = data[i]
      if (dict[phrase + currChar] != null) {
        phrase += currChar
      } else {
        out.push(phrase.length > 1 ? dict[phrase] : phrase.charCodeAt(0))
        dict[phrase + currChar] = code
        code++
        phrase = currChar
      }
    }
    out.push(phrase.length > 1 ? dict[phrase] : phrase.charCodeAt(0))
    for (var j = 0; j < out.length; j++) {
      out[j] = String.fromCharCode(out[j])
    }
    return utoa(out.join(''))
  } catch (e) {
    console.log('Failed to zip string return empty string', e)
    return ''
  }
}

// Decompress an LZW-encoded base64 string
export function unzip (base64ZippedString) {
  try {
    var s = atou(base64ZippedString)
    var dict = {}
    var data = (s + '').split('')
    var currChar = data[0]
    var oldPhrase = currChar
    var out = [currChar]
    var code = 256
    var phrase
    for (var i = 1; i < data.length; i++) {
      var currCode = data[i].charCodeAt(0)
      if (currCode < 256) {
        phrase = data[i]
      } else {
        phrase = dict[currCode] ? dict[currCode] : oldPhrase + currChar
      }
      out.push(phrase)
      currChar = phrase.charAt(0)
      dict[code] = oldPhrase + currChar
      code++
      oldPhrase = phrase
    }
    return out.join('')
  } catch (e) {
    console.log('Failed to unzip string return empty string', e)
    return ''
  }
}

// ucs-2 string to base64 encoded ascii
function utoa (str) {
  return window.btoa(unescape(encodeURIComponent(str)))
}
// base64 encoded ascii to ucs-2 string
function atou (str) {
  return decodeURIComponent(escape(window.atob(str)))
}

Usage:

用法：

import { zip, unzip } from './zip'

// Zip a string
const str = 'zip it'
const base64CompressedString = zip(str)

// Zip an object
const obj = { a: 123, b: 'zipit' }
const base64CompressedString = zip(JSON.stringify(obj))

// Unzip the base64 compressed string back to an object.
const originalObject = JSON.parse(unzip(base64CompressedString))

BTW... if you're concerned about escape/unescape being depreciated consider a polyfill

顺便说一句......如果你担心逃避/逃避被贬值，考虑一个polyfill

LZW algorithm from hereand base64 encoding from here

LZW算法从这里和编码的base64这里