Node.js / javascript minhash模块，为类似文本输出类似的hashstring

const str1 = "The quick brown fox jumps over the lazy dog";
const str2 = "The quick brown fox jumps over the lazy d";
console.log(str1);
console.log(str2);
var s1 = str1.split(' ');
var s2 = str2.split(' ');

// create a hash for each set of words to compare
// default numPerm is 128 but that gives very long hash
// below 8, almost similar string will give exactly the same hash
var m1 = new Minhash({numPerm: 8});
var m2 = new Minhash({numPerm: 8});

// update each hash
s1.map(function(w) { m1.update(w) });
s2.map(function(w) { m2.update(w) });


// estimate the jaccard similarity between two minhashes
console.log('jaccard similarity:', m1.jaccard(m2));

// Now to convert hashvalues to a string we use a kind of base64
// encode but since hasvalues is an array of 32bits integer we
// have to explode it into a array of 8bits integers first

// for a given int32 returns 4 bytes
function int32ToBytes(num) {
    // the hexadecimal representation of the largest 32bits unsigned integer is 0xFFFFFFFF
    // the hexadecimal representation of the largest unsigned integer (8bits === a byte) is 0xFF
    // so it is possible to think a 32bits uint (unsigned integer) as the concatenation of 4 8bits uint.
    // the bitwise & operator is the bitwise AND
    // its table of truth is 0 & 0 = 0, 0 & 1 = 0, 1 & 0 = 0 and 1 & 1 = 1
    // for instance 8 & 1 <=> 0b111 & 0b001 <=> 0b001 <=> 1

    // the same is possible with hex representation:
    // 65535 & 255 <=> 0xFFFF & 0x00FF <=> 0x0FF <=> 255
    // 65535 & 65280 <=> 0xFFFF & 0xFF00 <=> 0xFF00 <=> 65280
    // 255 + 65535 = 65535

    // now about the bitwise >> shift operator
    // a >> n shift the number a by n bits to the right
    // in hex FF is 8bits so `0xFF00 >> 8 = 0xFF`
    // this operation is reversible `0xFF << 8 = 0xFF00`

    // 0xFFFF needs 16 bits to be represented, as 0xFF00
    // but 0xFF only needs 8 bits
    // so its possible to split a 16 bits integer into two 8 bits integer this way:
    // int16 = (int16 & 0xFF00) >> 8 + (int16 & 0x00FF) >> 0
    // no information was lost because we're able to do the reverse operation

    // the same principle is used below to encode a 32 bits integer into 4 bytes (8bits integers)
   // max uint32 = 0xFFFFFFFF =
   // 0xFF << 24 + 0xFF << 16 + 0xFF << 8 + 0xFF << 0
    

  
    const arr = [
        (num & 0xff000000) >> 24,
        (num & 0x00ff0000) >> 16,
        (num & 0x0000ff00) >> 8,
        (num & 0x000000ff)
    ];
    return arr;
}

// tolerant base64 encode of 4 bytes
function Uint8ToString(u8a){
  var CHUNK_SZ = 0x8000;
  var c = [];
  for (var i=0; i < u8a.length; i+=CHUNK_SZ) {
    c.push(String.fromCharCode.apply(null, u8a.subarray(i, i+CHUNK_SZ)));
  }
  return c.join("");
}

// tolerant base64 encode of int32 array
function base64EncodeInt32Array(intArray) {
    let str = '';
    intArray.forEach((i) => {
        var u8 = new Uint8Array(int32ToBytes(i));
        var b64encoded = btoa(Uint8ToString(u8));
        str += b64encoded;
    });
    
    return str;
    
}

// replace non significant '==' to shorten hash
console.log(base64EncodeInt32Array(m1.hashvalues).replace(/==/g, ''));
console.log(base64EncodeInt32Array(m2.hashvalues).replace(/==/g, ''));

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