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Applying optimizations to the limited version
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@Yomguithereal
Yomguithereal committed Jan 19, 2017
1 parent 8082e25 commit 60996ef
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Showing 2 changed files with 199 additions and 51 deletions.
6 changes: 6 additions & 0 deletions benchmark/levenshtein.js
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,8 @@ require('babel-core/register');
var levenshtein = require('../src/metrics/distance/levenshtein');
var leven = require('leven');

var limited = levenshtein.limited;

function run(fn) {
fn('a', 'b');
fn('ab', 'ac');
Expand All @@ -29,4 +31,8 @@ suite('Levenshtein', function() {
bench('leven', function() {
run(leven);
});

bench('limited', function() {
run(limited.bind(null, 2));
});
});
244 changes: 193 additions & 51 deletions src/metrics/distance/levenshtein.js
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Original file line number Diff line number Diff line change
Expand Up @@ -13,22 +13,23 @@
*
* [Tags]: metric, string metric.
*/
const VECTOR = [],
CODES = [];

/**
* Function returning the Levenshtein distance between two sequences.
* Function returning the Levenshtein distance between two sequences. This
* version only works on strings and leverage the `.charCodeAt` method to
* perform fast comparisons between 16 bits integers.
*
* @param {mixed} a - The first sequence to process.
* @param {mixed} b - The second sequence to process.
* @return {number} - The Levenshtein distance between a & b.
* @param {string} a - The first string to process.
* @param {string} b - The second string to process.
* @return {number} - The Levenshtein distance between a & b.
*/
const vector = [],
codes = [];

function levenshteinForStrings(a, b) {
if (a === b)
return 0;

let tmp = a;
const tmp = a;

// Swapping the strings so that the shorter string is the first one.
if (a.length > b.length) {
Expand Down Expand Up @@ -66,12 +67,12 @@ function levenshteinForStrings(a, b) {
if (!la)
return lb;

const v0 = vector;
const v0 = VECTOR;

let i = 0;

while (i < lb) {
codes[i] = b.charCodeAt(i);
CODES[i] = b.charCodeAt(i);
v0[i] = ++i;
}

Expand All @@ -94,7 +95,7 @@ function levenshteinForStrings(a, b) {
current = left;
left = v0[j];

if (charA !== codes[j]) {
if (charA !== CODES[j]) {

// Insertion
if (left < current)
Expand All @@ -112,16 +113,25 @@ function levenshteinForStrings(a, b) {
}

return current;
};
}

/**
* Function returning the Levenshtein distance between two arbitrary sequences.
*
* @param {mixed} a - The first sequence to process.
* @param {mixed} b - The second sequence to process.
* @return {number} - The Levenshtein distance between a & b.
*/
export default function levenshtein(a, b) {

// If the sequences are string, we use the optimized version
if (typeof a === 'string')
return levenshteinForStrings(a, b);

if (a === b)
return 0;

let tmp = a;
const tmp = a;

// Swapping the strings so that the shorter string is the first one.
if (a.length > b.length) {
Expand Down Expand Up @@ -159,7 +169,7 @@ export default function levenshtein(a, b) {
if (!la)
return lb;

const v0 = vector;
const v0 = VECTOR;

let i = 0;

Expand Down Expand Up @@ -205,7 +215,129 @@ export default function levenshtein(a, b) {
return current;
}

// TODO: adapt the optimizations above
/**
* Function returning the Levenshtein distance between two sequences
* but with a twist: this version will stop its computation if distance
* exceed a given maximum and return Infinity. This version only works on
* strings and leverage the `.charCodeAt` method to perform fast comparisons
* between 16 bits integers.
*
* @param {number} max - Maximum distance.
* @param {string} a - The first string to process.
* @param {string} b - The second string to process.
* @return {number} - The Levenshtein distance between a & b or Infinity.
*/
function limitedLevenshteinForStrings(max, a, b) {
if (a === b)
return 0;

const tmp = a;

// Swapping the strings so that the shorter string is the first one.
if (a.length > b.length) {
a = b;
b = tmp;
}

let la = a.length,
lb = b.length;

if (!la)
return lb > max ? Infinity : lb;
if (!lb)
return la > max ? Infinity : la;

// Ignoring common suffix
// NOTE: ~- is a fast - 1 operation, it does not work on big number though
while (la > 0 && (a.charCodeAt(~-la) === b.charCodeAt(~-lb))) {
la--;
lb--;
}

if (!la)
return lb > max ? Infinity : lb;

let start = 0;

// Ignoring common prefix
while (start < la && (a.charCodeAt(start) === b.charCodeAt(start)))
start++;

la -= start;
lb -= start;

if (!la)
return lb > max ? Infinity : lb;

const diff = lb - la;

if (max > lb)
max = lb;
else if (diff > max)
return Infinity;

const v0 = VECTOR;

let i = 0;

while (i < max) {
CODES[i] = b.charCodeAt(i);
v0[i] = ++i;
}
while (i < lb) {
CODES[i] = b.charCodeAt(i++);
v0[i] = max + 1;
}

const offset = max - diff,
haveMax = max < lb;

let jStart = 0,
jEnd = max;

let current = 0,
left,
above,
charA,
j;

// Starting the nested loops
for (i = 0; i < la; i++) {
left = i;
current = i + 1;

charA = a.charCodeAt(start + i);
jStart += (i > offset) ? 1 : 0;
jEnd += (jEnd < lb) ? 1 : 0;

for (j = jStart; j < jEnd; j++) {
above = current;

current = left;
left = v0[j];

if (charA !== CODES[j]) {

// Insertion
if (left < current)
current = left;

// Deletion
if (above < current)
current = above;

current++;
}

v0[j] = current;
}

if (haveMax && v0[i + diff] > max)
return Infinity;
}

return current <= max ? current : Infinity;
}

/**
* Function returning the Levenshtein distance between two sequences
Expand All @@ -218,9 +350,22 @@ export default function levenshtein(a, b) {
* @return {number} - The Levenshtein distance between a & b or Infinity.
*/
export function limited(max, a, b) {

// If the sequences are string, we use the optimized version
if (typeof a === 'string')
return limitedLevenshteinForStrings(max, a, b);

if (a === b)
return 0;

const tmp = a;

// Swapping the strings so that the shorter string is the first one.
if (a.length > b.length) {
a = b;
b = tmp;
}

let la = a.length,
lb = b.length;

Expand All @@ -229,35 +374,27 @@ export function limited(max, a, b) {
if (!lb)
return la > max ? Infinity : la;

// Swapping the strings so that the shorter string is the first one.
if (la > lb) {
[a, b] = [b, a];
[la, lb] = [lb, la];
}

// Ignoring common suffix
while (la > 0 && (a[la - 1] === b[lb - 1])) {
// NOTE: ~- is a fast - 1 operation, it does not work on big number though
while (la > 0 && (a[~-la] === b[~-lb])) {
la--;
lb--;
}

let start = 0;

// If a common prefix exists of if a is a full b suffix
if (a[0] === b[0] || !la) {
if (!la)
return lb > max ? Infinity : lb;

// Common prefix can also be ignored
while (start < la && a[start] === b[start])
start++;
let start = 0;

la -= start;
lb -= start;
// Ignoring common prefix
while (start < la && (a[start] === b[start]))
start++;

if (!la)
return lb > max ? Infinity : lb;
la -= start;
lb -= start;

b = b.slice(start, start + lb);
}
if (!la)
return lb > max ? Infinity : lb;

const diff = lb - la;

Expand All @@ -266,40 +403,45 @@ export function limited(max, a, b) {
else if (diff > max)
return Infinity;

const v0 = new Array(lb),
v2 = new Array(lb);
const v0 = VECTOR;

let i;
for (i = 0; i < max; i++)
v0[i] = i + 1;
for (; i < lb; i++)
v0[i] = max + 1;
let i = 0;

while (i < max) {
v0[i] = ++i;
}
while (i < lb) {
v0[i++] = max + 1;
}

const offset = max - diff,
haveMax = max < lb;

let jStart = 0,
jEnd = max;

let current = 0;
let current = 0,
left,
above,
charA,
j;

// Starting the nested loops
for (i = 0; i < la; i++) {
let left = i;

left = i;
current = i + 1;
const charA = a[start + i];

charA = a[start + i];
jStart += (i > offset) ? 1 : 0;
jEnd += (jEnd < lb) ? 1 : 0;

for (let j = jStart; j < jEnd; j++) {
const above = current;
for (j = jStart; j < jEnd; j++) {
above = current;

const charB = b[j];
v2[j] = current = left;
current = left;
left = v0[j];

if (charA !== charB) {
if (charA !== b[j]) {

// Insertion
if (left < current)
Expand Down

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