diffsequences
Compare items in two sequences to find a longest common subsequence.
The items not in common are the items to delete or insert in a shortest edit script.
To maximize flexibility and minimize memory, you write callback functions as configuration:
Input function isCommon(aIndex, bIndex)
compares items at indexes in the sequences and returns a truthy/falsey value. This package might call your function more than once for some pairs of indexes.
 Because your function encapsulates comparison, this package can compare items according to
===
operator,Object.is
method, or other criterion.  Because your function encapsulates sequences, this package can find differences in arrays, strings, or other data.
Output function foundSubsequence(nCommon, aCommon, bCommon)
receives the number of adjacent items and starting indexes of each common subsequence. If sequences do not have common items, then this package does not call your function.
If N is the sum of lengths of sequences and L is length of a longest common subsequence, then D = N – 2L is the number of differences in the corresponding shortest edit script.
An O(ND) Difference Algorithm and Its Variations by Eugene W. Myers is fast when sequences have few differences.
This package implements the linear space variation with optimizations so it is fast even when sequences have many differences.
Usage
To add this package as a dependency of a project, do either of the following:
npm install diffsequences
yarn add diffsequences
To use diff
as the name of the default export from this package, do either of the following:
var diff = require('diffsequences').default; // CommonJS modules
import diff from 'diffsequences'; // ECMAScript modules
Call diff
with the lengths of sequences and your callback functions:
const a = ['a', 'b', 'c', 'a', 'b', 'b', 'a'];
const b = ['c', 'b', 'a', 'b', 'a', 'c'];
function isCommon(aIndex, bIndex) {
return a[aIndex] === b[bIndex];
}
function foundSubsequence(nCommon, aCommon, bCommon) {
// see examples
}
diff(a.length, b.length, isCommon, foundSubsequence);
Example of longest common subsequence
Some sequences (for example, a
and b
in the example of usage) have more than one longest common subsequence.
This package finds the following common items:
comparisons of common items  values  output arguments 

a[2] === b[0] 
'c' 
foundSubsequence(1, 2, 0) 
a[4] === b[1] 
'b' 
foundSubsequence(1, 4, 1) 
a[5] === b[3] && a[6] === b[4] 
'b', 'a' 
foundSubsequence(2, 5, 3) 
The “edit graph” analogy in the Myers paper shows the following common items:
comparisons of common items  values 

a[2] === b[0] 
'c' 
a[3] === b[2] && a[4] === b[3] 
'a', 'b' 
a[6] === b[4] 
'a' 
Various packages which implement the Myers algorithm will always agree on the length of a longest common subsequence, but might sometimes disagree on which items are in it.
Example of callback functions to count common items
// Return length of longest common subsequence according to === operator.
function countCommonItems(a, b) {
let n = 0;
function isCommon(aIndex, bIndex) {
return a[aIndex] === b[bIndex];
}
function foundSubsequence(nCommon) {
n += nCommon;
}
diff(a.length, b.length, isCommon, foundSubsequence);
return n;
}
const commonLength = countCommonItems(
['a', 'b', 'c', 'a', 'b', 'b', 'a'],
['c', 'b', 'a', 'b', 'a', 'c'],
);
category of items  expression  value 

in common  commonLength 
4 
to delete from a

a.length  commonLength 
3 
to insert from b

b.length  commonLength 
2 
If the length difference b.length  a.length
is:
 negative: its absolute value is the minimum number of items to delete from
a
 positive: it is the minimum number of items to insert from
b
 zero: there is an equal number of items to delete from
a
and insert fromb
 nonzero: there is an equal number of additional items to delete from
a
and insert fromb
In this example, 6  7
is:
 negative:
1
is the minimum number of items to delete froma
 nonzero:
2
is the number of additional items to delete froma
and insert fromb
Example of callback functions to find common items
// Return array of items in longest common subsequence according to Object.is method.
const findCommonItems = (a, b) => {
const array = [];
diff(
a.length,
b.length,
(aIndex, bIndex) => Object.is(a[aIndex], b[bIndex]),
(nCommon, aCommon) => {
for (; nCommon !== 0; nCommon = 1, aCommon += 1) {
array.push(a[aCommon]);
}
},
);
return array;
};
const commonItems = findCommonItems(
['a', 'b', 'c', 'a', 'b', 'b', 'a'],
['c', 'b', 'a', 'b', 'a', 'c'],
);
i 
commonItems[i] 
aIndex 

0 
'c' 
2 
1 
'b' 
4 
2 
'b' 
5 
3 
'a' 
6 
Example of callback functions to diff index intervals
Instead of slicing arraylike objects, you can adjust indexes in your callback functions.
// Diff index intervals that are half open [start, end) like array slice method.
const diffIndexIntervals = (a, aStart, aEnd, b, bStart, bEnd) => {
// Validate: 0 <= aStart and aStart <= aEnd and aEnd <= a.length
// Validate: 0 <= bStart and bStart <= bEnd and bEnd <= b.length
diff(
aEnd  aStart,
bEnd  bStart,
(aIndex, bIndex) => Object.is(a[aStart + aIndex], b[bStart + bIndex]),
(nCommon, aCommon, bCommon) => {
// aStart + aCommon, bStart + bCommon
},
);
// After the last common subsequence, do any remaining work.
};
Example of callback functions to emulate diff command
Linux or Unix has a diff
command to compare files line by line. Its output is a shortest edit script:
 change adjacent lines from the first file to lines from the second file
 delete lines from the first file
 append or insert lines from the second file
// Given zerobased halfopen range [start, end) of array indexes,
// return onebased closed range [start + 1, end] as string.
const getRange = (start, end) =>
start + 1 === end ? `${start + 1}` : `${start + 1},${end}`;
// Given index intervals of lines to delete or insert, or both, or neither,
// push formatted diff lines onto array.
const pushDelIns = (aLines, aIndex, aEnd, bLines, bIndex, bEnd, array) => {
const deleteLines = aIndex !== aEnd;
const insertLines = bIndex !== bEnd;
const changeLines = deleteLines && insertLines;
if (changeLines) {
array.push(`${getRange(aIndex, aEnd)}c${getRange(bIndex, bEnd)}`);
} else if (deleteLines) {
array.push(`${getRange(aIndex, aEnd)}d${String(bIndex)}`);
} else if (insertLines) {
array.push(`${String(aIndex)}a${getRange(bIndex, bEnd)}`);
} else {
return;
}
for (; aIndex !== aEnd; aIndex += 1) {
array.push(`< ${aLines[aIndex]}`); // delete is less than
}
if (changeLines) {
array.push('');
}
for (; bIndex !== bEnd; bIndex += 1) {
array.push(`> ${bLines[bIndex]}`); // insert is greater than
}
};
// Given content of two files, return emulated output of diff utility.
const findShortestEditScript = (a, b) => {
const aLines = a.split('\n');
const bLines = b.split('\n');
const aLength = aLines.length;
const bLength = bLines.length;
const isCommon = (aIndex, bIndex) => aLines[aIndex] === bLines[bIndex];
let aIndex = 0;
let bIndex = 0;
const array = [];
const foundSubsequence = (nCommon, aCommon, bCommon) => {
pushDelIns(aLines, aIndex, aCommon, bLines, bIndex, bCommon, array);
aIndex = aCommon + nCommon; // number of lines compared in a
bIndex = bCommon + nCommon; // number of lines compared in b
};
diff(aLength, bLength, isCommon, foundSubsequence);
// After the last common subsequence, push remaining change lines.
pushDelIns(aLines, aIndex, aLength, bLines, bIndex, bLength, array);
return array.length === 0 ? '' : `${array.join('\n')}\n`;
};
Example of callback functions to format diff lines
Here is simplified code to format changed and unchanged lines in expected and received values after a test fails in Jest:
// Format diff with minus or plus for change lines and space for common lines.
const formatDiffLines = (a, b) => {
// Jest depends on prettyformat package to serialize objects as strings.
// Unindented for comparison to avoid distracting differences:
const aLinesUn = format(a, {indent: 0 /*, other options*/}).split('\n');
const bLinesUn = format(b, {indent: 0 /*, other options*/}).split('\n');
// Indented to display changed and unchanged lines:
const aLinesIn = format(a, {indent: 2 /*, other options*/}).split('\n');
const bLinesIn = format(b, {indent: 2 /*, other options*/}).split('\n');
const aLength = aLinesIn.length; // Validate: aLinesUn.length === aLength
const bLength = bLinesIn.length; // Validate: bLinesUn.length === bLength
const isCommon = (aIndex, bIndex) => aLinesUn[aIndex] === bLinesUn[bIndex];
// Only because the GitHub Flavored Markdown doc collapses adjacent spaces,
// this example code and the following table represent spaces as middle dots.
let aIndex = 0;
let bIndex = 0;
const array = [];
const foundSubsequence = (nCommon, aCommon, bCommon) => {
for (; aIndex !== aCommon; aIndex += 1) {
array.push(`·${aLinesIn[aIndex]}`); // delete is minus
}
for (; bIndex !== bCommon; bIndex += 1) {
array.push(`+·${bLinesIn[bIndex]}`); // insert is plus
}
for (; nCommon !== 0; nCommon = 1, aIndex += 1, bIndex += 1) {
// For common lines, received indentation seems more intuitive.
array.push(`··${bLinesIn[bIndex]}`); // common is space
}
};
diff(aLength, bLength, isCommon, foundSubsequence);
// After the last common subsequence, push remaining change lines.
for (; aIndex !== aLength; aIndex += 1) {
array.push(`·${aLinesIn[aIndex]}`);
}
for (; bIndex !== bLength; bIndex += 1) {
array.push(`+·${bLinesIn[bIndex]}`);
}
return array;
};
const expected = {
searching: '',
sorting: {
ascending: true,
fieldKey: 'what',
},
};
const received = {
searching: '',
sorting: [
{
descending: false,
fieldKey: 'what',
},
],
};
const diffLines = formatDiffLines(expected, received);
If N is the sum of lengths of sequences and L is length of a longest common subsequence, then N – L is length of an array of diff lines. In this example, N is 7 + 9, L is 5, and N – L is 11.
i 
diffLines[i] 
aIndex 
bIndex 

0 
'··Object {' 
0 
0 
1 
'····"searching": "",' 
1 
1 
2 
'···"sorting": Object {' 
2 

3 
'·····"ascending": true,' 
3 

4 
'+·····"sorting": Array [' 
2 

5 
'+·······Object {' 
3 

6 
'+·········"descending": false,' 
4 

7 
'··········"fieldKey": "what",' 
4 
5 
8 
'········},' 
5 
6 
9 
'+·····],' 
7 

10 
'··}' 
6 
8 
Example of callback functions to find diff items
Here is simplified code to find changed and unchanged substrings within adjacent changed lines in expected and received values after a test fails in Jest:
// Return diff items for strings (compatible with diffmatchpatch package).
const findDiffItems = (a, b) => {
const isCommon = (aIndex, bIndex) => a[aIndex] === b[bIndex];
let aIndex = 0;
let bIndex = 0;
const array = [];
const foundSubsequence = (nCommon, aCommon, bCommon) => {
if (aIndex !== aCommon) {
array.push([1, a.slice(aIndex, aCommon)]); // delete is 1
}
if (bIndex !== bCommon) {
array.push([1, b.slice(bIndex, bCommon)]); // insert is 1
}
aIndex = aCommon + nCommon; // number of characters compared in a
bIndex = bCommon + nCommon; // number of characters compared in b
array.push([0, a.slice(aCommon, aIndex)]); // common is 0
};
diff(a.length, b.length, isCommon, foundSubsequence);
// After the last common subsequence, push remaining change items.
if (aIndex !== a.length) {
array.push([1, a.slice(aIndex)]);
}
if (bIndex !== b.length) {
array.push([1, b.slice(bIndex)]);
}
return array;
};
const expectedDeleted = ['"sorting": Object {', '"ascending": true,'].join(
'\n',
);
const receivedInserted = [
'"sorting": Array [',
'Object {',
'"descending": false,',
].join('\n');
const diffItems = findDiffItems(expectedDeleted, receivedInserted);
i 
diffItems[i][0] 
diffItems[i][1] 

0 
0 
'"sorting": ' 
1 
1 
'Array [\n' 
2 
0 
'Object {\n"' 
3 
1 
'a' 
4 
1 
'de' 
5 
0 
'scending": ' 
6 
1 
'tru' 
7 
1 
'fals' 
8 
0 
'e,' 
The length difference b.length  a.length
is equal to the sum of diffItems[i][0]
values times diffItems[i][1]
lengths. In this example, the difference 48  38
is equal to the sum 10
.
category of diff item  [0] 
[1] lengths 
subtotal 

in common  0 
11 + 10 + 11 + 2 
0 
to delete from a

–1 
1 + 3 
4 
to insert from b

1 
8 + 2 + 4 
14 
Instead of formatting the changed substrings with escape codes for colors in the foundSubsequence
function to save memory, this example spends memory to gain flexibility before formatting, so a separate heuristic algorithm might modify the generic array of diff items to show changes more clearly:
i 
diffItems[i][0] 
diffItems[i][1] 

6 
1 
'true' 
7 
1 
'false' 
8 
0 
',' 
For expected and received strings of serialized data, the result of finding changed lines, and then finding changed substrings within adjacent changed lines (as in the preceding two examples) sometimes displays the changes in a more intuitive way than the result of finding changed substrings, and then splitting them into changed and unchanged lines.