Line diff

Which lines of a file one side changed, from @textui/chat.

import { diffLines, toLines } from '@textui/chat';

const result = diffLines('const a = 1;', 'const a = 2;');
const rows = result.rows;
const lines = toLines('one\ntwo\n');

A host sends two whole files and a count of what changed between them. It does not send the diff itself, so somebody has to work out which lines those were, and this is that.

toLines

toLines splits a file into lines, with the trailing newline not counted as an empty last line. The empty string is no lines at all rather than one.

diffLines

diffLines(before, after, limit = 4000) lines the two sides up and returns a DiffResult.

It is deliberately the textbook algorithm rather than anything clever: the longest common subsequence of the two line arrays, with everything not in it marked as removed on the left or added on the right. The common head and tail are taken off first, because two files that differ in one line share everything either side of it and the quadratic part should only see the part that differs. A tie goes to the removal, so a replaced line reads -old then +new the way every other diff on the machine prints it.

A creation has no before and a deletion no after, and both are passed as an empty string rather than as a special case - “every line is an addition” is exactly the right diff for a new file.

The cost is quadratic in the number of lines, which is what limit is for. Over it, the result carries tooLarge with the line count and the limit instead of a diff, because the honest answer at that size is to say the files are too big rather than to spend a minute proving it.

DiffResult

Field	What it is
rows	The two sides, lined up, in file order.
added, removed	How many lines each way.
tooLarge	Set instead of a diff when the pair was over limit, with lines and limit.

Each DiffRow has a kind of same, added or removed, a text, and a 1-based before and after number - each absent on the side the row does not exist on.

See also

• FileDiff - how the rows are drawn
• CodeViewer - reading a whole file, rather than a change to it
• ScrollView - the viewport a diff is drawn in