import { Change, diffWords } from 'diff';

import { Context } from './context';

import { FormattedString } from './formattedString';

import { ThemeColor } from './themes';

import { zip } from './zip';

const HIGHLIGHT_CHANGE_RATIO = 1.0;

/**

* Given a pair of lines from a diff, returns more granular changes to the

* lines. Attempts to only return changes that are useful. The current heuristic

* is to only show granular changes if the ratio of change to unchanged parts in

* the line is below a threshold, otherwise the lines have changed substantially

* enough for the granular diffs to not be useful.

*/

function getChangesInLine(

context: Context,

lineA: string | null,

lineB: string | null

): Change[] | null {

const { HIGHLIGHT_LINE_CHANGES } = context;

if (!HIGHLIGHT_LINE_CHANGES || lineA === null || lineB === null) {

return null;

}

// Drop the prefix

const lineTextA = lineA.slice(1);

const lineTextB = lineB.slice(1);

const changes = diffWords(lineTextA, lineTextB, {

ignoreCase: false,

ignoreWhitespace: false,

});

// Count how many words changed vs total words. Note that a replacement gets

// double counted.

let changedWords = 0;

let totalWords = 0;

for (const { added, removed, count } of changes) {

if (added || removed) {

changedWords += count ?? 0;

} else {

totalWords += count ?? 0;

}

}

if (changedWords > totalWords * HIGHLIGHT_CHANGE_RATIO) {

return null;

}

return changes;

}

export function getChangesInLines(

context: Context,

linesA: (string | null)[],

linesB: (string | null)[]

): Array<Change[] | null> {

const changes = [];

for (const [lineA, lineB] of zip(linesA, linesB)) {

changes.push(getChangesInLine(context, lineA ?? null, lineB ?? null));

}

return changes;

}

export function highlightChangesInLine(

context: Context,

linePrefix: string,

formattedLine: FormattedString,

changes: Change[] | null

): void {

if (!changes) {

return;

}

const { DELETED_WORD_COLOR, INSERTED_WORD_COLOR, UNMODIFIED_LINE_COLOR } =

context;

let wordColor: ThemeColor;

switch (linePrefix) {

case '-':

wordColor = DELETED_WORD_COLOR;

break;

case '+':

wordColor = INSERTED_WORD_COLOR;

break;

default:

wordColor = UNMODIFIED_LINE_COLOR; // This is actually not used

break;

}

let lineIndex = 0;

for (const change of changes) {

// Skip changes that would not be present in the line

if (change.removed && linePrefix === '+') {

continue;

}

if (change.added && linePrefix === '-') {

continue;

}

if (change.removed || change.added) {

formattedLine.addSpan(

lineIndex,

lineIndex + change.value.length,

wordColor

);

}

lineIndex += change.value.length;

}

}