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// Copyright (C) 2025 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
// Qt-Security score:significant reason:default
// This code is adapted from QtCreator/src/libs/utils/differ.
/*
The main algorithm "diffMyers()" is based on "An O(ND) Difference Algorithm
Preprocessing and postprocessing functions inspired by "Diff Strategies"
*/
#include "qqmldiffer_p.h"
#include <QStringList>
#include <algorithm>
#include <vector>
QT_BEGIN_NAMESPACE
using namespace Qt::StringLiterals;
namespace QQmlLSUtils {
static qsizetype commonPrefix(const QString &text1, const QString &text2)
{
// mismatch returns first non-equal pair
const auto [it1, it2] = std::mismatch(text1.cbegin(), text1.cend(),
text2.cbegin(), text2.cend());
return qsizetype(std::distance(text1.cbegin(), it1)); // number of matching characters
}
static qsizetype commonSuffix(const QString &text1, const QString &text2)
{
// mismatch returns first non-equal pair
const auto [it1, it2] = std::mismatch(text1.crbegin(), text1.crend(),
text2.crbegin(), text2.crend());
return qsizetype(std::distance(text1.crbegin(), it1)); // number of matching characters
}
static QList<Diff> decode(const QList<Diff> &diffList, const QStringList &lines)
{
QList<Diff> newDiffList;
newDiffList.reserve(diffList.size());
for (const Diff &diff : diffList) {
QString text;
for (QChar c : diff.text) {
const qsizetype idx = static_cast<ushort>(c.unicode());
text += lines.value(idx);
}
newDiffList.append({diff.command, text});
}
return newDiffList;
}
static QList<Diff> squashEqualities(const QList<Diff> &diffList)
{
if (diffList.size() < 3) // we need at least 3 items
return diffList;
QList<Diff> newDiffList;
Diff prevDiff = diffList.at(0);
Diff thisDiff = diffList.at(1);
Diff nextDiff = diffList.at(2);
qsizetype i = 2;
while (i < diffList.size()) {
if (prevDiff.command == Diff::Equal
&& nextDiff.command == Diff::Equal) {
if (thisDiff.text.endsWith(prevDiff.text)) {
thisDiff.text = prevDiff.text
+ thisDiff.text.left(thisDiff.text.size()
- prevDiff.text.size());
nextDiff.text = prevDiff.text + nextDiff.text;
} else if (thisDiff.text.startsWith(nextDiff.text)) {
prevDiff.text += nextDiff.text;
thisDiff.text = thisDiff.text.mid(nextDiff.text.size())
+ nextDiff.text;
i++;
if (i < diffList.size())
nextDiff = diffList.at(i);
newDiffList.append(prevDiff);
} else {
newDiffList.append(prevDiff);
}
} else {
newDiffList.append(prevDiff);
}
prevDiff = thisDiff;
thisDiff = nextDiff;
i++;
if (i < diffList.size())
nextDiff = diffList.at(i);
}
newDiffList.append(prevDiff);
if (i == diffList.size())
newDiffList.append(thisDiff);
return newDiffList;
}
///////////////
Diff::Diff(Command com, const QString &txt) :
command(com),
text(txt)
{
}
bool Diff::operator==(const Diff &other) const
{
return command == other.command && text == other.text;
}
bool Diff::operator!=(const Diff &other) const
{
return !(operator == (other));
}
///////////////
Differ::Differ()
{
}
QList<Diff> Differ::diff(const QString &text1, const QString &text2)
{
m_currentDiffMode = m_diffMode;
return merge(preprocess1AndDiff(text1, text2));
}
void Differ::setDiffMode(Differ::DiffMode mode)
{
m_diffMode = mode;
}
Differ::DiffMode Differ::diffMode() const
{
return m_diffMode;
}
QList<Diff> Differ::preprocess1AndDiff(const QString &text1, const QString &text2)
{
if (text1.isNull() && text2.isNull())
return {};
if (text1 == text2) {
QList<Diff> diffList;
if (!text1.isEmpty())
diffList.append(Diff(Diff::Equal, text1));
return diffList;
}
QString newText1 = text1;
QString newText2 = text2;
QString prefix;
QString suffix;
const qsizetype prefixCount = commonPrefix(text1, text2);
if (prefixCount) {
prefix = text1.left(prefixCount);
newText1 = text1.mid(prefixCount);
newText2 = text2.mid(prefixCount);
}
const qsizetype suffixCount = commonSuffix(newText1, newText2);
if (suffixCount) {
suffix = newText1.right(suffixCount);
newText1 = newText1.left(newText1.size() - suffixCount);
newText2 = newText2.left(newText2.size() - suffixCount);
}
QList<Diff> diffList = preprocess2AndDiff(newText1, newText2);
if (prefixCount)
diffList.prepend(Diff(Diff::Equal, prefix));
if (suffixCount)
diffList.append(Diff(Diff::Equal, suffix));
return diffList;
}
QList<Diff> Differ::preprocess2AndDiff(const QString &text1, const QString &text2)
{
QList<Diff> diffList;
if (text1.isEmpty()) {
diffList.append(Diff(Diff::Insert, text2));
return diffList;
}
if (text2.isEmpty()) {
diffList.append(Diff(Diff::Delete, text1));
return diffList;
}
if (text1.size() != text2.size()) {
const QString longtext = text1.size() > text2.size() ? text1 : text2;
const QString shorttext = text1.size() > text2.size() ? text2 : text1;
const qsizetype i = longtext.indexOf(shorttext);
if (i != -1) {
const Diff::Command command = (text1.size() > text2.size())
? Diff::Delete : Diff::Insert;
diffList.append(Diff(command, longtext.left(i)));
diffList.append(Diff(Diff::Equal, shorttext));
diffList.append(Diff(command, longtext.mid(i + shorttext.size())));
return diffList;
}
if (shorttext.size() == 1) {
diffList.append(Diff(Diff::Delete, text1));
diffList.append(Diff(Diff::Insert, text2));
return diffList;
}
}
if (m_currentDiffMode != Differ::CharMode && text1.size() > 80 && text2.size() > 80)
return diffNonCharMode(text1, text2);
return diffMyers(text1, text2);
}
QList<Diff> Differ::diffMyers(const QString &text1, const QString &text2)
{
const qsizetype n = text1.size();
const qsizetype m = text2.size();
const bool odd = (n + m) % 2;
const qsizetype D = odd ? (n + m) / 2 + 1 : (n + m) / 2;
const qsizetype delta = n - m;
const qsizetype vShift = D;
std::vector<qsizetype> forwardV(2 * D + 1);
std::vector<qsizetype> reverseV(2 * D + 1);
for (qsizetype i = 0; i <= 2 * D; i++) {
forwardV[i] = -1;
reverseV[i] = -1;
}
forwardV[vShift + 1] = 0;
reverseV[vShift + 1] = 0;
qsizetype kMinForward = -D;
qsizetype kMaxForward = D;
qsizetype kMinReverse = -D;
qsizetype kMaxReverse = D;
for (qsizetype d = 0; d <= D; d++) {
// going forward
for (qsizetype k = qMax(-d, kMinForward + qAbs(d + kMinForward) % 2);
k <= qMin(d, kMaxForward - qAbs(d + kMaxForward) % 2);
k = k + 2) {
qsizetype x;
if (k == -d || (k < d && forwardV[k + vShift - 1] < forwardV[k + vShift + 1]))
x = forwardV[k + vShift + 1]; // copy vertically from diagonal k + 1, y increases, y may exceed the graph
else
x = forwardV[k + vShift - 1] + 1; // copy horizontally from diagonal k - 1, x increases, x may exceed the graph
qsizetype y = x - k;
if (x > n) {
kMaxForward = k - 1; // we are beyond the graph (right border), don't check diagonals >= current k anymore
} else if (y > m) {
kMinForward = k + 1; // we are beyond the graph (bottom border), don't check diagonals <= current k anymore
} else {
// find snake
while (x < n && y < m) {
if (text1.at(x) != text2.at(y))
break;
x++;
y++;
}
forwardV[k + vShift] = x;
if (odd) { // check if overlap
if (k >= delta - (d - 1) && k <= delta + (d - 1)) {
if (n - reverseV[delta - k + vShift] <= x) {
return diffMyersSplit(text1, x, text2, y);
}
}
}
}
}
// in reverse direction
for (qsizetype k = qMax(-d, kMinReverse + qAbs(d + kMinReverse) % 2);
k <= qMin(d, kMaxReverse - qAbs(d + kMaxReverse) % 2);
k = k + 2) {
qsizetype x;
if (k == -d || (k < d && reverseV[k + vShift - 1] < reverseV[k + vShift + 1]))
x = reverseV[k + vShift + 1];
else
x = reverseV[k + vShift - 1] + 1;
qsizetype y = x - k;
if (x > n) {
kMaxReverse = k - 1; // we are beyond the graph (right border), don't check diagonals >= current k anymore
} else if (y > m) {
kMinReverse = k + 1; // we are beyond the graph (bottom border), don't check diagonals <= current k anymore
} else {
// find snake
while (x < n && y < m) {
if (text1.at(n - x - 1) != text2.at(m - y - 1))
break;
x++;
y++;
}
reverseV[k + vShift] = x;
if (!odd) { // check if overlap
if (k >= delta - d && k <= delta + d) {
if (n - forwardV[delta - k + vShift] <= x) {
return diffMyersSplit(text1, n - x, text2, m - x + k);
}
}
}
}
}
}
// Completely different
QList<Diff> diffList;
diffList.append(Diff(Diff::Delete, text1));
diffList.append(Diff(Diff::Insert, text2));
return diffList;
}
QList<Diff> Differ::diffMyersSplit(
const QString &text1, qsizetype x,
const QString &text2, qsizetype y)
{
const QString text11 = text1.left(x);
const QString text12 = text1.mid(x);
const QString text21 = text2.left(y);
const QString text22 = text2.mid(y);
const QList<Diff> &diffList1 = preprocess1AndDiff(text11, text21);
const QList<Diff> &diffList2 = preprocess1AndDiff(text12, text22);
return diffList1 + diffList2;
}
QList<Diff> Differ::diffNonCharMode(const QString &text1, const QString &text2)
{
QString encodedText1;
QString encodedText2;
QStringList subtexts = encode(text1, text2, &encodedText1, &encodedText2);
DiffMode diffMode = m_currentDiffMode;
m_currentDiffMode = CharMode;
// Each different subtext is a separate symbol
// process these symbols as text with bigger alphabet
QList<Diff> diffList = preprocess1AndDiff(encodedText1, encodedText2);
diffList = decode(diffList, subtexts);
QString lastDelete;
QString lastInsert;
QList<Diff> newDiffList;
for (qsizetype i = 0; i <= diffList.size(); i++) {
const Diff diffItem = i < diffList.size()
? diffList.at(i)
: Diff(Diff::Equal); // dummy, ensure we process to the end
// even when diffList doesn't end with equality
if (diffItem.command == Diff::Delete) {
lastDelete += diffItem.text;
} else if (diffItem.command == Diff::Insert) {
lastInsert += diffItem.text;
} else { // Diff::Equal
if (!(lastDelete.isEmpty() && lastInsert.isEmpty())) {
// Rediff here on char basis
newDiffList += preprocess1AndDiff(lastDelete, lastInsert);
lastDelete.clear();
lastInsert.clear();
}
newDiffList.append(diffItem);
}
}
m_currentDiffMode = diffMode;
return newDiffList;
}
QStringList Differ::encode(const QString &text1,
const QString &text2,
QString *encodedText1,
QString *encodedText2)
{
QStringList lines{{}}; // don't use code: 0
QHash<QString, qsizetype> lineToCode;
*encodedText1 = encode(text1, &lines, &lineToCode);
*encodedText2 = encode(text2, &lines, &lineToCode);
return lines;
}
qsizetype Differ::findSubtextEnd(const QString &text,
qsizetype subtextStart)
{
if (m_currentDiffMode == Differ::LineMode) {
qsizetype subtextEnd = text.indexOf(u'\n', subtextStart);
if (subtextEnd == -1)
subtextEnd = text.size() - 1;
return ++subtextEnd;
} else if (m_currentDiffMode == Differ::WordMode) {
if (!text.at(subtextStart).isLetter())
return subtextStart + 1;
qsizetype i = subtextStart + 1;
const qsizetype count = text.size();
while (i < count && text.at(i).isLetter())
i++;
return i;
}
return subtextStart + 1; // CharMode
}
QString Differ::encode(const QString &text,
QStringList *lines,
QHash<QString, qsizetype> *lineToCode)
{
qsizetype subtextStart = 0;
qsizetype subtextEnd = -1;
QString codes;
while (subtextEnd < text.size()) {
subtextEnd = findSubtextEnd(text, subtextStart);
const QString line = text.mid(subtextStart, subtextEnd - subtextStart);
subtextStart = subtextEnd;
if (lineToCode->contains(line)) {
codes += QChar(static_cast<ushort>(lineToCode->value(line)));
} else {
lines->append(line);
lineToCode->insert(line, lines->size() - 1);
codes += QChar(static_cast<ushort>(lines->size() - 1));
}
}
return codes;
}
QList<Diff> Differ::merge(const QList<Diff> &diffList)
{
QString lastDelete;
QString lastInsert;
QList<Diff> newDiffList;
for (qsizetype i = 0; i <= diffList.size(); i++) {
Diff diff = i < diffList.size()
? diffList.at(i)
: Diff(Diff::Equal); // dummy, ensure we process to the end
// even when diffList doesn't end with equality
if (diff.command == Diff::Delete) {
lastDelete += diff.text;
} else if (diff.command == Diff::Insert) {
lastInsert += diff.text;
} else { // Diff::Equal
if (!(lastDelete.isEmpty() && lastInsert.isEmpty())) {
// common prefix
const qsizetype prefixCount = commonPrefix(lastDelete, lastInsert);
if (prefixCount) {
const QString prefix = lastDelete.left(prefixCount);
lastDelete = lastDelete.mid(prefixCount);
lastInsert = lastInsert.mid(prefixCount);
if (!newDiffList.isEmpty()
&& newDiffList.last().command == Diff::Equal) {
newDiffList.last().text += prefix;
} else {
newDiffList.append(Diff(Diff::Equal, prefix));
}
}
// common suffix
const qsizetype suffixCount = commonSuffix(lastDelete, lastInsert);
if (suffixCount) {
const QString suffix = lastDelete.right(suffixCount);
lastDelete = lastDelete.left(lastDelete.size() - suffixCount);
lastInsert = lastInsert.left(lastInsert.size() - suffixCount);
diff.text.prepend(suffix);
}
// append delete / insert / equal
if (!lastDelete.isEmpty())
newDiffList.append(Diff(Diff::Delete, lastDelete));
if (!lastInsert.isEmpty())
newDiffList.append(Diff(Diff::Insert, lastInsert));
if (!diff.text.isEmpty())
newDiffList.append(diff);
lastDelete.clear();
lastInsert.clear();
} else { // join with last equal diff
if (!newDiffList.isEmpty()
&& newDiffList.last().command == Diff::Equal) {
newDiffList.last().text += diff.text;
} else {
if (!diff.text.isEmpty())
newDiffList.append(diff);
}
}
}
}
QList<Diff> squashedDiffList = squashEqualities(newDiffList);
if (squashedDiffList.size() != newDiffList.size())
return merge(squashedDiffList);
return squashedDiffList;
}
} // namespace QQmlLSUtils
QT_END_NAMESPACE