juce_TextDiff.cpp

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 /*
  ==============================================================================
 
   This file is part of the JUCE library.
   Copyright (c) 2022 - Raw Material Software Limited
 
   JUCE is an open source library subject to commercial or open-source
   licensing.
 
   The code included in this file is provided under the terms of the ISC license
   http://www.isc.org/downloads/software-support-policy/isc-license. Permission
   To use, copy, modify, and/or distribute this software for any purpose with or
   without fee is hereby granted provided that the above copyright notice and
   this permission notice appear in all copies.
 
   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
   DISCLAIMED.
 
  ==============================================================================
*/
 
namespace juce
{
 
struct TextDiffHelpers
{
    enum { minLengthToMatch = 3,
           maxComplexity = 16 * 1024 * 1024 };
 
    struct StringRegion
    {
        StringRegion (const String& s) noexcept
            : text (s.getCharPointer()), start (0), length (s.length()) {}
 
        StringRegion (String::CharPointerType t, int s, int len) noexcept
            : text (t), start (s), length (len) {}
 
        void incrementStart() noexcept  { ++text; ++start; --length; }
 
        String::CharPointerType text;
        int start, length;
    };
 
    static void addInsertion (TextDiff& td, String::CharPointerType text, int index, int length)
    {
        TextDiff::Change c;
        c.insertedText = String (text, (size_t) length);
        c.start = index;
        c.length = 0;
        td.changes.add (c);
    }
 
    static void addDeletion (TextDiff& td, int index, int length)
    {
        TextDiff::Change c;
        c.start = index;
        c.length = length;
        td.changes.add (c);
    }
 
    static void diffSkippingCommonStart (TextDiff& td, StringRegion a, StringRegion b)
    {
        for (;;)
        {
            auto ca = *a.text;
            auto cb = *b.text;
 
            if (ca != cb || ca == 0)
                break;
 
            a.incrementStart();
            b.incrementStart();
        }
 
        diffRecursively (td, a, b);
    }
 
    static void diffRecursively (TextDiff& td, StringRegion a, StringRegion b)
    {
        int indexA = 0, indexB = 0;
        auto len = findLongestCommonSubstring (a.text, a.length, indexA,
                                               b.text, b.length, indexB);
 
        if (len >= minLengthToMatch)
        {
            if (indexA > 0 && indexB > 0)
                diffSkippingCommonStart (td, StringRegion (a.text, a.start, indexA),
                                             StringRegion (b.text, b.start, indexB));
            else if (indexA > 0)
                addDeletion (td, b.start, indexA);
            else if (indexB > 0)
                addInsertion (td, b.text, b.start, indexB);
 
            diffRecursively (td, StringRegion (a.text + (indexA + len), a.start + indexA + len, a.length - indexA - len),
                                 StringRegion (b.text + (indexB + len), b.start + indexB + len, b.length - indexB - len));
        }
        else
        {
            if (a.length > 0)   addDeletion (td, b.start, a.length);
            if (b.length > 0)   addInsertion (td, b.text, b.start, b.length);
        }
    }
 
    static int findLongestCommonSubstring (String::CharPointerType a, const int lenA, int& indexInA,
                                           String::CharPointerType b, const int lenB, int& indexInB) noexcept
    {
        if (lenA == 0 || lenB == 0)
            return 0;
 
        if (lenA * lenB > maxComplexity)
            return findCommonSuffix (a, lenA, indexInA,
                                     b, lenB, indexInB);
 
        auto scratchSpace = sizeof (int) * (2 + 2 * (size_t) lenB);
 
        if (scratchSpace < 4096)
        {
            JUCE_BEGIN_IGNORE_WARNINGS_MSVC (6255)
            auto* scratch = (int*) alloca (scratchSpace);
            JUCE_END_IGNORE_WARNINGS_MSVC
 
            return findLongestCommonSubstring (a, lenA, indexInA, b, lenB, indexInB, scratchSpace, scratch);
        }
 
        HeapBlock<int> scratch (scratchSpace);
        return findLongestCommonSubstring (a, lenA, indexInA, b, lenB, indexInB, scratchSpace, scratch);
    }
 
    static int findLongestCommonSubstring (String::CharPointerType a, const int lenA, int& indexInA,
                                           String::CharPointerType b, const int lenB, int& indexInB,
                                           const size_t scratchSpace, int* const lines) noexcept
    {
        zeromem (lines, scratchSpace);
 
        auto* l0 = lines;
        auto* l1 = l0 + lenB + 1;
 
        int loopsWithoutImprovement = 0;
        int bestLength = 0;
 
        for (int i = 0; i < lenA; ++i)
        {
            auto ca = a.getAndAdvance();
            auto b2 = b;
 
            for (int j = 0; j < lenB; ++j)
            {
                if (ca != b2.getAndAdvance())
                {
                    l1[j + 1] = 0;
                }
                else
                {
                    auto len = l0[j] + 1;
                    l1[j + 1] = len;
 
                    if (len > bestLength)
                    {
                        loopsWithoutImprovement = 0;
                        bestLength = len;
                        indexInA = i;
                        indexInB = j;
                    }
                }
            }
 
            if (++loopsWithoutImprovement > 100)
                break;
 
            std::swap (l0, l1);
        }
 
        indexInA -= bestLength - 1;
        indexInB -= bestLength - 1;
        return bestLength;
    }
 
    static int findCommonSuffix (String::CharPointerType a, int lenA, int& indexInA,
                                 String::CharPointerType b, int lenB, int& indexInB) noexcept
    {
        int length = 0;
        a += lenA - 1;
        b += lenB - 1;
 
        while (length < lenA && length < lenB && *a == *b)
        {
            --a;
            --b;
            ++length;
        }
 
        indexInA = lenA - length;
        indexInB = lenB - length;
        return length;
    }
};
 
TextDiff::TextDiff (const String& original, const String& target)
{
    TextDiffHelpers::diffSkippingCommonStart (*this, original, target);
}
 
String TextDiff::appliedTo (String text) const
{
    for (auto& c : changes)
        text = c.appliedTo (text);
 
    return text;
}
 
bool TextDiff::Change::isDeletion() const noexcept
{
    return insertedText.isEmpty();
}
 
String TextDiff::Change::appliedTo (const String& text) const noexcept
{
    return text.replaceSection (start, length, insertedText);
}
 
 
//==============================================================================
//==============================================================================
#if JUCE_UNIT_TESTS
 
class DiffTests final : public UnitTest
{
public:
    DiffTests()
        : UnitTest ("TextDiff class", UnitTestCategories::text)
    {}
 
    static String createString (Random& r)
    {
        juce_wchar buffer[500] = { 0 };
 
        for (int i = r.nextInt (numElementsInArray (buffer) - 1); --i >= 0;)
        {
            if (r.nextInt (10) == 0)
            {
                do
                {
                    buffer[i] = (juce_wchar) (1 + r.nextInt (0x10ffff - 1));
                }
                while (! CharPointer_UTF16::canRepresent (buffer[i]));
            }
            else
                buffer[i] = (juce_wchar) ('a' + r.nextInt (3));
        }
 
        return CharPointer_UTF32 (buffer);
    }
 
    void testDiff (const String& a, const String& b)
    {
        TextDiff diff (a, b);
        auto result = diff.appliedTo (a);
        expectEquals (result, b);
    }
 
    void runTest() override
    {
        beginTest ("TextDiff");
 
        auto r = getRandom();
 
        testDiff (String(), String());
        testDiff ("x", String());
        testDiff (String(), "x");
        testDiff ("x", "x");
        testDiff ("x", "y");
        testDiff ("xxx", "x");
        testDiff ("x", "xxx");
 
        for (int i = 1000; --i >= 0;)
        {
            auto s = createString (r);
            testDiff (s, createString (r));
            testDiff (s + createString (r), s + createString (r));
        }
    }
};
 
static DiffTests diffTests;
 
#endif
 
} // namespace juce