JUCE/modules/juce_core/text/juce_TextDiff.cpp

/*
  ==============================================================================

   This file is part of the JUCE library.
   Copyright (c) 2016 - ROLI Ltd.

   Permission is granted to use this software 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.

   THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH REGARD
   TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
   FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT,
   OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF
   USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
   TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE
   OF THIS SOFTWARE.

   -----------------------------------------------------------------------------

   To release a closed-source product which uses other parts of JUCE not
   licensed under the ISC terms, commercial licenses are available: visit
   www.juce.com for more information.

  ==============================================================================
*/

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 (const 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, const 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 (;;)
        {
            const juce_wchar ca = *a.text;
            const juce_wchar 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;
        const int 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);

        const size_t scratchSpace = sizeof (int) * (2 + 2 * (size_t) lenB);

        if (scratchSpace < 4096)
        {
            int* scratch = (int*) alloca (scratchSpace);
            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);

        int* l0 = lines;
        int* l1 = l0 + lenB + 1;

        int loopsWithoutImprovement = 0;
        int bestLength = 0;

        for (int i = 0; i < lenA; ++i)
        {
            const juce_wchar ca = a.getAndAdvance();
            String::CharPointerType b2 (b);

            for (int j = 0; j < lenB; ++j)
            {
                if (ca != b2.getAndAdvance())
                {
                    l1[j + 1] = 0;
                }
                else
                {
                    const int 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, const int lenA, int& indexInA,
                                 String::CharPointerType b, const 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 (int i = 0; i < changes.size(); ++i)
        text = changes.getReference(i).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  : public UnitTest
{
public:
    DiffTests() : UnitTest ("TextDiff class") {}

    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);
        const String result (diff.appliedTo (a));
        expectEquals (result, b);
    }

    void runTest() override
    {
        beginTest ("TextDiff");

        Random 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;)
        {
            String s (createString (r));
            testDiff (s, createString (r));
            testDiff (s + createString (r), s + createString (r));
        }
    }
};

static DiffTests diffTests;

#endif