JUCE/modules/juce_core/text/juce_TextDiff.cpp

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

   This file is part of the JUCE library.
   Copyright (c) 2017 - ROLI Ltd.

   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.

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

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