Add QTextMarkdownImporter

This provides the ability to read from a Markdown string or file into
a QTextDocument, such that the formatting will be recognized and can be
rendered.

- Add QTextDocument::setMarkdown(QString)
- Add QTextEdit::setMarkdown(QString)
- Add TextFormat::MarkdownText
- QWidgetTextControl::setContent() calls QTextDocument::setMarkdown()
  if that's the format

Fixes: QTBUG-72349
Change-Id: Ief2ad71bf840666c64145d58e9ca71d05fad5659
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

7 files changed, 663 insertions, 2 deletions

diff --git a/src/gui/configure.json b/src/gui/configure.json
index 7a1796041e..a4ea4375d1 100644
--- a/src/gui/configure.json
+++ b/src/gui/configure.json
@@ -28,6 +28,7 @@
             "lgmon": "boolean",
             "libinput": "boolean",
             "libjpeg": { "type": "enum", "values": [ "no", "qt", "system" ] },
+            "libmd4c": { "type": "enum", "values": [ "no", "qt", "system" ] },
             "libpng": { "type": "enum", "values": [ "no", "qt", "system" ] },
             "linuxfb": "boolean",
             "mtdev": "boolean",
@@ -376,6 +377,17 @@
                 "-ljpeg"
             ]
         },
+        "libmd4c": {
+            "label": "libmd4c",
+            "test": {
+                "main": "md_parse(\"hello\", 5, nullptr, nullptr);"
+            },
+            "headers": "md4c.h",
+            "sources": [
+                { "type": "pkgConfig", "args": "md4c" },
+                { "libs": "-lmd4c" }
+            ]
+        },
         "libpng": {
             "label": "libpng",
             "test": {
@@ -1583,6 +1595,22 @@
             "section": "Kernel",
             "output": [ "publicFeature", "feature" ]
         },
+        "textmarkdownreader": {
+            "label": "MarkdownReader",
+            "disable": "input.libmd4c == 'no'",
+            "enable": "input.libmd4c == 'system' || input.libmd4c == 'qt' || input.libmd4c == 'yes'",
+            "purpose": "Provides a Markdown (CommonMark and GitHub) reader",
+            "section": "Kernel",
+            "output": [ "publicFeature" ]
+        },
+        "system-textmarkdownreader": {
+            "label": "  Using system libmd4c",
+            "disable": "input.libmd4c == 'qt'",
+            "enable": "input.libmd4c == 'system'",
+            "section": "Kernel",
+            "condition": "libs.libmd4c",
+            "output": [ "publicFeature" ]
+        },
         "textodfwriter": {
             "label": "OdfWriter",
             "purpose": "Provides an ODF writer.",
@@ -1861,6 +1889,12 @@ QMAKE_LIBDIR_OPENGL[_ES2] and QMAKE_LIBS_OPENGL[_ES2] in the mkspec for your pla
                         "gif", "ico", "jpeg", "system-jpeg", "png", "system-png"
                     ]
                 },
+                {
+                    "section": "Text formats",
+                    "entries": [
+                        "texthtmlparser", "cssparser", "textodfwriter", "textmarkdownreader", "system-textmarkdownreader"
+                    ]
+                },
                 "egl",
                 "openvg",
                 {
diff --git a/src/gui/text/qtextdocument.cpp b/src/gui/text/qtextdocument.cpp
index fd3473b32e..87c8f1ba8a 100644
--- a/src/gui/text/qtextdocument.cpp
+++ b/src/gui/text/qtextdocument.cpp
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2019 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtGui module of the Qt Toolkit.
@@ -70,6 +70,9 @@
 #include <private/qabstracttextdocumentlayout_p.h>
 #include "qpagedpaintdevice.h"
 #include "private/qpagedpaintdevice_p.h"
+#if QT_CONFIG(textmarkdownreader)
+#include <private/qtextmarkdownimporter_p.h>
+#endif
 
 #include <limits.h>
 
@@ -3286,6 +3289,31 @@ QString QTextDocument::toHtml(const QByteArray &encoding) const
 #endif // QT_NO_TEXTHTMLPARSER
 
 /*!
+    Replaces the entire contents of the document with the given
+    Markdown-formatted text in the \a markdown string, with the given
+    \a features supported.  By default, all supported GitHub-style
+    Markdown features are included; pass \c MarkdownDialectCommonMark
+    for a more basic parse.
+
+    The Markdown formatting is respected as much as possible; for example,
+    "*bold* text" will produce text where the first word has a font weight that
+    gives it an emphasized appearance.
+
+    Parsing of HTML included in the \a markdown string is handled in the same
+    way as in \l setHtml; however, Markdown formatting inside HTML blocks is
+    not supported. The \c MarkdownNoHTML feature flag can be set to disable
+    HTML parsing.
+
+    The undo/redo history is reset when this function is called.
+*/
+#if QT_CONFIG(textmarkdownreader)
+void QTextDocument::setMarkdown(const QString &markdown, QTextDocument::MarkdownFeatures features)
+{
+    QTextMarkdownImporter(static_cast<QTextMarkdownImporter::Features>(int(features))).import(this, markdown);
+}
+#endif
+
+/*!
     Returns a vector of text formats for all the formats used in the document.
 */
 QVector<QTextFormat> QTextDocument::allFormats() const
diff --git a/src/gui/text/qtextdocument.h b/src/gui/text/qtextdocument.h
index c9b22e053b..ade67999ad 100644
--- a/src/gui/text/qtextdocument.h
+++ b/src/gui/text/qtextdocument.h
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2019 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtGui module of the Qt Toolkit.
@@ -151,6 +151,19 @@ public:
     void setHtml(const QString &html);
 #endif
 
+#if QT_CONFIG(textmarkdownreader)
+    // Must be in sync with QTextMarkdownImporter::Features, should be in sync with #define MD_FLAG_* in md4c
+    enum MarkdownFeature {
+        MarkdownNoHTML = 0x0020 | 0x0040,
+        MarkdownDialectCommonMark = 0,
+        MarkdownDialectGitHub = 0x0004 | 0x0008 | 0x0400 | 0x0100 | 0x0200 | 0x0800
+    };
+    Q_DECLARE_FLAGS(MarkdownFeatures, MarkdownFeature)
+    Q_FLAG(MarkdownFeatures)
+
+    void setMarkdown(const QString &markdown, MarkdownFeatures features = MarkdownDialectGitHub);
+#endif
+
     QString toRawText() const;
     QString toPlainText() const;
     void setPlainText(const QString &text);
diff --git a/src/gui/text/qtextformat.h b/src/gui/text/qtextformat.h
index 80d8e82694..1eb52a379c 100644
--- a/src/gui/text/qtextformat.h
+++ b/src/gui/text/qtextformat.h
@@ -176,6 +176,7 @@ public:
         BlockNonBreakableLines = 0x1050,
         BlockTrailingHorizontalRulerWidth = 0x1060,
         HeadingLevel = 0x1070,
+        BlockMarker = 0x1080,
 
         // character properties
         FirstFontProperty = 0x1FE0,
@@ -605,6 +606,12 @@ public:
         LineDistanceHeight = 4
     };
 
+    enum MarkerType {
+        NoMarker = 0,
+        Unchecked = 1,
+        Checked = 2
+    };
+
     QTextBlockFormat();
 
     bool isValid() const { return isBlockFormat(); }
@@ -668,6 +675,11 @@ public:
     void setTabPositions(const QList<QTextOption::Tab> &tabs);
     QList<QTextOption::Tab> tabPositions() const;
 
+    inline void setMarker(MarkerType marker)
+    { setProperty(BlockMarker, int(marker)); }
+    inline MarkerType marker() const
+    { return MarkerType(intProperty(BlockMarker)); }
+
 protected:
     explicit QTextBlockFormat(const QTextFormat &fmt);
     friend class QTextFormat;
diff --git a/src/gui/text/qtextmarkdownimporter.cpp b/src/gui/text/qtextmarkdownimporter.cpp
new file mode 100644
index 0000000000..6c053ac81a
--- /dev/null
+++ b/src/gui/text/qtextmarkdownimporter.cpp
@@ -0,0 +1,435 @@
+/****************************************************************************
+**
+** Copyright (C) 2019 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtGui module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include "qtextmarkdownimporter_p.h"
+#include "qtextdocumentfragment_p.h"
+#include <QLoggingCategory>
+#include <QRegularExpression>
+#include <QTextCursor>
+#include <QTextDocument>
+#include <QTextDocumentFragment>
+#include <QTextList>
+#include <QTextTable>
+
+QT_BEGIN_NAMESPACE
+
+Q_LOGGING_CATEGORY(lcMD, "qt.text.markdown")
+
+// --------------------------------------------------------
+// MD4C callback function wrappers
+
+static int CbEnterBlock(MD_BLOCKTYPE type, void *detail, void *userdata)
+{
+    QTextMarkdownImporter *mdi = static_cast<QTextMarkdownImporter *>(userdata);
+    return mdi->cbEnterBlock(type, detail);
+}
+
+static int CbLeaveBlock(MD_BLOCKTYPE type, void *detail, void *userdata)
+{
+    QTextMarkdownImporter *mdi = static_cast<QTextMarkdownImporter *>(userdata);
+    return mdi->cbLeaveBlock(type, detail);
+}
+
+static int CbEnterSpan(MD_SPANTYPE type, void *detail, void *userdata)
+{
+    QTextMarkdownImporter *mdi = static_cast<QTextMarkdownImporter *>(userdata);
+    return mdi->cbEnterSpan(type, detail);
+}
+
+static int CbLeaveSpan(MD_SPANTYPE type, void *detail, void *userdata)
+{
+    QTextMarkdownImporter *mdi = static_cast<QTextMarkdownImporter *>(userdata);
+    return mdi->cbLeaveSpan(type, detail);
+}
+
+static int CbText(MD_TEXTTYPE type, const MD_CHAR *text, MD_SIZE size, void *userdata)
+{
+    QTextMarkdownImporter *mdi = static_cast<QTextMarkdownImporter *>(userdata);
+    return mdi->cbText(type, text, size);
+}
+
+static void CbDebugLog(const char *msg, void *userdata)
+{
+    Q_UNUSED(userdata)
+    qCDebug(lcMD) << msg;
+}
+
+// MD4C callback function wrappers
+// --------------------------------------------------------
+
+static Qt::Alignment MdAlignment(MD_ALIGN a, Qt::Alignment defaultAlignment = Qt::AlignLeft | Qt::AlignVCenter)
+{
+    switch (a) {
+    case MD_ALIGN_LEFT:
+        return Qt::AlignLeft | Qt::AlignVCenter;
+    case MD_ALIGN_CENTER:
+        return Qt::AlignHCenter | Qt::AlignVCenter;
+    case MD_ALIGN_RIGHT:
+        return Qt::AlignRight | Qt::AlignVCenter;
+    default: // including MD_ALIGN_DEFAULT
+        return defaultAlignment;
+    }
+}
+
+QTextMarkdownImporter::QTextMarkdownImporter(QTextMarkdownImporter::Features features)
+  : m_monoFont(QFontDatabase::systemFont(QFontDatabase::FixedFont))
+  , m_features(features)
+{
+}
+
+void QTextMarkdownImporter::import(QTextDocument *doc, const QString &markdown)
+{
+    MD_PARSER callbacks = {
+        0, // abi_version
+        m_features,
+        &CbEnterBlock,
+        &CbLeaveBlock,
+        &CbEnterSpan,
+        &CbLeaveSpan,
+        &CbText,
+        &CbDebugLog,
+        nullptr // syntax
+    };
+    m_doc = doc;
+    m_cursor = new QTextCursor(doc);
+    doc->clear();
+    qCDebug(lcMD) << "default font" << doc->defaultFont() << "mono font" << m_monoFont;
+    QByteArray md = markdown.toUtf8();
+    md_parse(md.constData(), md.size(), &callbacks, this);
+    delete m_cursor;
+    m_cursor = nullptr;
+}
+
+int QTextMarkdownImporter::cbEnterBlock(MD_BLOCKTYPE type, void *det)
+{
+    m_blockType = type;
+    switch (type) {
+    case MD_BLOCK_P: {
+        QTextBlockFormat blockFmt;
+        int margin = m_doc->defaultFont().pointSize() / 2;
+        blockFmt.setTopMargin(margin);
+        blockFmt.setBottomMargin(margin);
+        m_cursor->insertBlock(blockFmt, QTextCharFormat());
+    } break;
+    case MD_BLOCK_CODE: {
+        QTextBlockFormat blockFmt;
+        QTextCharFormat charFmt;
+        charFmt.setFont(m_monoFont);
+        m_cursor->insertBlock(blockFmt, charFmt);
+    } break;
+    case MD_BLOCK_H: {
+        MD_BLOCK_H_DETAIL *detail = static_cast<MD_BLOCK_H_DETAIL *>(det);
+        QTextBlockFormat blockFmt;
+        QTextCharFormat charFmt;
+        int sizeAdjustment = 4 - detail->level; // H1 to H6: +3 to -2
+        charFmt.setProperty(QTextFormat::FontSizeAdjustment, sizeAdjustment);
+        charFmt.setFontWeight(QFont::Bold);
+        blockFmt.setHeadingLevel(detail->level);
+        m_cursor->insertBlock(blockFmt, charFmt);
+    } break;
+    case MD_BLOCK_LI: {
+        MD_BLOCK_LI_DETAIL *detail = static_cast<MD_BLOCK_LI_DETAIL *>(det);
+        QTextBlockFormat bfmt = m_cursor->blockFormat();
+        bfmt.setMarker(detail->is_task ?
+                           (detail->task_mark == ' ' ? QTextBlockFormat::Unchecked : QTextBlockFormat::Checked) :
+                           QTextBlockFormat::NoMarker);
+        if (!m_emptyList) {
+            m_cursor->insertBlock(bfmt, QTextCharFormat());
+            m_listStack.top()->add(m_cursor->block());
+        }
+        m_cursor->setBlockFormat(bfmt);
+        m_emptyList = false; // Avoid insertBlock for the first item (because insertList already did that)
+    } break;
+    case MD_BLOCK_UL: {
+        MD_BLOCK_UL_DETAIL *detail = static_cast<MD_BLOCK_UL_DETAIL *>(det);
+        QTextListFormat fmt;
+        fmt.setIndent(m_listStack.count() + 1);
+        switch (detail->mark) {
+        case '*':
+            fmt.setStyle(QTextListFormat::ListCircle);
+            break;
+        case '+':
+            fmt.setStyle(QTextListFormat::ListSquare);
+            break;
+        default: // including '-'
+            fmt.setStyle(QTextListFormat::ListDisc);
+            break;
+        }
+        m_listStack.push(m_cursor->insertList(fmt));
+        m_emptyList = true;
+    } break;
+    case MD_BLOCK_OL: {
+        MD_BLOCK_OL_DETAIL *detail = static_cast<MD_BLOCK_OL_DETAIL *>(det);
+        QTextListFormat fmt;
+        fmt.setIndent(m_listStack.count() + 1);
+        fmt.setNumberSuffix(QChar::fromLatin1(detail->mark_delimiter));
+        fmt.setStyle(QTextListFormat::ListDecimal);
+        m_listStack.push(m_cursor->insertList(fmt));
+        m_emptyList = true;
+    } break;
+    case MD_BLOCK_TD: {
+        MD_BLOCK_TD_DETAIL *detail = static_cast<MD_BLOCK_TD_DETAIL *>(det);
+        ++m_tableCol;
+        // absolute movement (and storage of m_tableCol) shouldn't be necessary, but
+        // movePosition(QTextCursor::NextCell) doesn't work
+        QTextTableCell cell = m_currentTable->cellAt(m_tableRowCount - 1, m_tableCol);
+        if (!cell.isValid()) {
+            qWarning("malformed table in Markdown input");
+            return 1;
+        }
+        *m_cursor = cell.firstCursorPosition();
+        QTextBlockFormat blockFmt = m_cursor->blockFormat();
+        blockFmt.setAlignment(MdAlignment(detail->align));
+        m_cursor->setBlockFormat(blockFmt);
+        qCDebug(lcMD) << "TD; align" << detail->align << MdAlignment(detail->align) << "col" << m_tableCol;
+    } break;
+    case MD_BLOCK_TH: {
+        ++m_tableColumnCount;
+        ++m_tableCol;
+        if (m_currentTable->columns() < m_tableColumnCount)
+            m_currentTable->appendColumns(1);
+        auto cell = m_currentTable->cellAt(m_tableRowCount - 1, m_tableCol);
+        if (!cell.isValid()) {
+            qWarning("malformed table in Markdown input");
+            return 1;
+        }
+        auto fmt = cell.format();
+        fmt.setFontWeight(QFont::Bold);
+        cell.setFormat(fmt);
+    } break;
+    case MD_BLOCK_TR: {
+        ++m_tableRowCount;
+        m_nonEmptyTableCells.clear();
+        if (m_currentTable->rows() < m_tableRowCount)
+            m_currentTable->appendRows(1);
+        m_tableCol = -1;
+        qCDebug(lcMD) << "TR" << m_currentTable->rows();
+    } break;
+    case MD_BLOCK_TABLE:
+        m_tableColumnCount = 0;
+        m_tableRowCount = 0;
+        m_currentTable = m_cursor->insertTable(1, 1); // we don't know the dimensions yet
+        break;
+    case MD_BLOCK_HR: {
+        QTextBlockFormat blockFmt = m_cursor->blockFormat();
+        blockFmt.setProperty(QTextFormat::BlockTrailingHorizontalRulerWidth, 1);
+        m_cursor->insertBlock(blockFmt, QTextCharFormat());
+    } break;
+    default:
+        break; // nothing to do for now
+    }
+    return 0; // no error
+}
+
+int QTextMarkdownImporter::cbLeaveBlock(MD_BLOCKTYPE type, void *detail)
+{
+    Q_UNUSED(detail)
+    switch (type) {
+    case MD_BLOCK_UL:
+    case MD_BLOCK_OL:
+        m_listStack.pop();
+        break;
+    case MD_BLOCK_TR: {
+        // https://github.com/mity/md4c/issues/29
+        // MD4C doesn't tell us explicitly which cells are merged, so merge empty cells
+        // with previous non-empty ones
+        int mergeEnd = -1;
+        int mergeBegin = -1;
+        for (int col = m_tableCol; col >= 0; --col) {
+            if (m_nonEmptyTableCells.contains(col)) {
+                if (mergeEnd >= 0 && mergeBegin >= 0) {
+                    qCDebug(lcMD) << "merging cells" << mergeBegin << "to" << mergeEnd << "inclusive, on row" << m_currentTable->rows() - 1;
+                    m_currentTable->mergeCells(m_currentTable->rows() - 1, mergeBegin - 1, 1, mergeEnd - mergeBegin + 2);
+                }
+                mergeEnd = -1;
+                mergeBegin = -1;
+            } else {
+                if (mergeEnd < 0)
+                    mergeEnd = col;
+                else
+                    mergeBegin = col;
+            }
+        }
+    } break;
+    case MD_BLOCK_QUOTE: {
+        QTextBlockFormat blockFmt = m_cursor->blockFormat();
+        blockFmt.setIndent(1);
+        m_cursor->setBlockFormat(blockFmt);
+    } break;
+    case MD_BLOCK_TABLE:
+        qCDebug(lcMD) << "table ended with" << m_currentTable->columns() << "cols and" << m_currentTable->rows() << "rows";
+        m_currentTable = nullptr;
+        m_cursor->movePosition(QTextCursor::End);
+        break;
+    default:
+        break;
+    }
+    return 0; // no error
+}
+
+int QTextMarkdownImporter::cbEnterSpan(MD_SPANTYPE type, void *det)
+{
+    QTextCharFormat charFmt;
+    switch (type) {
+    case MD_SPAN_EM:
+        charFmt.setFontItalic(true);
+        break;
+    case MD_SPAN_STRONG:
+        charFmt.setFontWeight(QFont::Bold);
+        break;
+    case MD_SPAN_A: {
+        MD_SPAN_A_DETAIL *detail = static_cast<MD_SPAN_A_DETAIL *>(det);
+        QString url = QString::fromLatin1(detail->href.text, detail->href.size);
+        QString title = QString::fromLatin1(detail->title.text, detail->title.size);
+        charFmt.setAnchorHref(url);
+        charFmt.setAnchorName(title);
+        charFmt.setForeground(m_palette.link());
+        qCDebug(lcMD) << "anchor" << url << title;
+        } break;
+    case MD_SPAN_IMG: {
+        m_imageSpan = true;
+        MD_SPAN_IMG_DETAIL *detail = static_cast<MD_SPAN_IMG_DETAIL *>(det);
+        QString src = QString::fromUtf8(detail->src.text, detail->src.size);
+        QString title = QString::fromUtf8(detail->title.text, detail->title.size);
+        QTextImageFormat img;
+        img.setName(src);
+        qCDebug(lcMD) << "image" << src << "title" << title << "relative to" << m_doc->baseUrl();
+        m_cursor->insertImage(img);
+        break;
+    }
+    case MD_SPAN_CODE:
+        charFmt.setFont(m_monoFont);
+        break;
+    case MD_SPAN_DEL:
+        charFmt.setFontStrikeOut(true);
+        break;
+    }
+    m_spanFormatStack.push(charFmt);
+    m_cursor->setCharFormat(charFmt);
+    return 0; // no error
+}
+
+int QTextMarkdownImporter::cbLeaveSpan(MD_SPANTYPE type, void *detail)
+{
+    Q_UNUSED(detail)
+    QTextCharFormat charFmt;
+    if (!m_spanFormatStack.isEmpty()) {
+        m_spanFormatStack.pop();
+        if (!m_spanFormatStack.isEmpty())
+            charFmt = m_spanFormatStack.top();
+    }
+    m_cursor->setCharFormat(charFmt);
+    if (type == MD_SPAN_IMG)
+        m_imageSpan = false;
+    return 0; // no error
+}
+
+int QTextMarkdownImporter::cbText(MD_TEXTTYPE type, const MD_CHAR *text, MD_SIZE size)
+{
+    if (m_imageSpan)
+        return 0; // it's the alt-text
+    static const QRegularExpression openingBracket(QStringLiteral("<[a-zA-Z]"));
+    static const QRegularExpression closingBracket(QStringLiteral("(/>|</)"));
+    QString s = QString::fromUtf8(text, size);
+
+    switch (type) {
+    case MD_TEXT_NORMAL:
+        if (m_htmlTagDepth) {
+            m_htmlAccumulator += s;
+            s = QString();
+        }
+        break;
+    case MD_TEXT_NULLCHAR:
+        s = QString(QChar(0xFFFD)); // CommonMark-required replacement for null
+        break;
+    case MD_TEXT_BR:
+        s = QLatin1String("\n");
+        break;
+    case MD_TEXT_SOFTBR:
+        s = QLatin1String(" ");
+        break;
+    case MD_TEXT_CODE:
+        // We'll see MD_SPAN_CODE too, which will set the char format, and that's enough.
+        break;
+    case MD_TEXT_ENTITY:
+        m_cursor->insertHtml(s);
+        s = QString();
+        break;
+    case MD_TEXT_HTML:
+        // count how many tags are opened and how many are closed
+        {
+            int startIdx = 0;
+            while ((startIdx = s.indexOf(openingBracket, startIdx)) >= 0) {
+                ++m_htmlTagDepth;
+                startIdx += 2;
+            }
+            startIdx = 0;
+            while ((startIdx = s.indexOf(closingBracket, startIdx)) >= 0) {
+                --m_htmlTagDepth;
+                startIdx += 2;
+            }
+        }
+        m_htmlAccumulator += s;
+        s = QString();
+        if (!m_htmlTagDepth) { // all open tags are now closed
+            qCDebug(lcMD) << "HTML" << m_htmlAccumulator;
+            m_cursor->insertHtml(m_htmlAccumulator);
+            if (m_spanFormatStack.isEmpty())
+                m_cursor->setCharFormat(QTextCharFormat());
+            else
+                m_cursor->setCharFormat(m_spanFormatStack.top());
+            m_htmlAccumulator = QString();
+        }
+        break;
+    }
+
+    switch (m_blockType) {
+    case MD_BLOCK_TD:
+        m_nonEmptyTableCells.append(m_tableCol);
+        break;
+    default:
+        break;
+    }
+
+    if (!s.isEmpty())
+        m_cursor->insertText(s);
+    return 0; // no error
+}
+
+QT_END_NAMESPACE
diff --git a/src/gui/text/qtextmarkdownimporter_p.h b/src/gui/text/qtextmarkdownimporter_p.h
new file mode 100644
index 0000000000..b73e744434
--- /dev/null
+++ b/src/gui/text/qtextmarkdownimporter_p.h
@@ -0,0 +1,127 @@
+/****************************************************************************
+**
+** Copyright (C) 2019 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtGui module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef QTEXTMARKDOWNIMPORTER_H
+#define QTEXTMARKDOWNIMPORTER_H
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists purely as an
+// implementation detail.  This header file may change from version to
+// version without notice, or even be removed.
+//
+// We mean it.
+//
+
+#include <QtGui/qfont.h>
+#include <QtGui/qtguiglobal.h>
+#include <QtGui/qpalette.h>
+#include <QtGui/qtextlist.h>
+#include <QtCore/qstack.h>
+
+#include "../../3rdparty/md4c/md4c.h"
+
+QT_BEGIN_NAMESPACE
+
+class QTextCursor;
+class QTextDocument;
+class QTextTable;
+
+class Q_GUI_EXPORT QTextMarkdownImporter
+{
+public:
+    enum Feature {
+        FeatureCollapseWhitespace = MD_FLAG_COLLAPSEWHITESPACE,
+        FeaturePermissiveATXHeaders = MD_FLAG_PERMISSIVEATXHEADERS,
+        FeaturePermissiveURLAutoLinks = MD_FLAG_PERMISSIVEURLAUTOLINKS,
+        FeaturePermissiveMailAutoLinks = MD_FLAG_PERMISSIVEEMAILAUTOLINKS,
+        FeatureNoIndentedCodeBlocks = MD_FLAG_NOINDENTEDCODEBLOCKS,
+        FeatureNoHTMLBlocks = MD_FLAG_NOHTMLBLOCKS,
+        FeatureNoHTMLSpans = MD_FLAG_NOHTMLSPANS,
+        FeatureTables = MD_FLAG_TABLES,
+        FeatureStrikeThrough = MD_FLAG_STRIKETHROUGH,
+        FeaturePermissiveWWWAutoLinks = MD_FLAG_PERMISSIVEWWWAUTOLINKS,
+        FeatureTasklists = MD_FLAG_TASKLISTS,
+        // composite flags
+        FeaturePermissiveAutoLinks = MD_FLAG_PERMISSIVEAUTOLINKS,
+        FeatureNoHTML = MD_FLAG_NOHTML,
+        DialectCommonMark = MD_DIALECT_COMMONMARK,
+        DialectGitHub = MD_DIALECT_GITHUB
+    };
+    Q_DECLARE_FLAGS(Features, Feature)
+
+    QTextMarkdownImporter(Features features);
+
+    void import(QTextDocument *doc, const QString &markdown);
+
+public:
+    // MD4C callbacks
+    int cbEnterBlock(MD_BLOCKTYPE type, void* detail);
+    int cbLeaveBlock(MD_BLOCKTYPE type, void* detail);
+    int cbEnterSpan(MD_SPANTYPE type, void* detail);
+    int cbLeaveSpan(MD_SPANTYPE type, void* detail);
+    int cbText(MD_TEXTTYPE type, const MD_CHAR* text, MD_SIZE size);
+
+private:
+    QTextDocument *m_doc = nullptr;
+    QTextCursor *m_cursor = nullptr;
+    QTextTable *m_currentTable = nullptr; // because m_cursor->currentTable() doesn't work
+    QString m_htmlAccumulator;
+    QVector<int> m_nonEmptyTableCells; // in the current row
+    QStack<QTextList *> m_listStack;
+    QStack<QTextCharFormat> m_spanFormatStack;
+    QFont m_monoFont;
+    QPalette m_palette;
+    int m_htmlTagDepth = 0;
+    int m_tableColumnCount = 0;
+    int m_tableRowCount = 0;
+    int m_tableCol = -1; // because relative cell movements (e.g. m_cursor->movePosition(QTextCursor::NextCell)) don't work
+    Features m_features;
+    MD_BLOCKTYPE m_blockType = MD_BLOCK_DOC;
+    bool m_emptyList = false; // true when the last thing we did was insertList
+    bool m_imageSpan = false;
+};
+
+Q_DECLARE_OPERATORS_FOR_FLAGS(QTextMarkdownImporter::Features)
+
+QT_END_NAMESPACE
+
+#endif // QTEXTMARKDOWNIMPORTER_H
diff --git a/src/gui/text/text.pri b/src/gui/text/text.pri
index abe20abe02..b35a231747 100644
--- a/src/gui/text/text.pri
+++ b/src/gui/text/text.pri
@@ -97,6 +97,18 @@ qtConfig(textodfwriter) {
         text/qzip.cpp
 }
 
+qtConfig(textmarkdownreader) {
+    qtConfig(system-textmarkdownreader) {
+        QMAKE_USE += libmd4c
+    } else {
+        include($$PWD/../../3rdparty/md4c.pri)
+    }
+    HEADERS += \
+        text/qtextmarkdownimporter_p.h
+    SOURCES += \
+        text/qtextmarkdownimporter.cpp
+}
+
 qtConfig(cssparser) {
     HEADERS += \
         text/qcssparser_p.h