MathInlineParser.cs

// Copyright (c) Alexandre Mutel. All rights reserved.
// This file is licensed under the BSD-Clause 2 license.
// See the license.txt file in the project root for more information.

using Markdig.Helpers;
using Markdig.Parsers;
using Markdig.Renderers.Html;
using Markdig.Syntax;

namespace Markdig.Extensions.Mathematics;

/// <summary>
/// An inline parser for <see cref="MathInline"/>.
/// </summary>
/// <seealso cref="InlineParser" />
/// <seealso cref="IPostInlineProcessor" />
public class MathInlineParser : InlineParser
{
    /// <summary>
    /// Initializes a new instance of the <see cref="MathInlineParser"/> class.
    /// </summary>
    public MathInlineParser()
    {
        OpeningCharacters = ['$'];
        DefaultClass = "math";
    }

    /// <summary>
    /// Gets or sets the default class to use when creating a math inline block.
    /// </summary>
    public string DefaultClass { get; set; }

    public override bool Match(InlineProcessor processor, ref StringSlice slice)
    {
        var match = slice.CurrentChar;
        var pc = slice.PeekCharExtra(-1);
        if (pc == match)
        {
            return false;
        }

        var startPosition = slice.Start;

        // Match the opened $ or $$
        int openDollars = 1; // we have at least a $
        var c = slice.NextChar();
        if (c == match)
        {
            openDollars++;
            c = slice.NextChar();
        }

        pc.CheckUnicodeCategory(out bool openPrevIsWhiteSpace, out bool openPrevIsPunctuation);
        c.CheckUnicodeCategory(out bool openNextIsWhiteSpace, out _);

        // Check that opening $/$$ is correct, using the different heuristics than for emphasis delimiters
        // If a $/$$ is not preceded by a whitespace or punctuation, this is a not a math block
        if ((!openPrevIsWhiteSpace && !openPrevIsPunctuation))
        {
            return false;
        }

        bool isMatching = false;
        int closeDollars = 0;

        // Eat any leading spaces
        while (c.IsSpaceOrTab())
        {
            c = slice.NextChar();
        }

        var start = slice.Start;
        var end = 0;

        pc = match;
        var lastWhiteSpace = -1;
        while (c != '\0')
        {
            // Don't allow newline in an inline math expression
            if (c == '\r' || c == '\n')
            {
                return false;
            }

            // Don't process sticks if we have a '\' as a previous char
            if (pc != '\\')
            {
                // Record continuous whitespaces at the end
                if (c.IsSpaceOrTab())
                {
                    if (lastWhiteSpace < 0)
                    {
                        lastWhiteSpace = slice.Start;
                    }
                }
                else
                {
                    bool hasClosingDollars = c == match;
                    if (hasClosingDollars)
                    {
                        closeDollars += slice.CountAndSkipChar(match);
                        c = slice.CurrentChar;
                    }

                    if (closeDollars >= openDollars)
                    {
                        break;
                    }

                    lastWhiteSpace = -1;
                    if (hasClosingDollars)
                    {
                        pc = match;
                        continue;
                    }
                }
            }

            if (closeDollars > 0)
            {
                closeDollars = 0;
            }
            else
            {
                pc = c;
                c = slice.NextChar();
            }
        }

        if (closeDollars >= openDollars)
        {
            pc.CheckUnicodeCategory(out bool closePrevIsWhiteSpace, out _);
            c.CheckUnicodeCategory(out bool closeNextIsWhiteSpace, out bool closeNextIsPunctuation);

            // A closing $/$$ should be followed by at least a punctuation or a whitespace
            // and if the character after an opening $/$$ was a whitespace, it should be
            // a whitespace as well for the character preceding the closing of $/$$
            if ((!closeNextIsPunctuation && !closeNextIsWhiteSpace) || (openNextIsWhiteSpace != closePrevIsWhiteSpace))
            {
                return false;
            }

            if (closePrevIsWhiteSpace && lastWhiteSpace > 0)
            {
                end = lastWhiteSpace + openDollars - 1;
            }
            else
            {
                end = slice.Start - 1;
            }

            // Create a new MathInline
            var inline = new MathInline()
            {
                Span = new SourceSpan(processor.GetSourcePosition(startPosition, out int line, out int column), processor.GetSourcePosition(slice.Start - 1)),
                Line = line,
                Column = column,
                Delimiter = match,
                DelimiterCount = openDollars,
                Content = slice
            };
            inline.Content.Start = start;
            // We substract the end to the number of opening $ to keep inside the block the additionals $
            inline.Content.End = end - openDollars;

            // Add the default class if necessary
            if (DefaultClass != null)
            {
                inline.GetAttributes().AddClass(DefaultClass);
            }
            processor.Inline = inline;
            isMatching = true;
        }

        return isMatching;
    }
}