add initial marp implementation with sample content and build configuration

node_modules/mathjax-full/ts/input/tex/ParseUtil.ts

@@ -0,0 +1,713 @@
+/*************************************************************
+ *
+ *  Copyright (c) 2009-2022 The MathJax Consortium
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+
+/**
+ * @fileoverview A namespace for utility functions for the TeX Parser.
+ *
+ * @author v.sorge@mathjax.org (Volker Sorge)
+ */
+
+import {TEXCLASS, MmlNode} from '../../core/MmlTree/MmlNode.js';
+import {EnvList} from './StackItem.js';
+import {ArrayItem} from './base/BaseItems.js';
+import ParseOptions from './ParseOptions.js';
+import NodeUtil from './NodeUtil.js';
+import TexParser from './TexParser.js';
+import TexError from './TexError.js';
+import {entities} from '../../util/Entities.js';
+import {MmlMunderover} from '../../core/MmlTree/MmlNodes/munderover.js';
+
+
+namespace ParseUtil {
+
+  // TODO (VS): Combine some of this with lengths in util.
+  const emPerInch = 7.2;
+  const pxPerInch = 72;
+  // Note, the following are TeX CM font values.
+  const UNIT_CASES: {[key: string]: ((m: number) => number)}  = {
+    'em': m => m,
+    'ex': m => m * .43,
+    'pt': m => m / 10,                    // 10 pt to an em
+    'pc': m => m * 1.2,                   // 12 pt to a pc
+    'px': m => m * emPerInch / pxPerInch,
+    'in': m => m * emPerInch,
+    'cm': m => m * emPerInch / 2.54, // 2.54 cm to an inch
+    'mm': m => m * emPerInch / 25.4, // 10 mm to a cm
+    'mu': m => m / 18,
+  };
+  const num = '([-+]?([.,]\\d+|\\d+([.,]\\d*)?))';
+  const unit = '(pt|em|ex|mu|px|mm|cm|in|pc)';
+  const dimenEnd = RegExp('^\\s*' + num + '\\s*' + unit + '\\s*$');
+  const dimenRest = RegExp('^\\s*' + num + '\\s*' + unit + ' ?');
+
+
+  /**
+   * Matches for a dimension argument.
+   * @param {string} dim The argument.
+   * @param {boolean} rest Allow for trailing garbage in the dimension string.
+   * @return {[string, string, number]} The match result as (Anglosaxon) value,
+   *     unit name, length of matched string. The latter is interesting in the
+   *     case of trailing garbage.
+   */
+  export function matchDimen(
+    dim: string, rest: boolean = false): [string, string, number] {
+      let match = dim.match(rest ? dimenRest : dimenEnd);
+      return match ?
+        muReplace([match[1].replace(/,/, '.'), match[4], match[0].length]) :
+        [null, null, 0];
+  }
+
+
+  /**
+   * Transforms mu dimension to em if necessary.
+   * @param {[string, string, number]} [value, unit, length] The dimension triple.
+   * @return {[string, string, number]} [value, unit, length] The transformed triple.
+   */
+  function muReplace([value, unit, length]: [string, string, number]): [string, string, number] {
+    if (unit !== 'mu') {
+      return [value, unit, length];
+    }
+    let em = Em(UNIT_CASES[unit](parseFloat(value || '1')));
+    return [em.slice(0, -2), 'em', length];
+  }
+
+
+  /**
+   * Convert a dimension string into standard em dimension.
+   * @param {string} dim The attribute string.
+   * @return {number} The numerical value.
+   */
+  export function dimen2em(dim: string): number {
+    let [value, unit] = matchDimen(dim);
+    let m = parseFloat(value || '1');
+    let func = UNIT_CASES[unit];
+    return func ? func(m) : 0;
+  }
+
+
+  /**
+   * Turns a number into an em value.
+   * @param {number} m The number.
+   * @return {string} The em dimension string.
+   */
+  export function Em(m: number): string {
+    if (Math.abs(m) < .0006) {
+      return '0em';
+    }
+    return m.toFixed(3).replace(/\.?0+$/, '') + 'em';
+  }
+
+
+  /**
+   * Takes an array of numbers and returns a space-separated string of em values.
+   * @param {number[]} W  The widths to be turned into em values
+   * @return {string}     The numbers with em units, separated by spaces.
+   */
+  export function cols(...W: number[]): string {
+    return W.map(n => Em(n)).join(' ');
+  }
+
+
+  /**
+   * Create an mrow that has stretchy delimiters at either end, as needed
+   * @param {ParseOptions} configuration Current parse options.
+   * @param {string} open The opening fence.
+   * @param {MmlNode} mml The enclosed node.
+   * @param {string} close The closing fence.
+   * @param {string=} big Bigg command.
+   */
+  export function fenced(configuration: ParseOptions, open: string, mml: MmlNode,
+                         close: string, big: string = '', color: string = '') {
+    // @test Fenced, Fenced3
+    let nf = configuration.nodeFactory;
+    let mrow = nf.create('node', 'mrow', [],
+                         {open: open, close: close, texClass: TEXCLASS.INNER});
+    let mo;
+    if (big) {
+      mo = new TexParser('\\' + big + 'l' + open, configuration.parser.stack.env, configuration).mml();
+    } else {
+      let openNode = nf.create('text', open);
+      mo = nf.create('node', 'mo', [],
+                     {fence: true, stretchy: true, symmetric: true, texClass: TEXCLASS.OPEN},
+                     openNode);
+    }
+    NodeUtil.appendChildren(mrow, [mo, mml]);
+    if (big) {
+      mo = new TexParser('\\' + big + 'r' + close, configuration.parser.stack.env, configuration).mml();
+    } else {
+      let closeNode = nf.create('text', close);
+      mo = nf.create('node', 'mo', [],
+                     {fence: true, stretchy: true, symmetric: true, texClass: TEXCLASS.CLOSE},
+                     closeNode);
+    }
+    color && mo.attributes.set('mathcolor', color);
+    NodeUtil.appendChildren(mrow, [mo]);
+    return mrow;
+  }
+
+
+  /**
+   *  Create an mrow that has \\mathchoice using \\bigg and \\big for the delimiters.
+   * @param {ParseOptions} configuration The current parse options.
+   * @param {string} open The opening fence.
+   * @param {MmlNode} mml The enclosed node.
+   * @param {string} close The closing fence.
+   * @return {MmlNode} The mrow node.
+   */
+  export function fixedFence(configuration: ParseOptions, open: string,
+                             mml: MmlNode, close: string): MmlNode {
+    // @test Choose, Over With Delims, Above with Delims
+    let mrow = configuration.nodeFactory.create('node',
+      'mrow', [], {open: open, close: close, texClass: TEXCLASS.ORD});
+    if (open) {
+      NodeUtil.appendChildren(mrow, [mathPalette(configuration, open, 'l')]);
+    }
+    if (NodeUtil.isType(mml, 'mrow')) {
+      NodeUtil.appendChildren(mrow, NodeUtil.getChildren(mml));
+    } else {
+      NodeUtil.appendChildren(mrow, [mml]);
+    }
+    if (close) {
+      NodeUtil.appendChildren(mrow, [mathPalette(configuration, close, 'r')]);
+    }
+    return mrow;
+  }
+
+
+  /**
+   * Generates a mathchoice element for fences. These will be resolved later,
+   * once the position, and therefore size, of the of the fenced expression is
+   * known.
+   * @param {ParseOptions} configuration The current parse otpions.
+   * @param {string} fence The fence.
+   * @param {string} side The side of the fence (l or r).
+   * @return {MmlNode} The mathchoice node.
+   */
+  export function mathPalette(configuration: ParseOptions, fence: string,
+                              side: string): MmlNode  {
+    if (fence === '{' || fence === '}') {
+      fence = '\\' + fence;
+    }
+    let D = '{\\bigg' + side + ' ' + fence + '}';
+    let T = '{\\big' + side + ' ' + fence + '}';
+    return new TexParser('\\mathchoice' + D + T + T + T, {}, configuration).mml();
+  }
+
+
+  /**
+   * If the initial child, skipping any initial space or
+   * empty braces (TeXAtom with child being an empty inferred row),
+   * is an <mo>, precede it by an empty <mi> to force the <mo> to
+   * be infix.
+   * @param {ParseOptions} configuration The current parse options.
+   * @param {MmlNode[]} nodes The row of nodes to scan for an initial <mo>
+   */
+  export function fixInitialMO(configuration: ParseOptions, nodes: MmlNode[]) {
+    for (let i = 0, m = nodes.length; i < m; i++) {
+      let child = nodes[i];
+      if (child && (!NodeUtil.isType(child, 'mspace') &&
+                    (!NodeUtil.isType(child, 'TeXAtom') ||
+                     (NodeUtil.getChildren(child)[0] &&
+                      NodeUtil.getChildren(NodeUtil.getChildren(child)[0]).length)))) {
+        if (NodeUtil.isEmbellished(child) ||
+            (NodeUtil.isType(child, 'TeXAtom') && NodeUtil.getTexClass(child) === TEXCLASS.REL)) {
+          let mi = configuration.nodeFactory.create('node', 'mi');
+          nodes.unshift(mi);
+        }
+        break;
+      }
+    }
+  }
+
+
+  /**
+   * Break up a string into text and math blocks.
+   * @param {TexParser} parser The calling parser.
+   * @param {string} text The text in the math expression to parse.
+   * @param {number|string=} level The scriptlevel.
+   * @param {string} font The mathvariant to use
+   * @return {MmlNode[]} The nodes corresponding to the internal math expression.
+   */
+  export function internalMath(parser: TexParser, text: string,
+                               level?: number | string, font?: string): MmlNode[] {
+    if (parser.configuration.options.internalMath) {
+      return parser.configuration.options.internalMath(parser, text, level, font);
+    }
+    let mathvariant = font || parser.stack.env.font;
+    let def = (mathvariant ? {mathvariant} : {});
+    let mml: MmlNode[] = [], i = 0, k = 0, c, node, match = '', braces = 0;
+    if (text.match(/\\?[${}\\]|\\\(|\\(eq)?ref\s*\{/)) {
+      while (i < text.length) {
+        c = text.charAt(i++);
+        if (c === '$') {
+          if (match === '$' && braces === 0) {
+            // @test Interspersed Text
+            node = parser.create(
+              'node', 'TeXAtom',
+              [(new TexParser(text.slice(k, i - 1), {}, parser.configuration)).mml()]);
+            mml.push(node);
+            match = '';
+            k = i;
+          } else if (match === '') {
+            // @test Interspersed Text
+            if (k < i - 1) {
+              // @test Interspersed Text
+              mml.push(internalText(parser, text.slice(k, i - 1), def));
+            }
+            match = '$';
+            k = i;
+          }
+        } else if (c === '{' && match !== '') {
+          // @test Mbox Mbox, Mbox Math
+          braces++;
+        } else if (c === '}') {
+          // @test Mbox Mbox, Mbox Math
+          if (match === '}' && braces === 0) {
+            // @test Mbox Eqref, Mbox Math
+            let atom = (new TexParser(text.slice(k, i), {}, parser.configuration)).mml();
+            node = parser.create('node', 'TeXAtom', [atom], def);
+            mml.push(node);
+            match = '';
+            k = i;
+          } else if (match !== '') {
+            // @test Mbox Math, Mbox Mbox
+            if (braces) {
+              // @test Mbox Math, Mbox Mbox
+              braces--;
+            }
+          }
+        } else if (c === '\\') {
+          // @test Mbox Eqref, Mbox CR
+          if (match === '' && text.substr(i).match(/^(eq)?ref\s*\{/)) {
+            // @test Mbox Eqref
+            let len = ((RegExp as any)['$&'] as string).length;
+            if (k < i - 1) {
+              // @test Mbox Eqref
+              mml.push(internalText(parser, text.slice(k, i - 1), def));
+            }
+            match = '}';
+            k = i - 1;
+            i += len;
+          } else {
+            // @test Mbox CR, Mbox Mbox
+            c = text.charAt(i++);
+            if (c === '(' && match === '') {
+              // @test Mbox Internal Display
+              if (k < i - 2) {
+                // @test Mbox Internal Display
+                mml.push(internalText(parser, text.slice(k, i - 2), def));
+              }
+              match = ')'; k = i;
+            } else if (c === ')' && match === ')' && braces === 0) {
+              // @test Mbox Internal Display
+              node = parser.create(
+                'node', 'TeXAtom',
+                [(new TexParser(text.slice(k, i - 2), {}, parser.configuration)).mml()]);
+              mml.push(node);
+              match = '';
+              k = i;
+            } else if (c.match(/[${}\\]/) && match === '')  {
+              // @test Mbox CR
+              i--;
+              text = text.substr(0, i - 1) + text.substr(i); // remove \ from \$, \{, \}, or \\
+            }
+          }
+        }
+      }
+      if (match !== '') {
+        // @test Internal Math Error
+        throw new TexError('MathNotTerminated', 'Math not terminated in text box');
+      }
+    }
+    if (k < text.length) {
+      // @test Interspersed Text, Mbox Mbox
+      mml.push(internalText(parser, text.slice(k), def));
+    }
+    if (level != null) {
+      // @test Label, Fbox, Hbox
+      mml = [parser.create('node', 'mstyle', mml, {displaystyle: false, scriptlevel: level})];
+    } else if (mml.length > 1) {
+      // @test Interspersed Text
+      mml = [parser.create('node', 'mrow', mml)];
+    }
+    return mml;
+  }
+
+
+  /**
+   * Parses text internal to boxes or labels.
+   * @param {TexParser} parser The current tex parser.
+   * @param {string} text The text to parse.
+   * @param {EnvList} def The attributes of the text node.
+   * @return {MmlNode} The text node.
+   */
+  export function internalText(parser: TexParser, text: string, def: EnvList): MmlNode {
+    // @test Label, Fbox, Hbox
+    text = text.replace(/^\s+/, entities.nbsp).replace(/\s+$/, entities.nbsp);
+    let textNode = parser.create('text', text);
+    return parser.create('node', 'mtext', [], def, textNode);
+  }
+
+  /**
+   * Create an munderover node with the given script position.
+   * @param {TexParser} parser   The current TeX parser.
+   * @param {MmlNode} base       The base node.
+   * @param {MmlNode} script     The under- or over-script.
+   * @param {string} pos         Either 'over' or 'under'.
+   * @param {boolean} stack      True if super- or sub-scripts should stack.
+   * @return {MmlNode}           The generated node (MmlMunderover or TeXAtom)
+   */
+  export function underOver(parser: TexParser, base: MmlNode, script: MmlNode, pos: string, stack: boolean): MmlNode {
+    // @test Overline
+    ParseUtil.checkMovableLimits(base);
+    if (NodeUtil.isType(base, 'munderover') && NodeUtil.isEmbellished(base)) {
+      // @test Overline Limits
+      NodeUtil.setProperties(NodeUtil.getCoreMO(base), {lspace: 0, rspace: 0});
+      const mo = parser.create('node', 'mo', [], {rspace: 0});
+      base = parser.create('node', 'mrow', [mo, base]);
+      // TODO? add an empty <mi> so it's not embellished any more
+    }
+    const mml = parser.create('node', 'munderover', [base]) as MmlMunderover;
+    NodeUtil.setChild(mml, pos === 'over' ?  mml.over : mml.under, script);
+    let node: MmlNode = mml;
+    if (stack) {
+      // @test Overbrace 1 2 3, Underbrace, Overbrace Op 1 2
+      node = parser.create('node', 'TeXAtom', [mml], {texClass: TEXCLASS.OP, movesupsub: true});
+    }
+    NodeUtil.setProperty(node, 'subsupOK', true);
+    return node;
+  }
+
+  /**
+   * Set movablelimits to false if necessary.
+   * @param {MmlNode} base   The base node being tested.
+   */
+  export function checkMovableLimits(base: MmlNode) {
+    const symbol = (NodeUtil.isType(base, 'mo') ? NodeUtil.getForm(base) : null);
+    if (NodeUtil.getProperty(base, 'movablelimits') || (symbol && symbol[3] && symbol[3].movablelimits)) {
+      // @test Overline Sum
+      NodeUtil.setProperties(base, {movablelimits: false});
+    }
+  }
+
+  /**
+   * Trim spaces from a string.
+   * @param {string} text The string to clean.
+   * @return {string} The string with leading and trailing whitespace removed.
+   */
+  export function trimSpaces(text: string): string {
+    if (typeof(text) !== 'string') {
+      return text;
+    }
+    let TEXT = text.trim();
+    if (TEXT.match(/\\$/) && text.match(/ $/)) {
+      TEXT += ' ';
+    }
+    return TEXT;
+  }
+
+
+  /**
+   * Sets alignment in array definitions.
+   * @param {ArrayItem} array The array item.
+   * @param {string} align The alignment string.
+   * @return {ArrayItem} The altered array item.
+   */
+  export function setArrayAlign(array: ArrayItem, align: string): ArrayItem {
+    // @test Array1, Array2, Array Test
+    align = ParseUtil.trimSpaces(align || '');
+    if (align === 't') {
+      array.arraydef.align = 'baseline 1';
+    } else if (align === 'b') {
+      array.arraydef.align = 'baseline -1';
+    } else if (align === 'c') {
+      array.arraydef.align = 'axis';
+    } else if (align) {
+      array.arraydef.align = align;
+    } // FIXME: should be an error?
+    return array;
+  }
+
+
+  /**
+   * Replace macro parameters with their values.
+   * @param {TexParser} parser The current TeX parser.
+   * @param {string[]} args A list of arguments for macro parameters.
+   * @param {string} str The macro parameter string.
+   * @return {string} The string with all parameters replaced by arguments.
+   */
+  export function substituteArgs(parser: TexParser, args: string[],
+                                 str: string): string {
+    let text = '';
+    let newstring = '';
+    let i = 0;
+    while (i < str.length) {
+      let c = str.charAt(i++);
+      if (c === '\\') {
+        text += c + str.charAt(i++);
+      }
+      else if (c === '#') {
+        c = str.charAt(i++);
+        if (c === '#') {
+          text += c;
+        } else {
+          if (!c.match(/[1-9]/) || parseInt(c, 10) > args.length) {
+            throw new TexError('IllegalMacroParam',
+                                'Illegal macro parameter reference');
+          }
+          newstring = addArgs(parser, addArgs(parser, newstring, text),
+                              args[parseInt(c, 10) - 1]);
+          text = '';
+        }
+      } else {
+        text += c;
+      }
+    }
+    return addArgs(parser, newstring, text);
+  }
+
+
+  /**
+   * Adds a new expanded argument to an already macro parameter string.  Makes
+   * sure that macros are followed by a space if their names could accidentally
+   * be continued into the following text.
+   * @param {TexParser} parser The current TeX parser.
+   * @param {string} s1 The already expanded string.
+   * @param {string} s2 The string to add.
+   * @return {string} The combined string.
+   */
+  export function addArgs(parser: TexParser, s1: string, s2: string): string {
+    if (s2.match(/^[a-z]/i) && s1.match(/(^|[^\\])(\\\\)*\\[a-z]+$/i)) {
+      s1 += ' ';
+    }
+    if (s1.length + s2.length > parser.configuration.options['maxBuffer']) {
+      throw new TexError('MaxBufferSize',
+                          'MathJax internal buffer size exceeded; is there a' +
+                          ' recursive macro call?');
+    }
+    return s1 + s2;
+  }
+
+  /**
+   * Report an error if there are too many macro substitutions.
+   * @param {TexParser} parser The current TeX parser.
+   * @param {boolean} isMacro  True if we are substituting a macro, false for environment.
+   */
+  export function checkMaxMacros(parser: TexParser, isMacro: boolean = true) {
+    if (++parser.macroCount <= parser.configuration.options['maxMacros']) {
+      return;
+    }
+    if (isMacro) {
+      throw new TexError('MaxMacroSub1',
+                         'MathJax maximum macro substitution count exceeded; ' +
+                         'is here a recursive macro call?');
+    } else {
+      throw new TexError('MaxMacroSub2',
+                         'MathJax maximum substitution count exceeded; ' +
+                         'is there a recursive latex environment?');
+    }
+  }
+
+
+  /**
+   *  Check for bad nesting of equation environments
+   */
+  export function checkEqnEnv(parser: TexParser) {
+    if (parser.stack.global.eqnenv) {
+      // @test ErroneousNestingEq
+      throw new TexError('ErroneousNestingEq', 'Erroneous nesting of equation structures');
+    }
+    parser.stack.global.eqnenv = true;
+  }
+
+  /**
+   * Copy an MmlNode and add it (and its children) to the proper lists.
+   *
+   * @param {MmlNode} node       The MmlNode to copy
+   * @param {TexParser} parser   The active tex parser
+   * @return {MmlNode}           The duplicate tree
+   */
+  export function copyNode(node: MmlNode, parser: TexParser): MmlNode  {
+    const tree = node.copy() as MmlNode;
+    const options = parser.configuration;
+    tree.walkTree((n: MmlNode) => {
+      options.addNode(n.kind, n);
+      const lists = (n.getProperty('in-lists') as string || '').split(/,/);
+      for (const list of lists) {
+        list && options.addNode(list, n);
+      }
+    });
+    return tree;
+  }
+
+  /**
+   * This is a placeholder for future security filtering of attributes.
+   * @param {TexParser} parser The current parser.
+   * @param {string} name The attribute name.
+   * @param {string} value The attribute value to filter.
+   * @return {string} The filtered value.
+   */
+  export function MmlFilterAttribute(_parser: TexParser, _name: string, value: string): string {
+    // TODO: Implement in security package.
+    return value;
+  }
+
+
+  /**
+   * Initialises an stack environment with current font definition in the parser.
+   * @param {TexParser} parser The current tex parser.
+   * @return {EnvList} The initialised environment list.
+   */
+  export function getFontDef(parser: TexParser): EnvList {
+    const font = parser.stack.env['font'];
+    return (font ? {mathvariant: font} : {});
+  }
+
+
+  /**
+   * Splits a package option list of the form [x=y,z=1] into an attribute list
+   * of the form {x: y, z: 1}.
+   * @param {string} attrib The attributes of the package.
+   * @param {{[key: string]: number}?} allowed A list of allowed options. If
+   *     given only allowed arguments are returned.
+   * @param {boolean?} error If true, raises an exception if not allowed options
+   *     are found.
+   * @return {EnvList} The attribute list.
+   */
+  export function keyvalOptions(attrib: string,
+                                allowed: {[key: string]: number} = null,
+                                error: boolean = false): EnvList {
+    let def: EnvList = readKeyval(attrib);
+    if (allowed) {
+      for (let key of Object.keys(def)) {
+        if (!allowed.hasOwnProperty(key)) {
+          if (error) {
+            throw new TexError('InvalidOption', 'Invalid option: %1', key);
+          }
+          delete def[key];
+        }
+      }
+    }
+    return def;
+  }
+
+
+  /**
+   * Implementation of the keyval function from https://www.ctan.org/pkg/keyval
+   * @param {string} text The optional parameter string for a package or
+   *     command.
+   * @return {EnvList} Set of options as key/value pairs.
+   */
+  function readKeyval(text: string): EnvList {
+    let options: EnvList = {};
+    let rest = text;
+    let end, key, val;
+    while (rest) {
+      [key, end, rest] = readValue(rest, ['=', ',']);
+      if (end === '=') {
+        [val, end, rest] = readValue(rest, [',']);
+        val = (val === 'false' || val === 'true') ?
+            JSON.parse(val) : val;
+        options[key] = val;
+      } else if (key) {
+        options[key] = true;
+      }
+    }
+    return options;
+  }
+
+
+  /**
+   * Removes pairs of outer braces.
+   * @param {string} text The string to clean.
+   * @param {number} count The number of outer braces to slice off.
+   * @return {string} The cleaned string.
+   */
+  function removeBraces(text: string, count: number): string {
+    while (count > 0) {
+      text = text.trim().slice(1, -1);
+      count--;
+    }
+    return text.trim();
+  }
+
+
+  /**
+   * Read a value from the given string until an end parameter is reached or
+   * string is exhausted.
+   * @param {string} text The string to process.
+   * @param {string[]} end List of possible end characters.
+   * @return {[string, string, string]} The collected value, the actual end
+   *     character, and the rest of the string still to parse.
+   */
+  function readValue(text: string, end: string[]): [string, string, string] {
+    let length = text.length;
+    let braces = 0;
+    let value = '';
+    let index = 0;
+    let start = 0;             // Counter for the starting left braces.
+    let startCount = true;     // Flag for counting starting left braces.
+    let stopCount = false;     // If true right braces are found directly
+                               // after starting braces, but no other char yet.
+    while (index < length) {
+      let c = text[index++];
+      switch (c) {
+      case ' ':                // Ignore spaces.
+        break;
+      case '{':
+        if (startCount) {      // Count start left braces at start.
+          start++;
+        } else {
+          stopCount = false;
+          if (start > braces) {   // Some start left braces have been closed.
+            start = braces;
+          }
+        }
+        braces++;
+        break;
+      case '}':
+        if (braces) {          // Closing braces.
+          braces--;
+        }
+        if (startCount || stopCount) {  // Closing braces at the start.
+          start--;
+          stopCount = true;    // Continue to close braces.
+        }
+        startCount = false;    // Stop counting start left braces.
+        break;
+      default:
+        if (!braces && end.indexOf(c) !== -1) {   // End character reached.
+          return [stopCount ? 'true' :            // If Stop count is true we
+                                                  // have balanced braces, only.
+                  removeBraces(value, start), c, text.slice(index)];
+        }
+        startCount = false;
+        stopCount = false;
+      }
+      value += c;
+    }
+    if (braces) {
+      throw new TexError('ExtraOpenMissingClose',
+                         'Extra open brace or missing close brace');
+    }
+    return [stopCount ? 'true' : removeBraces(value, start), '', text.slice(index)];
+  }
+
+}
+
+export default ParseUtil;