From ba1aece66eadc83614fa0f693b2c57cb7556260d Mon Sep 17 00:00:00 2001
From: Dmytro <30444137+swarty@users.noreply.github.com>
Date: Wed, 17 Jan 2024 16:02:11 +0200
Subject: [PATCH] Support for ? symbol and questioned equal operator (#160)

---
 src/functions.ts     | 6 +++---
 src/parser.ts        | 2 +-
 src/symbols.ts       | 4 +++-
 test/parsing-test.ts | 1 +
 4 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/functions.ts b/src/functions.ts
index 3d0651b9..4a03767d 100644
--- a/src/functions.ts
+++ b/src/functions.ts
@@ -8,7 +8,7 @@ import {flatten, isOneOf, join, repeat, unique, words} from '@mathigon/core';
 import {evaluate, interval, Interval} from './eval';
 import {collapseTerm} from './parser';
 import {BRACKETS, escape, isSpecialFunction, VOICE_STRINGS} from './symbols';
-import {CustomFunction, ExprElement, ExprMap, ExprNumber, MathMLMap, VarMap} from './elements';
+import {ExprElement, ExprMap, ExprNumber, MathMLMap, VarMap} from './elements';
 import {ExprError} from './errors';
 
 
@@ -119,7 +119,7 @@ export class ExprFunction extends ExprElement {
     if (this.fn === 'sub') return args.join('_');
     if (this.fn === 'subsup') return `${args[0]}_${args[1]}^${args[2]}`;
 
-    if (words('+ * × · / = < > ≤ ≥ ≈').includes(this.fn)) {
+    if (words('+ * × · / = < > ≤ ≥ ≈ ≟ ≠').includes(this.fn)) {
       return args.join(` ${this.fn} `);
     }
 
@@ -149,7 +149,7 @@ export class ExprFunction extends ExprElement {
       return argsF.length > 1 ? argsF.join('<mo value="−">−</mo>') : `<mo rspace="0" value="−">−</mo>${argsF[0]}`;
     }
 
-    if (isOneOf(this.fn, '+', '=', '<', '>', '≤', '≥', '≈')) {
+    if (isOneOf(this.fn, '+', '=', '<', '>', '≤', '≥', '≈', '≟', '≠')) {
       const fn = escape(this.fn);
       return argsF.join(`<mo value="${fn}">${fn}</mo>`);
     }
diff --git a/src/parser.ts b/src/parser.ts
index e2c65fe3..43b62708 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -262,7 +262,7 @@ export function collapseTerm(tokens: ExprElement[]): ExprElement {
   if (!tokens.length) throw ExprError.invalidExpression();
 
   // Match comparison operators first
-  const comp = tokens.findIndex(t => isOperator(t, '= < > ≤ ≥'));
+  const comp = tokens.findIndex(t => isOperator(t, '= < > ≤ ≥ ≟ ≠'));
   if (comp === 0) throw ExprError.startOperator(tokens[0]);
   if (comp === tokens.length - 1) throw ExprError.endOperator(tokens[0]);
   if (comp > 0) {
diff --git a/src/symbols.ts b/src/symbols.ts
index 7c5f5523..abc4a9c4 100644
--- a/src/symbols.ts
+++ b/src/symbols.ts
@@ -39,6 +39,7 @@ export const SPECIAL_OPERATORS: Obj<string> = {
   '\'': '’',
 
   '!=': '≠',
+  '?=': '≟',
   '<=': '≤',
   '>=': '≥',
   'in': '∈',
@@ -112,7 +113,7 @@ const UPPERCASE = ALPHABET.toUpperCase().split('');
 const GREEK = Object.values(SPECIAL_IDENTIFIERS);
 export const IDENTIFIER_SYMBOLS = [...LOWERCASE, ...UPPERCASE, ...GREEK, '$'];
 
-const SIMPLE_SYMBOLS = '|()[]{}÷,!<>=*/+-–−~^_…°•∥⊥\'∠:%∼△';
+const SIMPLE_SYMBOLS = '|()[]{}÷,!?<>=*/+-–−~^_…°•∥⊥\'∠:%∼△';
 const COMPLEX_SYMBOLS = Object.values(SPECIAL_OPERATORS);
 export const OPERATOR_SYMBOLS = [...SIMPLE_SYMBOLS, ...COMPLEX_SYMBOLS];
 
@@ -154,6 +155,7 @@ export const VOICE_STRINGS: Obj<string> = {
   '±': 'plus-minus',
   '=': 'equals',
   '≠': 'does not equal',
+  '≟': 'is it equal?',
   '<': 'is less than',
   '>': 'is greater than',
   '≤': 'is less than or equal to',
diff --git a/test/parsing-test.ts b/test/parsing-test.ts
index 736e082c..a3447aff 100644
--- a/test/parsing-test.ts
+++ b/test/parsing-test.ts
@@ -73,6 +73,7 @@ tape('symbols', (test) => {
   test.equal(str('x cup y'), 'x ∪ y');
   test.equal(str('x ∩ y'), 'x ∩ y');
   test.equal(str('a != b'), 'a ≠ b');
+  test.equal(str('a ?= b'), 'a ≟ b');
   test.end();
 });