From 01ae7f8eef09bcddc6e327c2cb5a3460800652d5 Mon Sep 17 00:00:00 2001
From: Erik Demaine <edemaine@mit.edu>
Date: Sat, 24 Apr 2021 15:33:01 -0400
Subject: [PATCH] fix: Respect catcode in macro expansion and set ~'s catcode
 correctly (#2949)

Support single characters having a catcode of 13 (active) or 12 (other),
and expand a macro defined for the character only if it's active.
This enables defining `~` as a macro, but avoiding expansion in URLs.

Fix #2924

Co-authored-by: ylemkimon <y@ylem.kim>
---
 src/Lexer.js         | 4 +++-
 src/MacroExpander.js | 8 ++++++++
 src/Parser.js        | 2 ++
 src/macros.js        | 2 ++
 src/symbols.js       | 2 --
 test/katex-spec.js   | 7 +++++++
 6 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/src/Lexer.js b/src/Lexer.js
index c9271c1f..7cf96edd 100644
--- a/src/Lexer.js
+++ b/src/Lexer.js
@@ -60,7 +60,8 @@ export default class Lexer implements LexerInterface {
     input: string;
     settings: Settings;
     tokenRegex: RegExp;
-    // category codes, only supports comment characters (14) for now
+    // Category codes. The lexer only supports comment characters (14) for now.
+    // MacroExpander additionally distinguishes active (13).
     catcodes: {[string]: number};
 
     constructor(input: string, settings: Settings) {
@@ -70,6 +71,7 @@ export default class Lexer implements LexerInterface {
         this.tokenRegex = new RegExp(tokenRegexString, 'g');
         this.catcodes = {
             "%": 14, // comment character
+            "~": 13, // active character
         };
     }
 
diff --git a/src/MacroExpander.js b/src/MacroExpander.js
index bcd5ec4f..94a809a8 100644
--- a/src/MacroExpander.js
+++ b/src/MacroExpander.js
@@ -395,6 +395,14 @@ export default class MacroExpander implements MacroContextInterface {
         if (definition == null) { // mainly checking for undefined here
             return definition;
         }
+        // If a single character has an associated catcode other than 13
+        // (active character), then don't expand it.
+        if (name.length === 1) {
+            const catcode = this.lexer.catcodes[name];
+            if (catcode != null && catcode !== 13) {
+                return;
+            }
+        }
         const expansion =
             typeof definition === "function" ? definition(this) : definition;
         if (typeof expansion === "string") {
diff --git a/src/Parser.js b/src/Parser.js
index c5448740..49ea4c5f 100644
--- a/src/Parser.js
+++ b/src/Parser.js
@@ -682,8 +682,10 @@ export default class Parser {
      */
     parseUrlGroup(optional: boolean): ?ParseNode<"url"> {
         this.gullet.lexer.setCatcode("%", 13); // active character
+        this.gullet.lexer.setCatcode("~", 12); // other character
         const res = this.parseStringGroup("url", optional);
         this.gullet.lexer.setCatcode("%", 14); // comment character
+        this.gullet.lexer.setCatcode("~", 13); // active character
         if (res == null) {
             return null;
         }
diff --git a/src/macros.js b/src/macros.js
index 986e31c5..0d8ec59c 100644
--- a/src/macros.js
+++ b/src/macros.js
@@ -327,10 +327,12 @@ defineMacro("\\bgroup", "{");
 defineMacro("\\egroup", "}");
 
 // Symbols from latex.ltx:
+// \def~{\nobreakspace{}}
 // \def\lq{`}
 // \def\rq{'}
 // \def \aa {\r a}
 // \def \AA {\r A}
+defineMacro("~", "\\nobreakspace");
 defineMacro("\\lq", "`");
 defineMacro("\\rq", "'");
 defineMacro("\\aa", "\\r a");
diff --git a/src/symbols.js b/src/symbols.js
index bd9f2544..ee71a65e 100644
--- a/src/symbols.js
+++ b/src/symbols.js
@@ -582,13 +582,11 @@ defineSymbol(math, main, rel, "\u2192", "\\to");
 defineSymbol(math, ams, rel, "\u2271", "\\ngeq", true);
 defineSymbol(math, ams, rel, "\u2270", "\\nleq", true);
 defineSymbol(math, main, spacing, "\u00a0", "\\ ");
-defineSymbol(math, main, spacing, "\u00a0", "~");
 defineSymbol(math, main, spacing, "\u00a0", "\\space");
 // Ref: LaTeX Source 2e: \DeclareRobustCommand{\nobreakspace}{%
 defineSymbol(math, main, spacing, "\u00a0", "\\nobreakspace");
 defineSymbol(text, main, spacing, "\u00a0", "\\ ");
 defineSymbol(text, main, spacing, "\u00a0", " ");
-defineSymbol(text, main, spacing, "\u00a0", "~");
 defineSymbol(text, main, spacing, "\u00a0", "\\space");
 defineSymbol(text, main, spacing, "\u00a0", "\\nobreakspace");
 defineSymbol(math, main, spacing, null, "\\nobreak");
diff --git a/test/katex-spec.js b/test/katex-spec.js
index d3df44f9..ba668201 100644
--- a/test/katex-spec.js
+++ b/test/katex-spec.js
@@ -3282,6 +3282,13 @@ describe("A macro expander", function() {
         expect('\\char"g').not.toParse();
     });
 
+    it("\\char escapes ~ correctly", () => {
+        const parsedBare = getParsed("~");
+        expect(parsedBare[0].type).toEqual("spacing");
+        const parsedChar = getParsed("\\char`\\~");
+        expect(parsedChar[0].type).toEqual("textord");
+    });
+
     it("should build Unicode private area characters", function() {
         expect`\gvertneqq\lvertneqq\ngeqq\ngeqslant\nleqq`.toBuild();
         expect`\nleqslant\nshortmid\nshortparallel\varsubsetneq`.toBuild();