Add support for nested comments

Unfortunately, the 'extras' array expects single tokens only, so one
can't really use a recursive parsing rule to express nesting comments...

grammar.js

@@ -25,6 +25,11 @@ module.exports = grammar({
     [$._lvalue, $.array_expression],
   ],
 
+  externals: ($) => [
+    // Nested comments need to be tokenized externally
+    $.comment,
+  ],
+
   extras: ($) => [
     /( |\n|\r|\t)+/,
     $.comment,
@@ -36,21 +41,6 @@ module.exports = grammar({
       optional($._declaration_chunks),
     ),
 
-    comment: ($) => token(
-      seq(
-        "/*",
-        repeat(
-          choice(
-            // Match anything but the end-delimiter
-            /(\*[^/]|[^*])+/,
-            // Comments can be nested
-            // $.comment,
-          ),
-        ),
-        "*/",
-      ),
-    ),
-
     // Expressions {{{
 
     _expr: ($) => choice(

src/grammar.json

@@ -23,34 +23,6 @@
         }
       ]
     },
-    "comment": {
-      "type": "TOKEN",
-      "content": {
-        "type": "SEQ",
-        "members": [
-          {
-            "type": "STRING",
-            "value": "/*"
-          },
-          {
-            "type": "REPEAT",
-            "content": {
-              "type": "CHOICE",
-              "members": [
-                {
-                  "type": "PATTERN",
-                  "value": "(\\*[^/]|[^*])+"
-                }
-              ]
-            }
-          },
-          {
-            "type": "STRING",
-            "value": "*/"
-          }
-        ]
-      }
-    },
     "_expr": {
       "type": "CHOICE",
       "members": [
@@ -1468,7 +1440,12 @@
     ]
   ],
   "precedences": [],
-  "externals": [],
+  "externals": [
+    {
+      "type": "SYMBOL",
+      "name": "comment"
+    }
+  ],
   "inline": [],
   "supertypes": []
 }

src/scanner.c

@@ -0,0 +1,98 @@
+#include <stdint.h>
+#include <string.h>
+
+#include <tree_sitter/parser.h>
+
+enum TokenType {
+    COMMENT,
+};
+
+static int32_t advance(TSLexer *lexer) {
+    int32_t last = lexer->lookahead;
+    lexer->advance(lexer, false);
+    return last;
+}
+
+static bool expect(TSLexer *lexer, int32_t expected) {
+    if (lexer->eof(lexer) || lexer->lookahead != expected) {
+        return false;
+    }
+    advance(lexer);
+    return true;
+}
+
+static void skip_whitespace(TSLexer *lexer) {
+    while (!lexer->eof(lexer)) {
+        switch (lexer->lookahead) {
+        case ' ':
+        case '\t':
+        case '\n':
+        case '\r':
+            lexer->advance(lexer, true);
+        default:
+            return;
+        }
+    }
+}
+
+// Comments start with "/*", end with "*/", and can be nested like OCaml
+static bool scan_comment(TSLexer *lexer) {
+    // '/' already consumed outside of the function
+    if (!expect(lexer, '*')) {
+        return false; // Division etc...
+    }
+
+    unsigned long level = 1;
+
+    while (level > 0 && !lexer->eof(lexer)) {
+        switch (advance(lexer)) {
+        case '/':
+            if (expect(lexer, '*')) {
+                ++level;
+            }
+            break;
+        case '*':
+            if (expect(lexer, '/')) {
+                --level;
+            }
+            break;
+        }
+    }
+
+    return level == 0;
+}
+
+void *tree_sitter_tiger_external_scanner_create() {
+    return NULL;
+}
+
+void tree_sitter_tiger_external_scanner_destroy(void *payload) {}
+
+unsigned tree_sitter_tiger_external_scanner_serialize(void *payload,
+                                                      char *buffer) {
+    return 0;
+}
+
+void tree_sitter_tiger_external_scanner_deserialize(void *payload,
+                                                    char const *buffer,
+                                                    unsigned length) {}
+
+bool tree_sitter_tiger_external_scanner_scan(void *payload,
+                                             TSLexer *lexer,
+                                             bool const *valid_symbols) {
+    // Only try to scan when appropriate
+    if (!valid_symbols[COMMENT]) {
+        return false;
+    }
+
+    // Apparently it is expected of us to skip all whitespace by ourselves...
+    skip_whitespace(lexer);
+
+    // Comments start with "/*", scan_comment expects '/' to have been consumed
+    if (expect(lexer, '/')) {
+        lexer->result_symbol = COMMENT;
+        return scan_comment(lexer);
+    }
+
+    return false;
+}

test/corpus/comments.txt

@@ -51,13 +51,7 @@ Nested comment
 --------------------------------------------------------------------------------
 
 (source_file
-  (comment)
-  (identifier)
-  (ERROR
-    (operator)
-    (operator)
-    (operator)
-    (operator)))
+  (comment))
 
 ================================================================================
 Unterminated comment