Keep a count of the number of open blocks.

extensions/table.c

@@ -311,12 +311,18 @@ static cmark_node *try_opening_table_header(cmark_syntax_extension *self,
     }
   }
 
+  assert(cmark_node_get_type(parent_container) == CMARK_NODE_PARAGRAPH);
   if (!cmark_node_set_type(parent_container, CMARK_NODE_TABLE)) {
     free_table_row(parser->mem, header_row);
     free_table_row(parser->mem, marker_row);
     return parent_container;
   }
 
+  // Update the node counts after parent_container changed type.
+  assert(parent_container->next == NULL);
+  decr_open_block_count(parser, CMARK_NODE_PARAGRAPH);
+  incr_open_block_count(parser, CMARK_NODE_TABLE);
+
   if (header_row->paragraph_offset) {
     try_inserting_table_header_paragraph(parser, parent_container, (unsigned char *)parent_string,
                                          header_row->paragraph_offset);

src/blocks.c

@@ -70,6 +70,22 @@ static void S_parser_feed(cmark_parser *parser, const unsigned char *buffer,
 static void S_process_line(cmark_parser *parser, const unsigned char *buffer,
                            bufsize_t bytes);
 
+static void subtract_open_block_counts(cmark_parser *parser, cmark_node *node) {
+  do {
+    decr_open_block_count(parser, S_type(node));
+    node->flags &= ~CMARK_NODE__OPEN_BLOCK;
+    node = node->last_child;
+  } while (node);
+}
+
+static void add_open_block_counts(cmark_parser *parser, cmark_node *node) {
+  do {
+    incr_open_block_count(parser, S_type(node));
+    node->flags |= CMARK_NODE__OPEN_BLOCK;
+    node = node->last_child;
+  } while (node);
+}
+
 static cmark_node *make_block(cmark_mem *mem, cmark_node_type tag,
                               int start_line, int start_column) {
   cmark_node *e;
@@ -129,6 +145,7 @@ static void cmark_parser_reset(cmark_parser *parser) {
   parser->refmap = cmark_reference_map_new(parser->mem);
   parser->root = document;
   parser->current = document;
+  add_open_block_counts(parser, document);
 
   parser->syntax_extensions = saved_exts;
   parser->inline_syntax_extensions = saved_inline_exts;
@@ -310,6 +327,12 @@ static cmark_node *finalize(cmark_parser *parser, cmark_node *b) {
     has_content = resolve_reference_link_definitions(parser, b);
     if (!has_content) {
       // remove blank node (former reference def)
+      if (b->flags & CMARK_NODE__OPEN_BLOCK) {
+        decr_open_block_count(parser, S_type(b));
+        if (b->prev) {
+          add_open_block_counts(parser, b->prev);
+        }
+      }
       cmark_node_free(b);
     }
     break;
@@ -382,6 +405,15 @@ static cmark_node *finalize(cmark_parser *parser, cmark_node *b) {
   return parent;
 }
 
+// Recalculates the number of open blocks. Returns true if it matches what's currently stored
+// in parser. (Used to check that the counts in parser, which are updated incrementally, are
+// correct.)
+bool check_open_block_counts(cmark_parser *parser) {
+  cmark_parser tmp_parser = {0}; // Only used for its open_block_counts field.
+  add_open_block_counts(&tmp_parser, parser->root);
+  return memcmp(tmp_parser.open_block_counts, parser->open_block_counts, sizeof(parser->open_block_counts)) == 0;
+}
+
 // Add a node as child of another.  Return pointer to child.
 static cmark_node *add_child(cmark_parser *parser, cmark_node *parent,
                              cmark_node_type block_type, int start_column) {
@@ -400,11 +432,14 @@ static cmark_node *add_child(cmark_parser *parser, cmark_node *parent,
   if (parent->last_child) {
     parent->last_child->next = child;
     child->prev = parent->last_child;
+    subtract_open_block_counts(parser, parent->last_child);
   } else {
     parent->first_child = child;
     child->prev = NULL;
   }
   parent->last_child = child;
+  add_open_block_counts(parser, child);
+
   return child;
 }
 
@@ -1048,6 +1083,8 @@ static cmark_node *check_open_blocks(cmark_parser *parser, cmark_chunk *input,
   cmark_node *container = parser->root;
   cmark_node_type cont_type;
 
+  assert(check_open_block_counts(parser));
+
   while (S_last_child_is_open(container)) {
     container = container->last_child;
     cont_type = S_type(container);
@@ -1193,8 +1230,9 @@ static void open_new_blocks(cmark_parser *parser, cmark_node **container,
       has_content = resolve_reference_link_definitions(parser, *container);
 
       if (has_content) {
-
-        (*container)->type = (uint16_t)CMARK_NODE_HEADING;
+        cmark_node_set_type(*container, CMARK_NODE_HEADING);
+        decr_open_block_count(parser, CMARK_NODE_PARAGRAPH);
+        incr_open_block_count(parser, CMARK_NODE_HEADING);
         (*container)->as.heading.level = lev;
         (*container)->as.heading.setext = true;
         S_advance_offset(parser, input, input->len - 1 - parser->offset, false);
@@ -1478,6 +1516,7 @@ static void S_process_line(cmark_parser *parser, const unsigned char *buffer,
 
   parser->line_number++;
 
+  assert(parser->current->next == NULL);
   last_matched_container = check_open_blocks(parser, &input, &all_matched);
 
   if (!last_matched_container)

src/cmark-gfm.h

@@ -37,6 +37,16 @@ char *cmark_markdown_to_html(const char *text, size_t len, int options);
 #define CMARK_NODE_TYPE_MASK (0xc000)
 #define CMARK_NODE_VALUE_MASK (0x3fff)
 
+/**
+ * This is the maximum number of block types (CMARK_NODE_DOCUMENT,
+ * CMARK_NODE_HEADING, ...). It needs to be bigger than the number of
+ * hardcoded block types (below) to allow for extensions (see
+ * cmark_syntax_extension_add_node). But it also determines the size of the
+ * open_block_counts array in the cmark_parser struct, so we don't want it
+ * to be excessively large.
+ */
+#define CMARK_NODE_TYPE_BLOCK_LIMIT 0x20
+
 typedef enum {
   /* Error status */
   CMARK_NODE_NONE = 0x0000,

src/node.h

@@ -50,12 +50,13 @@ typedef struct {
 
 enum cmark_node__internal_flags {
   CMARK_NODE__OPEN = (1 << 0),
-  CMARK_NODE__LAST_LINE_BLANK = (1 << 1),
-  CMARK_NODE__LAST_LINE_CHECKED = (1 << 2),
+  CMARK_NODE__OPEN_BLOCK = (1 << 1),
+  CMARK_NODE__LAST_LINE_BLANK = (1 << 2),
+  CMARK_NODE__LAST_LINE_CHECKED = (1 << 3),
 
   // Extensions can register custom flags by calling `cmark_register_node_flag`.
   // This is the starting value for the custom flags.
-  CMARK_NODE__REGISTER_FIRST = (1 << 3),
+  CMARK_NODE__REGISTER_FIRST = (1 << 4),
 };
 
 typedef uint16_t cmark_node_internal_flags;

src/parser.h

@@ -50,8 +50,37 @@ struct cmark_parser {
   cmark_llist *syntax_extensions;
   cmark_llist *inline_syntax_extensions;
   cmark_ispunct_func backslash_ispunct;
+
+  /**
+   * The "open" blocks are the blocks visited by the loop in
+   * check_open_blocks (blocks.c). I.e. the blocks in this list:
+   *
+   *   parser->root->last_child->...->last_child
+   *
+   * open_block_counts is used to keep track of how many of each type of
+   * node are currently in the open blocks list. Knowing these counts can
+   * sometimes help to end the loop in check_open_blocks early, improving
+   * efficiency.
+   *
+   * The count is stored at this offset: type - CMARK_NODE_TYPE_BLOCK - 1
+   * For example, CMARK_NODE_LIST (0x8003) is stored at offset 2.
+   */
+  size_t open_block_counts[CMARK_NODE_TYPE_BLOCK_LIMIT];
 };
 
+static CMARK_INLINE void incr_open_block_count(cmark_parser *parser, cmark_node_type type) {
+  assert(type > CMARK_NODE_TYPE_BLOCK);
+  assert(type <= CMARK_NODE_TYPE_BLOCK + CMARK_NODE_TYPE_BLOCK_LIMIT);
+  parser->open_block_counts[type - CMARK_NODE_TYPE_BLOCK - 1]++;
+}
+
+static CMARK_INLINE void decr_open_block_count(cmark_parser *parser, cmark_node_type type) {
+  assert(type > CMARK_NODE_TYPE_BLOCK);
+  assert(type <= CMARK_NODE_TYPE_BLOCK + CMARK_NODE_TYPE_BLOCK_LIMIT);
+  assert(parser->open_block_counts[type - CMARK_NODE_TYPE_BLOCK - 1] > 0);
+  parser->open_block_counts[type - CMARK_NODE_TYPE_BLOCK - 1]--;
+}
+
 #ifdef __cplusplus
 }
 #endif

src/syntax_extension.c

@@ -29,7 +29,10 @@ cmark_syntax_extension *cmark_syntax_extension_new(const char *name) {
 cmark_node_type cmark_syntax_extension_add_node(int is_inline) {
   cmark_node_type *ref = !is_inline ? &CMARK_NODE_LAST_BLOCK : &CMARK_NODE_LAST_INLINE;
 
-  if ((*ref & CMARK_NODE_VALUE_MASK) == CMARK_NODE_VALUE_MASK) {
+  if ((*ref & CMARK_NODE_VALUE_MASK) >= CMARK_NODE_TYPE_BLOCK_LIMIT) {
+    // This assertion will fail if you try to register more extensions than
+    // are currently allowed by CMARK_NODE_TYPE_BLOCK_MAXNUM. Try increasing
+    // the limit.
     assert(false);
     return (cmark_node_type) 0;
   }