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+/**
+ * @fileoverview Rule to prevent duplicate headings in Markdown.
+ * @author Nicholas C. Zakas
+ */
+//-----------------------------------------------------------------------------
+// Type Definitions
+//-----------------------------------------------------------------------------
+/**
+ * @import { MarkdownRuleDefinition } from "../types.js";
+ * @typedef {"duplicateHeading"} NoDuplicateHeadingsMessageIds
+ * @typedef {[{ checkSiblingsOnly?: boolean }]} NoDuplicateHeadingsOptions
+ * @typedef {MarkdownRuleDefinition<{ RuleOptions: NoDuplicateHeadingsOptions, MessageIds: NoDuplicateHeadingsMessageIds }>} NoDuplicateHeadingsRuleDefinition
+ */
+//-----------------------------------------------------------------------------
+// Rule Definition
+//-----------------------------------------------------------------------------
+export default /** @satisfies {NoDuplicateHeadingsRuleDefinition} */ ({
+    meta: {
+        type: "problem",
+        docs: {
+            description: "Disallow duplicate headings in the same document",
+            url: "https://github.com/eslint/markdown/blob/main/docs/rules/no-duplicate-headings.md",
+        },
+        messages: {
+            duplicateHeading: 'Duplicate heading "{{text}}" found.',
+        },
+        schema: [
+            {
+                type: "object",
+                properties: {
+                    checkSiblingsOnly: {
+                        type: "boolean",
+                    },
+                },
+                additionalProperties: false,
+            },
+        ],
+        defaultOptions: [{ checkSiblingsOnly: false }],
+    },
+    create(context) {
+        const [{ checkSiblingsOnly }] = context.options;
+        /** @type {Map<number, Set<string>>} */
+        const headingsByLevel = checkSiblingsOnly
+            ? new Map([
+                [1, new Set()],
+                [2, new Set()],
+                [3, new Set()],
+                [4, new Set()],
+                [5, new Set()],
+                [6, new Set()],
+            ])
+            : new Map([[1, new Set()]]);
+        let lastLevel = 1;
+        let currentLevelHeadings = headingsByLevel.get(lastLevel);
+        /** @type {string} */
+        let headingChildrenSequence;
+        /** @type {string} */
+        let headingText;
+        return {
+            heading(node) {
+                headingChildrenSequence = "";
+                headingText = "";
+                if (checkSiblingsOnly) {
+                    const currentLevel = node.depth;
+                    if (currentLevel < lastLevel) {
+                        for (let level = lastLevel; level > currentLevel; level--) {
+                            headingsByLevel.get(level).clear();
+                        }
+                    }
+                    lastLevel = currentLevel;
+                    currentLevelHeadings = headingsByLevel.get(currentLevel);
+                }
+            },
+            "heading *"({ type, value }) {
+                if (value) {
+                    headingChildrenSequence += `[${type},${value}]`; // We use a custom sequence representation to keep track of heading children.
+                    if (type !== "html") {
+                        headingText += value;
+                    }
+                }
+                else {
+                    headingChildrenSequence += `[${type}]`;
+                }
+            },
+            "heading:exit"(node) {
+                if (currentLevelHeadings.has(headingChildrenSequence)) {
+                    context.report({
+                        loc: node.position,
+                        messageId: "duplicateHeading",
+                        data: {
+                            text: headingText,
+                        },
+                    });
+                }
+                else {
+                    currentLevelHeadings.add(headingChildrenSequence);
+                }
+            },
+        };
+    },
+});