feat: add no-missing-atx-heading-space rule (#371)

Author: SwetaTanwar

README.md

@@ -82,6 +82,7 @@ export default defineConfig([
 | [`no-empty-links`](./docs/rules/no-empty-links.md) | Disallow empty links | yes |
 | [`no-html`](./docs/rules/no-html.md) | Disallow HTML tags | no |
 | [`no-invalid-label-refs`](./docs/rules/no-invalid-label-refs.md) | Disallow invalid label references | yes |
+| [`no-missing-atx-heading-space`](./docs/rules/no-missing-atx-heading-space.md) | Disallow headings without a space after the hash characters | yes |
 | [`no-missing-label-refs`](./docs/rules/no-missing-label-refs.md) | Disallow missing label references | yes |
 | [`require-alt-text`](./docs/rules/require-alt-text.md) | Require alternative text for images | yes |
 <!-- Rule Table End -->

docs/rules/no-missing-atx-heading-space.md

@@ -0,0 +1,52 @@
+# no-missing-atx-heading-space
+
+This rule warns when spaces are missing after the hash characters in an ATX style heading.
+
+## Rule Details
+
+In Markdown, headings can be created using ATX style (using hash (`#`) characters at the beginning of the line) or Setext style (using underlining with equals (`=`) or hyphens (`-`)).
+
+For ATX style headings, a space should be used after the hash characters to improve readability and ensure proper rendering across various Markdown parsers.
+
+This rule is automatically fixable by the `--fix` command line option.
+
+Examples of **incorrect** code for this rule:
+
+```md
+<!-- eslint markdown/no-missing-atx-heading-space: "error" -->
+
+#Heading 1
+##Heading 2
+###Heading 3
+```
+
+Examples of **correct** code for this rule:
+
+```md
+<!-- eslint markdown/no-missing-atx-heading-space: "error" -->
+
+# Heading 1
+## Heading 2
+### Heading 3
+
+<h1>#Some Heading</h1>
+
+[#123](link.com)
+
+![#alttext][link.png]
+
+This is a paragraph with a #hashtag, not a heading.
+```
+
+## When Not To Use It
+
+You might want to turn this rule off if you're working with a Markdown variant that doesn't require spaces after hash characters in headings.
+
+## Prior Art
+
+- [MD018 - No space after hash on atx style heading](https://github.com/DavidAnson/markdownlint/blob/main/doc/md018.md)
+
+## Further Reading
+
+- [Markdown Syntax: Headings](https://daringfireball.net/projects/markdown/syntax#header)
+- [CommonMark Spec: ATX Headings](https://spec.commonmark.org/0.30/#atx-headings) 

src/rules/no-missing-atx-heading-space.js

@@ -0,0 +1,102 @@
+/**
+ * @fileoverview Rule to ensure there is a space after hash on ATX style headings in Markdown.
+ * @author Sweta Tanwar (@SwetaTanwar)
+ */
+
+//-----------------------------------------------------------------------------
+// Type Definitions
+//-----------------------------------------------------------------------------
+
+/**
+ * @typedef {import("../types.ts").MarkdownRuleDefinition<{ RuleOptions: []; }>}
+ * NoMissingAtxHeadingSpaceRuleDefinition
+ */
+
+//-----------------------------------------------------------------------------
+// Helpers
+//-----------------------------------------------------------------------------
+
+const headingPattern = /^(#{1,6})(?:[^#\s]|$)/u;
+const newLinePattern = /\r?\n/u;
+
+//-----------------------------------------------------------------------------
+// Rule Definition
+//-----------------------------------------------------------------------------
+
+/** @type {NoMissingAtxHeadingSpaceRuleDefinition} */
+export default {
+	meta: {
+		type: "problem",
+
+		docs: {
+			recommended: true,
+			description:
+				"Disallow headings without a space after the hash characters",
+			url: "https://github.com/eslint/markdown/blob/main/docs/rules/no-missing-atx-heading-space.md",
+		},
+
+		fixable: "whitespace",
+
+		messages: {
+			missingSpace: "Missing space after hash(es) on ATX style heading.",
+		},
+	},
+
+	create(context) {
+		return {
+			paragraph(node) {
+				if (node.children && node.children.length > 0) {
+					const firstTextChild = node.children.find(
+						child => child.type === "text",
+					);
+					if (!firstTextChild) {
+						return;
+					}
+
+					const text = context.sourceCode.getText(firstTextChild);
+					const lines = text.split(newLinePattern);
+
+					lines.forEach((line, idx) => {
+						const lineNum =
+							firstTextChild.position.start.line + idx;
+
+						const match = headingPattern.exec(line);
+						if (!match) {
+							return;
+						}
+
+						const hashes = match[1];
+
+						const startColumn =
+							firstTextChild.position.start.column;
+
+						context.report({
+							loc: {
+								start: { line: lineNum, column: startColumn },
+								end: {
+									line: lineNum,
+									column: startColumn + hashes.length + 1,
+								},
+							},
+							messageId: "missingSpace",
+							fix(fixer) {
+								const offset =
+									firstTextChild.position.start.offset +
+									lines.slice(0, idx).join("\n").length +
+									(idx > 0 ? 1 : 0);
+
+								return fixer.insertTextAfterRange(
+									[
+										offset + hashes.length - 1,
+										offset + hashes.length,
+									],
+									" ",
+								);
+							},
+						});
+					});
+				}
+			},
+		};
+	},
+};