feat: support line number checking in link fragment (e.g: '#L50')

Fixes #6

src/index.js

@@ -8,6 +8,9 @@ const {
   convertHeadingToHTMLFragment,
   getMarkdownHeadings,
   getMarkdownIdOrAnchorNameFragments,
+  isValidIntegerString,
+  getNumberOfLines,
+  getLineNumberStringFromFragment,
 } = require("./utils.js")
 
 /** @typedef {import('markdownlint').Rule} MarkdownLintRule */
@@ -121,7 +124,35 @@ const customRule = {
 
         fragmentsHTML.push(...idOrAnchorNameHTMLFragments)
 
-        if (!fragmentsHTML.includes(url.hash)) {
+        if (!fragmentsHTML.includes(url.hash.toLowerCase())) {
+          if (url.hash.startsWith("#L")) {
+            const lineNumberFragmentString = getLineNumberStringFromFragment(
+              url.hash,
+            )
+
+            const hasOnlyDigits = isValidIntegerString(lineNumberFragmentString)
+            if (!hasOnlyDigits) {
+              onError({
+                lineNumber,
+                detail: `${detail} should have a valid fragment identifier`,
+              })
+              continue
+            }
+
+            const lineNumberFragment = Number.parseInt(
+              lineNumberFragmentString,
+              10,
+            )
+            const numberOfLines = getNumberOfLines(fileContent)
+            if (lineNumberFragment > numberOfLines) {
+              onError({
+                lineNumber,
+                detail: `${detail} should have a valid fragment identifier, ${detail} should have at least ${lineNumberFragment} lines to be valid`,
+              })
+              continue
+            }
+          }
+
           onError({
             lineNumber,
             detail: `${detail} should have a valid fragment identifier`,

src/utils.js

@@ -114,8 +114,47 @@ const getMarkdownIdOrAnchorNameFragments = (content) => {
   return result
 }
 
+/**
+ * Checks if a string is a valid integer.
+ *
+ * Using `Number.parseInt` combined with `Number.isNaN` will not be sufficient enough because `Number.parseInt("1abc", 10)` will return `1` (a valid number) instead of `NaN`.
+ *
+ * @param {string} value
+ * @returns {boolean}
+ * @example isValidIntegerString("1") // true
+ * @example isValidIntegerString("45") // true
+ * @example isValidIntegerString("1abc") // false
+ * @example isValidIntegerString("1.0") // false
+ */
+const isValidIntegerString = (value) => {
+  const regex = /^\d+$/
+  return regex.test(value)
+}
+
+/**
+ * Gets the number of lines in a string, based on the number of `\n` characters.
+ * @param {string} content
+ * @returns {number}
+ */
+const getNumberOfLines = (content) => {
+  return content.split("\n").length
+}
+
+/**
+ * Gets the line number string from a fragment.
+ * @param {string} fragment
+ * @returns {string}
+ * @example getLineNumberStringFromFragment("#L50") // 50
+ */
+const getLineNumberStringFromFragment = (fragment) => {
+  return fragment.slice(2)
+}
+
 module.exports = {
   convertHeadingToHTMLFragment,
   getMarkdownHeadings,
   getMarkdownIdOrAnchorNameFragments,
+  isValidIntegerString,
+  getNumberOfLines,
+  getLineNumberStringFromFragment,
 }