feat: validate relative links fragments

Similar to https://github.com/DavidAnson/markdownlint/blob/main/doc/md051.md

Fixes #2

BREAKING CHANGE: Validate links fragments in relative links

src/index.js

@@ -3,42 +3,12 @@
 const { pathToFileURL } = require('node:url')
 const fs = require('node:fs')
 
-/**
- * Calls the provided function for each matching token.
- *
- * @param {Object} params RuleParams instance.
- * @param {string} type Token type identifier.
- * @param {Function} handler Callback function.
- * @returns {void}
- */
-const filterTokens = (params, type, handler) => {
-  for (const token of params.tokens) {
-    if (token.type === type) {
-      handler(token)
-    }
-  }
-}
-
-/**
- * Adds a generic error object via the onError callback.
- *
- * @param {Object} onError RuleOnError instance.
- * @param {number} lineNumber Line number.
- * @param {string} [detail] Error details.
- * @param {string} [context] Error context.
- * @param {number[]} [range] Column and length of error.
- * @param {Object} [fixInfo] RuleOnErrorFixInfo instance.
- * @returns {void}
- */
-const addError = (onError, lineNumber, detail, context, range, fixInfo) => {
-  onError({
-    lineNumber,
-    detail,
-    context,
-    range,
-    fixInfo
-  })
-}
+const {
+  filterTokens,
+  addError,
+  convertHeadingToHTMLFragment,
+  getMarkdownHeadings
+} = require('./utils.js')
 
 const customRule = {
   names: ['relative-links'],
@@ -70,12 +40,37 @@ const customRule = {
 
         if (hrefSrc != null) {
           const url = new URL(hrefSrc, pathToFileURL(params.name))
-          url.hash = ''
           const isRelative =
             url.protocol === 'file:' && !hrefSrc.startsWith('/')
-          if (isRelative && !fs.existsSync(url)) {
-            const detail = `Link "${hrefSrc}" is dead`
-            addError(onError, lineNumber, detail)
+          if (isRelative) {
+            const detail = `Link "${hrefSrc}" is not valid`
+
+            if (!fs.existsSync(url)) {
+              addError(onError, lineNumber, detail)
+              return
+            }
+
+            if (type === 'link_open' && url.hash !== '') {
+              const fileContent = fs.readFileSync(url, { encoding: 'utf8' })
+              const headings = getMarkdownHeadings(fileContent)
+
+              /** @type {Map<string, number>} */
+              const fragments = new Map()
+
+              const headingsHTMLFragments = headings.map((heading) => {
+                const fragment = convertHeadingToHTMLFragment(heading)
+                const count = fragments.get(fragment) ?? 0
+                fragments.set(fragment, count + 1)
+                if (count !== 0) {
+                  return `${fragment}-${count}`
+                }
+                return fragment
+              })
+
+              if (!headingsHTMLFragments.includes(url.hash)) {
+                addError(onError, lineNumber, detail)
+              }
+            }
           }
         }
       })