Fix to match GH for HTML generated for backreferences

*   GH previously generated HTML for backreferences to repeated references
    that was not accessible, as it failed [WCAG 2.1 SC 2.4.4 — Link Purpose
    (In Context)](https://www.w3.org/TR/WCAG21/#link-purpose-in-context)
*   GH changed the text content they use in their backreferences from
    `Back to content` to `Back to reference i`, where `i` is either `x` or
    `x-y`, of which `x` is the reference index, and `y` the rereference index

This commit changes all HTML output for users that relied on the defaults, so
that it matches GH again, exactly.
The default handling is exposed as `defaultFootnoteBackLabel`.

Users who set `footnoteBackLabel` are not affected.
But these users can now provide a function instead of a `string`, to also solve
the WCAG issue.
The type for this function is exposed as `FootnoteBackLabelTemplate`.

Additionally, you can now pass `footnoteBackContent` to set the *content* of the
backreference.

Related-to: github/cmark-gfm#307.
Closes remarkjs/remark-rehype#32.

index.d.ts

@@ -1,10 +1,18 @@
 import type {Data, ElementContent, Literal, Properties} from 'hast'
 
 // Expose types.
+export type {
+  FootnoteBackContentTemplate,
+  FootnoteBackLabelTemplate
+} from './lib/footer.js'
 export type {Handler, Handlers, Options, State} from './lib/state.js'
 
 // Expose JS API.
 export {handlers as defaultHandlers} from './lib/handlers/index.js'
+export {
+  defaultFootnoteBackContent,
+  defaultFootnoteBackLabel
+} from './lib/footer.js'
 export {toHast} from './lib/index.js'
 
 /**

index.js

@@ -1,3 +1,7 @@
 // Note: types exposed from `index.d.ts`.
 export {handlers as defaultHandlers} from './lib/handlers/index.js'
 export {toHast} from './lib/index.js'
+export {
+  defaultFootnoteBackContent,
+  defaultFootnoteBackLabel
+} from './lib/footer.js'

lib/footer.js

@@ -5,9 +5,114 @@
  * @typedef {import('./state.js').State} State
  */
 
+/**
+ * @callback FootnoteBackContentTemplate
+ *   Generate content for the backreference dynamically.
+ *
+ *   For the following markdown:
+ *
+ *   ```markdown
+ *   Alpha[^micromark], bravo[^micromark], and charlie[^remark].
+ *
+ *   [^remark]: things about remark
+ *   [^micromark]: things about micromark
+ *   ```
+ *
+ *   This function will be called with:
+ *
+ *   *  `0` and `0` for the backreference from `things about micromark` to
+ *      `alpha`, as it is the first used definition, and the first call to it
+ *   *  `0` and `1` for the backreference from `things about micromark` to
+ *      `bravo`, as it is the first used definition, and the second call to it
+ *   *  `1` and `0` for the backreference from `things about remark` to
+ *      `charlie`, as it is the second used definition
+ * @param {number} referenceIndex
+ *   Index of the definition in the order that they are first referenced,
+ *   0-indexed.
+ * @param {number} rereferenceIndex
+ *   Index of calls to the same definition, 0-indexed.
+ * @returns {Array<ElementContent> | ElementContent | string}
+ *   Content for the backreference when linking back from definitions to their
+ *   reference.
+ *
+ * @callback FootnoteBackLabelTemplate
+ *   Generate a back label dynamically.
+ *
+ *   For the following markdown:
+ *
+ *   ```markdown
+ *   Alpha[^micromark], bravo[^micromark], and charlie[^remark].
+ *
+ *   [^remark]: things about remark
+ *   [^micromark]: things about micromark
+ *   ```
+ *
+ *   This function will be called with:
+ *
+ *   *  `0` and `0` for the backreference from `things about micromark` to
+ *      `alpha`, as it is the first used definition, and the first call to it
+ *   *  `0` and `1` for the backreference from `things about micromark` to
+ *      `bravo`, as it is the first used definition, and the second call to it
+ *   *  `1` and `0` for the backreference from `things about remark` to
+ *      `charlie`, as it is the second used definition
+ * @param {number} referenceIndex
+ *   Index of the definition in the order that they are first referenced,
+ *   0-indexed.
+ * @param {number} rereferenceIndex
+ *   Index of calls to the same definition, 0-indexed.
+ * @returns {string}
+ *   Back label to use when linking back from definitions to their reference.
+ */
+
 import structuredClone from '@ungap/structured-clone'
 import {normalizeUri} from 'micromark-util-sanitize-uri'
 
+/**
+ * Generate the default content that GitHub uses on backreferences.
+ *
+ * @param {number} _
+ *   Index of the definition in the order that they are first referenced,
+ *   0-indexed.
+ * @param {number} rereferenceIndex
+ *   Index of calls to the same definition, 0-indexed.
+ * @returns {Array<ElementContent>}
+ *   Content.
+ */
+export function defaultFootnoteBackContent(_, rereferenceIndex) {
+  /** @type {Array<ElementContent>} */
+  const result = [{type: 'text', value: '↩'}]
+
+  if (rereferenceIndex > 1) {
+    result.push({
+      type: 'element',
+      tagName: 'sup',
+      properties: {},
+      children: [{type: 'text', value: String(rereferenceIndex)}]
+    })
+  }
+
+  return result
+}
+
+/**
+ * Generate the default label that GitHub uses on backreferences.
+ *
+ * @param {number} referenceIndex
+ *   Index of the definition in the order that they are first referenced,
+ *   0-indexed.
+ * @param {number} rereferenceIndex
+ *   Index of calls to the same definition, 0-indexed.
+ * @returns {string}
+ *   Label.
+ */
+export function defaultFootnoteBackLabel(referenceIndex, rereferenceIndex) {
+  return (
+    'Back to reference ' +
+    (referenceIndex + 1) +
+    (rereferenceIndex > 1 ? '-' + rereferenceIndex : '')
+  )
+}
+
 /**
  * Generate a hast footer for called footnote definitions.
  *
@@ -16,23 +121,27 @@ import {normalizeUri} from 'micromark-util-sanitize-uri'
  * @returns {Element | undefined}
  *   `section` element or `undefined`.
  */
+// eslint-disable-next-line complexity
 export function footer(state) {
   const clobberPrefix =
     typeof state.options.clobberPrefix === 'string'
       ? state.options.clobberPrefix
       : 'user-content-'
-  const footnoteBackLabel = state.options.footnoteBackLabel || 'Back to content'
+  const footnoteBackContent =
+    state.options.footnoteBackContent || defaultFootnoteBackContent
+  const footnoteBackLabel =
+    state.options.footnoteBackLabel || defaultFootnoteBackLabel
   const footnoteLabel = state.options.footnoteLabel || 'Footnotes'
   const footnoteLabelTagName = state.options.footnoteLabelTagName || 'h2'
   const footnoteLabelProperties = state.options.footnoteLabelProperties || {
     className: ['sr-only']
   }
   /** @type {Array<ElementContent>} */
   const listItems = []
-  let index = -1
+  let referenceIndex = -1
 
-  while (++index < state.footnoteOrder.length) {
-    const def = state.footnoteById.get(state.footnoteOrder[index])
+  while (++referenceIndex < state.footnoteOrder.length) {
+    const def = state.footnoteById.get(state.footnoteOrder[referenceIndex])
 
     if (!def) {
       continue
@@ -41,15 +150,27 @@ export function footer(state) {
     const content = state.all(def)
     const id = String(def.identifier).toUpperCase()
     const safeId = normalizeUri(id.toLowerCase())
-    let referenceIndex = 0
+    let rereferenceIndex = 0
     /** @type {Array<ElementContent>} */
     const backReferences = []
     const counts = state.footnoteCounts.get(id)
 
     // eslint-disable-next-line no-unmodified-loop-condition
-    while (counts !== undefined && ++referenceIndex <= counts) {
-      /** @type {Element} */
-      const backReference = {
+    while (counts !== undefined && ++rereferenceIndex <= counts) {
+      if (backReferences.length > 0) {
+        backReferences.push({type: 'text', value: ' '})
+      }
+
+      let children =
+        typeof footnoteBackContent === 'string'
+          ? footnoteBackContent
+          : footnoteBackContent(referenceIndex, rereferenceIndex)
+
+      if (typeof children === 'string') {
+        children = {type: 'text', value: children}
+      }
+
+      backReferences.push({
         type: 'element',
         tagName: 'a',
         properties: {
@@ -58,28 +179,16 @@ export function footer(state) {
             clobberPrefix +
             'fnref-' +
             safeId +
-            (referenceIndex > 1 ? '-' + referenceIndex : ''),
-          dataFootnoteBackref: true,
-          className: ['data-footnote-backref'],
-          ariaLabel: footnoteBackLabel
+            (rereferenceIndex > 1 ? '-' + rereferenceIndex : ''),
+          dataFootnoteBackref: '',
+          ariaLabel:
+            typeof footnoteBackLabel === 'string'
+              ? footnoteBackLabel
+              : footnoteBackLabel(referenceIndex, rereferenceIndex),
+          className: ['data-footnote-backref']
         },
-        children: [{type: 'text', value: '↩'}]
-      }
-
-      if (referenceIndex > 1) {
-        backReference.children.push({
-          type: 'element',
-          tagName: 'sup',
-          properties: {},
-          children: [{type: 'text', value: String(referenceIndex)}]
-        })
-      }
-
-      if (backReferences.length > 0) {
-        backReferences.push({type: 'text', value: ' '})
-      }
-
-      backReferences.push(backReference)
+        children: Array.isArray(children) ? children : [children]
+      })
     }
 
     const tail = content[content.length - 1]