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import type { Element, Parent, Root } from 'hast';
import { fromHtml } from 'hast-util-from-html';
import { toText } from 'hast-util-to-text';
import { removePosition } from 'unist-util-remove-position';
import { visitParents } from 'unist-util-visit-parents';
type Highlighter = (code: string, language: string, options?: { meta?: string }) => Promise<string>;
const languagePattern = /\blanguage-(\S+)\b/;
/**
* A hast utility to syntax highlight code blocks with a given syntax highlighter.
*
* @param tree
* The hast tree in which to syntax highlight code blocks.
* @param highlighter
* A function which receives the code and language, and returns the HTML of a syntax
* highlighted `<pre>` element.
*/
export async function highlightCodeBlocks(tree: Root, highlighter: Highlighter) {
const nodes: Array<{
node: Element;
language: string;
parent: Element;
grandParent: Parent;
}> = [];
// We’re looking for `<code>` elements
visitParents(tree, { type: 'element', tagName: 'code' }, (node, ancestors) => {
const parent = ancestors.at(-1);
// Whose parent is a `<pre>`.
if (parent?.type !== 'element' || parent.tagName !== 'pre') {
return;
}
// Where the `<code>` is the only child.
if (parent.children.length !== 1) {
return;
}
// And the `<code>` has a class name that starts with `language-`.
let languageMatch: RegExpMatchArray | null | undefined;
let { className } = node.properties;
if (typeof className === 'string') {
languageMatch = className.match(languagePattern);
} else if (Array.isArray(className)) {
for (const cls of className) {
if (typeof cls !== 'string') {
continue;
}
languageMatch = cls.match(languagePattern);
if (languageMatch) {
break;
}
}
}
// Don’t mighlight math code blocks.
if (languageMatch?.[1] === 'math') {
return;
}
nodes.push({
node,
language: languageMatch?.[1] || 'plaintext',
parent,
grandParent: ancestors.at(-2)!,
});
});
for (const { node, language, grandParent, parent } of nodes) {
const meta = (node.data as any)?.meta ?? node.properties.metastring ?? undefined;
const code = toText(node, { whitespace: 'pre' });
// TODO: In Astro 5, have `highlighter()` return hast directly to skip expensive HTML parsing and serialization.
const html = await highlighter(code, language, { meta });
// The replacement returns a root node with 1 child, the `<pr>` element replacement.
const replacement = fromHtml(html, { fragment: true }).children[0] as Element;
// We just generated this node, so any positional information is invalid.
removePosition(replacement);
// We replace the parent in its parent with the new `<pre>` element.
const index = grandParent.children.indexOf(parent);
grandParent.children[index] = replacement;
}
}
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