docs: huge migration to runtime asset cache

packages/docs/src/asset-builder/asset-builder.ts

@@ -0,0 +1,85 @@
+import { RequestHandler } from "express"
+import { createHash } from "node:crypto"
+import { readFileSync } from "node:fs"
+import { mkdir, stat, writeFile } from "node:fs/promises"
+import { dirname, extname, join, parse } from "node:path"
+
+export type Asset = {
+  file: string
+  url: string
+  content: Buffer
+}
+
+export type AssetTransformer = {
+  transform: (asset: Asset) => Promise<AssetTransformResult | undefined>
+}
+
+export type AssetTransformResult = {
+  content: string
+  type: string
+}
+
+export class AssetBuilder {
+  // map of asset urls to asset objects
+  private library = new Map<string, Asset>()
+
+  constructor(
+    private cacheFolder: string,
+    private transformers: AssetTransformer[],
+  ) {}
+
+  // accepts a path to a file, then returns a url to where the built file will be served
+  // the url will include a hash of the file contents
+  file(file: string | URL): string {
+    if (file instanceof URL) {
+      file = file.pathname
+    }
+
+    const existing = this.library.get(file)
+    if (existing) {
+      return existing.url
+    }
+
+    const content = readFileSync(file)
+    const hash = createHash("sha256").update(content).digest("hex").slice(0, 8)
+
+    const { name, ext } = parse(file)
+    const url = `/${name}.${hash}${ext}`
+    this.library.set(url, { file, url, content })
+    return url
+  }
+
+  middleware(): RequestHandler {
+    return async (req, res, next) => {
+      try {
+        const asset = this.library.get(req.path)
+        if (!asset) return next()
+
+        const file = join(this.cacheFolder, asset.url)
+        const extension = extname(file)
+
+        const stats = await stat(file).catch(() => undefined)
+        if (stats?.isFile()) {
+          res
+            .status(200)
+            .type(extension.endsWith("tsx") ? "text/javascript" : extension)
+            .sendFile(file)
+          return
+        }
+
+        for (const transformer of this.transformers) {
+          const result = await transformer.transform(asset)
+          if (result) {
+            await mkdir(dirname(file), { recursive: true })
+            await writeFile(file, result.content)
+            return res.type(extension).send(result.content)
+          }
+        }
+
+        next()
+      } catch (error) {
+        next(error)
+      }
+    }
+  }
+}