Set up decoder infrastructure

src/codecs/decoders.ts

@@ -0,0 +1,42 @@
+import * as browserJPEG from './browser-jpeg/decoder';
+import * as browserPNG from './browser-png/decoder';
+import * as browserWebP from './browser-webp/decoder';
+import * as wasmWebP from './webp/decoder';
+
+export interface Decoder {
+  name: string;
+  decode(file: File): Promise<ImageBitmap>;
+  isSupported(): Promise<boolean>;
+  supportedMimeTypes: string[];
+  supportedExtensions: string[];
+}
+
+// We load all decoders and filter out the unsupported ones.
+export const decodersPromise: Promise<Decoder[]> = Promise.all(
+  [
+    browserPNG,
+    browserJPEG,
+    browserWebP,
+    wasmWebP,
+  ]
+  .map(async (encoder) => {
+    if (await encoder.isSupported()) {
+      return encoder;
+    }
+    return null;
+  }),
+// TypeScript is not smart enough to realized that I’m filtering all the falsy
+// values here.
+).then(list => list.filter(item => !!item)) as any as Promise<Decoder[]>;
+
+export async function findDecoder(file: File): Promise<Decoder | undefined> {
+  const decoders = await decodersPromise;
+  // Prefer a match on mime type over a match on file extension
+  const decoder = decoders.find(decoder => decoder.supportedMimeTypes.includes(file.type));
+  if (decoder) {
+    return decoder;
+  }
+  return decoders.find(decoder =>
+    decoder.supportedExtensions.some(extension =>
+       file.name.endsWith(`.${extension}`)));
+}