URN for image info (#78)

* Allow music service to return a URN identifying cover art for an entity

* Fix bug with playlist cover art rending same album multiple times

src/burn.ts

@@ -0,0 +1,90 @@
+import _ from "underscore";
+import { createUrnUtil } from "urn-lib";
+import randomstring from "randomstring";
+
+import jwsEncryption from "./encryption";
+
+const BURN = createUrnUtil("bnb", {
+  components: ["system", "resource"],
+  separator: ":",
+  allowEmpty: false,
+});
+
+export type BUrn = {
+  system: string;
+  resource: string;
+};
+
+const DEFAULT_FORMAT_OPTS = {
+  shorthand: false,
+  encrypt: false,
+}
+
+const SHORTHAND_MAPPINGS: Record<string, string> = {
+  "internal" : "i",
+  "external": "e",
+  "subsonic": "s",
+  "navidrome": "n",
+  "encrypted": "x"
+}
+const REVERSE_SHORTHAND_MAPPINGS: Record<string, string> = Object.keys(SHORTHAND_MAPPINGS).reduce((ret, key) => {
+  ret[SHORTHAND_MAPPINGS[key] as unknown as string] = key;
+  return ret;
+}, {} as Record<string, string>)
+if(SHORTHAND_MAPPINGS.length != REVERSE_SHORTHAND_MAPPINGS.length) {
+  throw `Invalid SHORTHAND_MAPPINGS, must be duplicate!`
+}
+
+export const BURN_SALT = randomstring.generate(5);
+const encryptor = jwsEncryption(BURN_SALT);
+
+export const format = (
+  burn: BUrn,
+  opts: Partial<{ shorthand: boolean; encrypt: boolean }> = {}
+): string => {
+  const o = { ...DEFAULT_FORMAT_OPTS, ...opts }
+  let toBurn = burn;
+  if(o.shorthand) {
+    toBurn = {
+      ...toBurn,
+      system: SHORTHAND_MAPPINGS[toBurn.system] || toBurn.system
+    }
+  }
+  if(o.encrypt) {
+    const encryptedToBurn = {
+      system: "encrypted",
+      resource: encryptor.encrypt(BURN.format(toBurn))
+    }
+    return format(encryptedToBurn, { ...opts, encrypt: false })
+  } else {
+    return BURN.format(toBurn);
+  }
+};
+
+export const formatForURL = (burn: BUrn) => {
+  if(burn.system == "external") return format(burn, { shorthand: true, encrypt: true })
+  else return format(burn, { shorthand: true })
+}
+
+export const parse = (burn: string): BUrn => {
+  const result = BURN.parse(burn)!;
+  const validationErrors = BURN.validate(result) || [];
+  if (validationErrors.length > 0) {
+    throw new Error(`Invalid burn: '${burn}'`);
+  }
+  const system = result.system as string;
+  const x = {
+    system: REVERSE_SHORTHAND_MAPPINGS[system] || system,
+    resource: result.resource as string,
+  };
+  if(x.system == "encrypted") {
+    return parse(encryptor.decrypt(x.resource));
+  } else {
+    return x;
+  }
+}
+
+export function assertSystem(urn: BUrn, system: string): BUrn {
+  if (urn.system != system) throw `Unsupported urn: '${format(urn)}'`;
+  else return urn;
+}