nip13: add minePow function

nip13.ts

@@ -1,3 +1,5 @@
+import { type UnsignedEvent, type Event, getEventHash } from './event.ts'
+
 /** Get POW difficulty from a Nostr hex ID. */
 export function getPow(hex: string): number {
   let count = 0
@@ -14,3 +16,37 @@ export function getPow(hex: string): number {
 
   return count
 }
+
+/**
+ * Mine an event with the desired POW. This function mutates the event.
+ * Note that this operation is synchronous and should be run in a worker context to avoid blocking the main thread.
+ *
+ * Adapted from Snort: https://git.v0l.io/Kieran/snort/src/commit/4df6c19248184218c4c03728d61e94dae5f2d90c/packages/system/src/pow-util.ts#L14-L36
+ */
+export function minePow<K extends number>(unsigned: UnsignedEvent<K>, difficulty: number): Omit<Event<K>, 'sig'> {
+  let count = 0
+
+  const event = unsigned as Omit<Event<K>, 'sig'>
+  const tag = ['nonce', count.toString(), difficulty.toString()]
+
+  event.tags.push(tag)
+
+  while (true) {
+    const now = Math.floor(new Date().getTime() / 1000)
+
+    if (now !== event.created_at) {
+      count = 0
+      event.created_at = now
+    }
+
+    tag[1] = (++count).toString()
+
+    event.id = getEventHash(event)
+
+    if (getPow(event.id) >= difficulty) {
+      break
+    }
+  }
+
+  return event
+}