Add NIP-13 (proof-of-work) module

nip13.ts

@@ -0,0 +1,42 @@
+import * as secp256k1 from '@noble/secp256k1'
+
+/** Get POW difficulty from a Nostr hex ID. */
+export function getPow(id: string): number {
+  return getLeadingZeroBits(secp256k1.utils.hexToBytes(id))
+}
+
+/**
+ * Get number of leading 0 bits. Adapted from nostream.
+ * https://github.com/Cameri/nostream/blob/fb6948fd83ca87ce552f39f9b5eb780ea07e272e/src/utils/proof-of-work.ts
+ */
+function getLeadingZeroBits(hash: Uint8Array): number {
+  let total: number, i: number, bits: number
+
+  for (i = 0, total = 0; i < hash.length; i++) {
+    bits = msb(hash[i])
+    total += bits
+    if (bits !== 8) {
+      break
+    }
+  }
+  return total
+}
+
+/**
+ * Adapted from nostream.
+ * https://github.com/Cameri/nostream/blob/fb6948fd83ca87ce552f39f9b5eb780ea07e272e/src/utils/proof-of-work.ts
+ */
+function msb(b: number) {
+  let n = 0
+
+  if (b === 0) {
+    return 8
+  }
+
+  // eslint-disable-next-line no-cond-assign
+  while (b >>= 1) {
+    n++
+  }
+
+  return 7 - n
+}