From a05506468dc1fb3ffe98161f5c8e130f1d2ec811 Mon Sep 17 00:00:00 2001
From: Alex Gleason <alex@alexgleason.me>
Date: Tue, 11 Apr 2023 15:49:08 -0500
Subject: [PATCH] Add NIP-13 (proof-of-work) module

---
 index.ts      |  1 +
 nip13.test.js |  8 ++++++++
 nip13.ts      | 42 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 51 insertions(+)
 create mode 100644 nip13.test.js
 create mode 100644 nip13.ts

diff --git a/index.ts b/index.ts
index 0295f02..8f45843 100644
--- a/index.ts
+++ b/index.ts
@@ -9,6 +9,7 @@ export * as nip04 from './nip04'
 export * as nip05 from './nip05'
 export * as nip06 from './nip06'
 export * as nip10 from './nip10'
+export * as nip13 from './nip13'
 export * as nip19 from './nip19'
 export * as nip26 from './nip26'
 export * as nip39 from './nip39'
diff --git a/nip13.test.js b/nip13.test.js
new file mode 100644
index 0000000..9f23fab
--- /dev/null
+++ b/nip13.test.js
@@ -0,0 +1,8 @@
+/* eslint-env jest */
+const {nip13} = require('./lib/nostr.cjs')
+
+test('identifies proof-of-work difficulty', async () => {
+  const id = '000006d8c378af1779d2feebc7603a125d99eca0ccf1085959b307f64e5dd358'
+  const difficulty = nip13.getPow(id)
+  expect(difficulty).toEqual(21)
+})
diff --git a/nip13.ts b/nip13.ts
new file mode 100644
index 0000000..f96c58b
--- /dev/null
+++ b/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
+}