From 437cdecfb1e5a1576e8681266e2f8019a7a17ea0 Mon Sep 17 00:00:00 2001
From: fiatjaf <fiatjaf@gmail.com>
Date: Tue, 20 Aug 2024 13:19:21 -0300
Subject: [PATCH] nip13: CommittedDifficulty()

---
 nip13/nip13.go      | 20 ++++++++++++++++++--
 nip13/nip13_test.go | 21 +++++++++++++++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/nip13/nip13.go b/nip13/nip13.go
index 53538e4..ee01d4c 100644
--- a/nip13/nip13.go
+++ b/nip13/nip13.go
@@ -1,5 +1,3 @@
-// Package nip13 implements NIP-13
-// See https://github.com/nostr-protocol/nips/blob/master/13.md for details.
 package nip13
 
 import (
@@ -18,6 +16,24 @@ var (
 	ErrMissingPubKey    = errors.New("nip13: attempting to work on an event without a pubkey, which makes no sense")
 )
 
+// CommittedDifficulty returns the Difficulty but checks the "nonce" tag for a target.
+//
+// if the target is smaller than the actual difficulty then the value of the target is used.
+// if the target is bigger than the actual difficulty then it returns 0.
+func CommittedDifficulty(event *nostr.Event) int {
+	work := 0
+	if nonceTag := event.Tags.GetFirst([]string{"nonce", ""}); nonceTag != nil && len(*nonceTag) >= 3 {
+		work = Difficulty(event.ID)
+		target, _ := strconv.Atoi((*nonceTag)[2])
+		if target <= work {
+			work = target
+		} else {
+			work = 0
+		}
+	}
+	return work
+}
+
 // Difficulty counts the number of leading zero bits in an event ID.
 func Difficulty(id string) int {
 	var zeros int
diff --git a/nip13/nip13_test.go b/nip13/nip13_test.go
index 93d6d94..3d1821d 100644
--- a/nip13/nip13_test.go
+++ b/nip13/nip13_test.go
@@ -8,6 +8,7 @@ import (
 	"time"
 
 	nostr "github.com/nbd-wtf/go-nostr"
+	"github.com/stretchr/testify/require"
 )
 
 func TestCheck(t *testing.T) {
@@ -31,6 +32,26 @@ func TestCheck(t *testing.T) {
 	}
 }
 
+func TestCommittedDifficulty(t *testing.T) {
+	tests := []struct {
+		result int
+		id     string
+		tags   nostr.Tags
+	}{
+		{18, "000000000e9d97a1ab09fc381030b346cdd7a142ad57e6df0b46dc9bef6c7e2d", nostr.Tags{{"-"}, {"nonce", "654", "18"}}},
+		{36, "000000000e9d97a1ab09fc381030b346cdd7a142ad57e6df0b46dc9bef6c7e2d", nostr.Tags{{"nonce", "12315", "36"}}},
+		{0, "000000000e9d97a1ab09fc381030b346cdd7a142ad57e6df0b46dc9bef6c7e2d", nostr.Tags{{"nonce", "12315", "37"}}},
+		{0, "000000000e9d97a1ab09fc381030b346cdd7a142ad57e6df0b46dc9bef6c7e2d", nostr.Tags{{"nonce", "654", "64"}, {"t", "spam"}}},
+		{0, "000000000e9d97a1ab09fc381030b346cdd7a142ad57e6df0b46dc9bef6c7e2d", nostr.Tags{}},
+	}
+	for i, tc := range tests {
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			work := CommittedDifficulty(&nostr.Event{ID: tc.id, Tags: tc.tags})
+			require.Equal(t, tc.result, work)
+		})
+	}
+}
+
 func TestGenerateShort(t *testing.T) {
 	event := &nostr.Event{
 		Kind:    nostr.KindTextNote,