From cf1a0c51856af48991593e601e1312a6ee375088 Mon Sep 17 00:00:00 2001
From: Stephen Buttolph <stephen@avalabs.org>
Date: Tue, 5 Nov 2024 11:47:00 -0500
Subject: [PATCH 1/2] ACP-77: Implement ids.ID#Append

---
 ids/id.go      | 16 ++++++++++
 ids/id_test.go | 86 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 102 insertions(+)

diff --git a/ids/id.go b/ids/id.go
index 91eacfdbd08f..4be8e7ecb1e8 100644
--- a/ids/id.go
+++ b/ids/id.go
@@ -107,6 +107,22 @@ func (id ID) Prefix(prefixes ...uint64) ID {
 	return hashing.ComputeHash256Array(packer.Bytes)
 }
 
+// Append this id to create a more selective id.
+//
+// This is used to generate the ACP-77 validationIDs.
+func (id ID) Append(suffixes ...uint32) ID {
+	packer := wrappers.Packer{
+		Bytes: make([]byte, IDLen+len(suffixes)*wrappers.IntLen),
+	}
+
+	packer.PackFixedBytes(id[:])
+	for _, suffix := range suffixes {
+		packer.PackInt(suffix)
+	}
+
+	return hashing.ComputeHash256Array(packer.Bytes)
+}
+
 // XOR this id and the provided id and return the resulting id.
 //
 // Note: this id is not modified.
diff --git a/ids/id_test.go b/ids/id_test.go
index 930a323e614c..b062aaa0b0c2 100644
--- a/ids/id_test.go
+++ b/ids/id_test.go
@@ -6,12 +6,14 @@ package ids
 import (
 	"encoding/json"
 	"fmt"
+	"slices"
 	"testing"
 
 	"github.com/stretchr/testify/require"
 
 	"github.com/ava-labs/avalanchego/utils"
 	"github.com/ava-labs/avalanchego/utils/cb58"
+	"github.com/ava-labs/avalanchego/utils/hashing"
 )
 
 func TestID(t *testing.T) {
@@ -25,6 +27,90 @@ func TestID(t *testing.T) {
 	require.Equal(prefixed, id.Prefix(0))
 }
 
+func TestIDPrefix(t *testing.T) {
+	id := GenerateTestID()
+	tests := []struct {
+		name             string
+		id               ID
+		prefix           []uint64
+		expectedPreimage []byte
+	}{
+		{
+			name:             "empty prefix",
+			id:               id,
+			prefix:           []uint64{},
+			expectedPreimage: id[:],
+		},
+		{
+			name:   "1 prefix",
+			id:     id,
+			prefix: []uint64{1},
+			expectedPreimage: slices.Concat(
+				[]byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01},
+				id[:],
+			),
+		},
+		{
+			name:   "multiple prefixes",
+			id:     id,
+			prefix: []uint64{1, 256},
+			expectedPreimage: slices.Concat(
+				[]byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01},
+				[]byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00},
+				id[:],
+			),
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			expected := ID(hashing.ComputeHash256Array(test.expectedPreimage))
+			require.Equal(t, expected, test.id.Prefix(test.prefix...))
+		})
+	}
+}
+
+func TestIDAppend(t *testing.T) {
+	id := GenerateTestID()
+	tests := []struct {
+		name             string
+		id               ID
+		suffix           []uint32
+		expectedPreimage []byte
+	}{
+		{
+			name:             "empty suffix",
+			id:               id,
+			suffix:           []uint32{},
+			expectedPreimage: id[:],
+		},
+		{
+			name:   "1 suffix",
+			id:     id,
+			suffix: []uint32{1},
+			expectedPreimage: slices.Concat(
+				id[:],
+				[]byte{0x00, 0x00, 0x00, 0x01},
+			),
+		},
+		{
+			name:   "multiple suffixes",
+			id:     id,
+			suffix: []uint32{1, 256},
+			expectedPreimage: slices.Concat(
+				id[:],
+				[]byte{0x00, 0x00, 0x00, 0x01},
+				[]byte{0x00, 0x00, 0x01, 0x00},
+			),
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			expected := ID(hashing.ComputeHash256Array(test.expectedPreimage))
+			require.Equal(t, expected, test.id.Append(test.suffix...))
+		})
+	}
+}
+
 func TestIDXOR(t *testing.T) {
 	require := require.New(t)
 

From 1b358781d605d81c45ff8abca509e8f9f0fc3a6e Mon Sep 17 00:00:00 2001
From: Stephen Buttolph <stephen@avalabs.org>
Date: Tue, 5 Nov 2024 12:01:33 -0500
Subject: [PATCH 2/2] update doc

---
 ids/id.go | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/ids/id.go b/ids/id.go
index 4be8e7ecb1e8..ea0993ef25d7 100644
--- a/ids/id.go
+++ b/ids/id.go
@@ -107,9 +107,12 @@ func (id ID) Prefix(prefixes ...uint64) ID {
 	return hashing.ComputeHash256Array(packer.Bytes)
 }
 
-// Append this id to create a more selective id.
+// Append this id with the provided suffixes and re-hash the result. This
+// returns a new ID and does not modify the original ID.
 //
-// This is used to generate the ACP-77 validationIDs.
+// This is used to generate ACP-77 validationIDs.
+//
+// Ref: https://github.com/avalanche-foundation/ACPs/tree/e333b335c34c8692d84259d21bd07b2bb849dc2c/ACPs/77-reinventing-subnets#convertsubnettol1tx
 func (id ID) Append(suffixes ...uint32) ID {
 	packer := wrappers.Packer{
 		Bytes: make([]byte, IDLen+len(suffixes)*wrappers.IntLen),