From d936b42e9820aa578af791b09865124104dd71dd Mon Sep 17 00:00:00 2001
From: Raphael Fakhri <153192858+RaphaelFakhri@users.noreply.github.com>
Date: Tue, 29 Sep 2026 10:58:53 +0000
Subject: [PATCH] hujson: reject Patch add operations with an out-of-range
 array index

RFC 6902, section 4.1 requires the index of an "add" operation on an
array to be no greater than the array length. Patch instead silently
appended the value for any larger index. Return an error instead.

Fixes #52

Signed-off-by: Raphael Fakhri <153192858+RaphaelFakhri@users.noreply.github.com>
---
 patch.go      |  6 ++++++
 patch_test.go | 13 +++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/patch.go b/patch.go
index 16bcbf6..0c8f7b0 100644
--- a/patch.go
+++ b/patch.go
@@ -9,6 +9,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"reflect"
+	"strconv"
 	"strings"
 )
 
@@ -156,6 +157,11 @@ func (v *Value) patchAdd(i int, op patchOperation) error {
 				comp.Members[s.idx].Name.Value = String(s.name)
 			}
 		case *Array:
+			// Per RFC 6902, section 4.1, the index must not be greater
+			// than the number of elements in the array.
+			if n, err := strconv.ParseUint(s.name, 10, 0); s.name != "-" && (err != nil || n != uint64(s.idx)) {
+				return fmt.Errorf("hujson: patch operation %d: array index out of range: %s", i, s.name)
+			}
 			insertAt(comp, s.idx, op.value)
 		}
 	}
diff --git a/patch_test.go b/patch_test.go
index 4808d6b..56d1580 100644
--- a/patch_test.go
+++ b/patch_test.go
@@ -179,6 +179,19 @@ var testdataPatch = []struct {
 	in:      `{"fizz":["buzz","wuzz"],"fizzy":"wizzy"}`,
 	patch:   `[{ "op": "test", "path": "/noexist", "value": null }]`,
 	wantErr: errors.New(`hujson: patch operation 0: value not found`),
+}, {
+	in:    `[1,2]`,
+	patch: `[{ "op": "add", "path": "/2", "value": 3 }]`,
+	want:  `[1,2,3]`,
+}, {
+	in:    `[1,2]`,
+	patch: `[{ "op": "add", "path": "/-", "value": 3 }]`,
+	want:  `[1,2,3]`,
+}, {
+	in:      `[1,2]`,
+	patch:   `[{ "op": "add", "path": "/5", "value": 3 }]`,
+	wantErr: errors.New(`hujson: patch operation 0: array index out of range: 5`),
+	want:    `[1,2]`,
 }, {
 	in:      `{}`,
 	patch:   `[{`,