From ea384230edc92a566330cf8108f9582ac7131257 Mon Sep 17 00:00:00 2001
From: William Poussier <william.poussier@gmail.com>
Date: Mon, 11 Nov 2024 18:22:58 +0100
Subject: [PATCH] fix: bound check on patch insert

Closes #26
---
 differ.go         | 4 ++--
 hash.go           | 3 +--
 operation.go      | 7 +++++--
 operation_test.go | 5 +++++
 4 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/differ.go b/differ.go
index 540fe7f..fe6111d 100644
--- a/differ.go
+++ b/differ.go
@@ -339,7 +339,7 @@ func (d *Differ) compareArraysLCS(ptr pointer, src, tgt []interface{}, doc strin
 
 	adjust := func(i int) int {
 		// Adjust indice considering add and remove
-		// operations that precede.
+		// operations that precede it.
 		return i + add - remove
 	}
 
@@ -483,7 +483,7 @@ func (d *Differ) add(path string, v interface{}, doc string, lcs bool) {
 			if !lcs {
 				d.patch = d.patch.append(OperationMove, op.Path, path, v, v, 0)
 			} else {
-				d.patch = d.patch.prepend(d.snapshotPatchLen, OperationMove, op.Path, path, v, v, 0)
+				d.patch = d.patch.insert(d.snapshotPatchLen, OperationMove, op.Path, path, v, v, 0)
 			}
 		}
 		return
diff --git a/hash.go b/hash.go
index 86ded75..4c52434 100644
--- a/hash.go
+++ b/hash.go
@@ -73,8 +73,7 @@ func (h *hasher) sortArray(a []interface{}) {
 			return 1
 		} else if d1 < d2 {
 			return -1
-		} else {
-			return 0
 		}
+		return 0
 	})
 }
diff --git a/operation.go b/operation.go
index 4859f6a..6c08835 100644
--- a/operation.go
+++ b/operation.go
@@ -118,7 +118,10 @@ func (p *Patch) append(typ string, from, path string, src, tgt interface{}, vl i
 	})
 }
 
-func (p *Patch) prepend(startIdx int, typ string, from, path string, src, tgt interface{}, vl int) Patch {
+func (p *Patch) insert(pos int, typ string, from, path string, src, tgt interface{}, vl int) Patch {
+	if pos > len(*p) {
+		return p.append(typ, from, path, src, tgt, vl)
+	}
 	op := Operation{
 		Type:     typ,
 		From:     from,
@@ -127,7 +130,7 @@ func (p *Patch) prepend(startIdx int, typ string, from, path string, src, tgt in
 		Value:    tgt,
 		valueLen: vl,
 	}
-	return append((*p)[:startIdx], append([]Operation{op}, (*p)[startIdx:]...)...)
+	return append((*p)[:pos], append([]Operation{op}, (*p)[pos:]...)...)
 }
 
 func (p *Patch) jsonLength() int {
diff --git a/operation_test.go b/operation_test.go
index e6fbe80..85550f6 100644
--- a/operation_test.go
+++ b/operation_test.go
@@ -152,6 +152,11 @@ func TestPatch_jsonLength(t *testing.T) {
 	})
 }
 
+func Test_issue26(_ *testing.T) {
+	p := new(Patch)
+	p.insert(6, OperationAdd, emptyPointer, "/a", nil, 42, 0)
+}
+
 func typeNilIface() interface{} {
 	var i *int
 	var p interface{}