fix: bound check on patch insert

Closes #26

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

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
 	})
 }

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 {

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{}