From 68c1334335ae7b66f79015c1425775d72cab5f3a Mon Sep 17 00:00:00 2001
From: "Chris (ChrisJr404)" <11917633+ChrisJr404@users.noreply.github.com>
Date: Tue, 18 Aug 2026 11:08:26 -0400
Subject: [PATCH] Fix panic in slice filter with negative length

The slice filter computed end = start + length without a lower bound. A
negative length produced end < start, so the runes[start:end] and
slice[start:end] expressions panicked with a slice-bounds-out-of-range
runtime error. Clamp end up to start so a negative length yields an empty
result, matching Ruby's String#slice behavior, and add regression tests
covering the string and array cases.
---
 filters/standard_filters.go      | 6 ++++++
 filters/standard_filters_test.go | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/filters/standard_filters.go b/filters/standard_filters.go
index 13b2ca1..4737a46 100644
--- a/filters/standard_filters.go
+++ b/filters/standard_filters.go
@@ -391,6 +391,9 @@ func AddStandardFilters(fd FilterDictionary) { //nolint: gocyclo
 				start = len(runes)
 			}
 			end := start + n
+			if end < start {
+				end = start
+			}
 			if end > len(runes) {
 				end = len(runes)
 			}
@@ -413,6 +416,9 @@ func AddStandardFilters(fd FilterDictionary) { //nolint: gocyclo
 					start = len(slice)
 				}
 				end := start + n
+				if end < start {
+					end = start
+				}
 				if end > len(slice) {
 					end = len(slice)
 				}
diff --git a/filters/standard_filters_test.go b/filters/standard_filters_test.go
index 9141c98..55d3591 100644
--- a/filters/standard_filters_test.go
+++ b/filters/standard_filters_test.go
@@ -169,6 +169,9 @@ Liquid" | slice: 2, 4`, "quid"},
 	{`"白鵬翔" | slice: -100`, "白"},
 	{`"白鵬翔" | slice: -100, 200`, "白鵬翔"},
 	{`">` + strings.Repeat(".", 10000) + `<" | slice: 1, 10000`, strings.Repeat(".", 10000)},
+	{`"Liquid" | slice: 2, -1`, ""},
+	{`"白鵬翔" | slice: 1, -1`, ""},
+	{`"a,b,c" | split: "," | slice: 1, -1 | join`, ""},
 	{`"a,b,c" | split: "," | slice: -1 | join`, "c"},
 	{`"a,b,c" | split: "," | slice: 1, 1 | join`, "b"},
 	{`"a,b,c" | split: "," | slice: 0, 2 | join`, "a b"},