From 6cf0edb662c172f835d09cf93f7e2977bda52a6b Mon Sep 17 00:00:00 2001
From: Anton Medvedev <anton@medv.io>
Date: Sun, 12 May 2024 11:55:15 +0200
Subject: [PATCH] Add better error messages for index out of bounds errors

---
 expr_test.go          | 10 +++++++++-
 vm/runtime/runtime.go |  6 +++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/expr_test.go b/expr_test.go
index e16d987a..234b9be6 100644
--- a/expr_test.go
+++ b/expr_test.go
@@ -1329,7 +1329,9 @@ func TestExpr(t *testing.T) {
 }
 
 func TestExpr_error(t *testing.T) {
-	env := mock.Env{}
+	env := mock.Env{
+		ArrayOfAny: []any{1, "2", 3, true},
+	}
 
 	tests := []struct {
 		code string
@@ -1341,6 +1343,12 @@ func TestExpr_error(t *testing.T) {
  | filter(1..9, # > 9)[0]
  | ...................^`,
 		},
+		{
+			`ArrayOfAny[-7]`,
+			`index out of range: -3 (array length is 4) (1:11)
+ | ArrayOfAny[-7]
+ | ..........^`,
+		},
 	}
 
 	for _, tt := range tests {
diff --git a/vm/runtime/runtime.go b/vm/runtime/runtime.go
index 7da1320d..cd48a280 100644
--- a/vm/runtime/runtime.go
+++ b/vm/runtime/runtime.go
@@ -35,8 +35,12 @@ func Fetch(from, i any) any {
 	switch v.Kind() {
 	case reflect.Array, reflect.Slice, reflect.String:
 		index := ToInt(i)
+		l := v.Len()
 		if index < 0 {
-			index = v.Len() + index
+			index = l + index
+		}
+		if index < 0 || index >= l {
+			panic(fmt.Sprintf("index out of range: %v (array length is %v)", index, l))
 		}
 		value := v.Index(index)
 		if value.IsValid() {