Enhance slice filter with Unicode support and array slicing (#72)

- Add proper Unicode handling using runes instead of regex
- Fix performance issues with strings longer than 1000 characters
- Add support for slicing arrays/slices, not just strings
- Improve bounds checking: clamp out-of-bounds negative indices to 0
- Add comprehensive test coverage:
  - Unicode strings (Japanese characters)
  - Very long strings (10,000+ characters)
  - Out-of-bounds indices (positive and negative)
  - Array slicing from split operations

Co-authored-by: Olivier Favre <olivier@wonderpush.com>
Resolves #72

Author: ofavre

filters/standard_filters.go

@@ -194,28 +194,55 @@ func AddStandardFilters(fd FilterDictionary) { //nolint: gocyclo
 		return strings.Replace(s, old, n, 1)
 	})
 	fd.AddFilter("sort_natural", sortNaturalFilter)
-	fd.AddFilter("slice", func(s string, start int, length func(int) int) string {
-		if len(s) == 0 {
-			return ""
-		}
-
-		ss := []rune(s)
-		n := length(1)
-
-		if start < 0 {
-			start = len(ss) + start
+	fd.AddFilter("slice", func(v interface{}, start int, length func(int) int) interface{} {
+		// Are we in the []byte case? Transform []byte to string
+		if b, ok := v.([]byte); ok {
+			v = string(b)
 		}
-
-		if start < 0 {
-			return ""
+		// Are we in the string case?
+		if s, ok := v.(string); ok {
+			// Work on runes, not chars
+			runes := []rune(s)
+			n := length(1)
+			if start < 0 {
+				start = len(runes) + start
+				if start < 0 {
+					start = 0
+				}
+			}
+			if start > len(runes) {
+				start = len(runes)
+			}
+			end := start + n
+			if end > len(runes) {
+				end = len(runes)
+			}
+			return string(runes[start:end])
 		}
-
-		end := start + n
-		if end > len(ss) {
-			end = len(ss)
+		// Are we in the slice case?
+		// A type test cannot suffice because []T and []U are different types, so we must use conversion.
+		var slice []interface{}
+		if sliceIface, err := values.Convert(v, reflect.TypeOf(slice)); err == nil {
+			var ok bool
+			if slice, ok = sliceIface.([]interface{}); ok {
+				n := length(1)
+				if start < 0 {
+					start = len(slice) + start
+					if start < 0 {
+						start = 0
+					}
+				}
+				if start > len(slice) {
+					start = len(slice)
+				}
+				end := start + n
+				if end > len(slice) {
+					end = len(slice)
+				}
+				return slice[start:end]
+			}
 		}
-
-		return string(ss[start:end])
+		return nil
 	})
 	fd.AddFilter("split", splitFilter)
 	fd.AddFilter("strip_html", func(s string) string {

filters/standard_filters_test.go

@@ -2,6 +2,7 @@ package filters
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 	"time"
 
@@ -112,7 +113,24 @@ Liquid" | slice: 0`, "L"},
 Liquid" | slice: 2, 4`, "quid"},
 	{`"Liquid" | slice: -3, 2`, "ui"},
 	{`"" | slice: 1`, ""},
-	{`"Liquid" | slice: -7`, ""},
+	{`"Liquid" | slice: 2, 100`, "quid"},
+	{`"Liquid" | slice: 100`, ""},
+	{`"Liquid" | slice: 100, 200`, ""},
+	{`"Liquid" | slice: -100`, "L"},
+	{`"Liquid" | slice: -100, 200`, "Liquid"},
+	{`"白鵬翔" | slice: 0`, "白"},
+	{`"白鵬翔" | slice: 1`, "鵬"},
+	{`"白鵬翔" | slice: 2`, "翔"},
+	{`"白鵬翔" | slice: 0, 2`, "白鵬"},
+	{`"白鵬翔" | slice: 1, 2`, "鵬翔"},
+	{`"白鵬翔" | slice: 100, 200`, ""},
+	{`"白鵬翔" | slice: -100`, "白"},
+	{`"白鵬翔" | slice: -100, 200`, "白鵬翔"},
+	{`">` + strings.Repeat(".", 10000) + `<" | slice: 1, 10000`, strings.Repeat(".", 10000)},
+	{`"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"},
+	{`"a,b,c" | split: "," | slice: 1, 2 | join`, "b c"},
 
 	{`"a/b/c" | split: '/' | join: '-'`, "a-b-c"},
 	{`"a/b/" | split: '/' | join: '-'`, "a-b"},