refactor: use an index normalization function instead of array_at()

This fixes a possible (untested) edge case where the $result could have
been null even if the index was valid, and also simplifies a possible
feature extension to return paths instead of node values.

Author: loilo

src/parsers/root.php

@@ -113,8 +113,12 @@ function apply_index_selector($node, $json)
 {
 	if (is_json_array($json)) {
 		if ($node->index < sizeof($json)) {
-			$result = array_at($json, $node->index);
-			return $result === null ? [] : [$result];
+			$index = normalize_index($node->index, sizeof($json));
+			if ($index === null) {
+				return [];
+			} else {
+				return [$json[$index]];
+			}
 		}
 		return [];
 	}

src/utils/helpers.php

@@ -84,14 +84,20 @@ function is_number($value): bool
 }
 
 /**
- * Get the value at a given index of an array or null if it does not exist
- * Allow to use negative indexes
+ * Normalize an index for a given array length to be positive
+ * or null if it is out of bounds
+ * An index is out of bounds if it is less than the negative length
+ * or greater than or equal to the length
  */
-function array_at(array $array, int $index)
+function normalize_index($index, int $length): ?int
 {
+	if ($index < -$length || $index >= $length) {
+		return null;
+	}
+
 	if ($index < 0) {
-		$index += sizeof($array);
+		$index += $length;
 	}
 
-	return $array[$index] ?? null;
+	return $index;
 }