From ab628eff7f2aad03d13dea8c0fe7a444c72884fb Mon Sep 17 00:00:00 2001
From: "A.J. Stein" <aj@gsa.gov>
Date: Sun, 6 Oct 2024 10:40:30 -0400
Subject: [PATCH] Return empty String sequence in kind for in #132

---
 .../core/metapath/function/library/FnSubstring.java       | 8 +++++++-
 .../core/metapath/function/library/FnSubstringTest.java   | 3 +++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/core/src/main/java/gov/nist/secauto/metaschema/core/metapath/function/library/FnSubstring.java b/core/src/main/java/gov/nist/secauto/metaschema/core/metapath/function/library/FnSubstring.java
index 8a9154ba..aaefd202 100644
--- a/core/src/main/java/gov/nist/secauto/metaschema/core/metapath/function/library/FnSubstring.java
+++ b/core/src/main/java/gov/nist/secauto/metaschema/core/metapath/function/library/FnSubstring.java
@@ -113,8 +113,14 @@ private static ISequence<IStringItem> execute(
    */
   @NonNull
   public static IStringItem substring(@NonNull IStringItem sourceString, @NonNull IDecimalItem start, @NonNull IDecimalItem length) {
+    String sourceStringData = sourceString.toString();
+
+    if (sourceStringData.length() == 0) {
+      return IStringItem.valueOf("");
+    }
+
     int startIndex = start.asInteger().intValue() - 1;
     int endIndex = startIndex + length.asInteger().intValue();
-    return IStringItem.valueOf(sourceString.toString().substring(startIndex, endIndex));
+    return IStringItem.valueOf(sourceStringData.substring(startIndex, endIndex));
   }
 }
diff --git a/core/src/test/java/gov/nist/secauto/metaschema/core/metapath/function/library/FnSubstringTest.java b/core/src/test/java/gov/nist/secauto/metaschema/core/metapath/function/library/FnSubstringTest.java
index 5f2d8e2b..de92f500 100644
--- a/core/src/test/java/gov/nist/secauto/metaschema/core/metapath/function/library/FnSubstringTest.java
+++ b/core/src/test/java/gov/nist/secauto/metaschema/core/metapath/function/library/FnSubstringTest.java
@@ -24,6 +24,9 @@ class FnSubstringTest
     extends ExpressionTestBase {
   private static Stream<Arguments> provideValues() { // NOPMD - false positive
     return Stream.of(
+        Arguments.of(
+            string(""),
+            "substring((), 1, 3)"),  
         Arguments.of(
             ISequence.of(string("ada")),
             "substring('metadata', 4, 3)"));