Fix StringIndexOutOfBounds when a snapshot has an "empty" before

jvm/CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Fixed
 - Off-by-one in the error message for a VCR key mismatch. ([#526](https://github.com/diffplug/selfie/pull/526))
+- Fix `StringIndexOutOfBoundsException` when an empty snapshot had a facet added. (fixes [#529](https://github.com/diffplug/selfie/issues/529))
 
 ## [2.5.1] - 2025-03-04
 ### Fixed

jvm/selfie-lib/src/commonMain/kotlin/com/diffplug/selfie/SelfieImplementations.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2024 DiffPlug
+ * Copyright (C) 2024-2025 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -212,7 +212,7 @@ class StringSelfie(
  * Returns a serialized form of only the given facets if they are available, silently omits missing
  * facets.
  */
-private fun serializeOnlyFacets(snapshot: Snapshot, keys: Collection<String>): String {
+internal fun serializeOnlyFacets(snapshot: Snapshot, keys: Collection<String>): String {
   val writer = StringBuilder()
   for (key in keys) {
     if (key.isEmpty()) {
@@ -226,7 +226,10 @@ private fun serializeOnlyFacets(snapshot: Snapshot, keys: Collection<String>): S
     // this codepath is triggered by the `key.isEmpty()` line above
     writer.subSequence(EMPTY_KEY_AND_FACET.length, writer.length - 1).toString()
   } else {
-    writer.setLength(writer.length - 1)
+    // Check if the writer is empty to avoid StringIndexOutOfBoundsException
+    if (writer.length > 0) {
+      writer.setLength(writer.length - 1)
+    }
     writer.toString()
   }
 }

jvm/selfie-lib/src/commonTest/kotlin/com/diffplug/selfie/SelfieImplementationsTest.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2025 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.selfie
+
+import io.kotest.matchers.shouldBe
+import kotlin.test.Test
+
+class SelfieImplementationsTest {
+  @Test
+  fun issue_529() {
+    val empty = Snapshot.of("")
+    val emptyPlusFacet = Snapshot.of("").plusFacet("new-facet", "new-facet-value")
+    val mismathedKeys = listOf("new-facet")
+    serializeOnlyFacets(empty, mismathedKeys) shouldBe ""
+    serializeOnlyFacets(emptyPlusFacet, mismathedKeys) shouldBe "╔═ [new-facet] ═╗\nnew-facet-value"
+  }
+}