Give a single-line diff for the line which has the mismatch.

Author: nedtwigg

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

@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2024 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.guts
+
+object SnapshotNotEqualErrorMsg {
+  const val REASONABLE_LINE_LENGTH = 120
+  fun forUnequalStrings(expected: String, actual: String): String {
+    var lineNumber = 1
+    var columnNumber = 1
+    var index = 0
+
+    while (index < expected.length && index < actual.length) {
+      val expectedChar = expected[index]
+      val actualChar = actual[index]
+      if (expectedChar != actualChar) {
+        val endOfLineExpected =
+            expected.indexOf('\n', index).let { if (it == -1) expected.length else it }
+        val endOfLineActual =
+            actual.indexOf('\n', index).let { if (it == -1) actual.length else it }
+        val expectedLine = expected.substring(index - columnNumber + 1, endOfLineExpected)
+        val actualLine = actual.substring(index - columnNumber + 1, endOfLineActual)
+        return "Snapshot mismatch at L$lineNumber:C$columnNumber\n-$expectedLine\n+$actualLine"
+      }
+      if (expectedChar == '\n') {
+        lineNumber++
+        columnNumber = 1
+      } else {
+        columnNumber++
+      }
+      index++
+    }
+    if (expected.length != actual.length) {
+      //            val diffString = if (expected.length < actual.length) "Actual is longer" else
+      // "Expected is longer"
+      return "Snapshot mismatch at L$lineNumber:C$columnNumber"
+    }
+    throw IllegalArgumentException("The strings were supposed to be unequal")
+  }
+}

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

@@ -0,0 +1,47 @@
+/*
+ * Copyright (C) 2024 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.guts
+
+import io.kotest.matchers.shouldBe
+import kotlin.test.Test
+
+class SnapshotNotEqualErrorMsgTest {
+  @Test
+  fun errorLine1() {
+    SnapshotNotEqualErrorMsg.forUnequalStrings("Testing 123", "Testing ABC") shouldBe
+        """Snapshot mismatch at L1:C9
+-Testing 123
++Testing ABC"""
+
+    SnapshotNotEqualErrorMsg.forUnequalStrings("123 Testing", "ABC Testing") shouldBe
+        """Snapshot mismatch at L1:C1
+-123 Testing
++ABC Testing"""
+  }
+
+  @Test
+  fun errorLine2() {
+    SnapshotNotEqualErrorMsg.forUnequalStrings("Line\nTesting 123", "Line\nTesting ABC") shouldBe
+        """Snapshot mismatch at L2:C9
+-Testing 123
++Testing ABC"""
+
+    SnapshotNotEqualErrorMsg.forUnequalStrings("Line\n123 Testing", "Line\nABC Testing") shouldBe
+        """Snapshot mismatch at L2:C1
+-123 Testing
++ABC Testing"""
+  }
+}