diff --git a/sjsonnet/src/sjsonnet/Importer.scala b/sjsonnet/src/sjsonnet/Importer.scala
index 8d7da363..882cdb75 100644
--- a/sjsonnet/src/sjsonnet/Importer.scala
+++ b/sjsonnet/src/sjsonnet/Importer.scala
@@ -57,7 +57,7 @@ final case class FileParserInput(file: File) extends ParserInput {
   override def checkTraceable(): Unit = {}
 
   private lazy val lineNumberLookup: Array[Int] = {
-    val lines = mutable.ArrayBuffer[Int]()
+    val lines = mutable.ArrayBuffer[Int](0)
     val bufferedStream = new BufferedInputStream(new FileInputStream(file))
     var byteRead: Int = 0
     var currentPosition = 0
@@ -74,14 +74,8 @@ final case class FileParserInput(file: File) extends ParserInput {
     lines.toArray
   }
 
-  def prettyIndex(index: Int): String = {
-    val line = lineNumberLookup.indexWhere(_ > index) match {
-      case -1 => lineNumberLookup.length - 1
-      case n  => math.max(0, n - 1)
-    }
-    val col = index - lineNumberLookup(line)
-    s"${line + 1}:${col + 1}"
-  }
+  def prettyIndex(index: Int): String =
+    Util.prettyIndex(lineNumberLookup, index)
 }
 
 class BufferedRandomAccessFile(fileName: String, bufferSize: Int) {
diff --git a/sjsonnet/test/src-jvm/sjsonnet/FileParserInputTests.scala b/sjsonnet/test/src-jvm/sjsonnet/FileParserInputTests.scala
new file mode 100644
index 00000000..b627cd85
--- /dev/null
+++ b/sjsonnet/test/src-jvm/sjsonnet/FileParserInputTests.scala
@@ -0,0 +1,91 @@
+package sjsonnet
+
+import utest._
+
+import java.io.{File, FileOutputStream}
+import java.nio.file.Files
+import scala.collection.mutable
+
+object FileParserInputTests extends TestSuite {
+  private val tempFiles = mutable.ArrayBuffer[File]()
+
+  def createTestFile(content: String): FileParserInput = {
+    val tempFile = Files.createTempFile("fileparser-", ".txt").toFile
+    tempFiles += tempFile
+    val fos = new FileOutputStream(tempFile)
+    try {
+      fos.write(content.getBytes("UTF-8"))
+    } finally {
+      fos.close()
+    }
+    FileParserInput(tempFile)
+  }
+
+  val tests: Tests = Tests {
+    test("prettyIndex - empty file") {
+      val input = createTestFile("")
+      val result = input.prettyIndex(0)
+      assert(result == "1:1")
+    }
+
+    test("prettyIndex - single line without newline") {
+      val input = createTestFile("hello world")
+
+      assert(input.prettyIndex(0) == "1:1")
+      assert(input.prettyIndex(1) == "1:2")
+      assert(input.prettyIndex(6) == "1:7")
+      assert(input.prettyIndex(10) == "1:11")
+    }
+
+    test("prettyIndex - single line with newline") {
+      val input = createTestFile("hello world\n")
+
+      assert(input.prettyIndex(0) == "1:1")
+      assert(input.prettyIndex(5) == "1:6")
+      assert(input.prettyIndex(11) == "1:12")
+    }
+
+    test("prettyIndex - multiple lines with different lengths") {
+      val input = createTestFile("a\nbb\nccc\n")
+
+      assert(input.prettyIndex(0) == "1:1")
+      assert(input.prettyIndex(1) == "1:2")
+
+      assert(input.prettyIndex(2) == "2:1")
+      assert(input.prettyIndex(3) == "2:2")
+      assert(input.prettyIndex(4) == "2:3")
+
+      assert(input.prettyIndex(5) == "3:1")
+      assert(input.prettyIndex(6) == "3:2")
+      assert(input.prettyIndex(7) == "3:3")
+      assert(input.prettyIndex(8) == "3:4")
+    }
+
+    test("prettyIndex - no trailing newline") {
+      val input = createTestFile("line 1\nline 2\nline 3")
+
+      assert(input.prettyIndex(0) == "1:1")
+      assert(input.prettyIndex(7) == "2:1")
+      assert(input.prettyIndex(14) == "3:1")
+      assert(input.prettyIndex(19) == "3:6")
+    }
+
+    test("prettyIndex - large file with many lines") {
+      val lines = (1 to 1000).map(i => s"Line $i").mkString("\n")
+      val input = createTestFile(lines)
+
+      val line1Bytes = 9 * 7
+      val line10Bytes = 90 * 8
+      val line100Bytes = 400 * 9
+      val offset500 = line1Bytes + line10Bytes + line100Bytes
+
+      val result = input.prettyIndex(offset500)
+      assert(result == "500:1")
+    }
+  }
+
+  override def utestAfterAll(): Unit = {
+    tempFiles.foreach(_.delete())
+    tempFiles.clear()
+  }
+}