Fixed prettyIndex for one-line files, use binarySearch (#554)

Pretty index was failing with following exception for files with one
line and no `\n` at the end. The `lines` array was empty:
```
java.lang.ArrayIndexOutOfBoundsException: Index -1 out of bounds for length 0
```
There are actually some generated files in our code based that follow
this pattern.

Also, for expressions in the very first line it was creating values with
negative columns, like:
```
x.libsonnet:2:-63
```

Fixed the issue and added tests, also for edge case with completely
empty file.

While changing it also optimized searching for line - previously
`indexWhere` was used which is linear search. Replaced with binary
search. It does not matter to much usually, but actually was causing
slowdown of profiler code that gets locations of all evaluated
expressions - this will be added in a subsequent PR.

Author: hauserx

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) {

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()
+  }
+}