diff --git a/src/main/scala/viper/silver/parser/FastParser.scala b/src/main/scala/viper/silver/parser/FastParser.scala
index 4378db354..2733a004e 100644
--- a/src/main/scala/viper/silver/parser/FastParser.scala
+++ b/src/main/scala/viper/silver/parser/FastParser.scala
@@ -34,12 +34,21 @@ object FastParser extends PosParser[Char, String] {
    */
   val standard_import_directory = "import"
 
-  var _lines: Array[Int] = null
+  var _line_offset: Array[Int] = null
 
   def parse(s: String, f: Path, plugins: Option[SilverPluginManager] = None) = {
     _file = f.toAbsolutePath
+
+    // Add an empty line at the end to make `computeFrom(s.length)` return `(lines.length, 1)`, as the old
+    // implementation of `computeFrom` used to do.
     val lines = s.linesWithSeparators
-    _lines = lines.map(_.length).toArray
+    _line_offset = (lines.map(_.length) ++ Seq(0)).toArray
+    var offset = 0
+    for (i <- _line_offset.indices) {
+      val line_length = _line_offset(i)
+      _line_offset(i) = offset
+      offset += line_length
+    }
 
     // Strategy to handle imports
     // Idea: Import every import reference and merge imported methods, functions, imports, .. into current program
@@ -155,7 +164,6 @@ object FastParser extends PosParser[Char, String] {
           column = pos.column
         }
         ParseError(msg, SourcePosition(_file, line, column))
-      case _:Throwable =>
     }
   }
 
diff --git a/src/main/scala/viper/silver/parser/ParserPositions.scala b/src/main/scala/viper/silver/parser/ParserPositions.scala
index 1170fb4fa..32d5a3bfc 100644
--- a/src/main/scala/viper/silver/parser/ParserPositions.scala
+++ b/src/main/scala/viper/silver/parser/ParserPositions.scala
@@ -30,15 +30,17 @@ case class FilePosition(file: Path, vline: Int, col: Int) extends util.parsing.i
 
 trait PosComputer {
   def computeFrom(index: Int) : (Int, Int) = {
-    var left = index
-    var i = 0
-    val arr = FastParser._lines
-    while (i < arr.length && left >= arr(i)){
-      left -= arr(i)
-      i += 1
+    val line_offset =  FastParser._line_offset
+    val result = java.util.Arrays.binarySearch(line_offset, index)
+    if (result >= 0) {
+      // Exact match
+      val line = result
+      (line + 1, index - line_offset(line) + 1)
+    } else {
+      // The binary search returned `- insertionPoint - 1`
+      val line = - result - 2
+      (line + 1, index - line_offset(line) + 1)
     }
-    val r1 = (i + 1, left + 1)
-    r1
   }
 }
 /**