diff --git a/tests/playground/filemap.lean b/tests/playground/filemap.lean
new file mode 100644
index 0000000000..c440da6004
--- /dev/null
+++ b/tests/playground/filemap.lean
@@ -0,0 +1,74 @@
+structure Position :=
+(line   : Nat := 1)
+(column : Nat := 0)
+
+instance : Inhabited Position :=
+⟨{}⟩
+
+structure FileMap :=
+(source    : String)
+(positions : Array String.Pos)
+(lines     : Array Nat)
+
+namespace FileMap
+
+instance : Inhabited FileMap :=
+⟨{ source := "", positions := Array.empty, lines := Array.empty }⟩
+
+private partial def ofStringAux (s : String) : String.Pos → Nat → Array String.Pos → Array Nat → FileMap
+| i line ps lines :=
+  if s.atEnd i then { source := s, positions := ps.push i, lines := lines.push line }
+  else
+    let c := s.get i in
+    let i := s.next i in
+    if c == '\n' then ofStringAux i (line+1) (ps.push i) (lines.push (line+1))
+    else ofStringAux i line ps lines
+
+def ofString (s : String) : FileMap :=
+ofStringAux s 0 1 (Array.empty.push 0) (Array.empty.push 1)
+
+private partial def toColumnAux (str : String) (lineBeginPos : String.Pos) (pos : String.Pos) : String.Pos → Nat → Nat
+| i c :=
+  if i == pos || str.atEnd i then c
+  else toColumnAux (str.next i) (c+1)
+
+/- Remark: `pos` is in `[ps.get b, ps.get e]` and `b < e` -/
+private partial def toPositionAux (str : String) (ps : Array Nat) (lines : Array Nat) (pos : String.Pos) : Nat → Nat → Position
+| b e :=
+  let posB := ps.get b in
+  if e == b + 1 then { line := lines.get b, column := toColumnAux str posB pos posB 0 }
+  else
+    let m := (b + e) / 2 in
+    let posM := ps.get m in
+    if pos == posM then { line := lines.get m, column := 0 }
+    else if pos > posM then toPositionAux m e
+    else toPositionAux b m
+
+def toPosition : FileMap → String.Pos → Position
+| { source := str, positions := ps, lines := lines } pos := toPositionAux str ps lines pos 0 (ps.size-1)
+
+end FileMap
+
+def String.toFileMap (s : String) : FileMap :=
+FileMap.ofString s
+
+partial def showLineColsAux (fmap : FileMap) : String.Pos → IO Unit
+| i :=
+  if fmap.source.atEnd i then pure ()
+  else do
+    let p := fmap.toPosition i,
+    IO.println ("[" ++ toString i ++ "] := (" ++ toString p.line ++ ", " ++ toString p.column ++ ")"),
+    showLineColsAux (fmap.source.next i)
+
+def showLineCols (str : String) : IO Unit :=
+let fmap := FileMap.ofString str in
+IO.println ("string: " ++ repr str) *>
+showLineColsAux fmap 0
+
+def main (xs : List String) : IO Unit :=
+do
+  showLineCols ("hello\nwoαld\ntβst"),
+  showLineCols "",
+  showLineCols "\n\n\n",
+  showLineCols "\n\n\nabc\n",
+  showLineCols "\n\n\nabc"