fix: utf8_prefix should take into account empty lines (#164)

* fix: utf8_prefix should take into account empty lines

If empty line is provided, the index gets out of bounds

Resolves #163

Signed-off-by: afterlook <[email protected]>

* test: add test for updates on empty document

* refactor: avoid pipe in min/max calc

---------

Signed-off-by: afterlook <[email protected]>
Co-authored-by: doorgan <[email protected]>

Author: afterlook

apps/expert/test/forge/document_test.exs

@@ -260,6 +260,12 @@ defmodule Forge.DocumentTest do
       assert doc.version == 0
     end
 
+    test "update on empty document" do
+      assert {:ok, doc} = run_changes("", [], version: 1)
+      assert "" == text(doc)
+      assert doc.version == 0
+    end
+
     test "setting the version" do
       assert {:ok, doc} = run_changes("abc123", [edit("mornin")], version: 3)
       assert "mornin" == text(doc)

apps/forge/lib/forge/document.ex

@@ -373,7 +373,10 @@ defmodule Forge.Document do
   end
 
   defp utf8_prefix(line(text: text), start_code_unit) do
-    length = max(0, start_code_unit - 1)
+    byte_count = byte_size(text)
+    desired_length = start_code_unit - 1
+    length = min(max(0, desired_length), byte_count)
+
     binary_part(text, 0, length)
   end