Handle multibyte strings

lib/syntax_tree/parser.rb

@@ -166,7 +166,8 @@ def char_pos
     # This represents the current column we're in relative to the beginning of
     # the current line.
     def current_column
-      column - line_counts[lineno - 1].start
+      line = line_counts[lineno - 1]
+      line[column].to_i - line.start
     end
 
     # As we build up a list of tokens, we'll periodically need to go backwards

test/node_test.rb

@@ -1009,6 +1009,16 @@ def test_column_positions
       assert_node(Command, "command", source, at: at)
     end
 
+    def test_multibyte_column_positions
+      source = <<~SOURCE
+        puts "Congrats"
+        puts "🎉 🎉"
+      SOURCE
+
+      at = location(lines: 2..2, chars: 16..26, columns: 0..10)
+      assert_node(Command, "command", source, at: at)
+    end
+
     private
 
     def location(lines: 1..1, chars: 0..0, columns: 0..0)