2 files changed, 69 insertions, 0 deletions

diff --git a/lib/error_highlight/formatter.rb b/lib/error_highlight/formatter.rb
index 20ca78d468..b4fce93e55 100644
--- a/lib/error_highlight/formatter.rb
+++ b/lib/error_highlight/formatter.rb
@@ -3,6 +3,8 @@ module ErrorHighlight
     def self.message_for(spot)
       # currently only a one-line code snippet is supported
       if spot[:first_lineno] == spot[:last_lineno]
+        spot = truncate(spot)
+
         indent = spot[:snippet][0...spot[:first_column]].gsub(/[^\t]/, " ")
         marker = indent + "^" * (spot[:last_column] - spot[:first_column])
 
@@ -11,6 +13,47 @@ module ErrorHighlight
         ""
       end
     end
+
+    def self.viewport_size
+      Ractor.current[:__error_highlight_viewport_size__] || terminal_columns
+    end
+
+    def self.viewport_size=(viewport_size)
+      Ractor.current[:__error_highlight_viewport_size__] = viewport_size
+    end
+
+    private
+    
+    def self.truncate(spot)
+      ellipsis = '...'
+      snippet = spot[:snippet]
+      diff = snippet.size - (viewport_size - ellipsis.size)
+
+      # snippet fits in the terminal
+      return spot if diff.negative?
+
+      if spot[:first_column] < diff
+        snippet = snippet[0...snippet.size - diff]
+        {
+          **spot,
+          snippet: snippet + ellipsis + "\n",
+          last_column: [spot[:last_column], snippet.size].min
+        }
+      else
+        {
+          **spot,
+          snippet: ellipsis + snippet[diff..-1],
+          first_column: spot[:first_column] - (diff - ellipsis.size),
+          last_column: spot[:last_column] - (diff - ellipsis.size)
+        }
+      end
+    end
+    
+    def self.terminal_columns
+      # lazy load io/console in case viewport_size is set
+      require "io/console"
+      IO.console.winsize[1]
+    end
   end
 
   def self.formatter
diff --git a/test/error_highlight/test_error_highlight.rb b/test/error_highlight/test_error_highlight.rb
index f0da5b5e62..fd5e32a549 100644
--- a/test/error_highlight/test_error_highlight.rb
+++ b/test/error_highlight/test_error_highlight.rb
@@ -12,6 +12,8 @@ class ErrorHighlightTest < Test::Unit::TestCase
   end
 
   def setup
+    ErrorHighlight::DefaultFormatter.viewport_size = 80
+
     if defined?(DidYouMean)
       @did_you_mean_old_formatter = DidYouMean.formatter
       DidYouMean.formatter = DummyFormatter
@@ -1285,6 +1287,30 @@ undefined method `time' for #{ ONE_RECV_MESSAGE }
     end
   end
 
+  def test_errors_on_small_viewports_when_error_lives_at_the_end
+    assert_error_message(NoMethodError, <<~END) do
+undefined method 'gsuub' for an instance of String
+
+...ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo".gsuub(//, "")
+                                                                 ^^^^^^
+    END
+
+      "fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo".gsuub(//, "")
+    end
+  end
+
+  def test_errors_on_small_viewports_when_error_lives_at_the_beginning
+    assert_error_message(NoMethodError, <<~END) do
+undefined method 'gsuub' for an instance of Integer
+
+      1.gsuub(//, "fooooooooooooooooooooooooooooooooooooooooooooooooooooooooo...
+       ^^^^^^
+    END
+
+      1.gsuub(//, "fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo")
+    end
+  end
+
   def test_simulate_funcallv_from_embedded_ruby
     assert_error_message(NoMethodError, <<~END) do
 undefined method `foo' for #{ NIL_RECV_MESSAGE }