2 files changed, 108 insertions, 22 deletions
diff --git a/lib/error_highlight/formatter.rb b/lib/error_highlight/formatter.rb
index 4d5c82f460..0656a88ef2 100644
--- a/lib/error_highlight/formatter.rb
+++ b/lib/error_highlight/formatter.rb
@@ -1,5 +1,7 @@
 module ErrorHighlight
   class DefaultFormatter
+    MIN_SNIPPET_WIDTH = 20
+
     def self.message_for(spot)
       # currently only a one-line code snippet is supported
       return "" unless spot[:first_lineno] == spot[:last_lineno]
@@ -7,22 +9,24 @@ module ErrorHighlight
       snippet      = spot[:snippet]
       first_column = spot[:first_column]
       last_column  = spot[:last_column]
+      ellipsis     = "..."
 
       # truncate snippet to fit in the viewport
-      if snippet.size > viewport_size
-        visible_start = [first_column - viewport_size / 2, 0].max
-        visible_end   = visible_start + viewport_size
+      if snippet_max_width && snippet.size > snippet_max_width
+        available_width = snippet_max_width - ellipsis.size
+        center          = first_column - snippet_max_width / 2
 
-        # avoid centering the snippet when the error is at the end of the line
-        visible_start = snippet.size - viewport_size if visible_end > snippet.size
+        visible_start  = last_column < available_width ? 0 : [center, 0].max
+        visible_end    = visible_start + snippet_max_width
+        visible_start  = snippet.size - snippet_max_width if visible_end > snippet.size
 
-        prefix = visible_start.positive?    ? "..." : ""
-        suffix = visible_end < snippet.size ? "..." : ""
+        prefix = visible_start.positive?    ? ellipsis : ""
+        suffix = visible_end < snippet.size ? ellipsis : ""
 
         snippet = prefix + snippet[(visible_start + prefix.size)...(visible_end - suffix.size)] + suffix
         snippet << "\n" unless snippet.end_with?("\n")
 
-        first_column = first_column - visible_start
+        first_column -= visible_start
         last_column  = [last_column - visible_start, snippet.size - 1].min
       end
 
@@ -32,18 +36,31 @@ module ErrorHighlight
       "\n\n#{ snippet }#{ marker }"
     end
 
-    def self.viewport_size
-      Ractor.current[:__error_highlight_viewport_size__] ||= terminal_columns
+    def self.snippet_max_width
+      return if Ractor.current[:__error_highlight_max_snippet_width__] == :disabled
+
+      Ractor.current[:__error_highlight_max_snippet_width__] ||= terminal_width
     end
 
-    def self.viewport_size=(viewport_size)
-      Ractor.current[:__error_highlight_viewport_size__] = viewport_size
+    def self.snippet_max_width=(width)
+      return Ractor.current[:__error_highlight_max_snippet_width__] = :disabled if width.nil?
+
+      width = width.to_i
+
+      if width < MIN_SNIPPET_WIDTH
+        warn "'snippet_max_width' adjusted to minimum value of #{MIN_SNIPPET_WIDTH}."
+        width = MIN_SNIPPET_WIDTH
+      end
+
+      Ractor.current[:__error_highlight_max_snippet_width__] = width
     end
 
-    def self.terminal_columns
-      # lazy load io/console, so it's not loaded when viewport_size is set
+    def self.terminal_width
+      # lazy load io/console, so it's not loaded when snippet_max_width is set
       require "io/console"
-      IO.console.winsize[1]
+      STDERR.winsize[1] if STDERR.tty?
+    rescue LoadError, NoMethodError, SystemCallError
+      # do not truncate when window size is not available
     end
   end
 
diff --git a/test/error_highlight/test_error_highlight.rb b/test/error_highlight/test_error_highlight.rb
index d00c04995d..be36fca260 100644
--- a/test/error_highlight/test_error_highlight.rb
+++ b/test/error_highlight/test_error_highlight.rb
@@ -5,7 +5,7 @@ require "did_you_mean"
 require "tempfile"
 
 class ErrorHighlightTest < Test::Unit::TestCase
-  ErrorHighlight::DefaultFormatter.viewport_size = 80
+  ErrorHighlight::DefaultFormatter.snippet_max_width = 80
 
   class DummyFormatter
     def self.message_for(corrections)
@@ -1287,7 +1287,7 @@ undefined method `time' for #{ ONE_RECV_MESSAGE }
     end
   end
 
-  def test_errors_on_small_viewports_at_the_end
+  def test_errors_on_small_terminal_window_at_the_end
     assert_error_message(NoMethodError, <<~END) do
 undefined method `time' for #{ ONE_RECV_MESSAGE }
 
@@ -1299,7 +1299,7 @@ undefined method `time' for #{ ONE_RECV_MESSAGE }
     end
   end
 
-  def test_errors_on_small_viewports_at_the_beginning
+  def test_errors_on_small_terminal_window_at_the_beginning
     assert_error_message(NoMethodError, <<~END) do
 undefined method `time' for #{ ONE_RECV_MESSAGE }
 
@@ -1312,7 +1312,19 @@ undefined method `time' for #{ ONE_RECV_MESSAGE }
     end
   end
 
-  def test_errors_on_small_viewports_at_the_middle
+  def test_errors_on_small_terminal_window_at_the_middle_near_beginning
+    assert_error_message(NoMethodError, <<~END) do
+undefined method `time' for #{ ONE_RECV_MESSAGE }
+
+    100000000000000000000000000000000000000 + 1.time { 1000000000000000000000...
+                                               ^^^^^
+    END
+
+    100000000000000000000000000000000000000 + 1.time { 100000000000000000000000000000000000000 }
+    end
+  end
+
+  def test_errors_on_small_terminal_window_at_the_middle
     assert_error_message(NoMethodError, <<~END) do
 undefined method `time' for #{ ONE_RECV_MESSAGE }
 
@@ -1320,11 +1332,68 @@ undefined method `time' for #{ ONE_RECV_MESSAGE }
                                         ^^^^^
     END
 
-    100000000000000000000000000000000000000 + 1.time { 100000000000000000000000000000000000000 }
+    10000000000000000000000000000000000000000000000000000000000000000000000 + 1.time { 1000000000000000000000000000000 }
+    end
+  end
+
+  def test_errors_on_extremely_small_terminal_window
+    custom_max_width = 30
+    original_max_width = ErrorHighlight::DefaultFormatter.snippet_max_width
+
+    ErrorHighlight::DefaultFormatter.snippet_max_width = custom_max_width
+
+    assert_error_message(NoMethodError, <<~END) do
+undefined method `time' for #{ ONE_RECV_MESSAGE }
+
+...00000000 + 1.time { 1000...
+               ^^^^^
+    END
+
+      100000000000000 + 1.time { 100000000000000 }
+    end
+  ensure
+    ErrorHighlight::DefaultFormatter.snippet_max_width = original_max_width
+  end
+
+  def test_errors_on_terminal_window_smaller_than_min_width
+    custom_max_width = 5
+    original_max_width = ErrorHighlight::DefaultFormatter.snippet_max_width
+
+    ErrorHighlight::DefaultFormatter.snippet_max_width = custom_max_width
+
+    assert_error_message(NoMethodError, <<~END) do
+undefined method `time' for #{ ONE_RECV_MESSAGE }
+
+...000 + 1.time {...
+          ^^^^^
+    END
+
+    100000000000000 + 1.time { 100000000000000 }
+    end
+  ensure
+    ErrorHighlight::DefaultFormatter.snippet_max_width = original_max_width
+  end
+
+  def test_errors_on_terminal_window_when_truncation_is_disabled
+    custom_max_width = nil
+    original_max_width = ErrorHighlight::DefaultFormatter.snippet_max_width
+
+    ErrorHighlight::DefaultFormatter.snippet_max_width = custom_max_width
+
+    assert_error_message(NoMethodError, <<~END) do
+undefined method `time' for #{ ONE_RECV_MESSAGE }
+
+      10000000000000000000000000000000000000000000000000000000000000000000000 + 1.time { 1000000000000000000000000000000 }
+                                                                                 ^^^^^
+    END
+
+      10000000000000000000000000000000000000000000000000000000000000000000000 + 1.time { 1000000000000000000000000000000 }
     end
+  ensure
+    ErrorHighlight::DefaultFormatter.snippet_max_width = original_max_width
   end
 
-  def test_errors_on_small_viewports_when_larger_than_viewport
+  def test_errors_on_small_terminal_window_when_larger_than_viewport
     assert_error_message(NoMethodError, <<~END) do
 undefined method `timessssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss!' for #{ ONE_RECV_MESSAGE }
 
@@ -1336,7 +1405,7 @@ undefined method `timessssssssssssssssssssssssssssssssssssssssssssssssssssssssss
     end
   end
 
-  def test_errors_on_small_viewports_when_exact_size_of_viewport
+  def test_errors_on_small_terminal_window_when_exact_size_of_viewport
     assert_error_message(NoMethodError, <<~END) do
 undefined method `timessssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss!' for #{ ONE_RECV_MESSAGE }