1 files changed, 27 insertions, 36 deletions
diff --git a/lib/error_highlight/formatter.rb b/lib/error_highlight/formatter.rb
index b4fce93e55..4d5c82f460 100644
--- a/lib/error_highlight/formatter.rb
+++ b/lib/error_highlight/formatter.rb
@@ -2,55 +2,46 @@ module ErrorHighlight
   class DefaultFormatter
     def self.message_for(spot)
       # currently only a one-line code snippet is supported
-      if spot[:first_lineno] == spot[:last_lineno]
-        spot = truncate(spot)
+      return "" unless spot[:first_lineno] == spot[:last_lineno]
 
-        indent = spot[:snippet][0...spot[:first_column]].gsub(/[^\t]/, " ")
-        marker = indent + "^" * (spot[:last_column] - spot[:first_column])
+      snippet      = spot[:snippet]
+      first_column = spot[:first_column]
+      last_column  = spot[:last_column]
 
-        "\n\n#{ spot[:snippet] }#{ marker }"
-      else
-        ""
+      # 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
+
+        # 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
+
+        prefix = visible_start.positive?    ? "..." : ""
+        suffix = visible_end < snippet.size ? "..." : ""
+
+        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
+        last_column  = [last_column - visible_start, snippet.size - 1].min
       end
+
+      indent = snippet[0...first_column].gsub(/[^\t]/, " ")
+      marker = indent + "^" * (last_column - first_column)
+
+      "\n\n#{ snippet }#{ marker }"
     end
 
     def self.viewport_size
-      Ractor.current[:__error_highlight_viewport_size__] || terminal_columns
+      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
+      # lazy load io/console, so it's not loaded when viewport_size is set
       require "io/console"
       IO.console.winsize[1]
     end