1 files changed, 74 insertions, 0 deletions
diff --git a/lib/error_highlight/formatter.rb b/lib/error_highlight/formatter.rb
new file mode 100644
index 0000000000..d2fad9e75c
--- /dev/null
+++ b/lib/error_highlight/formatter.rb
@@ -0,0 +1,74 @@
+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]
+
+      snippet      = spot[:snippet]
+      first_column = spot[:first_column]
+      last_column  = spot[:last_column]
+      ellipsis     = "..."
+
+      # truncate snippet to fit in the viewport
+      if max_snippet_width && snippet.size > max_snippet_width
+        available_width = max_snippet_width - ellipsis.size
+        center          = first_column - max_snippet_width / 2
+
+        visible_start  = last_column < available_width ? 0 : [center, 0].max
+        visible_end    = visible_start + max_snippet_width
+        visible_start  = snippet.size - max_snippet_width if 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 -= 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.max_snippet_width
+      return if Ractor.current[:__error_highlight_max_snippet_width__] == :disabled
+
+      Ractor.current[:__error_highlight_max_snippet_width__] ||= terminal_width
+    end
+
+    def self.max_snippet_width=(width)
+      return Ractor.current[:__error_highlight_max_snippet_width__] = :disabled if width.nil?
+
+      width = width.to_i
+
+      if width < MIN_SNIPPET_WIDTH
+        warn "'max_snippet_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_width
+      # lazy load io/console to avoid loading it when 'max_snippet_width' is manually set
+      require "io/console"
+      $stderr.winsize[1] if $stderr.tty?
+    rescue LoadError, NoMethodError, SystemCallError
+      # skip truncation when terminal window size is unavailable
+    end
+  end
+
+  def self.formatter
+    Ractor.current[:__error_highlight_formatter__] || DefaultFormatter
+  end
+
+  def self.formatter=(formatter)
+    Ractor.current[:__error_highlight_formatter__] = formatter
+  end
+end