Experimental: assume naive line pairings are homologous

Experimental feature controlled by environment variable
DELTA_EXPERIMENTAL_MAX_LINE_DISTANCE_FOR_NAIVELY_PAIRED_LINES.

src/config.rs

@@ -14,6 +14,7 @@ pub struct Config<'a> {
     pub theme: Option<&'a Theme>,
     pub theme_name: String,
     pub max_line_distance: f64,
+    pub max_line_distance_for_naively_paired_lines: f64,
     pub minus_style_modifier: StyleModifier,
     pub minus_emph_style_modifier: StyleModifier,
     pub plus_style_modifier: StyleModifier,
@@ -130,10 +131,16 @@ pub fn get_config<'a>(
     let minus_line_marker = if keep_plus_minus_markers { "-" } else { " " };
     let plus_line_marker = if keep_plus_minus_markers { "+" } else { " " };
 
+    let max_line_distance_for_naively_paired_lines =
+        env::get_env_var("DELTA_EXPERIMENTAL_MAX_LINE_DISTANCE_FOR_NAIVELY_PAIRED_LINES")
+            .map(|s| s.parse::<f64>().unwrap_or(0.0))
+            .unwrap_or(0.0);
+
     Config {
         theme,
         theme_name,
         max_line_distance: opt.max_line_distance,
+        max_line_distance_for_naively_paired_lines,
         minus_style_modifier,
         minus_emph_style_modifier,
         plus_style_modifier,

src/edits.rs

@@ -16,6 +16,7 @@ pub fn infer_edits<'a, EditOperation>(
     noop_insertion: EditOperation,
     insertion: EditOperation,
     max_line_distance: f64,
+    max_line_distance_for_naively_paired_lines: f64,
 ) -> (
     Vec<Vec<(EditOperation, &'a str)>>, // annotated minus lines
     Vec<Vec<(EditOperation, &'a str)>>, // annotated plus lines
@@ -42,7 +43,10 @@ where
                 minus_line,
                 plus_line,
             );
-            if distance <= max_line_distance {
+            if minus_lines.len() == plus_lines.len()
+                && distance <= max_line_distance_for_naively_paired_lines
+                || distance <= max_line_distance
+            {
                 // minus_line and plus_line are inferred to be a homologous pair.
 
                 // Emit as unpaired the plus lines already considered and rejected
@@ -597,6 +601,7 @@ mod tests {
             PlusNoop,
             Insertion,
             max_line_distance,
+            0.0,
         );
         assert_eq!(actual_edits, expected_edits);
     }

src/paint.rs

@@ -219,6 +219,7 @@ impl<'a> Painter<'a> {
             config.plus_style_modifier,
             config.plus_emph_style_modifier,
             config.max_line_distance,
+            config.max_line_distance_for_naively_paired_lines,
         )
     }
 }