Explore two graph nodes for each parenthesis position

This produces substantially better diff results, and fixes the 'last
item in the list shown as changed' problem.

This can produce slower diffing. typing_before.ml takes 10% more
instructions and slow_before.rs takes 110% more instructions.

CHANGELOG.md

@@ -1,5 +1,11 @@
 ## 0.33 (unreleased)
 
+### Diffing
+
+Difftastic now explores 2x more parenthesis nesting
+possibilities. This can make diffing slower, but produces
+substantially better results.
+
 ### Parsing
 
 Added support for CMake.

sample_files/compare.expected

@@ -140,13 +140,13 @@ sample_files/ruby_before.rb sample_files/ruby_after.rb
 4a9847a91e32ec6afdc2f0b01e28d2d6  -
 
 sample_files/scala_before.scala sample_files/scala_after.scala
-b276168d30704b4242fc82dc1f8382d8  -
+200c6ae867dfbfb4337efd5432d81431  -
 
 sample_files/Session_before.kt sample_files/Session_after.kt
 46994b58bb24200f82866951013f03ce  -
 
 sample_files/simple_before.js sample_files/simple_after.js
-4fa7016f884e76dddba4cab12cab8c61  -
+45fef08587e93e19a3bd76299aa7c142  -
 
 sample_files/simple_before.txt sample_files/simple_after.txt
 4b653ebe89321835c35722dd065cf6a2  -
@@ -155,7 +155,7 @@ sample_files/slider_before.rs sample_files/slider_after.rs
 50e1df5af0bf4a1fa7211e079196f1a9  -
 
 sample_files/slow_before.rs sample_files/slow_after.rs
-e8e7521e32766879f7219788a113005a  -
+1090ccf266b6e9fd22f2585f9fa350a9  -
 
 sample_files/small_before.js sample_files/small_after.js
 27bcac13aa17141718a3e6b8c0ac8f47  -
@@ -179,7 +179,7 @@ sample_files/toml_before.toml sample_files/toml_after.toml
 1e2de7235c339b07a0784498453e896c  -
 
 sample_files/typing_before.ml sample_files/typing_after.ml
-ceba89e4ffc8406454d337638c7d45e6  -
+75d0b1bec67a58b17c17a6438c447252  -
 
 sample_files/whitespace_before.tsx sample_files/whitespace_after.tsx
 c4151c5a44b11e04fd11c2594597ed33  -

src/diff/graph.rs

@@ -101,7 +101,7 @@ impl<'a, 'b> Hash for Vertex<'a, 'b> {
 }
 
 /// Tracks entering syntax List nodes.
-#[derive(Clone)]
+#[derive(Clone, PartialEq)]
 enum EnteredDelimiter<'a> {
     /// If we've entered the LHS or RHS separately, we can pop either
     /// side independently.
@@ -340,13 +340,35 @@ impl Edge {
 fn allocate_if_new<'syn, 'b>(
     v: Vertex<'syn, 'b>,
     alloc: &'b Bump,
-    seen: &mut FxHashMap<&Vertex<'syn, 'b>, &'b Vertex<'syn, 'b>>,
+    seen: &mut FxHashMap<&Vertex<'syn, 'b>, Vec<&'b Vertex<'syn, 'b>>>,
 ) -> &'b Vertex<'syn, 'b> {
-    match seen.get(&v) {
-        Some(existing) => *existing,
+    match seen.get_mut(&v) {
+        Some(existing) => {
+            // Don't explore more than two possible parenthesis
+            // nestings for each syntax node pair.
+            if let Some(allocated) = existing.last() {
+                if existing.len() >= 2 {
+                    return *allocated;
+                }
+            }
+
+            // If we have seen exactly this graph node before, even
+            // considering parenthesis matching, return it.
+            for existing_node in existing.iter() {
+                if existing_node.parents == v.parents {
+                    return existing_node;
+                }
+            }
+
+            // We haven't reached the graph node limit yet, allocate a
+            // new one.
+            let allocated = alloc.alloc(v);
+            existing.push(allocated);
+            allocated
+        }
         None => {
             let allocated = alloc.alloc(v);
-            seen.insert(allocated, allocated);
+            seen.insert(allocated, vec![allocated]);
             allocated
         }
     }
@@ -357,7 +379,7 @@ fn allocate_if_new<'syn, 'b>(
 pub fn get_set_neighbours<'syn, 'b>(
     v: &Vertex<'syn, 'b>,
     alloc: &'b Bump,
-    seen: &mut FxHashMap<&Vertex<'syn, 'b>, &'b Vertex<'syn, 'b>>,
+    seen: &mut FxHashMap<&Vertex<'syn, 'b>, Vec<&'b Vertex<'syn, 'b>>>,
 ) -> Vec<(Edge, &'b Vertex<'syn, 'b>)> {
     match &*v.neighbours.borrow() {
         Some(neighbours) => return neighbours.clone(),