Implement a KPatternStack to reduce allocations during IDFS.

Performance impact is small but seems to be roughly a 3% for 3x3x3:

    git checkout [this commit]
    cargo build
    cp ./target/release/twsearch /tmp/twsearch-with-stack

    git checkout 1ec95de
    cargo build
    cp ./target/release/twsearch /tmp/twsearch-without-stack

    hyperfine \
      '/tmp/twsearch-with-stack    search --generator-moves F,R,D,B,L --scramble-alg U "samples/json/3x3x3/3x3x3-Reid.def.json"' \
      '/tmp/twsearch-without-stack search --generator-moves F,R,D,B,L --scramble-alg U "samples/json/3x3x3/3x3x3-Reid.def.json"'

src/rs/_internal/search/idf_search.rs

@@ -15,6 +15,8 @@ use crate::_internal::{
     SearchGenerators, SearchLogger, CANONICAL_FSM_START_STATE,
 };
 
+use super::KPatternStack;
+
 const MAX_SUPPORTED_SEARCH_DEPTH: usize = 500; // TODO: increase
 
 #[allow(clippy::enum_variant_names)]
@@ -220,6 +222,7 @@ impl IDFSearch {
 
         let search_pattern = search_pattern.clone();
 
+        let mut kpattern_stack = KPatternStack::new(search_pattern);
         for remaining_depth in individual_search_data
             .individual_search_options
             .get_min_depth()
@@ -239,7 +242,7 @@ impl IDFSearch {
                 .start_depth(remaining_depth, Some("Starting search…"));
             let recursion_result = self.recurse(
                 &mut individual_search_data,
-                &search_pattern,
+                &mut kpattern_stack,
                 CANONICAL_FSM_START_STATE,
                 remaining_depth,
                 SolutionMoves(None),
@@ -257,14 +260,15 @@ impl IDFSearch {
     fn recurse(
         &self,
         individual_search_data: &mut IndividualSearchData,
-        current_pattern: &KPattern,
+        kpattern_stack: &mut KPatternStack,
         current_state: CanonicalFSMState,
         remaining_depth: usize,
         solution_moves: SolutionMoves,
     ) -> SearchRecursionResult {
         individual_search_data
             .recursive_work_tracker
             .record_recursive_call();
+        let current_pattern = kpattern_stack.current_pattern();
         if remaining_depth == 0 {
             if let Some(previous_moves) = solution_moves.0 {
                 if is_move_disallowed(
@@ -333,16 +337,21 @@ impl IDFSearch {
                     // TODO: is it always safe to `break` here?
                     continue;
                 }
-                match self.recurse(
+
+                kpattern_stack.push(&move_transformation_info.transformation);
+                let recursive_result = self.recurse(
                     individual_search_data,
-                    &current_pattern.apply_transformation(&move_transformation_info.transformation),
+                    kpattern_stack,
                     next_state,
                     remaining_depth - 1,
                     SolutionMoves(Some(&SolutionPreviousMoves {
                         latest_move: &move_transformation_info.r#move,
                         previous_moves: &solution_moves,
                     })),
-                ) {
+                );
+                kpattern_stack.pop();
+
+                match recursive_result {
                     SearchRecursionResult::DoneSearching() => {
                         return SearchRecursionResult::DoneSearching();
                     }

src/rs/_internal/search/kpattern_stack.rs

@@ -0,0 +1,38 @@
+use cubing::kpuzzle::{KPattern, KTransformation};
+
+pub(crate) struct KPatternStack {
+    stack: Vec<KPattern>,
+    current_idx: usize,
+}
+
+impl KPatternStack {
+    pub fn new(root_kpattern: KPattern) -> Self {
+        Self {
+            stack: vec![root_kpattern],
+            current_idx: 0,
+        }
+    }
+
+    pub fn push(&mut self, transformation: &KTransformation) {
+        self.current_idx += 1;
+        if self.current_idx >= self.stack.len() {
+            self.stack
+                .push(self.stack[self.current_idx - 1].apply_transformation(transformation))
+        } else {
+            // We have to use `split_at_mut` so that we can borrow both the read and write entries at the same time: https://doc.rust-lang.org/nomicon/borrow-splitting.html
+            let (left, right) = self.stack.split_at_mut(self.current_idx);
+
+            left.last()
+                .unwrap()
+                .apply_transformation_into(transformation, right.first_mut().unwrap());
+        }
+    }
+
+    pub fn current_pattern(&self) -> &KPattern {
+        &self.stack[self.current_idx]
+    }
+
+    pub fn pop(&mut self) {
+        self.current_idx -= 1;
+    }
+}

src/rs/_internal/search/mod.rs

@@ -10,3 +10,6 @@ pub(crate) use recursive_work_tracker::*;
 
 mod search_logger;
 pub use search_logger::*;
+
+mod kpattern_stack;
+pub(crate) use kpattern_stack::*;