From 2f4b016a8b673a49fe476e9b7b8a55b08acccf95 Mon Sep 17 00:00:00 2001
From: Mikolaj Wielgus <wielgusmikolaj@gmail.com>
Date: Fri, 7 Nov 2025 02:25:58 +0100
Subject: [PATCH] fix(autorouter/planar_reconfigurer): Use heap-based
 best-first search for ratline permutations

This fixes an infinite loop bug.
---
 src/autorouter/mod.rs                 |   1 +
 src/autorouter/permsearch.rs          | 106 ++++++++++++++++++++++++++
 src/autorouter/planar_reconfigurer.rs |  20 +++--
 src/autorouter/scc.rs                 |   8 ++
 4 files changed, 130 insertions(+), 5 deletions(-)
 create mode 100644 src/autorouter/permsearch.rs

diff --git a/src/autorouter/mod.rs b/src/autorouter/mod.rs
index c12113e..1954aac 100644
--- a/src/autorouter/mod.rs
+++ b/src/autorouter/mod.rs
@@ -14,6 +14,7 @@ pub mod multilayer_autoroute;
 pub mod multilayer_preconfigurer;
 pub mod multilayer_reconfigurator;
 pub mod multilayer_reconfigurer;
+pub mod permsearch;
 pub mod place_via;
 pub mod planar_autoroute;
 pub mod planar_preconfigurer;
diff --git a/src/autorouter/permsearch.rs b/src/autorouter/permsearch.rs
new file mode 100644
index 0000000..481dc4d
--- /dev/null
+++ b/src/autorouter/permsearch.rs
@@ -0,0 +1,106 @@
+// SPDX-FileCopyrightText: 2025 Topola contributors
+//
+// SPDX-License-Identifier: MIT
+
+use std::{cmp::Ordering, collections::BinaryHeap, iter::Skip, iter::Take};
+
+use itertools::{Itertools, Permutations};
+
+#[derive(Clone, Debug)]
+struct PermsearchNode<T> {
+    curr_permutation: Vec<T>,
+    permutations: Skip<Permutations<Take<std::vec::IntoIter<T>>>>,
+    length: usize,
+}
+
+impl<T: Eq> PartialEq for PermsearchNode<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.curr_permutation == other.curr_permutation
+    }
+}
+
+impl<T: Eq> Eq for PermsearchNode<T> {}
+
+impl<T: Eq> Ord for PermsearchNode<T> {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.length.cmp(&other.length)
+    }
+}
+
+impl<T: Eq> PartialOrd for PermsearchNode<T> {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        self.length.partial_cmp(&other.length)
+    }
+}
+
+impl<T: Clone> PermsearchNode<T> {
+    fn permute(mut self) -> Option<Self> {
+        for (i, element) in self.permutations.next()?.iter().enumerate() {
+            self.curr_permutation[i] = element.clone();
+        }
+
+        Some(self)
+    }
+
+    fn resize(self, length: usize) -> Option<Self> {
+        if length == self.length {
+            return None;
+        }
+
+        // TODO: Get rid of `self.curr_permutation` clone somehow?
+
+        Some(Self {
+            curr_permutation: self.curr_permutation.clone(),
+            permutations: self
+                .curr_permutation
+                .into_iter()
+                .take(length)
+                .permutations(length)
+                .skip(1),
+            length,
+        })
+    }
+}
+
+pub struct Permsearch<T> {
+    curr_node: PermsearchNode<T>,
+    frontier: BinaryHeap<PermsearchNode<T>>,
+}
+
+impl<T: Eq + Clone> Permsearch<T> {
+    pub fn new(original: Vec<T>) -> Self {
+        let len = original.len();
+
+        // TODO: Get rid of `original` clone somehow?
+
+        Self {
+            curr_node: PermsearchNode {
+                curr_permutation: original.clone(),
+                permutations: original.into_iter().take(len).permutations(0).skip(0),
+                length: 0,
+            },
+            frontier: BinaryHeap::new(),
+        }
+    }
+
+    pub fn step(&mut self, len: usize) -> Option<&[T]> {
+        // TODO: Get rid of `self.curr_node` clones somehow?
+
+        if let Some(resized_curr_node) = self.curr_node.clone().resize(len) {
+            if let Some(permuted_resized_curr_node) = resized_curr_node.permute() {
+                self.frontier.push(permuted_resized_curr_node);
+            }
+        }
+
+        if let Some(permuted_curr_node) = self.curr_node.clone().permute() {
+            self.frontier.push(permuted_curr_node);
+        }
+
+        self.curr_node = self.frontier.pop()?;
+        Some(&self.curr_node.curr_permutation)
+    }
+
+    pub fn curr_permutation(&self) -> &[T] {
+        &self.curr_node.curr_permutation
+    }
+}
diff --git a/src/autorouter/planar_reconfigurer.rs b/src/autorouter/planar_reconfigurer.rs
index 7ff318e..b420553 100644
--- a/src/autorouter/planar_reconfigurer.rs
+++ b/src/autorouter/planar_reconfigurer.rs
@@ -9,6 +9,7 @@ use itertools::{Itertools, Permutations};
 use specctra_core::mesadata::AccessMesadata;
 
 use crate::autorouter::{
+    permsearch::Permsearch,
     planar_autoroute::{PlanarAutorouteConfiguration, PlanarAutorouteExecutionStepper},
     planar_preconfigurer::SccIntersectionsAndLengthRatlinePlanarAutoroutePreconfigurer,
     scc::Scc,
@@ -52,7 +53,7 @@ impl PlanarAutorouteReconfigurer {
 }
 
 pub struct SccPermutationsPlanarAutorouteReconfigurer {
-    sccs_permutations_iter: Skip<Permutations<std::vec::IntoIter<Scc>>>,
+    sccs_permsearch: Permsearch<Scc>,
     preconfiguration: PlanarAutorouteConfiguration,
 }
 
@@ -66,10 +67,9 @@ impl SccPermutationsPlanarAutorouteReconfigurer {
         // TODO: Instead of instantiating presorter again here, get it from
         // an argument.
         let sccs = presorter.dissolve();
-        let sccs_len = sccs.len();
 
         Self {
-            sccs_permutations_iter: sccs.into_iter().permutations(sccs_len).skip(1),
+            sccs_permsearch: Permsearch::new(sccs),
             preconfiguration,
         }
     }
@@ -79,9 +79,19 @@ impl MakeNextPlanarAutorouteConfiguration for SccPermutationsPlanarAutorouteReco
     fn next_configuration(
         &mut self,
         autorouter: &Autorouter<impl AccessMesadata>,
-        _stepper: &PlanarAutorouteExecutionStepper,
+        stepper: &PlanarAutorouteExecutionStepper,
     ) -> Option<PlanarAutorouteConfiguration> {
-        let scc_permutation = self.sccs_permutations_iter.next()?;
+        let scc_index = self
+            .sccs_permsearch
+            .curr_permutation()
+            .iter()
+            .position(|scc| {
+                scc.scc_ref(autorouter)
+                    .contains(self.preconfiguration.ratlines[*stepper.curr_ratline_index()])
+            })
+            .unwrap();
+
+        let scc_permutation = self.sccs_permsearch.step(scc_index + 1)?;
         let mut ratlines = vec![];
 
         for scc in scc_permutation {
diff --git a/src/autorouter/scc.rs b/src/autorouter/scc.rs
index c7e176e..153f86a 100644
--- a/src/autorouter/scc.rs
+++ b/src/autorouter/scc.rs
@@ -26,6 +26,14 @@ pub struct Scc {
     length: f64,
 }
 
+impl PartialEq for Scc {
+    fn eq(&self, other: &Self) -> bool {
+        self.node_indices == other.node_indices
+    }
+}
+
+impl Eq for Scc {}
+
 impl Scc {
     pub fn new(
         autorouter: &mut Autorouter<impl AccessMesadata>,