From 4b71607215eefe15012fe5b7ae81ece79adca79d Mon Sep 17 00:00:00 2001
From: Mark Thom <markjordanthom@gmail.com>
Date: Mon, 31 Jan 2022 18:23:16 -0700
Subject: [PATCH] count cycle lengths in skip_max_list_cycle (#1260)

---
 src/machine/system_calls.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/machine/system_calls.rs b/src/machine/system_calls.rs
index a9f1b91f..4f3ed651 100644
--- a/src/machine/system_calls.rs
+++ b/src/machine/system_calls.rs
@@ -462,12 +462,21 @@ impl MachineState {
             hare = step(&self.heap, self.heap[hare]);
         }
 
+        let mut count = 1;
+
         while hare != tortoise {
             hare = step(&self.heap, self.heap[hare]);
             tortoise = step(&self.heap, self.heap[tortoise]);
+
+            count += 1;
         }
 
-        unify!(self, self.registers[4], self.heap[hare]);
+        let target_n = self.store(self.deref(self.registers[1]));
+        self.unify_fixnum(Fixnum::build_with(count), target_n);
+
+        if !self.fail {
+            unify!(self, self.registers[4], self.heap[hare]);
+        }
     }
 
     fn finalize_skip_max_list(&mut self, n: i64, value: HeapCellValue) {
-- 
2.54.0