Issue #143 - Track recursion depth of equalp

This prevents the possibility of segfaulting when traversing arbitrarily complex circular structures.

CHANGELOG.md

@@ -4,7 +4,7 @@
 
 Bug Fixes
 
-- Enforce a maximum recursion depth when printing an object via `display` and `write`. This prevents segmentation faults when printing circular data structures.
+- Enforce a maximum recursion depth when printing an object via `display` and `write`, and when comparing objects via `equal?`. This prevents segmentation faults when working with circular data structures.
 
 ## 0.34.0 - January 2, 2022
 

include/cyclone/runtime.h

@@ -574,7 +574,6 @@ double MRG32k3a (double seed);
 //object Cyc_eq(object x, object y);
 object Cyc_eqv(object x, object y);
 #define Cyc_eq(x, y) (make_boolean(x == y))
-int equal(object, object);
 object equalp(object, object);
 object Cyc_has_cycle(object lst);
 object Cyc_is_list(object lst);

runtime.c

@@ -785,7 +785,8 @@ void Cyc_rt_raise_msg(void *data, const char *err)
 
 /* END exception handler */
 
-int equal(object x, object y)
+object _equalp(object x, object y, int depth);
+static int equal(object x, object y, int depth)
 {
   if (x == y)
     return 1;
@@ -818,7 +819,7 @@ int equal(object x, object y)
       int i;
       if (x == y) return 1;
       for (i = 0; i < ((vector) x)->num_elements; i++) {
-        if (equalp(((vector) x)->elements[i], ((vector) y)->elements[i]) ==
+        if (_equalp(((vector) x)->elements[i], ((vector) y)->elements[i], depth + 1) ==
             boolean_f)
           return 0;
       }
@@ -1217,7 +1218,8 @@ done:
   return quote_void;
 }
 
-object Cyc_display(void *data, object x, FILE * port) {
+object Cyc_display(void *data, object x, FILE * port) 
+{
   return _Cyc_display(data, x, port, 0);
 }
 
@@ -1624,7 +1626,7 @@ object Cyc_heap_alloc_port(void *data, port_type *stack_p)
 /**
  * Check two objects for deep equality
  */
-object equalp(object x, object y)
+object _equalp(object x, object y, int depth)
 {
   int second_cycle = 0;
   object slow_lis = x, fast_lis = NULL;
@@ -1636,7 +1638,7 @@ object equalp(object x, object y)
   }
 
   for (;; x = cdr(x), y = cdr(y)) {
-    if (equal(x, y))
+    if (depth == MAX_DEPTH || equal(x, y, depth))
       return boolean_t;
     if (is_value_type(x) || is_value_type(y) ||
         (x == NULL) || (y == NULL) ||
@@ -1648,7 +1650,7 @@ object equalp(object x, object y)
         pcar_y == car(y)) {
       // do nothing, already equal
     } else {
-      if (boolean_f == equalp(car(x), car(y)))
+      if (boolean_f == _equalp(car(x), car(y), depth + 1))
         return boolean_f;
       pcar_x = car(x);
       pcar_y = car(y);
@@ -1682,6 +1684,11 @@ object equalp(object x, object y)
   }
 }
 
+object equalp(object x, object y)
+{
+  return _equalp(x, y, 0);
+}
+
 object Cyc_num_cmp_va_list(void *data, int argc,
                            int (fn_op(void *, object, object)), object n,
                            va_list ns)