feat: enforce maximum evolved tree-GP size with max_len

CHANGELOG.md

@@ -1,9 +1,14 @@
 # Changelog
 
+## Unreleased
+
+Changes:
+*   Feat: enforce maximum evolved tree-GP size with `max_len` instead of just the initial size ([#245](https://github.com/xcsf-dev/xcsf/pull/245))
+
 ## Version 1.5.1 (Aug 24, 2026)
 
 Changes:
-*   CI fix macosx_x86_64 wheel building ([#244](https://github.com/xcsf-dev/xcsf/pull/244))
+*   CI: fix macosx_x86_64 wheel building ([#244](https://github.com/xcsf-dev/xcsf/pull/244))
 
 ## Version 1.5.0 (Aug 24, 2026)
 

test/cond_gp_test.cpp

@@ -17,7 +17,7 @@
  * @file cond_gp_test.cpp
  * @author Richard Preen <rpreen@gmail.com>
  * @copyright The Authors.
- * @date 2023--2024.
+ * @date 2023--2026.
  * @brief GP condition tests.
  */
 
@@ -114,6 +114,10 @@ TEST_CASE("COND_GP")
     // check trees are different
     CHECK(!check_array_eq_int(dest_cond->gp.tree, src_cond->gp.tree, n));
 
+    /* Test max_len enforced during crossover */
+    tree_param_set_max_len(targs, 2);
+    CHECK(!cond_gp_crossover(&xcsf, c1, c2));
+
     /* Smoke test export */
     char *json_str = cond_gp_json_export(&xcsf, c1);
     CHECK(json_str != NULL);

xcsf/cond_gp.c

@@ -17,13 +17,12 @@
  * @file cond_gp.c
  * @author Richard Preen <rpreen@gmail.com>
  * @copyright The Authors.
- * @date 2016--2022.
+ * @date 2016--2026.
  * @brief Tree GP condition functions.
  */
 
 #include "cond_gp.h"
 #include "ea.h"
-#include "sam.h"
 #include "utils.h"
 
 /**
@@ -149,8 +148,7 @@ cond_gp_crossover(const struct XCSF *xcsf, const struct Cl *c1,
     struct CondGP *cond1 = c1->cond;
     struct CondGP *cond2 = c2->cond;
     if (rand_uniform(0, 1) < xcsf->ea->p_crossover) {
-        tree_crossover(&cond1->gp, &cond2->gp);
-        return true;
+        return tree_crossover(&cond1->gp, &cond2->gp, xcsf->cond->targs);
     }
     return false;
 }

xcsf/gp.c

@@ -17,7 +17,7 @@
  * @file gp.c
  * @author Richard Preen <rpreen@gmail.com>
  * @copyright The Authors.
- * @date 2016--2022.
+ * @date 2016--2026.
  * @brief An implementation of GP trees based upon TinyGP.
  * @see Poli, Langdon, and McPhee (2008) "A Field Guide to Genetic Programming"
  */
@@ -26,20 +26,20 @@
 #include "sam.h"
 #include "utils.h"
 
-#define GP_NUM_FUNC (4) //!< Number of selectable GP functions
-#define ADD (0) //!< Addition function
-#define SUB (1) //!< Subtraction function
-#define MUL (2) //!< Multiplication function
-#define DIV (3) //!< Division function
+#define GP_NUM_FUNC 4 //!< Number of selectable GP functions
+#define ADD 0 //!< Addition function
+#define SUB 1 //!< Subtraction function
+#define MUL 2 //!< Multiplication function
+#define DIV 3 //!< Division function
 
-#define STRING_ADD ("+\0") //!< Addition
-#define STRING_SUB ("-\0") //!< Subtraction
-#define STRING_MUL ("*\0") //!< Multiplication
-#define STRING_DIV ("/\0") //!< Division
+#define STRING_ADD "+\0" //!< Addition
+#define STRING_SUB "-\0" //!< Subtraction
+#define STRING_MUL "*\0" //!< Multiplication
+#define STRING_DIV "/\0" //!< Division
 
-#define N_MU (1) //!< Number of tree-GP mutation rates
-#define RET_MIN (-1000) //!< Minimum tree return value
-#define RET_MAX (1000) //!< Maximum tree return value
+#define N_MU 1 //!< Number of tree-GP mutation rates
+#define RET_MIN -1000 //!< Minimum tree return value
+#define RET_MAX 1000 //!< Maximum tree return value
 
 /**
  * @brief Self-adaptation method for mutating GP trees.
@@ -313,9 +313,11 @@ tree_copy(struct GPTree *dest, const struct GPTree *src)
  * @brief Performs sub-tree crossover.
  * @param [in] p1 The first GP tree to perform crossover.
  * @param [in] p2 The second GP tree to perform crossover.
+ * @param [in] args Tree GP parameters.
  */
-void
-tree_crossover(struct GPTree *p1, struct GPTree *p2)
+bool
+tree_crossover(struct GPTree *p1, struct GPTree *p2,
+               const struct ArgsGPTree *args)
 {
     const int len1 = p1->len;
     const int len2 = p2->len;
@@ -324,12 +326,17 @@ tree_crossover(struct GPTree *p1, struct GPTree *p2)
     const int start2 = rand_uniform_int(0, len2);
     const int end2 = tree_traverse(p2->tree, start2);
     const int nlen1 = start1 + (end2 - start2) + (len1 - end1);
+    const int nlen2 = start2 + (end1 - start1) + (len2 - end2);
+
+    if (nlen1 > args->max_len || nlen2 > args->max_len) {
+        return false;
+    }
+
     int *new1 = malloc(sizeof(int) * nlen1);
     memcpy(&new1[0], &p1->tree[0], sizeof(int) * start1);
     memcpy(&new1[start1], &p2->tree[start2], sizeof(int) * (end2 - start2));
     memcpy(&new1[start1 + (end2 - start2)], &p1->tree[end1],
            sizeof(int) * (len1 - end1));
-    const int nlen2 = start2 + (end1 - start1) + (len2 - end2);
     int *new2 = malloc(sizeof(int) * nlen2);
     memcpy(&new2[0], &p2->tree[0], sizeof(int) * start2);
     memcpy(&new2[start2], &p1->tree[start1], sizeof(int) * (end1 - start1));
@@ -341,6 +348,7 @@ tree_crossover(struct GPTree *p1, struct GPTree *p2)
     p2->tree = new2;
     p1->len = tree_traverse(p1->tree, 0);
     p2->len = tree_traverse(p2->tree, 0);
+    return true;
 }
 
 /**

xcsf/gp.h

@@ -17,7 +17,7 @@
  * @file gp.h
  * @author Richard Preen <rpreen@gmail.com>
  * @copyright The Authors.
- * @date 2016--2022.
+ * @date 2016--2026.
  * @brief An implementation of GP trees based upon TinyGP.
  */
 
@@ -34,7 +34,7 @@ struct ArgsGPTree {
     int n_inputs; //!< Number of inputs
     int n_constants; //!< Number of constants available
     int init_depth; //!< Initial depth
-    int max_len; //!< Maximum initial length
+    int max_len; //!< Maximum length
     double *constants; //!< Constants available for GP trees
 };
 
@@ -70,8 +70,9 @@ tree_json_import(struct GPTree *gp, const struct ArgsGPTree *args,
 double
 tree_eval(struct GPTree *gp, const struct ArgsGPTree *args, const double *x);
 
-void
-tree_crossover(struct GPTree *p1, struct GPTree *p2);
+bool
+tree_crossover(struct GPTree *p1, struct GPTree *p2,
+               const struct ArgsGPTree *args);
 
 bool
 tree_mutate(struct GPTree *gp, const struct ArgsGPTree *args);