Some modifications to the regression experiment.

offchain/src/bin/symreg_experiment.rs

@@ -2,20 +2,25 @@
 
 use anyhow::Result;
 use rand::{thread_rng, Rng};
+
+// Our GP + compiler modules (adjust paths as needed)
 use offchain::compiler::ast::{UntypedAst, Push3Ast};
 use offchain::helpers::artifact::get_creation_code;
 use offchain::runner::revm_runner::{
     EvmRunner, 
     Push3InterpreterInputs,
-    Push3InterpreterOutputs
 };
-use offchain::gp::generate_spec::{ranmdom_code_fixed};
-use offchain::gp::mutation::mutate_by_index;
 use offchain::compiler::push3_describtor::make_sublist_descriptor;
+
+// Our GP operators
+use offchain::gp::generate_spec::ranmdom_code_fixed;
+use offchain::gp::mutation::{mutate_by_index, crossover_by_index};
+
+// Ethers + ABI
 use ethers::types::U256;
 use ethers::abi::{encode, Token};
 
-// 1) Generate (x, y) samples for f(x) = 3x^2 + x + 3
+/// 1) Generate (x, y) samples for f(x)=3x^2 + x + 3
 fn generate_samples() -> Vec<(i32, i32)> {
     let mut samples = Vec::new();
     for x in -5..=5 {
@@ -25,8 +30,9 @@ fn generate_samples() -> Vec<(i32, i32)> {
     samples
 }
 
-// 2) Evaluate a single AST on a single input `x`, returning the integer
-//    that ends up on top of the final int stack (or 0 if empty).
+/// 2) Evaluate a single AST on a single input `x` by 
+/// pushing `x` onto the int stack, running in EVM, 
+/// and reading the final top of the int stack.
 fn evaluate_ast_on_x(
     runner: &mut EvmRunner,
     ast: &UntypedAst,
@@ -64,7 +70,7 @@ fn evaluate_ast_on_x(
     }
 }
 
-// 3) Evaluate an AST on all (x, y) samples => compute MSE
+/// 3) Evaluate an AST on all (x, y) samples => compute MSE
 fn evaluate_fitness(
     runner: &mut EvmRunner,
     ast: &UntypedAst,
@@ -80,30 +86,33 @@ fn evaluate_fitness(
 }
 
 fn main() -> Result<()> {
-    // A) Generate samples for f(x)=3x^2 + x + 3
+    // 1) Generate samples for f(x)=3x^2 + x + 3
     let samples = generate_samples();
 
-    // B) Create an ephemeral EVM runner (Push3Interpreter)
+    // 2) Create ephemeral EVM runner for the interpreter
     let creation_hex_filename = "../onchain/out/Push3Interpreter.sol/Push3Interpreter.json";
     let creation_bytes = get_creation_code(creation_hex_filename)?;
     let mut runner = EvmRunner::new(creation_bytes)?;
 
-    // C) Setup GP parameters
-    let pop_size = 1000;       // population size
-    let max_points = 15;      // max "size" for random code
-    let generations = 100;    // how many generations we run
-    let early_stop_threshold = 1.0; // if error < this => we can stop
+    // 3) GP parameters
+    let pop_size = 1000;  // must be divisible by 4 for our scheme
+    let generations = 30;
+    let early_stop_threshold = 1.0;
+    let max_points = 15;
+
+    assert_eq!(pop_size % 4, 0, "pop_size must be multiple of 4 for our scheme");
+    let quarter = pop_size / 4;
 
     let mut rng = thread_rng();
 
-    // D) Initialize random population
+    // 4) Initialize population (random)
     let mut population: Vec<UntypedAst> = (0..pop_size)
         .map(|_| ranmdom_code_fixed(&mut rng, max_points))
         .collect();
 
-    // E) Main GP Loop
+    // 5) Main GP loop
     for gen in 0..generations {
-        // 1) Evaluate each => store (ast, error)
+        // (a) Evaluate
         let mut scored: Vec<(UntypedAst, f64)> = population.into_iter()
             .map(|ast| {
                 let err = evaluate_fitness(&mut runner, &ast, &samples);
@@ -111,59 +120,77 @@ fn main() -> Result<()> {
             })
             .collect();
 
-        // 2) Sort ascending by error
+        // (b) Sort ascending by error
         scored.sort_by(|a, b| a.1.partial_cmp(&b.1).unwrap());
 
-        // 3) Print best error
+        // (c) Print generation summary
         let best_err = scored[0].1;
-        println!("Generation {gen}, best error = {best_err}");
+        println!("\n=== Generation {gen} ===");
+        println!("Best error = {best_err}");
 
-        // 4) Check if best is below threshold => early stop
+        // Print top 5 subjects in pretty format
+        let top_n = 5.min(scored.len());
+        println!("Top {top_n} subjects:");
+        for i in 0..top_n {
+            println!(
+                "  #{i}, err={}, AST:\n{:#?}",
+                scored[i].1,
+                scored[i].0
+            );
+        }
+
+        // early stop?
         if best_err < early_stop_threshold {
             println!("Best error < {early_stop_threshold}, stopping early!");
+            // we can keep 'scored' for final if we want
             population = scored.into_iter().map(|(a, _)| a).collect();
             break;
         }
 
-        // 5) Keep best half
-        let half = scored.len() / 2;
-        let survivors = &scored[..half];
+        // (d) Reproduction => build new population
+        let mut new_pop = Vec::with_capacity(pop_size);
 
-        // 6) Refill population: half mutated, half fresh random
-        let mut new_pop = Vec::with_capacity(scored.len());
-        // a) keep survivors
-        for &(ref ast, _) in survivors {
-            new_pop.push(ast.clone());
+        // keep best quarter
+        for i in 0..quarter {
+            new_pop.push(scored[i].0.clone());
         }
 
-        // b) how many to add?
-        let remainder_size = pop_size - new_pop.len();
-        // we do half mutated, half new random
-        let mutated_count = remainder_size / 2;
-        let random_count = remainder_size - mutated_count;
-
-        // c) add mutated
-        for _ in 0..mutated_count {
-            let idx = rng.gen_range(0..half);
-            let mutated = mutate_by_index(&survivors[idx].0, &mut rng, max_points);
-            new_pop.push(mutated);
-        }
-
-        // d) add brand-new random
-        for _ in 0..random_count {
+        // add random quarter
+        for _ in 0..quarter {
             let fresh = ranmdom_code_fixed(&mut rng, max_points);
             new_pop.push(fresh);
         }
 
-        // store for next generation
+        // crossover quarter
+        let top_half_count = scored.len() / 2;
+        let crossover_pairs = quarter / 2;  // each yields 2 kids => quarter total
+        for _ in 0..crossover_pairs {
+            let idx1 = rng.gen_range(0..top_half_count);
+            let idx2 = rng.gen_range(0..top_half_count);
+            let (child1, child2) = crossover_by_index(
+                &scored[idx1].0,
+                &scored[idx2].0,
+                &mut rng
+            );
+            new_pop.push(child1);
+            new_pop.push(child2);
+        }
+
+        // mutate quarter
+        while new_pop.len() < pop_size {
+            let idx = rng.gen_range(0..top_half_count);
+            let mutated = mutate_by_index(&scored[idx].0, &mut rng, max_points);
+            new_pop.push(mutated);
+        }
+
         population = new_pop;
     }
 
-    // F) Final Print
+    // 6) Final population => we can evaluate & print
     println!("\n=== Final Population ===");
     for (i, ast) in population.iter().enumerate() {
         let err = evaluate_fitness(&mut runner, ast, &samples);
-        println!("Subject #{i}, err={err}, ast={:?}", ast);
+        println!("Subject #{i}, err={err}, AST:\n{:#?}", ast);
     }
 
     Ok(())

offchain/src/gp/generate_spec.rs

@@ -47,7 +47,7 @@ impl InstructionSet {
             InstructionAtom::Opcode(op) => UntypedAst::Instruction(op.clone()),
             InstructionAtom::EphemeralInt => {
                 // For ephemeral int, produce a random literal in some range
-                let val = rng.gen_range(-10..10);
+                let val = rng.gen_range(-100..100);
                 UntypedAst::IntLiteral(val)
             }
         }

offchain/src/gp/mutation.rs

@@ -84,3 +84,64 @@ pub fn mutate_by_index(
     replace_subtree(original, chosen_path, new_subtree)
 }
 
+/// Return the subtree of `original` at `path`, 
+/// cloning it as a `UntypedAst`. 
+/// If `path` is empty => returns the entire `original`.
+pub fn get_subtree(original: &UntypedAst, path: &[usize]) -> UntypedAst {
+    if path.is_empty() {
+        // The entire AST is the subtree
+        return original.clone();
+    }
+
+    match original {
+        UntypedAst::Sublist(children) => {
+            let (first_idx, tail_path) = (path[0], &path[1..]);
+            if first_idx >= children.len() {
+                // If path index is out of range, just clone the entire AST or 
+                // do something fallback
+                return original.clone();
+            }
+            get_subtree(&children[first_idx], tail_path)
+        }
+        UntypedAst::IntLiteral(_) | UntypedAst::Instruction(_) => {
+            // If path is non-empty but this is a leaf, 
+            // fallback: just return the leaf.
+            original.clone()
+        }
+    }
+}
+
+/// Perform a subtree crossover between two ASTs.
+/// 1) We pick a random node in `a` and a random node in `b`,
+/// 2) We swap those subtrees,
+/// 3) Return the two new ASTs.
+///
+/// Example usage:
+/// ```rust
+/// let (childA, childB) = crossover_by_index(&parentA, &parentB, &mut rng);
+/// ```
+pub fn crossover_by_index(
+    a: &UntypedAst,
+    b: &UntypedAst,
+    rng: &mut impl Rng,
+) -> (UntypedAst, UntypedAst) {
+    // 1) enumerate nodes in a
+    let paths_a = enum_nodes_dfs(a);
+    let idx_a = rng.gen_range(0..paths_a.len());
+    let chosen_a = &paths_a[idx_a];
+
+    // 2) enumerate nodes in b
+    let paths_b = enum_nodes_dfs(b);
+    let idx_b = rng.gen_range(0..paths_b.len());
+    let chosen_b = &paths_b[idx_b];
+
+    // 3) get subtree from a, from b
+    let subtree_a = get_subtree(a, chosen_a);
+    let subtree_b = get_subtree(b, chosen_b);
+
+    // 4) replace them
+    let new_a = replace_subtree(a, chosen_a, subtree_b);
+    let new_b = replace_subtree(b, chosen_b, subtree_a);
+
+    (new_a, new_b)
+}