From 944bef5f91e6a509a496111259b61d79d7df3e31 Mon Sep 17 00:00:00 2001
From: Rutherther <rutherther@ditigal.xyz>
Date: Sat, 1 Nov 2025 17:17:49 +0100
Subject: [PATCH] feat(tsp): add nearest neighbor initializer

---
 codes/tsp_hw01/src/initializers.rs | 147 ++++++++++++++++++++++++++++-
 codes/tsp_hw01/src/tsp.rs          |   6 +-
 2 files changed, 148 insertions(+), 5 deletions(-)

diff --git a/codes/tsp_hw01/src/initializers.rs b/codes/tsp_hw01/src/initializers.rs
index 2875a2013a9312237ff62a3ef6729903a69ef8a3..995af405ba297369e24878550732806e8ce5152d 100644
--- a/codes/tsp_hw01/src/initializers.rs
+++ b/codes/tsp_hw01/src/initializers.rs
@@ -1,8 +1,8 @@
-use std::marker::PhantomData;
+use std::{cmp::Ordering, marker::PhantomData};
 use nalgebra::{allocator::Allocator, DefaultAllocator, Dim, OVector, U1};
-use rand::{prelude::SliceRandom, RngCore};
+use rand::{prelude::SliceRandom, Rng, RngCore};
 use eoa_lib::initializer::Initializer;
-use crate::tsp::NodePermutation;
+use crate::tsp::{NodePermutation, TSPInstance};
 
 pub struct TSPRandomInitializer<D>
 where
@@ -36,3 +36,144 @@ where
         NodePermutation { permutation: indices }
     }
 }
+
+pub struct NearestNeighborInitializer<'a, D: Dim>
+where
+    DefaultAllocator: nalgebra::allocator::Allocator<D>,
+    DefaultAllocator: nalgebra::allocator::Allocator<D, D>
+{
+    instance: &'a TSPInstance<D>,
+    neighbors_order: Vec<Vec<usize>>,
+    _phantom: PhantomData<D>,
+}
+
+impl<'a, D: Dim> NearestNeighborInitializer<'a, D>
+where
+    DefaultAllocator: nalgebra::allocator::Allocator<D>,
+    DefaultAllocator: nalgebra::allocator::Allocator<D, D>
+{
+    pub fn new(instance: &'a TSPInstance<D>) -> Self {
+        let mut neighbors_order = vec![
+            (0..instance.cities.len()).collect::<Vec<_>>();
+            instance.cities.len()
+        ];
+
+        for node in 0..instance.cities.len() {
+            let neighbor_order = &mut neighbors_order[node];
+            neighbor_order.sort_by(
+                |&neighbor1, &neighbor2| instance.distances(node, neighbor1)
+                    .partial_cmp(&instance.distances(node, neighbor2))
+                    .unwrap_or(Ordering::Less)
+            );
+        }
+
+        Self {
+            neighbors_order,
+            instance,
+            _phantom: PhantomData
+        }
+    }
+
+    fn initialize_from(
+        &self,
+        size: D,
+        index: usize,
+        p_choose_second: f64,
+        rng: &mut dyn RngCore
+    ) -> NodePermutation<D> {
+        let mut used_neighbors = vec![false; size.value()];
+        let mut individual =
+            OVector::zeros_generic(size, U1);
+
+        let mut current_node = index;
+        individual[0] = current_node;
+        used_neighbors[current_node] = true;
+
+        for i in 1..size.value() {
+            let neighbor_order = &self.neighbors_order[current_node];
+
+            let mut next_node = None;
+            for &neighbor in neighbor_order {
+                if !used_neighbors[neighbor] {
+                    next_node = Some(neighbor);
+
+                    // Do not choose third, ...
+                    if next_node.is_some() {
+                        break;
+                    }
+
+                    // Possibility to choose second nearest neighbor
+                    if !rng.random_bool(p_choose_second) {
+                        break;
+                    }
+                }
+            }
+
+            current_node = next_node.unwrap();
+            used_neighbors[current_node] = true;
+            individual[i] = current_node;
+        }
+
+        NodePermutation { permutation: individual }
+    }
+}
+
+impl<'a, D: Dim> Initializer<D, NodePermutation<D>> for NearestNeighborInitializer<'a, D>
+where
+    DefaultAllocator: nalgebra::allocator::Allocator<D>,
+    DefaultAllocator: nalgebra::allocator::Allocator<D, D>
+{
+    fn initialize_single(&self, size: D, rng: &mut dyn RngCore) -> NodePermutation<D> {
+        let starting_node = rng.random_range(0..size.value());
+        self.initialize_from(size, starting_node, rng.random_range(0.0..=0.7), rng)
+    }
+
+    fn initialize(
+        &self,
+        size: D,
+        mut count: usize,
+        rng: &mut dyn RngCore
+    ) -> Vec<NodePermutation<D>> {
+        let mut population = Vec::with_capacity(count);
+
+        // 1. Initialize from all starting nodes with nearest (p_choose_second == 0)
+        for i in 0..size.value() {
+            population.push(self.initialize_from(
+                size,
+                i,
+                0.0,
+                rng
+            ));
+
+            count -= 1;
+
+            if count == 0 {
+                return population;
+            }
+        }
+
+        // 2. Initialize from all starting nodes with second nearest (p_choose_second == 1)
+        for i in 0..size.value() {
+            population.push(self.initialize_from(
+                size,
+                i,
+                1.0,
+                rng
+            ));
+
+            count -= 1;
+
+            if count == 0 {
+                return population;
+            }
+        }
+
+        // 3. Initialize randomly with random p_choose_second (call initialize_single)
+        for i in 0..count {
+            population.push(self.initialize_single(size, rng))
+        }
+
+
+        population
+    }
+}
diff --git a/codes/tsp_hw01/src/tsp.rs b/codes/tsp_hw01/src/tsp.rs
index a639f900c60774ab3c32615f5b187989f6dffb29..13d9fa4218f0b0ee81cdbcd67f5c80b2f2d1f5e2 100644
--- a/codes/tsp_hw01/src/tsp.rs
+++ b/codes/tsp_hw01/src/tsp.rs
@@ -27,8 +27,8 @@ where
     D: Dim,
     DefaultAllocator: Allocator<D, D>
 {
-    cities: Vec<TSPCity>,
-    distances: OMatrix<f64, D, D>
+    pub cities: Vec<TSPCity>,
+    pub distances: OMatrix<f64, D, D>
 }
 
 impl TSPInstance<Dyn>
@@ -199,6 +199,8 @@ where
     type Err = Infallible;
 
     fn fit(self: &Self, inp: &Self::In) -> Result<Self::Out, Self::Err> {
+        assert_eq!(inp.permutation.len(), self.cities.len());
+        assert!(TSPInstance::verify_solution(inp));
         Ok(self.solution_cost(inp))
     }
 }