From 29dd7ee6ffae1650f9b86df26eb9baffe0d5567c Mon Sep 17 00:00:00 2001
From: sherlockyyc <994986061@qq.com>
Date: Tue, 7 Jul 2020 11:04:03 +0800
Subject: [PATCH] v1

---
 .../search_methods/genetic_algorithm.py       | 45 +++++++++++++++++++
 .../improved_genetic_algorithm.py             |  3 ++
 2 files changed, 48 insertions(+)

diff --git a/textattack/search_methods/genetic_algorithm.py b/textattack/search_methods/genetic_algorithm.py
index c3fda273..ef1baf9e 100644
--- a/textattack/search_methods/genetic_algorithm.py
+++ b/textattack/search_methods/genetic_algorithm.py
@@ -36,16 +36,61 @@ class GeneticAlgorithm(SearchMethod):
         temp=0.3,
         give_up_if_no_improvement=False,
         max_crossover_retries=20,
+        improved_genetic_algorithm = False,
+        max_replace_time_per_index = 5,
     ):
         self.max_iters = max_iters
         self.pop_size = pop_size
         self.temp = temp
         self.give_up_if_no_improvement = give_up_if_no_improvement
         self.max_crossover_retries = max_crossover_retries
+        self.improved_genetic_algorithm = improved_genetic_algorithm
+        # The maximum times words at the same index can be replaced in improved genetic algorithm
+        self.max_replace_time_per_index = max_replace_time_per_index
 
         # internal flag to indicate if search should end immediately
         self._search_over = False
 
+    def _replace_at_index(self, pop_member, idx, original_result):
+        """
+        Select the best replacement for word at position (idx) 
+        in (pop_member) to maximize score.
+
+        Args:
+            pop_member: The population member being perturbed.
+            idx: The index at which to replace a word.
+
+        Returns:
+            Whether a replacement which increased the score was found.
+        """
+        transformations = self.get_transformations(
+            pop_member.attacked_text,
+            original_text=original_result.attacked_text,
+            indices_to_modify=[idx],
+        )
+        if not len(transformations):
+            return False
+        new_results, self._search_over = self.get_goal_results(
+            transformations
+        )
+        if self._search_over:
+            return False
+        diff_scores = (
+            torch.Tensor([r.score for r in new_results]) - pop_member.result.score
+        )
+        if len(diff_scores) and diff_scores.max() > 0:
+            idx_with_max_score = diff_scores.argmax()
+            pop_member.attacked_text = transformations[idx_with_max_score]
+            # For genetic algorithms, the word has been replaced cannot be modified again. 
+            # For improved genetic algorithm, it allows to substitute words at the same index multiple times but not more than `max_replace_time_per_index`.
+            if self.improved_genetic_algorithm:
+                pop_member.num_candidates_per_word[idx_with_max_score] -= -1
+            else:
+                pop_member.num_candidates_per_word[idx_with_max_score] = 0
+            pop_member.results = new_results[idx_with_max_score]
+            return True
+        return False
+
     def _perturb(self, pop_member, original_result):
         """
         Replaces a word in pop_member that has not been modified in place.
diff --git a/textattack/search_methods/improved_genetic_algorithm.py b/textattack/search_methods/improved_genetic_algorithm.py
index dfc911aa..af767bed 100644
--- a/textattack/search_methods/improved_genetic_algorithm.py
+++ b/textattack/search_methods/improved_genetic_algorithm.py
@@ -41,10 +41,13 @@ class ImprovedGeneticAlgorithm(SearchMethod):
         self.max_replaced_times = max_replaced_times
         self.give_up_if_no_improvement = give_up_if_no_improvement
         self.max_crossover_retries = max_crossover_retries
+        # flag to indicate if it is the improved genetic algorithm proposed in Natural Language Adversarial Attacks and Defenses in Word Level by Wang et al.
 
         # internal flag to indicate if search should end immediately
         self._search_over = False
 
+
+
     def _replace_at_index(self, pop_member, idx):
         """
         Select the best replacement for word at position (idx)