Fix SM2 interval calculation to match standard algorithm

Gemfile.lock

@@ -2,8 +2,8 @@ PATH
   remote: .
   specs:
     active_recall (2.0.2)
-      activerecord (>= 6.0, <= 7.2)
-      activesupport (>= 6.0, <= 7.2)
+      activerecord (>= 6.0, <= 8.0.0.beta1)
+      activesupport (>= 6.0, <= 8.0.0.beta1)
 
 GEM
   remote: https://rubygems.org/
@@ -248,7 +248,7 @@ PLATFORMS
 DEPENDENCIES
   active_recall!
   appraisal
-  rails (>= 6.0, <= 7.2)
+  rails (>= 6.0, <= 8.0.0.beta1)
   rake (>= 12.0)
   rdoc
   rspec (>= 3.0)

active_recall.gemspec

@@ -33,13 +33,13 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_development_dependency "appraisal"
-  spec.add_development_dependency "rails", ">= 6.0", "<= 7.2"
+  spec.add_development_dependency "rails", ">= 6.0", "< 9.0"
   spec.add_development_dependency "rake", ">= 12.0"
   spec.add_development_dependency "rdoc"
   spec.add_development_dependency "rspec", ">= 3.0"
   spec.add_development_dependency "sqlite3"
   spec.add_development_dependency "standard"
-  spec.add_runtime_dependency "activerecord", ">= 6.0", "<= 7.2"
-  spec.add_runtime_dependency "activesupport", ">= 6.0", "<= 7.2"
+  spec.add_runtime_dependency "activerecord", ">= 6.0", "< 9.0"
+  spec.add_runtime_dependency "activesupport", ">= 6.0", "< 9.0"
   spec.required_ruby_version = ">= 3.0"
 end

lib/active_recall/algorithms/sm2.rb

@@ -81,7 +81,7 @@ def update_repetition_and_interval
         when 2
           6
         else
-          (@interval || 1) * @easiness_factor
+          6 * (@easiness_factor ** (@box - 1))
         end
       else
         @box = 0
@@ -90,6 +90,7 @@ def update_repetition_and_interval
       end
     end
 
+
     def next_review
       @current_time + @interval.days
     end

spec/active_recall/algorithms/sm2_spec.rb

@@ -78,5 +78,30 @@
         expect { subject }.to raise_error
       end
     end
+
+    context "with different box values" do
+          [
+            { box: 1, expected_interval: 1 },
+            { box: 2, expected_interval: 6 },
+            { box: 3, expected_interval: 15 },
+            { box: 4, expected_interval: 37 },
+            { box: 5, expected_interval: 93 },
+          ].each do |test_case|
+            context "when box is #{test_case[:box]}" do
+              before do
+                params[:box] = test_case[:box]
+                params[:times_right] = test_case[:box] - 1
+              end
+
+              it "calculates the correct interval" do
+                result = subject
+                expect(result[:box]).to eq(test_case[:box] + 1)
+                expect(result[:next_review]).to be_within(1.day).of(current_time + test_case[:expected_interval].days)
+              end
+            end
+          end
+        end
+
+
   end
 end