Unify calculation so we can remove case statement

app/models/case_log.rb

@@ -40,17 +40,7 @@ class CaseLog < ApplicationRecord
   RENT_TYPE_MAPPING_LABELS = { 1 => "Social Rent", 2 => "Affordable Rent", 3 => "Intermediate Rent" }.freeze
   HAS_BENEFITS_OPTIONS = [0, 1, 2, 3].freeze
   STATUS = { "not_started" => 0, "in_progress" => 1, "completed" => 2 }.freeze
-  RENT_AND_CHARGES_PERIOD_LABEL = {
-    0 => "Every 2 weeks",
-    1 => "Every 4 weeks",
-    2 => "Every calendar month",
-    3 => "Weekly for 50 weeks",
-    4 => "Weekly for 49 weeks",
-    5 => "Weekly for 48 weeks",
-    6 => "Weekly for 47 weeks",
-    7 => "Weekly for 46 weeks",
-    8 => "Weekly for 52 weeks",
-  }.freeze
+  NUM_OF_WEEKS_FROM_PERIOD = { 0 => 26, 1 => 13, 2 => 12, 3 => 50, 4 => 49, 5 => 48, 6 => 47, 7 => 46, 8 => 52 }.freeze
   enum status: STATUS
 
   def form
@@ -97,28 +87,10 @@ class CaseLog < ApplicationRecord
   end
 
   def weekly_value(field_value)
-    return unless field_value && period
-
-    case RENT_AND_CHARGES_PERIOD_LABEL[period]
-    when "Every 2 weeks"
-      field_value / 2
-    when "Every 4 weeks"
-      field_value / 4
-    when "Every calendar month"
-      field_value * 12 / 52
-    when "Weekly for 50 weeks"
-      field_value / 52 * 50
-    when "Weekly for 49 weeks"
-      field_value / 52 * 49
-    when "Weekly for 48 weeks"
-      field_value / 52 * 48
-    when "Weekly for 47 weeks"
-      field_value / 52 * 47
-    when "Weekly for 46 weeks"
-      field_value / 52 * 46
-    when "Weekly for 52 weeks"
-      field_value
-    end
+    num_of_weeks = NUM_OF_WEEKS_FROM_PERIOD[period]
+    return unless field_value && num_of_weeks
+
+    field_value / 52 * num_of_weeks
   end
 
   def applicable_income_range