Add standard deduction to federal tax calculator.

app/services/federal_tax_calculator.rb

@@ -14,9 +14,14 @@ def call
   attr_accessor :income
 
   def calculate
-    bracket = FederalTaxBracket.where("bottom_range_cents <= ?", income.fractional).order(:bottom_range_cents).last
-    taxable_at_bracket_rate = Money.new(income - bracket.bottom_range)
+    bracket = FederalTaxBracket.where("bottom_range_cents <= ?", taxable_income.fractional).order(:bottom_range_cents).last
+    taxable_at_bracket_rate = Money.new(taxable_income - bracket.bottom_range)
     rated = bracket.rate * taxable_at_bracket_rate
     rated + bracket.cumulative
   end
+
+  def taxable_income
+    # 2024 standard deduction = 13,850
+    @taxable_income ||= income - Money.new(13_850_00)
+  end
 end

spec/services/federal_tax_calculator_spec.rb

@@ -16,9 +16,10 @@
 
   it "calculates federal tax" do
     # salary_income = $50,000
+    # standard_deduction = $13,850
     # tax_brackets = 10% on first $1,000, 15% from $1,001 to $100,000, 25% from $100,001 to $500,000
 
-    expect(service.format).to eq("$7,449.85")
-    expect((salary_income.rate - service).format).to eq("$42,550.15")
+    expect(service.format).to eq("$5,372.35")
+    expect((salary_income.rate - service).format).to eq("$44,627.65")
   end
 end