Flat tax on gross income (#4867)

* Flat tax on gross income
Fixes #4830

* add positive gross income and neew param

* remove gross income bool

---------

Co-authored-by: Max Ghenis <mghenis@gmail.com>

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    added:
+    - Flat tax on gross income.

policyengine_us/parameters/gov/contrib/ubi_center/flat_tax/rate/gross_income.yaml

@@ -0,0 +1,7 @@
+description: Flat tax rate applied to federal gross income.
+values:
+  2010-01-01: 0
+metadata:
+  label: Flat tax rate on gross income
+  unit: /1
+  period: year

policyengine_us/tests/policy/baseline/gov/irs/income/taxable_income/adjusted_gross_income/positive_gross_income.yaml

@@ -0,0 +1,13 @@
+- name: Positive gross income
+  period: 2022
+  input:
+    irs_gross_income: 1_000
+  output:
+    positive_gross_income: 1_000
+
+- name: Negative gross income
+  period: 2022
+  input:
+    irs_gross_income: -1_000
+  output:
+    positive_gross_income: 0

policyengine_us/tests/policy/contrib/ubi_center/flat_tax.yaml

@@ -0,0 +1,9 @@
+- name: 10% flat tax on AGI and 20% flat tax on gross income
+  period: 2023
+  input:
+    gov.contrib.ubi_center.flat_tax.rate.agi: 0.1
+    gov.contrib.ubi_center.flat_tax.rate.gross_income: 0.2
+    positive_agi: 2_000
+    positive_gross_income: 5_000
+  output:
+    flat_tax: 1_200

policyengine_us/variables/contrib/ubi_center/flat_tax.py

@@ -6,10 +6,15 @@ class flat_tax(Variable):
     entity = TaxUnit
     label = "Flat tax"
     unit = USD
-    documentation = "Flat income tax on federal AGI."
+    documentation = "Flat income tax on federal AGI or gross income."
     definition_period = YEAR
 
     def formula(tax_unit, period, parameters):
+        p = parameters(period).gov.contrib.ubi_center.flat_tax.rate
+        # Gross income flat tax.
+        gross_income = add(tax_unit, period, ["positive_gross_income"])
+        gross_income_flat_tax = gross_income * p.gross_income
+        # AGI flat tax.
         agi = tax_unit("positive_agi", period)
-        p = parameters(period).gov.contrib.ubi_center.flat_tax
-        return p.rate * agi
+        agi_flat_tax = agi * p.agi
+        return gross_income_flat_tax + agi_flat_tax

policyengine_us/variables/gov/irs/income/taxable_income/adjusted_gross_income/positive_gross_income.py

@@ -0,0 +1,14 @@
+from policyengine_us.model_api import *
+
+
+class positive_gross_income(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "Positive Gross Income"
+    unit = USD
+    documentation = "Negative gross income values capped at zero"
+    definition_period = YEAR
+
+    def formula(tax_unit, period, parameters):
+        gross_income = add(tax_unit, period, ["irs_gross_income"])
+        return max_(gross_income, 0)