austax: Port full income tax implementation from Python

libdrcr/plugins/austax/calc.luau

@@ -20,6 +20,7 @@ local tax_tables = require('../austax/tax_tables')
 
 local calc = {}
 
+-- Get the amount of base income tax
 function calc.base_income_tax(net_taxable: number, context: libdrcr.ReportingContext): number
 	local year, _, _ = libdrcr.parse_date(context.eofy_date)
 	local base_tax_table = tax_tables.base_tax[year]
@@ -36,11 +37,93 @@ function calc.base_income_tax(net_taxable: number, context: libdrcr.ReportingCon
 		end
 		
 		if net_taxable <= upper_limit then
-			return flat_amount + marginal_rate * (net_taxable - lower_limit)
+			return flat_amount + math.floor(marginal_rate * (net_taxable - lower_limit))
 		end
 	end
 	
 	error('Taxable income not within any tax bracket')
 end
 
+-- Get the amount of low income tax offset
+-- https://www.ato.gov.au/forms-and-instructions/low-and-middle-income-earner-tax-offsets
+-- https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/cth/consol_act/itaa1997240/s61.115.html
+function calc.lito(net_taxable: number, tax_total: number, context: libdrcr.ReportingContext): number
+	if net_taxable <= 37500 * (10 ^ context.dps) then
+		-- LITO is non-refundable
+		-- FIXME: This will not work if we implement multiple non-refundable tax offsets
+		if tax_total <= 700 * (10 ^ context.dps) then
+			return tax_total
+		else
+			return 700 * (10 ^ context.dps)
+		end
+	elseif net_taxable <= 45000 * (10 ^ context.dps) then
+		return 700 * (10 ^ context.dps) - math.floor(0.05 * (net_taxable - 37500 * (10 ^ context.dps)))
+	elseif net_taxable <= 66667 * (10 ^ context.dps) then
+		return 325 * (10 ^ context.dps) - math.floor(0.015 * (net_taxable - 45000 * (10 ^ context.dps)))
+	else
+		return 0
+	end
+end
+
+-- Get the amount of Medicare levy
+function calc.medicare_levy(net_taxable: number, context: libdrcr.ReportingContext): number
+	local year, _, _ = libdrcr.parse_date(context.eofy_date)
+	local threshold_table = tax_tables.medicare_levy_threshold[year]
+	local lower_threshold = threshold_table[1] * (10 ^ context.dps)
+	local upper_threshold = threshold_table[2] * (10 ^ context.dps)
+	
+	if net_taxable < lower_threshold then
+		return 0
+	elseif net_taxable < upper_threshold then
+		-- Medicare levy is 10% of the amount above the lower threshold
+		return math.floor((net_taxable - lower_threshold) * 0.1)
+	else
+		-- Normal Medicare levy
+		return math.floor(net_taxable * 0.02)
+	end
+end
+
+-- Get the amount of Medicare levy surcharge
+function calc.medicare_levy_surcharge(net_taxable: number, rfb_grossedup: number, context: libdrcr.ReportingContext): number
+	local mls_income = net_taxable + rfb_grossedup
+	
+	local year, _, _ = libdrcr.parse_date(context.eofy_date)
+	local mls_table = tax_tables.medicare_levy_surcharge_single[year]
+	
+	for _, row in ipairs(mls_table) do
+		local upper_limit = row[1] * (10 ^ context.dps)
+		local rate = row[2]
+		
+		if mls_income <= upper_limit then
+			return math.floor(rate * mls_income)
+		end
+	end
+	
+	error('MLS income not within any MLS bracket')
+end
+
+-- Calculate the grossed-up reportable fringe benefit
+function calc.rfb_grossup(rfb_taxable: number, context: libdrcr.ReportingContext): number
+	return math.floor(rfb_taxable * tax_tables.fbt_grossup)
+end
+
+-- Get the amount of mandatory study loan repayment
+function calc.study_loan_repayment(net_taxable: number, rfb_grossedup: number, context: libdrcr.ReportingContext): number
+	local repayment_income = net_taxable + rfb_grossedup
+	
+	local year, _, _ = libdrcr.parse_date(context.eofy_date)
+	local repayment_table = tax_tables.study_loan_repayment_rates[year]
+	
+	for _, row in ipairs(repayment_table) do
+		local upper_limit = row[1] * (10 ^ context.dps)
+		local rate = row[2]
+		
+		if repayment_income < upper_limit then
+			return math.floor(rate * repayment_income)
+		end
+	end
+	
+	error('HELP repayment income not within any repayment bracket')
+end
+
 return calc

libdrcr/plugins/austax/reporting.luau

@@ -111,7 +111,7 @@ function reporting.CalculateIncomeTax.execute(args, context, kinds_for_account,
 			visible = true,
 			link = nil,
 			heading = true,
-			bordered = true,
+			bordered = false,
 		}})
 		table.insert(report.entries, { Section = section })
 		table.insert(report.entries, 'Spacer')
@@ -159,7 +159,7 @@ function reporting.CalculateIncomeTax.execute(args, context, kinds_for_account,
 			visible = true,
 			link = nil,
 			heading = true,
-			bordered = true,
+			bordered = false,
 		}})
 		table.insert(report.entries, { Section = section })
 		table.insert(report.entries, 'Spacer')
@@ -202,8 +202,45 @@ function reporting.CalculateIncomeTax.execute(args, context, kinds_for_account,
 		bordered = false,
 	}})
 	
+	-- Medicare levy row
+	local tax_ml = calc.medicare_levy(net_taxable, context)
+	if tax_ml ~= 0 then
+		table.insert(report.entries, { Row = {
+			text = 'Medicare levy',
+			quantity = {tax_ml},
+			id = 'tax_ml',
+			visible = true,
+			link = nil,
+			heading = false,
+			bordered = false,
+		}})
+	end
+	
+	-- Precompute RFB amount as this is required for MLS
+	local rfb_taxable = 0
+	for account, kinds in pairs(kinds_for_account) do
+		if libdrcr.arr_contains(kinds, 'austax.rfb') then
+			rfb_taxable -= balances[account] or 0  -- Invert as income = credit balances
+		end
+	end
+	local rfb_grossedup = calc.rfb_grossup(rfb_taxable, context)
+	
+	-- Medicare levy surcharge row
+	local tax_mls = calc.medicare_levy_surcharge(net_taxable, rfb_grossedup, context)
+	if tax_mls ~= 0 then
+		table.insert(report.entries, { Row = {
+			text = 'Medicare levy surcharge',
+			quantity = {tax_mls},
+			id = 'tax_mls',
+			visible = true,
+			link = nil,
+			heading = false,
+			bordered = false,
+		}})
+	end
+	
 	-- Total income tax row
-	local tax_total = tax_base
+	local tax_total = tax_base + tax_ml + tax_mls
 	table.insert(report.entries, { Row = {
 		text = 'Total income tax',
 		quantity = {tax_total},
@@ -213,10 +250,126 @@ function reporting.CalculateIncomeTax.execute(args, context, kinds_for_account,
 		heading = true,
 		bordered = true,
 	}})
+	table.insert(report.entries, 'Spacer')
 	
-	-- Generate income tax transaction
+	-- Low income tax offset row
+	local offset_lito = calc.lito(net_taxable, tax_total, context)
+	if offset_lito ~= 0 then
+		table.insert(report.entries, { Row = {
+			text = 'Low income tax offset',
+			quantity = {offset_lito},
+			id = nil,
+			visible = true,
+			link = nil,
+			heading = false,
+			bordered = false,
+		}})
+	end
+	
+	-- Total tax offsets row
+	local offset_total = offset_lito
+	if offset_total ~= 0 then
+		table.insert(report.entries, { Row = {
+			text = 'Total tax offsets',
+			quantity = {offset_total},
+			id = nil,
+			visible = true,
+			link = nil,
+			heading = true,
+			bordered = false,
+		}})
+		table.insert(report.entries, 'Spacer')
+	end
+	
+	-- Calculate mandatory study loan repayment
+	local study_loan_repayment = calc.study_loan_repayment(net_taxable, rfb_grossedup, context)
+	
+	-- Mandatory study loan repayment section
+	if study_loan_repayment ~= 0 then
+		-- Taxable value of reportable fringe benefits row
+		if rfb_taxable ~= 0 then
+			table.insert(report.entries, { Row = {
+				text = 'Taxable value of reportable fringe benefits',
+				quantity = {rfb_taxable},
+				id = 'rfb_taxable',
+				visible = true,
+				link = nil,
+				heading = false,
+				bordered = false,
+			}})
+		end
+		
+		-- Grossed-up value row
+		if rfb_grossedup ~= 0 then
+			table.insert(report.entries, { Row = {
+				text = 'Grossed-up value',
+				quantity = {rfb_grossedup},
+				id = 'rfb_grossedup',
+				visible = true,
+				link = nil,
+				heading = false,
+				bordered = false,
+			}})
+		end
+		
+		-- Mandatory study loan repayment row
+		table.insert(report.entries, { Row = {
+			text = 'Mandatory study loan repayment',
+			quantity = {study_loan_repayment},
+			id = 'study_loan_repayment',
+			visible = true,
+			link = nil,
+			heading = true,
+			bordered = false,
+		}})
+		table.insert(report.entries, 'Spacer')
+	end
+	
+	-- Add PAYGW entries
+	local total_paygw = 0
+	local entries = entries_for_kind('austax.paygw', false, balances, kinds_for_account)
+	
+	if #entries ~= 0 then
+		local section: libdrcr.Section = {
+			text = 'PAYG withheld amounts',
+			id = nil,
+			visible = true,
+			entries = entries,
+		}
+		table.insert(report.entries, { Section = section })
+		total_paygw = math.floor(entries_subtotal(entries) / 100) * 100
+	end
+	
+	-- Total PAYGW row
+	if total_paygw ~= 0 then
+		table.insert(report.entries, { Row = {
+			text = 'Total withheld amounts',
+			quantity = {total_paygw},
+			id = 'total_paygw',
+			visible = true,
+			link = nil,
+			heading = true,
+			bordered = false,
+		}})
+		table.insert(report.entries, 'Spacer')
+	end
+	
+	-- ATO liability row
+	local ato_payable = tax_total - offset_total - total_paygw + study_loan_repayment
+	table.insert(report.entries, { Row = {
+		text = 'ATO liability payable (refundable)',
+		quantity = {ato_payable},
+		id = 'ato_payable',
+		visible = true,
+		link = nil,
+		heading = true,
+		bordered = true,
+	}})
+	
+	-- Generate income tax transactions
 	local transactions: {libdrcr.Transaction} = {
 		{
+			-- # Estimated tax payable
 			id = nil,
 			dt = libdrcr.date_to_dt(context.eofy_date),
 			description = 'Estimated income tax',
@@ -226,23 +379,83 @@ function reporting.CalculateIncomeTax.execute(args, context, kinds_for_account,
 					transaction_id = nil,
 					description = nil,
 					account = 'Income Tax',
-					quantity = tax_total,
+					quantity = (tax_total - offset_total),
 					commodity = context.reporting_commodity,
-					quantity_ascost = tax_total,
+					quantity_ascost = (tax_total - offset_total),
 				},
 				{
 					id = nil,
 					transaction_id = nil,
 					description = nil,
 					account = 'Income Tax Control',
-					quantity = -tax_total,
+					quantity = -(tax_total - offset_total),
 					commodity = context.reporting_commodity,
-					quantity_ascost = -tax_total,
+					quantity_ascost = -(tax_total - offset_total),
-				}
+				},
+			},
+		},
+		{
+			-- Mandatory study loan repayment
+			id = nil,
+			dt = libdrcr.date_to_dt(context.eofy_date),
+			description = 'Mandatory study loan repayment payable',
+			postings = {
+				{
+					id = nil,
+					transaction_id = nil,
+					description = nil,
+					account = 'HELP',
+					quantity = study_loan_repayment,
+					commodity = context.reporting_commodity,
+					quantity_ascost = study_loan_repayment,
+				},
+				{
+					id = nil,
+					transaction_id = nil,
+					description = nil,
+					account = 'Income Tax Control',
+					quantity = -study_loan_repayment,
+					commodity = context.reporting_commodity,
+					quantity_ascost = -study_loan_repayment,
+				},
 			},
 		}
 	}
 	
+	-- Transfer PAYGW balances to Income Tax Control
+	for account, kinds in pairs(kinds_for_account) do
+		if libdrcr.arr_contains(kinds, 'austax.paygw') then
+			local balance = balances[account] or 0
+			if balance ~= 0 then
+				table.insert(transactions, {
+					id = nil,
+					dt = libdrcr.date_to_dt(context.eofy_date),
+					description = 'PAYG withheld amounts',
+					postings = {
+						{
+							id = nil,
+							transaction_id = nil,
+							description = nil,
+							account = 'Income Tax Control',
+							quantity = balance,
+							commodity = context.reporting_commodity,
+							quantity_ascost = balance,
+						},
+						{
+							id = nil,
+							transaction_id = nil,
+							description = nil,
+							account = account,
+							quantity = -balance,
+							commodity = context.reporting_commodity,
+							quantity_ascost = -balance,
+						},
+					},
+				})
+			end
+		end
+	end
+	
 	return {
 		[{ name = 'CalculateIncomeTax', kind = 'Transactions', args = 'VoidArgs' }] = {
 			Transactions = {

libdrcr/plugins/austax/tax_tables.luau

@@ -15,10 +15,14 @@
 --  You should have received a copy of the GNU Affero General Public License
 --  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
+local tax_tables = {}
+
 -- Base income tax
 -- https://www.ato.gov.au/rates/individual-income-tax-rates/
+-- https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/cth/consol_act/itra1986174/sch7.html
+-- https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/cth/consol_act/itra1986174/s3.html (tax-free threshold)
 -- Maps each financial year to list of (upper limit (INclusive), flat amount, marginal rate)
-local base_tax = {
+tax_tables.base_tax = {
 	[2025] = {
 		{18200, 0, 0},
 		{45000, 0, 0.16},
@@ -42,8 +46,114 @@ local base_tax = {
 	}
 }
 
-local tax_tables = {
+-- FBT type 1 gross-up factor
-	base_tax = base_tax,
+-- https://www.ato.gov.au/rates/fbt/#GrossupratesforFBT
+-- https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/cth/consol_act/fbtaa1986312/s5b.html
+tax_tables.fbt_grossup = 2.0802
+
+-- Medicare levy thresholds
+-- https://www.ato.gov.au/Individuals/Medicare-and-private-health-insurance/Medicare-levy/Medicare-levy-reduction/Medicare-levy-reduction-for-low-income-earners/
+-- https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/cth/consol_act/mla1986131/s3.html
+-- Maps each financial year to list of (lower threshold, upper threshold)
+tax_tables.medicare_levy_threshold = {
+	[2025] = {27222, 34027},
+	[2024] = {26000, 32500},
+	[2023] = {24276, 30345},
+	[2022] = {23365, 29207}
+}
+
+-- Medicare levy surcharge rates (singles)
+-- https://www.ato.gov.au/individuals-and-families/medicare-and-private-health-insurance/medicare-levy-surcharge/medicare-levy-surcharge-income-thresholds-and-rates
+-- https://www.austlii.edu.au/cgi-bin/viewdb/au/legis/cth/consol_act/mla1986131/s8b.html
+-- https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/cth/consol_act/phia2007248/s22.35.html
+-- https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/cth/consol_act/phia2007248/s22.45.html
+-- Maps each financial year to list of (upper limit (INclusive), MLS rate)
+-- FIXME: Only supports singles
+tax_tables.medicare_levy_surcharge_single = {
+	[2025] = {
+		{97000, 0},
+		{113000, 0.01},
+		{151000, 0.0125},
+		{math.huge, 0.015}
+	},
+	[2024] = {
+		{93000, 0},
+		{108000, 0.01},
+		{144000, 0.0125},
+		{math.huge, 0.015}
+	}
+}
+
+-- Study and training loan (HELP, etc.) repayment thresholds and rates
+-- https://www.ato.gov.au/Rates/HELP,-TSL-and-SFSS-repayment-thresholds-and-rates/
+-- https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/cth/consol_act/hesa2003271/s154.20.html
+-- https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/cth/consol_act/hesa2003271/s154.25.html
+-- https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/cth/consol_act/hesa2003271/s154.30.html
+-- Maps each financial year to list of (upper limit (EXclusive), repayment rate)
+tax_tables.study_loan_repayment_rates = {
+	[2025] = {
+		{54435, 0},
+		{62851, 0.01},
+		{66621, 0.02},
+		{70619, 0.025},
+		{74856, 0.03},
+		{79347, 0.035},
+		{84108, 0.04},
+		{89155, 0.045},
+		{94504, 0.05},
+		{100175, 0.055},
+		{106186, 0.06},
+		{112557, 0.065},
+		{119310, 0.07},
+		{126468, 0.075},
+		{134057, 0.08},
+		{142101, 0.085},
+		{150627, 0.09},
+		{159664, 0.095},
+		{math.huge, 0.1}
+	},
+	[2024] = {
+		{51550, 0},
+		{59519, 0.01},
+		{63090, 0.02},
+		{66876, 0.025},
+		{70889, 0.03},
+		{75141, 0.035},
+		{79650, 0.04},
+		{84430, 0.045},
+		{89495, 0.05},
+		{94866, 0.055},
+		{100558, 0.06},
+		{106591, 0.065},
+		{112986, 0.07},
+		{119765, 0.075},
+		{126951, 0.08},
+		{134569, 0.085},
+		{142643, 0.09},
+		{151201, 0.095},
+		{math.huge, 0.1}
+	},
+	[2023] = {
+		{48361, 0},
+		{55837, 0.01},
+		{59187, 0.02},
+		{62739, 0.025},
+		{66503, 0.03},
+		{70493, 0.035},
+		{74723, 0.04},
+		{79207, 0.045},
+		{83959, 0.05},
+		{88997, 0.055},
+		{94337, 0.06},
+		{99997, 0.065},
+		{105997, 0.07},
+		{112356, 0.075},
+		{119098, 0.08},
+		{126244, 0.085},
+		{133819, 0.09},
+		{141848, 0.095},
+		{math.huge, 0.1}
+	}
 }
 
 return tax_tables

libdrcr/src/lib.rs

@@ -13,6 +13,4 @@ pub type QuantityInt = i64;
 // Magic strings
 // TODO: Make this configurable
 pub const CURRENT_YEAR_EARNINGS: &'static str = "Current Year Earnings";
-pub const INCOME_TAX: &'static str = "Income Tax";
-pub const INCOME_TAX_CONTROL: &'static str = "Income Tax Control";
 pub const RETAINED_EARNINGS: &'static str = "Retained Earnings";