feat: Withholding Tax Certificate print format for Purchase Invoice

The Revenue Department's Sec. 50 bis form (approve_wh3_081156.pdf) as
the page background, with the invoice's data positioned in the form's
fields; two copies per certificate. Its AcroForm fields carry no Thai
font, so the PDF is not filled directly.

get_withholding_certificate(doc) resolves the payer and payee (13-digit
tax ID, one-line address), the certificate row from the category's
income_type, the P.N.D. return from the supplier type and income, and
the amounts in company currency with the tax in Thai words. Dates are
Buddhist Era.

default_thai_company/tax_withholding.py

@@ -11,8 +11,9 @@ from erpnext.accounts.doctype.tax_withholding_category.tax_withholding_category
 	normal_round,
 )
 from frappe import _
+from frappe.contacts.doctype.address.address import get_default_address
 from frappe.modules.utils import sync_customizations_for_doctype
-from frappe.utils import cint, flt
+from frappe.utils import cint, flt, fmt_money, getdate
 
 from default_thai_company.utils import money_in_words
 
@@ -389,3 +390,128 @@ def get_customer_withholding(doc):
 	else:
 		description = _("Withholding tax deducted by customer")
 	return flt(total, precision), description
+
+
+# Withholding tax certificate (หนังสือรับรองการหักภาษี ณ ที่จ่าย, Sec. 50 bis): the income
+# row the payment is reported on, from Tax Withholding Category.income_type (custom field).
+# Rows: 1 = Sec. 40(1), 2 = 40(2), 3 = 40(3), 4a = 40(4)(a), 4b = 40(4)(b) dividends
+# paid from profits taxed at the standard 20% rate (form row 4(b)(1)(1.3)), 5 = payments
+# withheld under Revenue Department orders issued under Sec. 3 tera, 6 = other (specified).
+CERTIFICATE_ROWS = {
+	"Salary and Wages - Sec. 40(1)": "1",
+	"Fees and Commissions - Sec. 40(2)": "2",
+	"Royalties - Sec. 40(3)": "3",
+	"Interest - Sec. 40(4)(a)": "4a",
+	"Dividends - Sec. 40(4)(b)": "4b",
+	"Sec. 3 Tera (Services, Rent, Contract Work etc.)": "5",
+}
+
+THAI_MONTHS = (
+	"มกราคม",
+	"กุมภาพันธ์",
+	"มีนาคม",
+	"เมษายน",
+	"พฤษภาคม",
+	"มิถุนายน",
+	"กรกฎาคม",
+	"สิงหาคม",
+	"กันยายน",
+	"ตุลาคม",
+	"พฤศจิกายน",
+	"ธันวาคม",
+)
+
+
+def pnd_form(row, supplier_type):
+	"""P.N.D. return the certificate row is filed on, as numbered on the form:
+	1 = ภ.ง.ด.1ก, 2 = ภ.ง.ด.2, 3 = ภ.ง.ด.3, 53 = ภ.ง.ด.53.
+
+	Juristic payees (Company; Partnership, taken as registered) file on
+	ภ.ง.ด.53 whatever the income; individuals by income type: salary on
+	ภ.ง.ด.1ก, Sec. 40(3)/(4) on ภ.ง.ด.2, the rest on ภ.ง.ด.3.
+	"""
+	if supplier_type != "Individual":
+		return "53"
+	if row == "1":
+		return "1"
+	if row in ("3", "4a", "4b"):
+		return "2"
+	return "3"
+
+
+def one_line_address(address_name):
+	"""Address in the order of the Thailand Address Template, on one line."""
+	if not address_name:
+		return None
+	address = frappe.get_cached_doc("Address", address_name)
+	parts = [
+		address.address_line1,
+		address.address_line2,
+		address.county,
+		address.city,
+		address.state,
+		address.pincode,
+	]
+	if address.country and address.country != "Thailand":
+		parts.append(address.country)
+	return " ".join(part.strip() for part in parts if part and part.strip())
+
+
+def tax_id_digits(tax_id):
+	"""The 13 digits of a Thai tax ID for the form's boxes, or None when it is not one."""
+	digits = "".join(ch for ch in (tax_id or "") if ch.isdigit())
+	return digits if len(digits) == 13 else None
+
+
+def get_withholding_certificate(doc):
+	"""Certificate data for a Purchase Invoice with Apply Tax Withholding Amount.
+
+	Amounts are in company currency; the form is Thai, so dates are Buddhist Era
+	and the tax in words is Thai whatever the print language.
+	"""
+	company = frappe.get_cached_doc("Company", doc.company)
+	currency = erpnext.get_company_currency(doc.company)
+	precision = doc.precision("base_grand_total")
+
+	category = (
+		frappe.get_cached_doc("Tax Withholding Category", doc.tax_withholding_category)
+		if doc.tax_withholding_category
+		else None
+	)
+	row = CERTIFICATE_ROWS.get(category.income_type if category else None, "6")
+	supplier_type = frappe.get_cached_value("Supplier", doc.supplier, "supplier_type")
+
+	amount = flt(doc.base_tax_withholding_net_total, precision)
+	tax = flt(
+		sum(abs(flt(t.base_tax_amount)) for t in doc.get("taxes") if cint(t.is_tax_withholding_account)),
+		precision,
+	)
+	posting_date = getdate(doc.posting_date)
+
+	return frappe._dict(
+		payer=frappe._dict(
+			name=company.company_name,
+			tax_id=company.tax_id,
+			tax_id_digits=tax_id_digits(company.tax_id),
+			address=one_line_address(get_default_address("Company", doc.company)),
+		),
+		payee=frappe._dict(
+			name=doc.supplier_name,
+			tax_id=doc.tax_id,
+			tax_id_digits=tax_id_digits(doc.tax_id),
+			address=one_line_address(doc.supplier_address or get_default_address("Supplier", doc.supplier)),
+		),
+		row=row,
+		# row 6 prints what was paid for; translations/th.csv carries the fixture categories
+		row_note=_(category.category_name) if row == "6" and category else None,
+		pnd=pnd_form(row, supplier_type),
+		date=f"{posting_date.day:02d}/{posting_date.month:02d}/{posting_date.year + 543}",
+		amount=fmt_money(amount, precision),
+		tax=fmt_money(tax, precision),
+		tax_in_words=money_in_words(tax, currency, lang="th"),
+		issued=frappe._dict(
+			day=posting_date.day,
+			month=THAI_MONTHS[posting_date.month - 1],
+			year=posting_date.year + 543,
+		),
+	)