fix: corrected banker's rounding

closes https://github.com/frappe/frappe/issues/19570

cypress/integration/rounding.js

@@ -4,7 +4,7 @@ context("Rounding behaviour", () => {
 		cy.visit("/app/");
 	});
 
-	it("Rounds floats accurately", () => {
+	it("Commercial Rounding", () => {
 		cy.window()
 			.its("flt")
 			.then((flt) => {
@@ -41,4 +41,64 @@ context("Rounding behaviour", () => {
 				expect(flt(-0.15, 1, null, rounding_method)).eq(-0.2);
 			});
 	});
+
+	it("Banker's Rounding", () => {
+		cy.window()
+			.its("flt")
+			.then((flt) => {
+				let rounding_method = "Banker's Rounding";
+
+				expect(flt("0.5", 0, null, rounding_method)).eq(0);
+				expect(flt("0.3", null, rounding_method)).eq(0.3);
+
+				expect(flt("1.5", 0, null, rounding_method)).eq(2);
+
+				// positive rounding to integers
+				expect(flt(0.4, 0, null, rounding_method)).eq(0);
+				expect(flt(0.5, 0, null, rounding_method)).eq(0);
+				expect(flt(1.455, 0, null, rounding_method)).eq(1);
+				expect(flt(1.5, 0, null, rounding_method)).eq(2);
+
+				// negative rounding to integers
+				expect(flt(-0.5, 0, null, rounding_method)).eq(0);
+				expect(flt(-1.5, 0, null, rounding_method)).eq(-2);
+
+				// negative precision i.e. round to nearest 10th
+				expect(flt(123, -1, null, rounding_method)).eq(120);
+				expect(flt(125, -1, null, rounding_method)).eq(120);
+				expect(flt(134.45, -1, null, rounding_method)).eq(130);
+				expect(flt(135, -1, null, rounding_method)).eq(140);
+
+				// positive multiple digit rounding
+				expect(flt(1.25, 1, null, rounding_method)).eq(1.2);
+				expect(flt(0.15, 1, null, rounding_method)).eq(0.2);
+				expect(flt(2.675, 2, null, rounding_method)).eq(2.68);
+				expect(flt(-2.675, 2, null, rounding_method)).eq(-2.68);
+
+				// negative multiple digit rounding
+				expect(flt(-1.25, 1, null, rounding_method)).eq(-1.2);
+				expect(flt(-0.15, 1, null, rounding_method)).eq(-0.2);
+
+				// Nearest number and not even (the default behaviour)
+				expect(flt(0.5, 0, null, rounding_method)).eq(0);
+				expect(flt(1.5, 0, null, rounding_method)).eq(2);
+				expect(flt(2.5, 0, null, rounding_method)).eq(2);
+				expect(flt(3.5, 0, null, rounding_method)).eq(4);
+
+				expect(flt(0.05, 1, null, rounding_method)).eq(0.0);
+				expect(flt(1.15, 1, null, rounding_method)).eq(1.2);
+				expect(flt(2.25, 1, null, rounding_method)).eq(2.2);
+				expect(flt(3.35, 1, null, rounding_method)).eq(3.4);
+
+				expect(flt(-0.5, 0, null, rounding_method)).eq(0);
+				expect(flt(-1.5, 0, null, rounding_method)).eq(-2);
+				expect(flt(-2.5, 0, null, rounding_method)).eq(-2);
+				expect(flt(-3.5, 0, null, rounding_method)).eq(-4);
+
+				expect(flt(-0.05, 1, null, rounding_method)).eq(0.0);
+				expect(flt(-1.15, 1, null, rounding_method)).eq(-1.2);
+				expect(flt(-2.25, 1, null, rounding_method)).eq(-2.2);
+				expect(flt(-3.35, 1, null, rounding_method)).eq(-3.4);
+			});
+	});
 });

frappe/core/doctype/system_settings/system_settings.json

@@ -527,7 +527,7 @@
    "fieldname": "rounding_method",
    "fieldtype": "Select",
    "label": "Rounding Method",
-   "options": "Banker's Rounding (legacy)\nCommercial Rounding"
+   "options": "Banker's Rounding (legacy)\nBanker's Rounding\nCommercial Rounding"
   }
  ],
  "icon": "fa fa-cog",

frappe/public/js/frappe/utils/number_format.js

@@ -188,6 +188,25 @@ function _round(num, precision, rounding_method) {
 		var r = !precision && f == 0.5 ? (i % 2 == 0 ? i : i + 1) : Math.round(n);
 		r = d ? r / m : r;
 		return is_negative ? -r : r;
+	} else if (rounding_method == "Banker's Rounding") {
+		precision = cint(precision);
+
+		let multiplier = Math.pow(10, precision);
+		num = Math.abs(num) * multiplier;
+
+		let floor_num = Math.floor(num);
+		let decimal_part = num - floor_num;
+
+		// For explanation of this method read python flt implementation notes.
+		let epsilon = 2.0 ** (Math.log2(Math.abs(num)) - 52.0);
+
+		if (Math.abs(decimal_part - 0.5) < epsilon) {
+			num = floor_num % 2 == 0 ? floor_num : floor_num + 1;
+		} else {
+			num = Math.round(num);
+		}
+		num = num / multiplier;
+		return is_negative ? -num : num;
 	} else if (rounding_method == "Commercial Rounding") {
 		if (num == 0) return 0.0;
 

frappe/tests/test_utils.py

@@ -1073,9 +1073,80 @@ class TestRounding(FrappeTestCase):
 		self.assertEqual(flt(-1.25, 1, rounding_method=rounding_method), -1.3)
 		self.assertEqual(flt(-0.15, 1, rounding_method=rounding_method), -0.2)
 
+		# Nearest number and not even (the default behaviour)
+		self.assertEqual(flt(0.5, 0, rounding_method=rounding_method), 1)
+		self.assertEqual(flt(1.5, 0, rounding_method=rounding_method), 2)
+		self.assertEqual(flt(2.5, 0, rounding_method=rounding_method), 3)
+		self.assertEqual(flt(3.5, 0, rounding_method=rounding_method), 4)
+
+		self.assertEqual(flt(0.05, 1, rounding_method=rounding_method), 0.1)
+		self.assertEqual(flt(1.15, 1, rounding_method=rounding_method), 1.2)
+		self.assertEqual(flt(2.25, 1, rounding_method=rounding_method), 2.3)
+		self.assertEqual(flt(3.35, 1, rounding_method=rounding_method), 3.4)
+
 	@change_settings("System Settings", {"rounding_method": "Commercial Rounding"})
 	@given(st.decimals(min_value=-1e8, max_value=1e8), st.integers(min_value=-2, max_value=4))
 	def test_normal_rounding_property(self, number, precision):
 		with localcontext() as ctx:
 			ctx.rounding = ROUND_HALF_UP
 			self.assertEqual(Decimal(str(flt(float(number), precision))), round(number, precision))
+
+	def test_bankers_rounding(self):
+		rounding_method = "Banker's Rounding"
+
+		self.assertEqual(flt("0.5", 0, rounding_method=rounding_method), 0)
+		self.assertEqual(flt("0.3", rounding_method=rounding_method), 0.3)
+
+		self.assertEqual(flt("1.5", 0, rounding_method=rounding_method), 2)
+
+		# positive rounding to integers
+		self.assertEqual(flt(0.4, 0, rounding_method=rounding_method), 0)
+		self.assertEqual(flt(0.5, 0, rounding_method=rounding_method), 0)
+		self.assertEqual(flt(1.455, 0, rounding_method=rounding_method), 1)
+		self.assertEqual(flt(1.5, 0, rounding_method=rounding_method), 2)
+
+		# negative rounding to integers
+		self.assertEqual(flt(-0.5, 0, rounding_method=rounding_method), 0)
+		self.assertEqual(flt(-1.5, 0, rounding_method=rounding_method), -2)
+
+		# negative precision i.e. round to nearest 10th
+		self.assertEqual(flt(123, -1, rounding_method=rounding_method), 120)
+		self.assertEqual(flt(125, -1, rounding_method=rounding_method), 120)
+		self.assertEqual(flt(134.45, -1, rounding_method=rounding_method), 130)
+		self.assertEqual(flt(135, -1, rounding_method=rounding_method), 140)
+
+		# positive multiple digit rounding
+		self.assertEqual(flt(1.25, 1, rounding_method=rounding_method), 1.2)
+		self.assertEqual(flt(0.15, 1, rounding_method=rounding_method), 0.2)
+		self.assertEqual(flt(2.675, 2, rounding_method=rounding_method), 2.68)
+		self.assertEqual(flt(-2.675, 2, rounding_method=rounding_method), -2.68)
+
+		# negative multiple digit rounding
+		self.assertEqual(flt(-1.25, 1, rounding_method=rounding_method), -1.2)
+		self.assertEqual(flt(-0.15, 1, rounding_method=rounding_method), -0.2)
+
+		# Nearest number and not even (the default behaviour)
+		self.assertEqual(flt(0.5, 0, rounding_method=rounding_method), 0)
+		self.assertEqual(flt(1.5, 0, rounding_method=rounding_method), 2)
+		self.assertEqual(flt(2.5, 0, rounding_method=rounding_method), 2)
+		self.assertEqual(flt(3.5, 0, rounding_method=rounding_method), 4)
+
+		self.assertEqual(flt(0.05, 1, rounding_method=rounding_method), 0.0)
+		self.assertEqual(flt(1.15, 1, rounding_method=rounding_method), 1.2)
+		self.assertEqual(flt(2.25, 1, rounding_method=rounding_method), 2.2)
+		self.assertEqual(flt(3.35, 1, rounding_method=rounding_method), 3.4)
+
+		self.assertEqual(flt(-0.5, 0, rounding_method=rounding_method), 0)
+		self.assertEqual(flt(-1.5, 0, rounding_method=rounding_method), -2)
+		self.assertEqual(flt(-2.5, 0, rounding_method=rounding_method), -2)
+		self.assertEqual(flt(-3.5, 0, rounding_method=rounding_method), -4)
+
+		self.assertEqual(flt(-0.05, 1, rounding_method=rounding_method), 0.0)
+		self.assertEqual(flt(-1.15, 1, rounding_method=rounding_method), -1.2)
+		self.assertEqual(flt(-2.25, 1, rounding_method=rounding_method), -2.2)
+		self.assertEqual(flt(-3.35, 1, rounding_method=rounding_method), -3.4)
+
+	@change_settings("System Settings", {"rounding_method": "Banker's Rounding"})
+	@given(st.decimals(min_value=-1e8, max_value=1e8), st.integers(min_value=-2, max_value=4))
+	def test_bankers_rounding_property(self, number, precision):
+		self.assertEqual(Decimal(str(flt(float(number), precision))), round(number, precision))

frappe/utils/data.py

@@ -1059,7 +1059,9 @@ def rounded(num, precision=0, rounding_method=None):
 	)
 
 	if rounding_method == "Banker's Rounding (legacy)":
-		return _round_half_even(num, precision)
+		return _bankers_rounding_legacy(num, precision)
+	elif rounding_method == "Banker's Rounding":
+		return _bankers_rounding(num, precision)
 	elif rounding_method == "Commercial Rounding":
 		return _round_away_from_zero(num, precision)
 	else:
@@ -1069,7 +1071,7 @@ def rounded(num, precision=0, rounding_method=None):
 		)
 
 
-def _round_half_even(num, precision):
+def _bankers_rounding_legacy(num, precision):
 	# avoid rounding errors
 	multiplier = 10**precision
 	num = round(num * multiplier if precision else num, 8)
@@ -1114,6 +1116,22 @@ def _round_away_from_zero(num, precision):
 	return round(num + math.copysign(epsilon, num), precision)
 
 
+def _bankers_rounding(num, precision):
+	multiplier = 10**precision
+	num = round(num * multiplier, 12)
+
+	floor_num = math.floor(num)
+	decimal_part = num - floor_num
+
+	epsilon = 2.0 ** (math.log(abs(num), 2) - 52.0)
+	if abs(decimal_part - 0.5) < epsilon:
+		num = floor_num if (floor_num % 2 == 0) else floor_num + 1
+	else:
+		num = round(num)
+
+	return num / multiplier
+
+
 def remainder(numerator: NumericType, denominator: NumericType, precision: int = 2) -> NumericType:
 	precision = cint(precision)
 	multiplier = 10**precision