Utils: SI: fix rounding problems (#895)

* Utils: SI: fix rounding problems

999.999 would give 1000.00 instead of 1.00k

* Tests: add Utils: SI tests

Utils.py

@@ -504,11 +504,12 @@ def chaining_prefix(index: int, labels: typing.Tuple[str]) -> str:
 
 
 # noinspection PyPep8Naming
-def format_SI_prefix(value, power=1000, power_labels=('', 'k', 'M', 'G', 'T', "P", "E", "Z", "Y")) -> str:
+def format_SI_prefix(value, power=1000, power_labels=("", "k", "M", "G", "T", "P", "E", "Z", "Y")) -> str:
     """Formats a value into a value + metric/si prefix. More info at https://en.wikipedia.org/wiki/Metric_prefix"""
     n = 0
     value = decimal.Decimal(value)
-    while value >= power:
+    limit = power - decimal.Decimal("0.005")
+    while value >= limit:
         value /= power
         n += 1
 

test/utils/TestSIPrefix.py

@@ -0,0 +1,44 @@
+# Tests for SI prefix in Utils.py
+
+import unittest
+from decimal import Decimal
+from Utils import format_SI_prefix
+
+
+class TestGenerateMain(unittest.TestCase):
+    """This tests SI prefix formatting in Utils.py"""
+    def assertEqual(self, first, second, msg=None):
+        # we strip spaces everywhere because that is an undefined implementation detail
+        super().assertEqual(first.replace(" ", ""), second.replace(" ", ""), msg)
+
+    def test_rounding(self):
+        # we don't care if float(999.995) would fail due to error in precision
+        self.assertEqual(format_SI_prefix(999.999), "1.00k")
+        self.assertEqual(format_SI_prefix(1000.001), "1.00k")
+        self.assertEqual(format_SI_prefix(Decimal("999.995")), "1.00k")
+        self.assertEqual(format_SI_prefix(Decimal("1000.004")), "1.00k")
+
+    def test_letters(self):
+        self.assertEqual(format_SI_prefix(0e0), "0.00")
+        self.assertEqual(format_SI_prefix(1e3), "1.00k")
+        self.assertEqual(format_SI_prefix(2e6), "2.00M")
+        self.assertEqual(format_SI_prefix(3e9), "3.00G")
+        self.assertEqual(format_SI_prefix(4e12), "4.00T")
+        self.assertEqual(format_SI_prefix(5e15), "5.00P")
+        self.assertEqual(format_SI_prefix(6e18), "6.00E")
+        self.assertEqual(format_SI_prefix(7e21), "7.00Z")
+        self.assertEqual(format_SI_prefix(8e24), "8.00Y")
+
+    def test_multiple_letters(self):
+        self.assertEqual(format_SI_prefix(9e27), "9.00kY")
+
+    def test_custom_power(self):
+        self.assertEqual(format_SI_prefix(1023.99, 1024), "1023.99")
+        self.assertEqual(format_SI_prefix(1034.24, 1024), "1.01k")
+
+    def test_custom_labels(self):
+        labels = ("E", "da", "h", "k")
+        self.assertEqual(format_SI_prefix(1, 10, labels), "1.00E")
+        self.assertEqual(format_SI_prefix(10, 10, labels), "1.00da")
+        self.assertEqual(format_SI_prefix(100, 10, labels), "1.00h")
+        self.assertEqual(format_SI_prefix(1000, 10, labels), "1.00k")