Fix formula precedence bug in the FormulaEngine (#141)

Formulas with repeated operators like `#1 - #2 - #3` were getting
calculated incorrectly as `#1 - (#2 - #3)`.

The correct post-fix notation would be `[#1, #2, -, #3, -]`, but the
formula engine was generating this instead: `[#1, #2, #3, -, -]`.

This is fixed by flushing the operators from the build stack not only if
a previous operator has a higher precedence, but also if a previous
operator has the same precedence as the current operator.

Author: sahas-subramanian-frequenz

RELEASE_NOTES.md

@@ -14,4 +14,6 @@
 
 ## Bug Fixes
 
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+- Formulas with repeated operators like `#1 - #2 - #3` were getting
+  calculated incorrectly as `#1 - (#2 - #3)`.  This has been fixed in
+  https://github.com/frequenz-floss/frequenz-sdk-python/pull/141

src/frequenz/sdk/timeseries/logical_meter/_formula_engine.py

@@ -179,7 +179,7 @@ def push_oper(self, oper: str) -> None:
             op_prec = _operator_precedence[oper]
             while self._build_stack:
                 prev_step = self._build_stack[-1]
-                if op_prec <= _operator_precedence[repr(prev_step)]:
+                if op_prec < _operator_precedence[repr(prev_step)]:
                     break
                 if oper == ")" and repr(prev_step) == "(":
                     self._build_stack.pop()

tests/timeseries/test_formula_engine.py

@@ -122,6 +122,30 @@ async def test_simple(self) -> None:
                 ([15.0, 20.0, 20.0], -19.25),
             ],
         )
+        await self.run_test(
+            "#2 - #4 - #5",
+            "[#2, #4, -, #5, -]",
+            [
+                ([6.0, 12.0, 15.0], -21.0),
+                ([15.0, 20.0, 20.0], -25.0),
+            ],
+        )
+        await self.run_test(
+            "#2 + #4 + #5",
+            "[#2, #4, +, #5, +]",
+            [
+                ([6.0, 12.0, 15.0], 33.0),
+                ([15.0, 20.0, 20.0], 55.0),
+            ],
+        )
+        await self.run_test(
+            "#2 / #4 / #5",
+            "[#2, #4, /, #5, /]",
+            [
+                ([30.0, 3.0, 5.0], 2.0),
+                ([15.0, 3.0, 2.0], 2.5),
+            ],
+        )
 
     async def test_compound(self) -> None:
         """Test compound formulas."""
@@ -141,6 +165,38 @@ async def test_compound(self) -> None:
                 ([15.0, 17.0, 20.0, 1.5], 10.5),
             ],
         )
+        await self.run_test(
+            "#2 + (#4 - #5 * #6)",
+            "[#2, #4, #5, #6, *, -, +]",
+            [
+                ([10.0, 12.0, 15.0, 2.0], -8.0),
+                ([15.0, 17.0, 20.0, 1.5], 2.0),
+            ],
+        )
+        await self.run_test(
+            "#2 + (#4 - #5 - #6)",
+            "[#2, #4, #5, -, #6, -, +]",
+            [
+                ([10.0, 12.0, 15.0, 2.0], 5.0),
+                ([15.0, 17.0, 20.0, 1.5], 10.5),
+            ],
+        )
+        await self.run_test(
+            "#2 + #4 - #5 - #6",
+            "[#2, #4, #5, -, #6, -, +]",
+            [
+                ([10.0, 12.0, 15.0, 2.0], 5.0),
+                ([15.0, 17.0, 20.0, 1.5], 10.5),
+            ],
+        )
+        await self.run_test(
+            "#2 + #4 - (#5 - #6)",
+            "[#2, #4, #5, #6, -, -, +]",
+            [
+                ([10.0, 12.0, 15.0, 2.0], 9.0),
+                ([15.0, 17.0, 20.0, 1.5], 13.5),
+            ],
+        )
         await self.run_test(
             "(#2 + #4 - #5) * #6",
             "[#2, #4, #5, -, +, #6, *]",