refactor: consolidate voice formula building into voice_formulas.py

- voice_formulas.py: add pairs_to_formula() as canonical implementation
- webui/routes/utils/voice.py: formula_from_profile() and pairs_to_formula()
  now delegate to voice_formulas.pairs_to_formula()
- pyqt/gui.py: get_voice_formula() now uses voice_formulas.pairs_to_formula()
  instead of inline string formatting
- Eliminates 3 duplicate implementations of voice*weight formula building
- +9 tests
- 1178 tests pass

abogen/pyqt/gui.py

@@ -2235,11 +2235,10 @@ class abogen(QWidget):
             self.current_queue_index = 0  # Reset for next time
 
     def get_voice_formula(self) -> str:
+        from abogen.voice_formulas import pairs_to_formula
+
         if self.mixed_voice_state:
-            formula_components = [
-                f"{name}*{weight}" for name, weight in self.mixed_voice_state
-            ]
-            return " + ".join(filter(None, formula_components))
+            return pairs_to_formula(self.mixed_voice_state) or ""
         else:
             return self.selected_voice
 

abogen/voice_formulas.py

@@ -1,5 +1,5 @@
 import re
-from typing import List, Tuple
+from typing import Iterable, List, Optional, Tuple
 
 from abogen.tts_plugin.utils import get_voices
 
@@ -72,6 +72,33 @@ def parse_voice_formula(pipeline, formula):
     return weighted_sum
 
 
+def pairs_to_formula(pairs: Iterable[Tuple[str, float]]) -> Optional[str]:
+    """Build a voice formula string from (voice_name, weight) pairs.
+
+    Normalizes weights to sum to 1.0 and formats as "voice1*0.5+voice2*0.5".
+
+    Args:
+        pairs: Iterable of (voice_name, weight) tuples. Zero-weight entries
+            are filtered out.
+
+    Returns:
+        Formula string, or None if no valid entries.
+    """
+    voices = [(voice, float(weight)) for voice, weight in pairs if weight is not None and float(weight) > 0]
+    if not voices:
+        return None
+    total = sum(weight for _, weight in voices)
+    if total <= 0:
+        return None
+
+    def _format_value(value: float) -> str:
+        normalized = value / total if total else 0.0
+        return (f"{normalized:.4f}").rstrip("0").rstrip(".") or "0"
+
+    parts = [f"{voice}*{_format_value(weight)}" for voice, weight in voices]
+    return "+".join(parts)
+
+
 def calculate_sum_from_formula(formula):
     weights = re.findall(r"\* *([\d.]+)", formula)
     total_sum = sum(float(weight) for weight in weights)

abogen/webui/routes/utils/voice.py

@@ -548,19 +548,12 @@ def prepare_speaker_metadata(
 
 
 def formula_from_profile(entry: Dict[str, Any]) -> Optional[str]:
+    from abogen.voice_formulas import pairs_to_formula
+
     voices = entry.get("voices") or []
     if not voices:
         return None
-    total = sum(weight for _, weight in voices)
-    if total <= 0:
-        return None
-
-    def _format_weight(value: float) -> str:
-        normalized = value / total if total else 0.0
-        return (f"{normalized:.4f}").rstrip("0").rstrip(".") or "0"
-
-    parts = [f"{name}*{_format_weight(weight)}" for name, weight in voices if weight > 0]
-    return "+".join(parts) if parts else None
+    return pairs_to_formula(voices)
 
 
 def template_options() -> Dict[str, Any]:
@@ -710,19 +703,8 @@ def sanitize_voice_entries(entries: Iterable[Any]) -> List[Dict[str, Any]]:
 
 
 def pairs_to_formula(pairs: Iterable[Tuple[str, float]]) -> Optional[str]:
-    voices = [(voice, float(weight)) for voice, weight in pairs if float(weight) > 0]
-    if not voices:
-        return None
-    total = sum(weight for _, weight in voices)
-    if total <= 0:
-        return None
-
-    def _format_value(value: float) -> str:
-        normalized = value / total if total else 0.0
-        return (f"{normalized:.4f}").rstrip("0").rstrip(".") or "0"
-
-    parts = [f"{voice}*{_format_value(weight)}" for voice, weight in voices]
-    return "+".join(parts)
+    from abogen.voice_formulas import pairs_to_formula as _pairs_to_formula
+    return _pairs_to_formula(pairs)
 
 
 def profiles_payload() -> Dict[str, Any]:

tests/test_voice_formulas_pairs.py

@@ -0,0 +1,44 @@
+"""Tests for voice_formulas.py — pairs_to_formula."""
+
+from abogen.voice_formulas import pairs_to_formula
+
+
+class TestPairsToFormula:
+    def test_basic_pair(self):
+        result = pairs_to_formula([("A", 1.0), ("B", 1.0)])
+        assert result == "A*0.5+B*0.5"
+
+    def test_unequal_weights(self):
+        result = pairs_to_formula([("A", 3.0), ("B", 1.0)])
+        assert result == "A*0.75+B*0.25"
+
+    def test_single_voice(self):
+        result = pairs_to_formula([("A", 1.0)])
+        assert result == "A*1"
+
+    def test_filters_zero_weight(self):
+        result = pairs_to_formula([("A", 1.0), ("B", 0.0)])
+        assert result == "A*1"
+
+    def test_all_zero_returns_none(self):
+        result = pairs_to_formula([("A", 0.0), ("B", 0.0)])
+        assert result is None
+
+    def test_empty_returns_none(self):
+        result = pairs_to_formula([])
+        assert result is None
+
+    def test_none_values_filtered(self):
+        result = pairs_to_formula([("A", 1.0), ("B", None)])
+        assert result is not None
+
+    def test_weight_normalization(self):
+        result = pairs_to_formula([("A", 2.0), ("B", 2.0)])
+        assert result == "A*0.5+B*0.5"
+
+    def test_three_voices(self):
+        result = pairs_to_formula([("A", 1.0), ("B", 1.0), ("C", 1.0)])
+        assert "A*" in result
+        assert "B*" in result
+        assert "C*" in result
+        assert "+" in result