fix(stats): refuse a zero-probability sequence in Viterbi

Assisted-by: GLM 5.3 Flash

stats/hmm.go

@@ -354,6 +354,12 @@ func (m *HiddenMarkovModel) Viterbi(observations []int) (states []int, logProbab
 			arg = k
 		}
 	}
+	if math.IsInf(best, -1) {
+		// Every path has probability zero, the structural zero an
+		// emission or transition carries: the same refusal Forward makes,
+		// not a meaningless path beside a −Inf log probability.
+		return nil, 0, base.Errf("%s: the observation sequence has probability zero under the model", name)
+	}
 	path := make([]int, len(observations))
 	path[len(observations)-1] = arg
 	for t := len(observations) - 1; t > 0; t-- {

stats/hmm_test.go

@@ -375,3 +375,29 @@ func TestHiddenMarkovFitSingleObservation(t *testing.T) {
 		}
 	}
 }
+
+func TestViterbiZeroProbabilitySequence(t *testing.T) {
+	// Symbol 2 is emitted by no state, so every path through the sequence
+	// has probability zero. Forward and Smooth refuse such a sequence, and
+	// Viterbi documents the same refusals: a meaningless path beside a
+	// −Inf log probability is not an answer.
+	model, err := NewHiddenMarkovModel(
+		[]float64{0.5, 0.5},
+		[]float64{0.5, 0.5, 0.5, 0.5},
+		[]float64{0.5, 0.5, 0, 0.5, 0.5, 0},
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, _, err := model.Viterbi([]int{0, 2, 1}); err == nil {
+		t.Fatal("a zero-probability sequence was accepted by Viterbi")
+	}
+	// The live symbols of the same model still decode.
+	path, lp, err := model.Viterbi([]int{0, 1})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if path[0] != path[1] || lp >= 0 {
+		t.Fatalf("Viterbi([0, 1]) = (%v, %g), want a coherent path with a negative log probability", path, lp)
+	}
+}