convert utility factor to policy

ve/variable.go

@@ -2,6 +2,7 @@ package ve
 
 import (
 	"fmt"
+	"math"
 	"slices"
 )
 
@@ -141,6 +142,73 @@ func (v *Variables) SumOut(f *Factor, variable Variable) Factor {
 	return fNew
 }
 
+func (v *Variables) Policy(f *Factor, variable Variable) Factor {
+	newVars := []Variable{}
+	idx := -1
+
+	for i := range f.variables {
+		if f.variables[i].id == variable.id {
+			idx = i
+		} else {
+			newVars = append(newVars, f.variables[i])
+		}
+	}
+
+	if idx < 0 {
+		panic(fmt.Sprintf("variable %d not in this factor", variable.id))
+	}
+	newVars = append(newVars, f.variables[idx])
+
+	fNew := v.CreateFactor(newVars, nil)
+
+	oldIndex := make([]int, len(f.variables))
+	newIndex := make([]int, len(newVars))
+	idxNew := len(newVars) - 1
+
+	cols := int(f.variables[idx].outcomes)
+	rows := len(f.data) / cols
+
+	rowData := make([]float64, cols)
+	for row := 0; row < rows; row++ {
+		fNew.Outcomes(row*cols, newIndex)
+		for c := 0; c < cols; c++ {
+			newIndex[idxNew] = c
+
+			for j := 0; j < idx; j++ {
+				oldIndex[j] = newIndex[j]
+			}
+			for j := idx + 1; j < len(oldIndex); j++ {
+				oldIndex[j] = newIndex[j-1]
+			}
+			oldIndex[idx] = newIndex[idxNew]
+			rowData[c] = f.Get(oldIndex)
+		}
+		maxUtility := math.Inf(-1)
+		maxIdx := -1
+		for c, u := range rowData {
+			if u > maxUtility {
+				maxUtility = u
+				maxIdx = c
+			}
+		}
+
+		if maxIdx < 0 {
+			panic("no utility values to derive policy")
+		}
+
+		for c := 0; c < cols; c++ {
+			newIndex[idxNew] = c
+			if c == maxIdx {
+				fNew.Set(newIndex, 1)
+			} else {
+				fNew.Set(newIndex, 0)
+			}
+		}
+	}
+
+	return fNew
+}
+
 func (v *Variables) Product(factors ...*Factor) Factor {
 	if len(factors) == 1 {
 		return *factors[0]

ve/variable_test.go

@@ -208,3 +208,31 @@ func TestVariablesProductScalar(t *testing.T) {
 
 	assert.Equal(t, []float64{2, 18, 10, 10, 16, 4}, f3.data)
 }
+
+func TestVariablesPolicy(t *testing.T) {
+	v := NewVariables()
+
+	v1 := v.Add(ChanceNode, 3)
+	v2 := v.Add(ChanceNode, 2)
+
+	f1 := v.CreateFactor([]Variable{v1, v2}, []float64{
+		0.4, 0.6,
+		0.9, 0.1,
+		0.2, 0.8,
+	})
+
+	p := v.Policy(&f1, v2)
+	assert.Equal(t, variables{v1, v2}, p.variables)
+	assert.Equal(t, []float64{
+		0, 1,
+		1, 0,
+		0, 1,
+	}, p.data)
+
+	p = v.Policy(&f1, v1)
+	assert.Equal(t, variables{v2, v1}, p.variables)
+	assert.Equal(t, []float64{
+		0, 1, 0,
+		0, 0, 1,
+	}, p.data)
+}