Add convert rules.

rules/rules.go

@@ -3,6 +3,7 @@ package rules
 
 import (
 	"errors"
+	"fmt"
 
 	"github.com/ichiban/prolog"
 )
@@ -14,6 +15,9 @@ type Engine struct {
 
 var (
 	prologKinds = `
+		unit_kind(f, temperature).
+		unit_kind(k, temperature).
+		unit_kind(c, temperature).
 		unit_kind(m2, area).
 		unit_kind(cm2, area).
 		unit_kind(cm, distance).
@@ -28,9 +32,22 @@ var (
 		unit_kind(butter, density).
 		unit_kind(water, density).
 `
-	prologKindsConvert = `
-		convert_kind(mass, volume, density).
-		convert_kind(volume, area, distance).
+	prologConvert = `
+rule(GivenQuant, f, TargetQuant, c) :-
+  TargetQuant is (GivenQuant - 32) * 5 / 9.
+rule(GivenQuant, c, TargetQuant, f) :-
+  TargetQuant is (GivenQuant / 5 * 9) + 32.
+rule(GivenQuant, c, TargetQuant, k) :-
+  TargetQuant is GivenQuant + 273.15.
+rule(GivenQuant, k, TargetQuant, c) :-
+  TargetQuant is GivenQuant - 273.15.
+rule(GivenQuant, GivenUnit, TargetQuant, TargetUnit) :-
+	rule(GivenQuant, GivenUnit, X, Y),
+	rule(X, Y, TargetQuant, TargetUnit).
+convert(GivenQuant, GivenUnit, TargetQuant, TargetUnit) :-
+  unit_kind(GivenUnit, temperature),
+  unit_kind(TargetUnit, temperature),
+	rule(GivenQuant, GivenUnit, TargetQuant, TargetUnit).
 `
 )
 
@@ -48,6 +65,10 @@ func New() (*Engine, error) {
 		return nil, err
 	}
 
+	if err := ngn.p.Exec(prologConvert); err != nil {
+		return nil, err
+	}
+
 	return ngn, nil
 }
 
@@ -74,3 +95,25 @@ func (ngn *Engine) Kind(unit string) (string, error) {
 	}
 	return result, nil
 }
+
+// Convert converts units.
+func (ngn *Engine) Convert(qty float64, givenUnit, targetUnit string) (float64, error) {
+	sols, err := ngn.p.Query("convert(?, ?, Qty, ?).", qty, givenUnit, targetUnit)
+	if err != nil {
+		fmt.Println("Huh?")
+		return 0, err
+	}
+	defer sols.Close()
+
+	for sols.Next() {
+		var quantity struct {
+			Qty float64
+		}
+		if err := sols.Scan(&quantity); err != nil {
+			fmt.Println("Scan?")
+			return 0, err
+		}
+		return quantity.Qty, err
+	}
+	return 0, err
+}

rules/rules_test.go

@@ -30,3 +30,32 @@ func TestKinds(t *testing.T) {
 		})
 	}
 }
+
+func TestConverts(t *testing.T) {
+	cases := []struct {
+		name   string
+		qty    float64
+		given  string
+		target string
+		result float64
+	}{
+		{
+			"c to f",
+			22.0,
+			"c",
+			"f",
+			71.6,
+		},
+	}
+
+	ngn, err := New()
+	assert.NoError(t, err)
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			result, err := ngn.Convert(tc.qty, tc.given, tc.target)
+			assert.NoError(t, err)
+
+			assert.InDelta(t, tc.result, result, 0.1)
+		})
+	}
+}