add heat index & warning level calculations

libwx.go

@@ -148,3 +148,111 @@ func WetBulbC(temp TempC, rh RelHumidity) (TempC, error) {
 		),
 		nil
 }
+
+func heatIndexConstantsF() [9]float64 {
+	// from https://en.wikipedia.org/wiki/Heat_index#Formula
+	// captured on 2024-07-17
+	return [9]float64{
+		-42.379,
+		2.04901523,
+		10.14333127,
+		-0.22475541,
+		-6.83783e-3,
+		-5.481717e-2,
+		1.22874e-3,
+		8.5282e-4,
+		-1.99e-6,
+	}
+}
+
+func heatIndexConstantsC() [9]float64 {
+	// from https://en.wikipedia.org/wiki/Heat_index#Formula
+	// captured on 2024-07-17
+	return [9]float64{
+		-8.78469475556,
+		1.61139411,
+		2.33854883889,
+		-0.14611605,
+		-0.012308094,
+		-0.0164248277778,
+		2.211732e-3,
+		7.2546e-4,
+		-3.582e-6,
+	}
+}
+
+func rawHeatIndex(c [9]float64, rawTemp, rawRelH float64) float64 {
+	// from https://en.wikipedia.org/wiki/Heat_index#Formula
+	// captured on 2024-07-17
+	// note that constants on that page are 1-indexed, while the constants
+	// array here is 0-indexed
+	return c[0] +
+		c[1]*rawTemp +
+		c[2]*rawRelH +
+		c[3]*rawTemp*rawRelH +
+		c[4]*math.Pow(rawTemp, 2) +
+		c[5]*math.Pow(rawRelH, 2) +
+		c[6]*math.Pow(rawTemp, 2)*rawRelH +
+		c[7]*rawTemp*math.Pow(rawRelH, 2) +
+		c[8]*math.Pow(rawTemp, 2)*math.Pow(rawRelH, 2)
+}
+
+// HeatIndexF calculates the heat index for the given temperature (in Fahrenheit)
+// and relative humidity percentage.
+func HeatIndexF(temp TempF, rh RelHumidity) TempF {
+	return TempF(rawHeatIndex(
+		heatIndexConstantsF(),
+		temp.Unwrap(),
+		rh.Clamped().UnwrapFloat64(),
+	))
+}
+
+// HeatIndexC calculates the heat index for the given temperature (in Celsius)
+// and relative humidity percentage.
+func HeatIndexC(temp TempC, rh RelHumidity) TempC {
+	return TempC(rawHeatIndex(
+		heatIndexConstantsC(),
+		temp.Unwrap(),
+		rh.Clamped().UnwrapFloat64(),
+	))
+}
+
+// HeatIndexWarningF returns a heat index warning level for the
+// given heat index temperature (in Fahrenheit) per
+// https://en.wikipedia.org/wiki/Heat_index#Table_of_values
+// captured on 2024-07-17.
+func HeatIndexWarningF(heatIndex TempF) HeatIndexWarning {
+	if heatIndex.Unwrap() < 80 {
+		return HeatIndexWarningNone
+	}
+	if heatIndex.Unwrap() < 91 {
+		return HeatIndexWarningCaution
+	}
+	if heatIndex.Unwrap() < 104 {
+		return HeatIndexWarningExtremeCaution
+	}
+	if heatIndex.Unwrap() < 125 {
+		return HeatIndexWarningDanger
+	}
+	return HeatIndexWarningExtremeDanger
+}
+
+// HeatIndexWarningC returns a heat index warning level for the
+// given heat index temperature (in Celsius) per
+// https://en.wikipedia.org/wiki/Heat_index#Table_of_values
+// captured on 2024-07-17.
+func HeatIndexWarningC(heatIndex TempC) HeatIndexWarning {
+	if heatIndex.Unwrap() < 27 {
+		return HeatIndexWarningNone
+	}
+	if heatIndex.Unwrap() < 33 {
+		return HeatIndexWarningCaution
+	}
+	if heatIndex.Unwrap() < 40 {
+		return HeatIndexWarningExtremeCaution
+	}
+	if heatIndex.Unwrap() < 52 {
+		return HeatIndexWarningDanger
+	}
+	return HeatIndexWarningExtremeDanger
+}

libwx_test.go

@@ -8,7 +8,7 @@ import (
 
 // TODO(cdzombak): testing for calculations in libwx.go
 
-func Test_WetBulbC(t *testing.T) {
+func Test_WetBulb_C(t *testing.T) {
 	r := require.New(t)
 	eq := CurriedFloat64Equal(Tolerance0)
 
@@ -52,3 +52,85 @@ func Test_WetBulbC(t *testing.T) {
 		r.True(eq(result.Unwrap(), c.expected.Unwrap()), msgAndArgs...)
 	}
 }
+
+func Test_HeatIndex_All(t *testing.T) {
+	r := require.New(t)
+	eq := CurriedFloat64Equal(0.5)
+
+	cases := []struct {
+		t        TempF
+		rh       RelHumidity
+		expected TempF
+	}{
+		{TempF(80), RelHumidity(40), TempF(80)},
+		{TempF(80), RelHumidity(60), TempF(82)},
+		{TempF(80), RelHumidity(80), TempF(84)},
+		{TempF(80), RelHumidity(100), TempF(87)},
+		{TempF(86), RelHumidity(40), TempF(85)},
+		{TempF(86), RelHumidity(60), TempF(91)},
+		{TempF(86), RelHumidity(80), TempF(100)},
+		{TempF(86), RelHumidity(100), TempF(112)},
+		{TempF(90), RelHumidity(40), TempF(91)},
+		{TempF(90), RelHumidity(60), TempF(100)},
+		{TempF(90), RelHumidity(80), TempF(113)},
+		{TempF(90), RelHumidity(100), TempF(132)},
+		{TempF(104), RelHumidity(40), TempF(119)},
+		{TempF(104), RelHumidity(45), TempF(124)},
+		{TempF(104), RelHumidity(50), TempF(131)},
+		{TempF(104), RelHumidity(55), TempF(137)},
+	}
+
+	for _, c := range cases {
+		resultF := HeatIndexF(c.t, c.rh)
+		msgAndArgsF := []interface{}{
+			"given t %v degF + rh %v: expected %v, got %v",
+			c.t,
+			c.rh,
+			c.expected,
+			resultF,
+		}
+		r.True(eq(resultF.Unwrap(), c.expected.Unwrap()), msgAndArgsF...)
+
+		resultC := HeatIndexC(c.t.C(), c.rh)
+		msgAndArgsC := []interface{}{
+			"given t %v degC + rh %v: expected %v, got %v",
+			c.t.C(),
+			c.rh,
+			c.expected.C(),
+			resultC,
+		}
+		r.True(eq(resultC.Unwrap(), c.expected.C().Unwrap()), msgAndArgsC...)
+	}
+}
+
+func Test_HeatIndexWarning_F(t *testing.T) {
+	r := require.New(t)
+
+	cases := []struct {
+		t        TempF
+		expected HeatIndexWarning
+	}{
+		{TempF(-19), HeatIndexWarningNone},
+		{TempF(79), HeatIndexWarningNone},
+		{TempF(80), HeatIndexWarningCaution},
+		{TempF(87), HeatIndexWarningCaution},
+		{TempF(90), HeatIndexWarningCaution},
+		{TempF(91), HeatIndexWarningExtremeCaution},
+		{TempF(103), HeatIndexWarningExtremeCaution},
+		{TempF(104), HeatIndexWarningDanger},
+		{TempF(124), HeatIndexWarningDanger},
+		{TempF(126), HeatIndexWarningExtremeDanger},
+		{TempF(135), HeatIndexWarningExtremeDanger},
+	}
+
+	for _, c := range cases {
+		result := HeatIndexWarningF(c.t)
+		msgAndArgs := []interface{}{
+			"given t %v degF: expected %v, got %v",
+			c.t,
+			c.expected,
+			result,
+		}
+		r.Equal(c.expected, result, msgAndArgs...)
+	}
+}

temperature_types.go

@@ -8,3 +8,13 @@ type TempC float64
 
 func (t TempF) Unwrap() float64 { return float64(t) }
 func (t TempC) Unwrap() float64 { return float64(t) }
+
+type HeatIndexWarning int
+
+const (
+	HeatIndexWarningNone = iota
+	HeatIndexWarningCaution
+	HeatIndexWarningExtremeCaution
+	HeatIndexWarningDanger
+	HeatIndexWarningExtremeDanger
+)