Correct derived briefing weather semantics

internal/briefing/derived_daypart_summaries_module.go

@@ -301,18 +301,8 @@ func celsiusToFahrenheit(value float64) float64 {
 }
 
 func temperatureBandIndex(value int) int {
-	decade := (value / 10) * 10
-	remainder := value - decade
-	if remainder < 0 {
-		remainder = -remainder
-	}
-	band := 1
-	switch {
-	case remainder <= 3:
-		band = 0
-	case remainder >= 7:
-		band = 2
-	}
+	decade, remainder := temperatureBandParts(value)
+	band := temperatureBandQualifierIndex(remainder)
 	return decade*3 + band
 }
 
@@ -348,19 +338,47 @@ func temperaturePhraseF(value forecast.Range) string {
 }
 
 func temperatureBandPhrase(value int) string {
-	decade := (value / 10) * 10
-	remainder := value - decade
-	if remainder < 0 {
-		remainder = -remainder
+	if value < 0 {
+		decade, remainder := temperatureBandParts(-value)
+		qualifier := temperatureBandQualifier(remainder)
+		if decade == 0 {
+			return fmt.Sprintf("%s single digits below zero", qualifier)
+		}
+		return fmt.Sprintf("%s %ds below zero", qualifier, decade)
 	}
-	qualifier := "mid"
+	decade, remainder := temperatureBandParts(value)
+	qualifier := temperatureBandQualifier(remainder)
+	return fmt.Sprintf("%s %ds", qualifier, decade)
+}
+
+func temperatureBandParts(value int) (int, int) {
+	decade := value / 10
+	if value < 0 && value%10 != 0 {
+		decade--
+	}
+	return decade * 10, value - decade*10
+}
+
+func temperatureBandQualifierIndex(remainder int) int {
 	switch {
 	case remainder <= 3:
-		qualifier = "low"
+		return 0
 	case remainder >= 7:
-		qualifier = "upper"
+		return 2
+	default:
+		return 1
+	}
+}
+
+func temperatureBandQualifier(remainder int) string {
+	switch temperatureBandQualifierIndex(remainder) {
+	case 0:
+		return "low"
+	case 2:
+		return "upper"
+	default:
+		return "mid"
 	}
-	return fmt.Sprintf("%s %ds", qualifier, decade)
 }
 
 func sentenceCase(value string) string {