Centralize daypart key canonicalization

internal/forecast/dayparts.go

@@ -2,7 +2,9 @@ package forecast
 
 import (
 	"fmt"
+	"strings"
 	"time"
+	"unicode"
 
 	"gitea.maximumdirect.net/eric/weatherreporter/internal/timeutil"
 	"gitea.maximumdirect.net/eric/weatherreporter/internal/weatherdata"
@@ -14,6 +16,25 @@ type DaypartDefinition struct {
 	End   string `json:"end"`
 }
 
+// CanonicalDaypartKey returns the stable identity for a configured daypart name.
+func CanonicalDaypartKey(value string) string {
+	lower := strings.ToLower(strings.TrimSpace(value))
+	var out strings.Builder
+	lastUnderscore := false
+	for _, r := range lower {
+		if unicode.IsLetter(r) || unicode.IsDigit(r) {
+			out.WriteRune(r)
+			lastUnderscore = false
+			continue
+		}
+		if !lastUnderscore {
+			out.WriteByte('_')
+			lastUnderscore = true
+		}
+	}
+	return strings.Trim(out.String(), "_")
+}
+
 type DaypartWindow struct {
 	Name   string          `json:"name"`
 	Start  time.Time       `json:"start"`