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"`

internal/forecast/derive_test.go

@@ -658,3 +658,26 @@ func assertTimedValueClose(t *testing.T, name string, got *TimedValue, want floa
 		t.Fatalf("%s = %#v, want %v", name, got, want)
 	}
 }
+
+func TestCanonicalDaypartKey(t *testing.T) {
+	for _, tt := range []struct {
+		name  string
+		value string
+		want  string
+	}{
+		{name: "blank", value: " \t ", want: ""},
+		{name: "casing", value: "MORNING", want: "morning"},
+		{name: "punctuation runs", value: " Morning--Commute / School ", want: "morning_commute_school"},
+		{name: "Unicode", value: "Déjà Vu", want: "déjà_vu"},
+		{name: "letters and digits", value: "Day 1 Outlook", want: "day_1_outlook"},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := CanonicalDaypartKey(tt.value); got != tt.want {
+				t.Fatalf("CanonicalDaypartKey(%q) = %q, want %q", tt.value, got, tt.want)
+			}
+		})
+	}
+	if CanonicalDaypartKey("Morning commute") != CanonicalDaypartKey("MORNING--COMMUTE") {
+		t.Fatal("equivalent configured names must have the same canonical daypart key")
+	}
+}