Normalize NWS forecast discussion preambles

internal/providers/nws/forecast_discussion.go

@@ -315,8 +315,9 @@ func parseForecastDiscussionHeader(lines []string) (string, time.Time, error) {
 
 func parseForecastDiscussionIssueTime(line string) (time.Time, error) {
 	line = strings.TrimSpace(line)
-	line = strings.TrimPrefix(line, "Issued at ")
-	line = strings.TrimSpace(line)
+	if isForecastDiscussionIssuedAtLine(line) {
+		line = strings.TrimSpace(line[len("Issued at"):])
+	}
 
 	parts := strings.Fields(line)
 	if len(parts) != 7 {
@@ -547,11 +548,14 @@ func parseForecastDiscussionSectionBlocks(lines []string) []forecastDiscussionSe
 }
 
 func parseForecastDiscussionKeyMessages(body []string) []string {
+	body = removeForecastDiscussionPresentationMarkers(body)
+	body = trimBlankLines(body)
+	if len(body) > 0 && isForecastDiscussionKeyMessageMetadataLine(body[0]) {
+		body = trimBlankLines(body[1:])
+	}
 	if len(body) == 0 {
 		return nil
 	}
-
-	body = trimBlankLines(body)
 	var messages []string
 	var current strings.Builder
 
@@ -589,16 +593,19 @@ func parseForecastDiscussionTextSection(block forecastDiscussionSectionBlock) (F
 		Qualifier: block.heading.qualifier,
 	}
 
-	body := trimBlankLines(block.body)
+	body := trimBlankLines(removeForecastDiscussionPresentationMarkers(block.body))
+	if section.Qualifier == "" && len(body) > 0 && isForecastDiscussionStandaloneParenthetical(body[0]) {
+		section.Qualifier = strings.TrimSpace(body[0])
+		body = trimBlankLines(body[1:])
+	}
 	if len(body) == 0 {
 		return section, nil
 	}
 
-	first := strings.TrimSpace(body[0])
-	if strings.HasPrefix(first, "Issued at ") {
-		issuedAt, err := parseForecastDiscussionIssueTime(first)
+	if isForecastDiscussionIssuedAtLine(body[0]) {
+		issuedAt, err := parseForecastDiscussionIssueTime(body[0])
 		if err != nil {
-			return ForecastDiscussionSection{}, fmt.Errorf("parse section issuedAt %q: %w", first, err)
+			return ForecastDiscussionSection{}, fmt.Errorf("parse section issuedAt %q: %w", strings.TrimSpace(body[0]), err)
 		}
 		tt := issuedAt.UTC()
 		section.IssuedAt = &tt
@@ -610,6 +617,48 @@ func parseForecastDiscussionTextSection(block forecastDiscussionSectionBlock) (F
 	return section, nil
 }
 
+func isForecastDiscussionPresentationMarker(line string) bool {
+	switch {
+	case strings.EqualFold(strings.TrimSpace(line), "-- Changed Discussion --"):
+		return true
+	case strings.EqualFold(strings.TrimSpace(line), "-- End Changed Discussion --"):
+		return true
+	default:
+		return false
+	}
+}
+
+func removeForecastDiscussionPresentationMarkers(lines []string) []string {
+	body := make([]string, 0, len(lines))
+	for _, line := range lines {
+		if !isForecastDiscussionPresentationMarker(line) {
+			body = append(body, line)
+		}
+	}
+	return body
+}
+
+func isForecastDiscussionStandaloneParenthetical(line string) bool {
+	line = strings.TrimSpace(line)
+	return len(line) > 2 && line[0] == '(' && line[len(line)-1] == ')' && strings.TrimSpace(line[1:len(line)-1]) != ""
+}
+
+func isForecastDiscussionIssuedAtLine(line string) bool {
+	line = strings.TrimSpace(line)
+	return len(line) > len("Issued at") &&
+		strings.EqualFold(line[:len("Issued at")], "Issued at") &&
+		isForecastDiscussionHorizontalWhitespace(line[len("Issued at")])
+}
+
+func isForecastDiscussionKeyMessageMetadataLine(line string) bool {
+	line = strings.TrimSpace(line)
+	return hasForecastDiscussionASCIIPrefix(line, "Issued at") || hasForecastDiscussionASCIIPrefix(line, "Updated at")
+}
+
+func hasForecastDiscussionASCIIPrefix(line, prefix string) bool {
+	return len(line) >= len(prefix) && strings.EqualFold(line[:len(prefix)], prefix)
+}
+
 func trimBlankLines(lines []string) []string {
 	start := 0
 	for start < len(lines) && strings.TrimSpace(lines[start]) == "" {