reactive-mail-server/backend/src/main/java/com/mailserver/config/MailDomains.java

package com.mailserver.config;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

import java.util.Arrays;
import java.util.Locale;
import java.util.Set;
import java.util.stream.Collectors;

/**
 * Domains whose mailboxes live on this server ({@code mailserver.domains}), and the optional catch-all mailbox
 * ({@code mailserver.catch-all}): when set, mail to ANY address is stored there and nothing is delivered outside —
 * a mail trap for dev/staging.
 */
@Component
public class MailDomains {

    private final Set<String> domains;
    private final String catchAll;

    public MailDomains(@Value("${mailserver.domains:localhost}") String domains,
                       @Value("${mailserver.catch-all:}") String catchAll) {
        this.domains = Arrays.stream(domains.split(","))
                .map(d -> d.trim().toLowerCase(Locale.ROOT))
                .filter(d -> !d.isEmpty())
                .collect(Collectors.toUnmodifiableSet());
        this.catchAll = catchAll.trim().toLowerCase(Locale.ROOT);
        if (this.domains.isEmpty()) {
            throw new IllegalStateException("mailserver.domains is empty");
        }
    }

    public boolean isLocal(String address) {
        return domains.contains(domainOf(address));
    }

    /** Mailbox that receives everything, or null. */
    public String catchAll() {
        return catchAll.isEmpty() ? null : catchAll;
    }

    public Set<String> domains() {
        return domains;
    }

    /**
     * Address without its "+tag": {@code bob+prod@x} → {@code bob@x}. Mailbox and list names never contain "+",
     * so the part after it is a label chosen by the sender — the folder the message goes to.
     */
    public static String baseOf(String address) {
        String lower = address.toLowerCase(Locale.ROOT);
        int at = lower.lastIndexOf('@');
        int plus = lower.indexOf('+');
        return plus > 0 && plus < at ? lower.substring(0, plus) + lower.substring(at) : lower;
    }

    /** The "+tag" of an address, or null: {@code bob+prod@x} → {@code prod}. */
    public static String tagOf(String address) {
        int at = address.lastIndexOf('@');
        int plus = address.indexOf('+');
        return plus > 0 && plus < at - 1 ? address.substring(plus + 1, at) : null;
    }

    public static String domainOf(String address) {
        int at = address == null ? -1 : address.lastIndexOf('@');
        return at < 0 ? "" : address.substring(at + 1).toLowerCase(Locale.ROOT);
    }
}