export interface DigestResult {
  text: string;
  messageIds: string[];
}

const MAX_MESSAGES = 20;
const PREVIEW_LENGTH = 120;

function formatDate(date: Date): string {
  return date.toLocaleDateString('en-US', {
    year: 'numeric',
    month: 'long',
    day: 'numeric',
    timeZone: 'UTC',
  });
}

export async function buildTemplateDigest(
  tenantId: string,
  tenantName: string,
  prisma: any,
  digestDate: Date,
): Promise<DigestResult | null> {
  const from = new Date(digestDate);
  const to = new Date(digestDate.getTime() + 24 * 60 * 60 * 1000);

  const messages = await prisma.message.findMany({
    where: {
      tenantId,
      status: { in: ['APPROVED', 'DISTRIBUTED'] },
      updatedAt: { gte: from, lt: to },
    },
    include: {
      sourceGroup: { select: { name: true } },
    },
    orderBy: { updatedAt: 'asc' },
    take: MAX_MESSAGES,
  });

  if (messages.length === 0) return null;

  // Group messages by first tag; untagged go under "General"
  const groups = new Map<string, typeof messages>();
  for (const msg of messages) {
    const tag = msg.tags?.[0] ?? 'General';
    if (!groups.has(tag)) groups.set(tag, []);
    groups.get(tag)!.push(msg);
  }

  const dateLabel = formatDate(digestDate);
  const lines: string[] = [
    `📋 *Daily Digest — ${tenantName}*`,
    `_${dateLabel}_`,
    '',
  ];

  for (const [tag, msgs] of groups) {
    const sectionHeader = tag === 'General' ? '*📌 General*' : `*🏷️ ${tag}*`;
    lines.push(sectionHeader);
    for (const msg of msgs) {
      const preview = msg.content.slice(0, PREVIEW_LENGTH).replace(/\n+/g, ' ');
      const groupName = msg.sourceGroup?.name ?? 'Unknown Group';
      lines.push(`• ${preview} _(${groupName})_`);
    }
    lines.push('');
  }

  const groupCount = new Set(messages.map((m: any) => m.sourceGroupId)).size;
  lines.push(`_${messages.length} message${messages.length !== 1 ? 's' : ''} from ${groupCount} group${groupCount !== 1 ? 's' : ''} | TOWER_`);

  return {
    text: lines.join('\n'),
    messageIds: messages.map((m: any) => m.id),
  };
}