// lambdas/invoices/index.js
// Invoices API
//   GET    /invoices           — list (paginated, filterable by status/poId)
//   POST   /invoices           — create invoice (bills against a PO)
//   GET    /invoices/:id       — get one invoice
//   PUT    /invoices/:id       — update invoice
//   PATCH  /invoices/:id/status — update status only (draft→pending→paid)
//   DELETE /invoices/:id       — delete (drafts only)

const {
  ScanCommand, GetCommand, PutCommand, UpdateCommand, DeleteCommand,
} = require("@aws-sdk/lib-dynamodb");
const {
  ok, created, noContent, badRequest, notFound, conflict, forbidden, serverError,
  getDocClient, TABLES, genId, parseBody, require_fields, handler,
  parsePagination, paginatedResponse, decodeCursor,
} = require("@ledgerflow/shared");

const INV_TABLE = TABLES.INVOICES;
const POS_TABLE = TABLES.POS;

// ─── Router ──────────────────────────────────────────────────────────────────

exports.handler = handler(async (event, _ctx, user) => {
  const method  = event.requestContext?.http?.method || event.httpMethod;
  const rawPath = event.rawPath || event.path || "";
  const segments = rawPath.replace(/^\/invoices\/?/, "").split("/").filter(Boolean);
  const id  = segments[0] || null;
  const sub = segments[1] || null;
  const qs  = event.queryStringParameters || {};

  if (!id) {
    if (method === "GET")  return listInvoices(qs);
    if (method === "POST") return createInvoice(event, user);
  } else {
    if (method === "GET"  && !sub)           return getInvoice(id);
    if (method === "PUT"  && !sub)           return updateInvoice(id, event, user);
    if (method === "PATCH" && sub === "status") return updateStatus(id, event, user);
    if (method === "DELETE" && !sub)         return deleteInvoice(id, user);
  }

  return { statusCode: 405, body: JSON.stringify({ error: "Method Not Allowed" }) };
});

// ─── List ─────────────────────────────────────────────────────────────────────

async function listInvoices(qs) {
  const { limit, cursor } = parsePagination(qs);
  const db = getDocClient();
  const params = { TableName: INV_TABLE, Limit: limit, ExclusiveStartKey: decodeCursor(cursor) };

  const filters = [];
  const names   = {};
  const values  = {};

  if (qs.status) {
    filters.push("#status = :status");
    names["#status"] = "status"; values[":status"] = qs.status;
  }
  if (qs.poId) {
    filters.push("poId = :poId");
    values[":poId"] = qs.poId;
  }
  if (qs.vendor) {
    filters.push("contains(vendor, :vendor)");
    values[":vendor"] = qs.vendor;
  }
  if (filters.length) {
    params.FilterExpression = filters.join(" AND ");
    if (Object.keys(names).length)  params.ExpressionAttributeNames = names;
    if (Object.keys(values).length) params.ExpressionAttributeValues = values;
  }

  const result = await db.send(new ScanCommand(params));
  // Sort newest first
  const items = (result.Items || []).sort((a, b) => b.createdAt?.localeCompare(a.createdAt || "") || 0);
  return ok(paginatedResponse(items, result.LastEvaluatedKey));
}

// ─── Get One ──────────────────────────────────────────────────────────────────

async function getInvoice(id) {
  const db = getDocClient();
  const result = await db.send(new GetCommand({ TableName: INV_TABLE, Key: { id } }));
  if (!result.Item) return notFound("Invoice");
  return ok(result.Item);
}

// ─── Create ───────────────────────────────────────────────────────────────────

async function createInvoice(event, user) {
  const body = parseBody(event);
  require_fields(body, ["number", "vendor", "lineItems"]);

  if (!Array.isArray(body.lineItems) || body.lineItems.length === 0) {
    return badRequest("lineItems must be a non-empty array");
  }

  const db  = getDocClient();
  const now = new Date().toISOString();

  // Validate & calculate line items
  const lineItems = body.lineItems.map((li, i) => {
    if (!li.description) throw { statusCode: 400, message: `lineItems[${i}].description required` };
    const qty       = parseFloat(li.qty || li.quantity || 1);
    const unitPrice = parseFloat(li.unitPrice || li.price || 0);
    return { description: li.description, qty, unitPrice, total: qty * unitPrice };
  });

  const subtotal = lineItems.reduce((s, li) => s + li.total, 0);
  const taxRate  = parseFloat(body.taxRate ?? 0);
  const tax      = subtotal * (taxRate / 100);
  const total    = subtotal + tax;

  // If billing against a PO, verify it exists and has remaining balance
  let poRecord = null;
  if (body.poId) {
    const poResult = await db.send(new GetCommand({ TableName: POS_TABLE, Key: { id: body.poId } }));
    if (!poResult.Item) return notFound("Referenced Purchase Order");
    poRecord = poResult.Item;

    const remaining = poRecord.amount - (poRecord.billed || 0);
    if (total > remaining + 0.01) {
      return badRequest(
        `Invoice total ${fmt(total)} exceeds PO remaining balance ${fmt(remaining)} for ${poRecord.poNumber}`
      );
    }
  }

  // Prevent duplicate invoice numbers
  const existing = await db.send(new ScanCommand({
    TableName: INV_TABLE,
    FilterExpression: "#num = :num",
    ExpressionAttributeNames: { "#num": "number" },
    ExpressionAttributeValues: { ":num": body.number.trim() },
    Limit: 1,
  }));
  if (existing.Count > 0) return conflict(`Invoice number ${body.number} already exists`);

  const id = genId("INV");
  const invoice = {
    id,
    number:    body.number.trim(),
    vendor:    body.vendor.trim(),
    poId:      body.poId     || null,
    poNumber:  poRecord?.poNumber || body.poNumber || null,
    issueDate: body.issueDate || now.split("T")[0],
    dueDate:   body.dueDate   || null,
    terms:     body.terms     || "Net 30",
    lineItems,
    subtotal,
    taxRate,
    tax,
    total,
    notes:     body.notes || "",
    status:    body.status === "draft" ? "draft" : "pending",
    ediStatus: "not_submitted",
    createdBy: user?.email || "system",
    createdAt: now,
    updatedAt: now,
  };

  await db.send(new PutCommand({ TableName: INV_TABLE, Item: invoice }));

  // Update PO billed amount
  if (poRecord) {
    await db.send(new UpdateCommand({
      TableName: POS_TABLE,
      Key: { id: body.poId },
      UpdateExpression: "SET billed = billed + :amt, updatedAt = :now",
      ExpressionAttributeValues: { ":amt": total, ":now": now },
    }));
  }

  return created(invoice);
}

// ─── Update ───────────────────────────────────────────────────────────────────

async function updateInvoice(id, event, user) {
  const body = parseBody(event);
  const db   = getDocClient();

  const existing = await db.send(new GetCommand({ TableName: INV_TABLE, Key: { id } }));
  if (!existing.Item) return notFound("Invoice");
  if (existing.Item.status === "paid") return forbidden("Cannot edit a paid invoice");

  const UPDATABLE = ["vendor", "issueDate", "dueDate", "terms", "notes"];
  const now       = new Date().toISOString();
  const expressions = ["#updatedAt = :now"];
  const names  = { "#updatedAt": "updatedAt" };
  const values = { ":now": now };

  UPDATABLE.forEach(key => {
    if (body[key] !== undefined) {
      expressions.push(`#${key} = :${key}`);
      names[`#${key}`] = key;
      values[`:${key}`] = body[key];
    }
  });

  // Handle line items update (recalculate totals)
  if (body.lineItems && Array.isArray(body.lineItems) && body.lineItems.length > 0) {
    const lineItems = body.lineItems.map((li, i) => {
      if (!li.description) throw { statusCode: 400, message: `lineItems[${i}].description required` };
      const qty       = parseFloat(li.qty || li.quantity || 1);
      const unitPrice = parseFloat(li.unitPrice || li.price || 0);
      return { description: li.description, qty, unitPrice, total: qty * unitPrice };
    });

    const subtotal = lineItems.reduce((s, li) => s + li.total, 0);
    const taxRate  = parseFloat(body.taxRate ?? existing.Item.taxRate ?? 0);
    const tax      = subtotal * (taxRate / 100);
    const total    = subtotal + tax;

    // If linked to a PO, check remaining balance (excluding current invoice's amount)
    if (existing.Item.poId) {
      const poResult = await db.send(new GetCommand({ TableName: POS_TABLE, Key: { id: existing.Item.poId } }));
      if (poResult.Item) {
        const billedWithoutThis = (poResult.Item.billed || 0) - existing.Item.total;
        const remaining = poResult.Item.amount - billedWithoutThis;
        if (total > remaining + 0.01) {
          return badRequest(
            `Invoice total ${fmt(total)} exceeds PO remaining balance ${fmt(remaining)} for ${poResult.Item.poNumber}`
          );
        }
        // Update PO billed amount: remove old total, add new total
        const diff = total - existing.Item.total;
        if (Math.abs(diff) > 0.001) {
          await db.send(new UpdateCommand({
            TableName: POS_TABLE,
            Key: { id: existing.Item.poId },
            UpdateExpression: "SET billed = billed + :diff, updatedAt = :now",
            ExpressionAttributeValues: { ":diff": diff, ":now": now },
          }));
        }
      }
    }

    expressions.push("#lineItems = :lineItems", "#subtotal = :subtotal", "#taxRate = :taxRate", "#tax = :tax", "#total = :total");
    names["#lineItems"] = "lineItems"; values[":lineItems"] = lineItems;
    names["#subtotal"]  = "subtotal";  values[":subtotal"]  = subtotal;
    names["#taxRate"]   = "taxRate";   values[":taxRate"]   = taxRate;
    names["#tax"]       = "tax";       values[":tax"]       = tax;
    names["#total"]     = "total";     values[":total"]     = total;
  }

  // If invoice was already submitted via EDI, mark as modified after send
  if (existing.Item.ediStatus === "submitted") {
    expressions.push("#ediStatus = :ediStatus");
    names["#ediStatus"] = "ediStatus";
    values[":ediStatus"] = "modified_after_send";
  }

  const result = await db.send(new UpdateCommand({
    TableName: INV_TABLE,
    Key: { id },
    UpdateExpression: "SET " + expressions.join(", "),
    ExpressionAttributeNames: names,
    ExpressionAttributeValues: values,
    ReturnValues: "ALL_NEW",
  }));

  return ok(result.Attributes);
}

// ─── Status Update ────────────────────────────────────────────────────────────

const VALID_TRANSITIONS = {
  draft:   ["pending", "cancelled"],
  pending: ["paid", "cancelled", "draft"],
  paid:    [],
  cancelled: [],
};

async function updateStatus(id, event, user) {
  const { status } = parseBody(event);
  if (!status) return badRequest("status required");

  const db = getDocClient();
  const existing = await db.send(new GetCommand({ TableName: INV_TABLE, Key: { id } }));
  if (!existing.Item) return notFound("Invoice");

  const current = existing.Item.status;
  const allowed = VALID_TRANSITIONS[current] || [];
  if (!allowed.includes(status)) {
    return badRequest(`Cannot transition invoice from '${current}' to '${status}'`);
  }

  const now = new Date().toISOString();
  const updateExpr = ["#status = :status", "#updatedAt = :now"];
  const names  = { "#status": "status", "#updatedAt": "updatedAt" };
  const values = { ":status": status, ":now": now };

  if (status === "paid") {
    updateExpr.push("#paidAt = :paidAt", "#paidBy = :paidBy");
    names["#paidAt"] = "paidAt"; names["#paidBy"] = "paidBy";
    values[":paidAt"] = now; values[":paidBy"] = user?.email || "system";
  }

  const result = await db.send(new UpdateCommand({
    TableName: INV_TABLE,
    Key: { id },
    UpdateExpression: "SET " + updateExpr.join(", "),
    ExpressionAttributeNames: names,
    ExpressionAttributeValues: values,
    ReturnValues: "ALL_NEW",
  }));

  return ok(result.Attributes);
}

// ─── Delete ───────────────────────────────────────────────────────────────────

async function deleteInvoice(id, user) {
  const db = getDocClient();
  const existing = await db.send(new GetCommand({ TableName: INV_TABLE, Key: { id } }));
  if (!existing.Item) return notFound("Invoice");
  if (existing.Item.status === "paid") {
    return forbidden("Cannot delete a paid invoice");
  }

  await db.send(new DeleteCommand({ TableName: INV_TABLE, Key: { id } }));

  // Reverse PO billed amount if invoice was tied to a PO
  if (existing.Item.poId) {
    await getDocClient().send(new UpdateCommand({
      TableName: POS_TABLE,
      Key: { id: existing.Item.poId },
      UpdateExpression: "SET billed = billed - :amt, updatedAt = :now",
      ExpressionAttributeValues: {
        ":amt": existing.Item.total,
        ":now": new Date().toISOString(),
      },
    }));
  }

  return noContent();
}

// ─── Helper ───────────────────────────────────────────────────────────────────

function fmt(n) {
  return "$" + parseFloat(n || 0).toFixed(2);
}