import { uuidv7 } from '@sims/domain'
import { writeAudit } from './audit'
import type { DB } from './db'
import { getClient } from './repos-clients'
import { generateReceipt, getDocument, listDocuments, type Doc } from './repos-documents'

/**
 * Payments — allocation, TDS, advances, settlement (D12 portable-repo pattern).
 *
 * Settlement per invoice: outstanding = payable − Σ allocations − Σ credit-note
 * payable (CNs whose ref_doc_id = the invoice). TDS spreads with the payment:
 * the effective settling power of a payment is amount + tds, allocated as one
 * pool. Leftover pool stays unallocated — the advance is computed on read as
 * Σ(amount + tds) − Σ allocations; no extra table.
 */

export type PaymentMode = 'bank' | 'upi' | 'cheque' | 'cash' | 'other'

const ALL_MODES: PaymentMode[] = ['bank', 'upi', 'cheque', 'cash', 'other']

export interface Payment {
  id: string; clientId: string; receivedOn: string; mode: PaymentMode
  reference: string; amountPaise: number; tdsPaise: number
  createdBy: string; createdAt: string
}

interface PaymentRow {
  id: string; client_id: string; received_on: string; mode: string
  reference: string; amount_paise: number; tds_paise: number
  created_by: string; created_at: string
}

function toPayment(r: PaymentRow): Payment {
  return {
    id: r.id, clientId: r.client_id, receivedOn: r.received_on,
    mode: r.mode as PaymentMode, reference: r.reference,
    amountPaise: r.amount_paise, tdsPaise: r.tds_paise,
    createdBy: r.created_by, createdAt: r.created_at,
  }
}

export async function getPayment(db: DB, id: string): Promise<Payment | null> {
  const row = await db.get<PaymentRow>(`SELECT * FROM payment WHERE id=?`, id)
  return row === undefined ? null : toPayment(row)
}

export async function listPayments(db: DB, clientId: string): Promise<Payment[]> {
  const rows = await db.all<PaymentRow>(
    `SELECT * FROM payment WHERE client_id=? ORDER BY id`, // uuidv7 ids sort by creation time
    clientId)
  return rows.map(toPayment)
}

// ---------- settlement math ----------

async function allocatedTo(db: DB, documentId: string): Promise<number> {
  const row = (await db.get<{ total: number }>(
    `SELECT COALESCE(SUM(amount_paise), 0) AS total FROM payment_allocation WHERE document_id=?`,
    documentId))!
  return row.total
}

/** Credit notes are "negative" only here, in settlement math (Task 8 stores them positive). */
async function creditedTo(db: DB, invoiceId: string): Promise<number> {
  const row = (await db.get<{ total: number }>(
    `SELECT COALESCE(SUM(payable_paise), 0) AS total FROM document
     WHERE doc_type='CREDIT_NOTE' AND ref_doc_id=? AND status != 'cancelled'`,
    invoiceId))!
  return row.total
}

async function outstandingOf(db: DB, doc: Doc): Promise<number> {
  return doc.payablePaise - await allocatedTo(db, doc.id) - await creditedTo(db, doc.id)
}

/** Exported outstanding read for the scheduler and dashboard. */
export async function outstandingPaise(db: DB, docId: string): Promise<number> {
  const doc = await getDocument(db, docId)
  return doc === null ? 0 : outstandingOf(db, doc)
}

/** Status flips: any allocation > 0 → part_paid; outstanding ≤ 0 → paid. */
async function refreshSettlementStatus(db: DB, userId: string, docId: string): Promise<void> {
  const doc = await getDocument(db, docId)
  if (doc === null || doc.status === 'cancelled') return
  const allocated = await allocatedTo(db, docId)
  const outstanding = doc.payablePaise - allocated - await creditedTo(db, docId)
  const status = outstanding <= 0 ? 'paid' : allocated > 0 ? 'part_paid' : doc.status
  if (status === doc.status) return
  await db.run(`UPDATE document SET status=? WHERE id=?`, status, docId)
  await writeAudit(db, userId, 'update', 'document', docId, { status: doc.status }, { status })
}

/** Issued, unsettled invoices for the client, oldest first. */
async function unsettledInvoices(db: DB, clientId: string): Promise<Doc[]> {
  const ids = await db.all<{ id: string }>(
    `SELECT id FROM document
     WHERE client_id=? AND doc_type='INVOICE' AND doc_no IS NOT NULL
       AND status NOT IN ('cancelled', 'paid')
     ORDER BY doc_date, doc_no`,
    clientId)
  const docs: Doc[] = []
  for (const r of ids) docs.push((await getDocument(db, r.id))!)
  return docs
}

// ---------- recording ----------

export interface AllocationInput { documentId: string; amountPaise: number }

export interface RecordPaymentInput {
  clientId: string; receivedOn: string; mode: PaymentMode; reference?: string
  amountPaise: number; tdsPaise?: number; allocations?: AllocationInput[]
  issueReceipt?: boolean
}

export interface RecordPaymentResult {
  payment: Payment
  allocated: { documentId: string; amountPaise: number }[]
  receipt?: Doc
}

export async function recordPayment(db: DB, userId: string, input: RecordPaymentInput): Promise<RecordPaymentResult> {
  if (await getClient(db, input.clientId) === null) throw new Error('Client not found')
  if (!ALL_MODES.includes(input.mode)) throw new Error(`Unknown payment mode: ${input.mode}`)
  if (!Number.isInteger(input.amountPaise) || input.amountPaise <= 0) {
    throw new Error('amountPaise must be a positive integer (paise)')
  }
  const tdsPaise = input.tdsPaise ?? 0
  if (!Number.isInteger(tdsPaise) || tdsPaise < 0) {
    throw new Error('tdsPaise must be a non-negative integer (paise)')
  }
  // D24 (red-team): TDS settles invoices from the same pool as cash, so an unbounded TDS
  // could 'pay' an invoice with no money. TDS is a fraction withheld from what's billed;
  // it can never exceed the cash actually received (a generous ceiling — real TDS on
  // professional services is 10%). Blocks fabricated-TDS settlement.
  if (tdsPaise > input.amountPaise) {
    throw new Error('TDS cannot exceed the cash amount received on the payment')
  }
  return db.transaction(async () => {
    const id = uuidv7()
    await db.run(
      `INSERT INTO payment (id, client_id, received_on, mode, reference, amount_paise, tds_paise,
         created_by, created_at)
       VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)`,
      id, input.clientId, input.receivedOn, input.mode, input.reference ?? '',
      input.amountPaise, tdsPaise, userId, new Date().toISOString())
    await writeAudit(db, userId, 'create', 'payment', id, undefined, {
      clientId: input.clientId, receivedOn: input.receivedOn, mode: input.mode,
      amountPaise: input.amountPaise, tdsPaise,
    })

    // One pool: TDS certificates settle invoices just like money received.
    let pool = input.amountPaise + tdsPaise
    const allocated: { documentId: string; amountPaise: number }[] = []
    const insertAllocation = async (documentId: string, amountPaise: number): Promise<void> => {
      const allocId = uuidv7()
      await db.run(
        `INSERT INTO payment_allocation (id, payment_id, document_id, amount_paise) VALUES (?, ?, ?, ?)`,
        allocId, id, documentId, amountPaise)
      await writeAudit(db, userId, 'create', 'payment_allocation', allocId, undefined, {
        paymentId: id, documentId, amountPaise,
      })
      allocated.push({ documentId, amountPaise })
      await refreshSettlementStatus(db, userId, documentId)
    }

    if (input.allocations !== undefined) {
      // Explicit allocations bypass the oldest-first loop, but never exceed
      // an invoice's outstanding or the payment's settling power.
      for (const a of input.allocations) {
        if (!Number.isInteger(a.amountPaise) || a.amountPaise <= 0) {
          throw new Error('Allocation amountPaise must be a positive integer (paise)')
        }
        const doc = await getDocument(db, a.documentId)
        if (doc === null) throw new Error(`Document not found: ${a.documentId}`)
        if (doc.docType !== 'INVOICE') throw new Error(`Can only allocate payments to invoices: ${a.documentId}`)
        if (doc.docNo === null) throw new Error(`Invoice is not issued: ${a.documentId}`)
        if (doc.status === 'cancelled') throw new Error(`Invoice is cancelled: ${doc.docNo}`)
        if (doc.clientId !== input.clientId) throw new Error(`Invoice ${doc.docNo} belongs to another client`)
        const outstanding = await outstandingOf(db, doc)
        if (a.amountPaise > outstanding) {
          throw new Error(`Allocation ${a.amountPaise} exceeds outstanding ${outstanding} on ${doc.docNo}`)
        }
        if (a.amountPaise > pool) {
          throw new Error(`Allocations exceed the payment's settling power (amount + TDS)`)
        }
        pool -= a.amountPaise
        await insertAllocation(doc.id, a.amountPaise)
      }
    } else {
      // Default: oldest-invoice-first, each up to its outstanding; leftover
      // stays unallocated (an advance).
      for (const doc of await unsettledInvoices(db, input.clientId)) {
        if (pool <= 0) break
        const take = Math.min(pool, await outstandingOf(db, doc))
        if (take <= 0) continue
        pool -= take
        await insertAllocation(doc.id, take)
      }
    }

    const receipt = input.issueReceipt === true ? await receiptForPayment(db, userId, id) : undefined
    return { payment: (await getPayment(db, id))!, allocated, ...(receipt !== undefined ? { receipt } : {}) }
  })
}

/** Build a RECEIPT for an already-recorded payment from its stored allocations. */
export async function receiptForPayment(db: DB, userId: string, paymentId: string): Promise<Doc> {
  const p = await getPayment(db, paymentId)
  if (p === null) throw new Error('Payment not found')
  const allocs = await db.all<{ amount_paise: number; doc_no: string | null }>(
    `SELECT a.amount_paise, d.doc_no FROM payment_allocation a
     JOIN document d ON d.id = a.document_id WHERE a.payment_id=?`,
    paymentId)
  return generateReceipt(db, userId, {
    clientId: p.clientId, paymentId: p.id, receivedOn: p.receivedOn, mode: p.mode,
    reference: p.reference, amountPaise: p.amountPaise, tdsPaise: p.tdsPaise,
    allocations: allocs.map((a) => ({ docNo: a.doc_no ?? '—', amountPaise: a.amount_paise })),
  })
}

// ---------- read views ----------

export interface ClientLedger { documents: Doc[]; payments: Payment[]; advancePaise: number }

export async function clientLedger(db: DB, clientId: string): Promise<ClientLedger> {
  const payments = await listPayments(db, clientId)
  const settling = payments.reduce((sum, p) => sum + p.amountPaise + p.tdsPaise, 0)
  const row = (await db.get<{ total: number }>(
    `SELECT COALESCE(SUM(a.amount_paise), 0) AS total
     FROM payment_allocation a JOIN payment p ON p.id = a.payment_id
     WHERE p.client_id=?`,
    clientId))!
  // The ledger must show EVERY document for the client (rule 8): walk the
  // paginated repo in max-size batches instead of trusting one page.
  const documents: Doc[] = []
  for (let page = 1; ; page++) {
    const batch = await listDocuments(db, { clientId, page, pageSize: 200 })
    documents.push(...batch.documents)
    if (page * batch.pageSize >= batch.total) break
  }
  return {
    documents,
    payments,
    advancePaise: settling - row.total,
  }
}

/**
 * Largest-remainder pro-rata split (same technique as allocateProRata in
 * packages/billing-engine/src/compute.ts — reimplemented locally so engine
 * internals stay unexported). Shares sum to exactly `amount`.
 */
export function splitProRata(amount: number, weights: number[]): number[] {
  const totalWeight = weights.reduce((a, w) => a + w, 0)
  if (amount === 0) return weights.map(() => 0)
  if (totalWeight === 0) throw new Error('Cannot allocate a nonzero amount over zero total weight')
  const raw = weights.map((w) => (amount * w) / totalWeight)
  const shares = raw.map(Math.floor)
  let remainder = amount - shares.reduce((a, s) => a + s, 0)
  const order = raw
    .map((r, i) => ({ frac: r - Math.floor(r), i }))
    .sort((a, b) => b.frac - a.frac || a.i - b.i)
  for (let k = 0; remainder > 0; k = (k + 1) % order.length, remainder--) shares[order[k]!.i]! += 1
  return shares
}

export interface ModulePaidRow { moduleId: string; billedPaise: number; settledPaise: number }

/**
 * Per-module billed vs settled: invoice-level settlement spread pro-rata
 * across the invoice's lines by lineTotalPaise (largest-remainder, so the
 * split sums exactly).
 */
export async function modulePaidView(db: DB, clientId: string): Promise<ModulePaidRow[]> {
  const invoices = await db.all<{ id: string }>(
    `SELECT id FROM document
     WHERE client_id=? AND doc_type='INVOICE' AND doc_no IS NOT NULL AND status != 'cancelled'
     ORDER BY doc_date, doc_no`,
    clientId)
  const acc = new Map<string, { billedPaise: number; settledPaise: number }>()
  for (const { id } of invoices) {
    const inv = (await getDocument(db, id))!
    const settledTotal = Math.min(
      inv.payablePaise, await allocatedTo(db, inv.id) + await creditedTo(db, inv.id),
    )
    const shares = splitProRata(settledTotal, inv.payload.lines.map((l) => l.lineTotalPaise))
    inv.payload.lines.forEach((line, i) => {
      const entry = acc.get(line.itemId) ?? { billedPaise: 0, settledPaise: 0 }
      entry.billedPaise += line.lineTotalPaise
      entry.settledPaise += shares[i]!
      acc.set(line.itemId, entry)
    })
  }
  return [...acc.entries()].map(([moduleId, v]) => ({ moduleId, ...v }))
}