import type { Doc, Payment } from './api'

/**
 * One line of a client account statement: an invoice (debit), a credit note (credit)
 * or a payment (credit), with the running balance AFTER the line is applied. Kept as a
 * pure data shape so the running-balance maths can be unit-tested without the DOM.
 */
export interface StatementRow {
  date: string
  kind: 'invoice' | 'credit_note' | 'payment'
  particulars: string
  ref: string
  /** Charge that raises what the client owes (0 when this row is a credit). */
  debitPaise: number
  /** Credit that lowers what the client owes (0 when this row is a debit). */
  creditPaise: number
  /** Cumulative balance after this row — positive = owed by client, negative = in credit. */
  balancePaise: number
}

export interface Statement {
  rows: StatementRow[]
  totalDebitPaise: number
  totalCreditPaise: number
  /** Final running balance: > 0 the client owes us, < 0 they are in credit (advance). */
  closingPaise: number
}

/**
 * Build a statement of account from the client's ledger. Only posted receivables move
 * the balance: issued (numbered), non-cancelled INVOICEs are debits and CREDIT_NOTEs are
 * credits; every payment is a credit (amount + TDS, matching how the ledger settles).
 * Quotations, proformas, receipts and drafts are not account postings and are excluded.
 * Rows are ordered by date, debits before credits on the same day (conventional).
 */
export function buildStatement(documents: Doc[], payments: Payment[]): Statement {
  type Entry = { date: string; order: number; row: Omit<StatementRow, 'balancePaise'> }
  const entries: Entry[] = []

  for (const d of documents) {
    if (d.docNo === null || d.status === 'cancelled') continue
    if (d.docType === 'INVOICE') {
      entries.push({
        date: d.docDate, order: 0,
        row: { date: d.docDate, kind: 'invoice', particulars: 'Invoice', ref: d.docNo, debitPaise: d.payablePaise, creditPaise: 0 },
      })
    } else if (d.docType === 'CREDIT_NOTE') {
      entries.push({
        date: d.docDate, order: 0,
        row: { date: d.docDate, kind: 'credit_note', particulars: 'Credit note', ref: d.docNo, debitPaise: 0, creditPaise: d.payablePaise },
      })
    }
  }
  for (const p of payments) {
    entries.push({
      date: p.receivedOn, order: 1,
      row: {
        date: p.receivedOn, kind: 'payment',
        particulars: `Payment (${p.mode})`, ref: p.reference,
        debitPaise: 0, creditPaise: p.amountPaise + p.tdsPaise,
      },
    })
  }

  entries.sort((a, b) => (a.date < b.date ? -1 : a.date > b.date ? 1 : a.order - b.order))

  let balance = 0
  let totalDebitPaise = 0
  let totalCreditPaise = 0
  const rows: StatementRow[] = entries.map((e) => {
    balance += e.row.debitPaise - e.row.creditPaise
    totalDebitPaise += e.row.debitPaise
    totalCreditPaise += e.row.creditPaise
    return { ...e.row, balancePaise: balance }
  })

  return { rows, totalDebitPaise, totalCreditPaise, closingPaise: balance }
}