sims-hq/packages/billing-engine/src/compute.ts

import type { BillLine, BillTotals, Paise, IsoDate } from '@sims/domain'
import { resolveTaxClass, type TaxClassRow } from './tax'

export interface LineInput {
  itemId: string
  name: string
  hsn: string
  qty: number
  unitCode: string
  unitPricePaise: Paise
  priceIncludesTax: boolean
  taxClassCode: string
  mrpPaise?: Paise
  discount?: { kind: 'percentBp'; value: number } | { kind: 'amount'; paise: Paise }
  batchId?: string
}

export interface BillContext {
  businessDate: IsoDate
  /** Store's GST state code. */
  supplyStateCode: string
  /** Customer/delivery state; equal to supply state ⇒ CGST+SGST, else IGST. */
  placeOfSupplyStateCode: string
  roundToRupee: boolean
  billDiscountPaise?: Paise
}

export interface ComputedBill {
  lines: BillLine[]
  totals: BillTotals
}

/**
 * Deterministic money rules — the same engine must produce byte-identical
 * results on POS, store server, and cloud:
 * - integer paise throughout; each rounding step is explicit
 * - inclusive price: line total stays the sticker amount; taxable is derived
 * - intra-state split: cgst = floor(tax/2), sgst = tax − cgst (sum exact)
 * - bill discount allocates pro-rata by line gross, largest-remainder method
 */
export function computeBill(inputs: LineInput[], ctx: BillContext, rates: TaxClassRow[]): ComputedBill {
  if (inputs.length === 0) throw new Error('Cannot compute an empty bill')
  for (const line of inputs) {
    // Returns are separate documents (immutability rule) — never negative lines.
    if (!(line.qty > 0)) throw new Error(`Quantity must be positive for "${line.name}"`)
    if (!Number.isInteger(line.unitPricePaise) || line.unitPricePaise < 0) {
      throw new Error(`Unit price must be non-negative integer paise for "${line.name}"`)
    }
    if (line.discount !== undefined) {
      const v = line.discount.kind === 'percentBp' ? line.discount.value : line.discount.paise
      if (v < 0) throw new Error(`Discount must be non-negative for "${line.name}"`)
      if (line.discount.kind === 'percentBp' && line.discount.value > 10_000) {
        throw new Error(`Discount above 100% for "${line.name}"`)
      }
    }
  }
  if ((ctx.billDiscountPaise ?? 0) < 0) throw new Error('Bill discount must be non-negative')
  const interState = ctx.supplyStateCode !== ctx.placeOfSupplyStateCode

  const grossAfterLineDisc = inputs.map((line) => {
    const gross = Math.round(line.unitPricePaise * line.qty)
    const disc =
      line.discount === undefined
        ? 0
        : line.discount.kind === 'percentBp'
          ? Math.round((gross * line.discount.value) / 10_000)
          : line.discount.paise
    if (disc > gross) throw new Error(`Discount exceeds line amount for "${line.name}"`)
    return { gross, disc, net: gross - disc }
  })

  const billDiscShares = allocateProRata(
    ctx.billDiscountPaise ?? 0,
    grossAfterLineDisc.map((g) => g.net),
  )

  const lines: BillLine[] = inputs.map((line, i) => {
    const { gross, disc } = grossAfterLineDisc[i]!
    const discount = disc + billDiscShares[i]!
    const net = gross - discount
    const rate = resolveTaxClass(rates, line.taxClassCode, ctx.businessDate)
    const totalRateBp = rate.ratePctBp + rate.cessPctBp

    let taxable: Paise
    let taxPlusCess: Paise
    let lineTotal: Paise
    if (line.priceIncludesTax) {
      taxable = Math.round((net * 10_000) / (10_000 + totalRateBp))
      taxPlusCess = net - taxable
      lineTotal = net
    } else {
      taxable = net
      taxPlusCess = Math.round((taxable * totalRateBp) / 10_000)
      lineTotal = taxable + taxPlusCess
    }

    const cess = totalRateBp === 0 ? 0 : Math.round((taxPlusCess * rate.cessPctBp) / totalRateBp)
    const tax = taxPlusCess - cess
    const cgst = interState ? 0 : Math.floor(tax / 2)
    const sgst = interState ? 0 : tax - cgst
    const igst = interState ? tax : 0

    return {
      itemId: line.itemId,
      name: line.name,
      hsn: line.hsn,
      qty: line.qty,
      unitCode: line.unitCode,
      unitPricePaise: line.unitPricePaise,
      ...(line.mrpPaise !== undefined ? { mrpPaise: line.mrpPaise } : {}),
      grossPaise: gross,
      discountPaise: discount,
      taxablePaise: taxable,
      taxRateBp: rate.ratePctBp,
      cgstPaise: cgst,
      sgstPaise: sgst,
      igstPaise: igst,
      cessPaise: cess,
      lineTotalPaise: lineTotal,
      ...(line.batchId !== undefined ? { batchId: line.batchId } : {}),
    }
  })

  const sum = (f: (l: BillLine) => number) => lines.reduce((a, l) => a + f(l), 0)
  const total = sum((l) => l.lineTotalPaise)
  const payable = ctx.roundToRupee ? Math.round(total / 100) * 100 : total
  const savings = lines.reduce((acc, l) => {
    if (l.mrpPaise === undefined) return acc
    return acc + Math.max(0, Math.round(l.mrpPaise * l.qty) - l.lineTotalPaise)
  }, 0)

  return {
    lines,
    totals: {
      grossPaise: sum((l) => l.grossPaise),
      discountPaise: sum((l) => l.discountPaise),
      taxablePaise: sum((l) => l.taxablePaise),
      cgstPaise: sum((l) => l.cgstPaise),
      sgstPaise: sum((l) => l.sgstPaise),
      igstPaise: sum((l) => l.igstPaise),
      cessPaise: sum((l) => l.cessPaise),
      roundOffPaise: payable - total,
      payablePaise: payable,
      savingsVsMrpPaise: savings,
    },
  }
}

/** Largest-remainder allocation; shares always sum exactly to `amount`. */
export function allocateProRata(amount: Paise, weights: number[]): Paise[] {
  const totalWeight = weights.reduce((a, w) => a + w, 0)
  if (amount === 0) return weights.map(() => 0)
  if (totalWeight === 0) {
    // Never silently drop money: allocating a nonzero amount needs weight somewhere.
    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
}