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

import { describe, expect, it } from 'vitest'
import {
  allocateProRata, computeBill, resolveTaxClass,
  type LineInput, type TaxClassRow,
} from '@sims/billing-engine'

// Rates as dated config — includes a GST-2.0-style change on 2025-09-22.
const RATES: TaxClassRow[] = [
  { classCode: 'STD18', ratePctBp: 1800, cessPctBp: 0, effectiveFrom: '2017-07-01' },
  { classCode: 'ZERO', ratePctBp: 0, cessPctBp: 0, effectiveFrom: '2017-07-01' },
  { classCode: 'FMCG', ratePctBp: 1200, cessPctBp: 0, effectiveFrom: '2017-07-01', effectiveTo: '2025-09-21' },
  { classCode: 'FMCG', ratePctBp: 500, cessPctBp: 0, effectiveFrom: '2025-09-22' },
  { classCode: 'DEMERIT', ratePctBp: 2800, cessPctBp: 1200, effectiveFrom: '2017-07-01' },
]

const INTRA = {
  businessDate: '2026-07-09',
  supplyStateCode: '32',
  placeOfSupplyStateCode: '32',
  roundToRupee: true,
}

const line = (over: Partial<LineInput>): LineInput => ({
  itemId: 'i1',
  name: 'Item',
  hsn: '1101',
  qty: 1,
  unitCode: 'PCS',
  unitPricePaise: 10_000,
  priceIncludesTax: false,
  taxClassCode: 'STD18',
  ...over,
})

describe('resolveTaxClass — dated rates', () => {
  it('resolves the rate in force on the business date', () => {
    expect(resolveTaxClass(RATES, 'FMCG', '2025-09-21').ratePctBp).toBe(1200)
    expect(resolveTaxClass(RATES, 'FMCG', '2025-09-22').ratePctBp).toBe(500)
  })
  it('fails loudly when no rate covers the date', () => {
    expect(() => resolveTaxClass(RATES, 'MISSING', '2026-01-01')).toThrow(/No tax rate/)
  })
})

describe('computeBill — golden cases (exact paise)', () => {
  it('A: MRP-inclusive B2C intra-state, 18% — sticker total is preserved', () => {
    // ₹285 incl. 18%: taxable = round(28500·10000/11800) = 24153, tax = 4347
    const { lines, totals } = computeBill(
      [line({ unitPricePaise: 28_500, priceIncludesTax: true, mrpPaise: 28_500 })],
      INTRA,
      RATES,
    )
    expect(lines[0]).toMatchObject({
      taxablePaise: 24_153,
      cgstPaise: 2_173,
      sgstPaise: 2_174,
      igstPaise: 0,
      lineTotalPaise: 28_500,
    })
    expect(totals.payablePaise).toBe(28_500)
    expect(totals.roundOffPaise).toBe(0)
    expect(totals.savingsVsMrpPaise).toBe(0)
  })

  it('B: tax-exclusive B2B inter-state — IGST, no split', () => {
    const ctx = { ...INTRA, placeOfSupplyStateCode: '27', roundToRupee: false }
    const { lines, totals } = computeBill([line({})], ctx, RATES)
    expect(lines[0]).toMatchObject({
      taxablePaise: 10_000,
      igstPaise: 1_800,
      cgstPaise: 0,
      sgstPaise: 0,
      lineTotalPaise: 11_800,
    })
    expect(totals.payablePaise).toBe(11_800)
  })

  it('C: fractional weighed qty at 0% — 1.25 kg × ₹32/kg', () => {
    const { lines } = computeBill(
      [line({ qty: 1.25, unitCode: 'KG', unitPricePaise: 3_200, taxClassCode: 'ZERO', priceIncludesTax: true })],
      INTRA,
      RATES,
    )
    expect(lines[0]!.lineTotalPaise).toBe(4_000)
    expect(lines[0]!.taxablePaise).toBe(4_000)
  })

  it('D: bill discount allocates pro-rata (largest remainder), taxes recompute per line', () => {
    const { lines, totals } = computeBill(
      [
        line({ unitPricePaise: 28_500, priceIncludesTax: true }),
        line({ itemId: 'i2', unitPricePaise: 4_000, taxClassCode: 'ZERO', priceIncludesTax: true }),
      ],
      { ...INTRA, billDiscountPaise: 1_000 },
      RATES,
    )
    // shares of 1000 over weights 28500:4000 → 877 + 123
    expect(lines[0]!.discountPaise).toBe(877)
    expect(lines[1]!.discountPaise).toBe(123)
    // line 1 net 27623 incl 18% → taxable 23409, tax 4214 split 2107/2107
    expect(lines[0]).toMatchObject({ taxablePaise: 23_409, cgstPaise: 2_107, sgstPaise: 2_107 })
    expect(totals.payablePaise).toBe(31_500)
    expect(totals.discountPaise).toBe(1_000)
  })

  it('E: rupee round-off — ₹99.50 + 18% = ₹117.41 → payable ₹117.00', () => {
    const { totals } = computeBill([line({ unitPricePaise: 9_950 })], INTRA, RATES)
    expect(totals.roundOffPaise).toBe(-41)
    expect(totals.payablePaise).toBe(11_700)
  })

  it('F: cess rides on top and splits out exactly (28% + 12% cess)', () => {
    const { lines } = computeBill([line({ taxClassCode: 'DEMERIT' })], INTRA, RATES)
    expect(lines[0]).toMatchObject({
      taxablePaise: 10_000,
      cessPaise: 1_200,
      cgstPaise: 1_400,
      sgstPaise: 1_400,
      lineTotalPaise: 14_000,
    })
  })

  it('G: savings vs MRP reported when selling below sticker', () => {
    const { totals } = computeBill(
      [line({ unitPricePaise: 27_000, priceIncludesTax: true, mrpPaise: 28_500 })],
      INTRA,
      RATES,
    )
    expect(totals.savingsVsMrpPaise).toBe(1_500)
  })

  it('rejects empty bills, non-positive qty, and negative discounts', () => {
    expect(() => computeBill([], INTRA, RATES)).toThrow(/empty/)
    expect(() => computeBill([line({ qty: 0 })], INTRA, RATES)).toThrow(/positive/)
    expect(() =>
      computeBill([line({ discount: { kind: 'amount', paise: -5 } })], INTRA, RATES),
    ).toThrow(/non-negative/)
  })
})

describe('allocateProRata', () => {
  it('always sums exactly to the amount', () => {
    for (const [amount, weights] of [
      [1000, [28_500, 4_000]],
      [999, [1, 1, 1]],
      [7, [100, 200, 300, 400]],
    ] as const) {
      const shares = allocateProRata(amount, [...weights])
      expect(shares.reduce((a, s) => a + s, 0)).toBe(amount)
    }
  })
  it('refuses to drop money on zero total weight', () => {
    expect(() => allocateProRata(100, [0, 0])).toThrow(/zero total weight/)
  })
})