Apartment Water Bill

Problem

Design ApartmentWaterBill for one apartment and one fixed 30-day month. The apartment receives ordinary water from corporation and borewell sources in a fixed ratio. Guests add demand that must be supplied by progressively priced tanker water.

Requirements

Select the base allowance. A 2BHK apartment has 900 base litres for the month, and a 3BHK apartment has 1,500 base litres.
Split base water by the exact ratio. If the ratio is corporationPart:borewellPart, corporation litres are baseLitres * corporationPart / (corporationPart + borewellPart), and borewell litres are the corresponding borewell share. Keep both shares exact and do not round them.
Accumulate guests. The guest count starts at zero. Each addGuests(count) call increases the same month's count by count.
Supply guest demand from tankers. Every guest adds exactly 300 litres, all supplied as tanker water. Guest additions do not change the corporation or borewell quantities.
Price contracted sources. Corporation water costs 1 charge unit per litre. Borewell water costs 1.5 charge units per litre. Apply each rate to its exact share.
Price tanker water progressively. Charge the first 500 tanker litres at 2 per litre, the next 1,000 at 3 per litre, the next 1,500 at 5 per litre, and every litre above 3,000 at 8 per litre. Each rate applies only to litres inside its band.
Calculate total litres. getBill() returns totalLitres equal to the base litres plus 300 times the accumulated guest count.
Aggregate the raw charge exactly. Add the exact corporation, borewell, and tanker charges. Do not round any source quantity or line-item charge before this sum.
Round only the final charge. getBill() returns totalCharge by rounding the raw aggregate charge to the nearest integer. An exact half rounds upward.
Accept positive guest additions. addGuests(count) returns true for a positive count and adds it to the cumulative guest count.
Reject nonpositive guest additions. addGuests(count) returns false when count is zero or negative.
Keep rejected additions atomic. A rejected addGuests call does not change cumulative guests. A later getBill() returns the same result it would have returned if the rejected call had not occurred.

API

Signature	Returns	Behavior
`ApartmentWaterBill(apartmentType: string, corporationPart: integer, borewellPart: integer)`	Not applicable	Creates the apartment-month with zero guests.
`addGuests(count: integer)`	`boolean`	Returns `true` and adds a positive count. Returns `false` without changing state for zero or a negative count.
`getBill()`	`BillResult`	Returns an object with exactly `totalLitres: integer` and `totalCharge: integer`.

Examples

For ApartmentWaterBill("2BHK", 3, 7):

Step	Operation	Result
1	`addGuests(2)`	`true`
2	`addGuests(3)`	`true`
3	`getBill()`	`{"totalLitres":2400,"totalCharge":5215}`

The 900 base litres split into 270 corporation litres and 630 borewell litres, costing 270 and 945. Five guests add 1,500 tanker litres, costing 500 * 2 + 1,000 * 3 = 4,000. The total is 2,400 litres and 5,215 charge units.

Constraints

apartmentType is exactly 2BHK or 3BHK.
1 <= corporationPart <= 1,000,000
1 <= borewellPart <= 1,000,000
Each addGuests call satisfies -1,000,000 <= count <= 1,000,000.
At most 100,000 public method calls occur per testcase.
The cumulative guest count is at most 1,000,000,000.
All exact intermediate numerators fit in a signed 64-bit integer.
Only inputs satisfying these constraints are supplied and judged.

Notes

Calls are sequential on one object. Meter readings, corrections, tariff changes, statements, payment, month reset, invalid constructor behavior, out-of-range guest counts, persistence, and concurrent calls are outside scope.