Design the object model and metering APIs for a smart electricity meter that accumulates consumption, prices it under a time-of-use tariff, and closes billing periods.

Operating context. One meter measures a single household supply. A metrology sensor emits a stream of interval readings (for example, watt-hours consumed in the last few seconds). The meter accumulates these into registers, and prices consumption against a tariff whose rate depends on the time of day (peak, shoulder, off-peak) and possibly the day type (weekday vs weekend). At the end of a billing period the meter snapshots the registers, produces a period bill, and rolls over cleanly to the next period. It also detects a stuck-or-reversed reading as a tamper signal. Everything runs in-process on the meter.

Out of scope. The utility head-end/back-office and its network protocols (the WAN uplink), the display firmware, prepayment/top-up token handling, disconnect-relay hardware, and cryptographic signing of exported readings.

What to produce. The class hierarchy (meter, reading, register/accumulator, tariff, rate schedule, billing period), the public API each class exposes, and the state transitions of a billing period. Be explicit about: how the tariff is a pluggable Strategy so a new rate plan drops in without touching accumulation, how period rollover is atomic (no reading is double-counted or lost across the boundary), and how the register update stays O(1) per reading.