89321e617d
generate/rate_comprehension/conversion.py: convert_time(value, from_unit, to_unit) -> Fraction for known time units (minute<->hour), exact rational (fractions.Fraction), NO floats. is_convertible gates it. 30 minute -> Fraction(1,2) hour; non-time/non-convertible (minute<->mile, dollar<->hour) raises ConversionError so the caller still refuses rate_unit_mismatch. Tiny: only minute<->hour in v1 (length/currency/compound deferred). 6 tests incl. never-a-float guard.
49 lines
1.9 KiB
Python
49 lines
1.9 KiB
Python
"""Exact rational time-unit conversion for R3.2 (single-rate only).
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The single new capability of R3.2: a duration whose unit differs from the rate's denominator may be
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**converted** when both are known time units (``minute`` ↔ ``hour``), instead of refusing. The
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conversion is **exact rational** (``fractions.Fraction``) — never a float, never a decimal:
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```text
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30 minutes -> Fraction(1, 2) hour
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90 minutes -> Fraction(3, 2) hour
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2 hours -> Fraction(120) minute
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```
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So ``60 mile/hour × 30 minute`` becomes ``60 × 1/2 = 30 mile`` (exact). A non-time / non-convertible
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pair (``minute`` ↔ ``mile``) raises ``ConversionError`` and the caller still refuses
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``rate_unit_mismatch`` — convertibility is the only thing R3.2 adds.
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Deliberately tiny: ONLY ``minute`` ↔ ``hour`` in v1. No length (mile↔km), no currency
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(dollar↔cent), no compound conversions, no clock-time intervals. Deterministic.
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"""
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from __future__ import annotations
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from fractions import Fraction
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class ConversionError(ValueError):
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"""The units are not a known convertible pair — the caller must refuse."""
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#: Each known time unit as an exact rational number of the base unit (one hour).
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_TIME_IN_HOURS: dict[str, Fraction] = {
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"hour": Fraction(1),
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"minute": Fraction(1, 60),
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}
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def is_convertible(from_unit: str, to_unit: str) -> bool:
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"""Whether *from_unit* and *to_unit* are both known time units (so a conversion exists)."""
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return from_unit in _TIME_IN_HOURS and to_unit in _TIME_IN_HOURS
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def convert_time(value: int, from_unit: str, to_unit: str) -> Fraction:
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"""Exact rational conversion of *value* ``from_unit`` into *to_unit*. Refuses unknown units."""
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if not is_convertible(from_unit, to_unit):
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raise ConversionError(f"no exact conversion {from_unit!r} -> {to_unit!r}")
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return Fraction(value) * _TIME_IN_HOURS[from_unit] / _TIME_IN_HOURS[to_unit]
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__all__ = ["ConversionError", "convert_time", "is_convertible"]
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