Design constraints

The rules every module follows: purity, exact arithmetic with one float boundary and a named rounding for every cut, determinism, one string per code, the serde representation, and the non_exhaustive policy.

The rules every module follows. Guides link here rather than restate them.

Purity

No I/O, no async, no clock, no environment. Where an instant matters — the now of a validation — it is an argument. Clippy's disallowed-methods (clippy.toml, run by just purity) refuses the clock, std::env and std::fs in the library. Runtime dependencies are rust_decimal, thiserror, time and time-tz, whose db feature embeds the IANA database; serde is optional.

Exactness

Every quantity is a Decimal, from input to result. There is one float crossing: the SigLinDe sigmoid raises to a non-integer power, so SigLinDe::h_value evaluates it in f64 and returns a Decimal. A source scan refuses an f64 ↔ Decimal conversion anywhere else.

One cut, named

Sums, differences and products do not round, so conservation holds to the digit: a register split reconstructs its Arbeitsmenge, a Lastgang sums to the difference of its outer readings, Σ allocated + residual = pool.

A quotient that does not terminate is cut once, where it is formed, to a named *_DP constant with a named *_STRATEGY from metering::precision; a bare round_dp( in src/ fails a source scan. The default mode is kaufmännisch wherever no source states one:

The SLP-Gas h-value is not rounded (Anlage 5: no rounding after the computation). Where the rounding is the market's choice — the G 685 final amount — it is a parameter.

Multiply before you divide. a × c ÷ b rounds once; a ÷ b × c scales the quotient's error by c. A source scan rejects a division by a simple term followed by a multiplication.

Determinism

Equal inputs give equal outputs, independent of input order (proptests shuffle and draw ties on purpose). Where a value cannot be determined the answer is None or an error, never a benign default: no Default for a Brennwert, no Spitzenleistung across a coarse grid, no market code where the market has none, no dynamisation factor for day 400.

One string per code

Where EDI@Energy publishes a code list, the crate is that list — SubstitutionReason is STS+Z40, Method::market_code is STS+Z32 per Sparte, QualityFlag::market_code is the QTY qualifier. Every coded enum has ALL, CODES, as_str, Display, FromStr, and a serde tag that is the as_str code, so a database CHECK generated from CODES cannot drift. A code (BUSS_UND_BETTAG) and a description (Buß- und Bettag) stay apart; parsing accepts input aliases that are never written back.

Every value with a string form has one canonical spelling:

use metering::{ObisCode, Resolution};

let a: ObisCode = "1-0:1.8.0*255".parse().unwrap();
let b: ObisCode = " 1-0:01.8.0 ".parse().unwrap();
assert_eq!(a, b);
assert_eq!(a.to_string(), "1-0:1.8.0");
assert_eq!(format!("{a:#}"), "1-0:1.8.0*255");

assert_eq!(Resolution::from_seconds(900), Some(Resolution::QUARTER_HOUR));
assert_eq!(Resolution::QUARTER_HOUR.to_string().parse::<Resolution>(), Ok(Resolution::QUARTER_HOUR));

Serde

The serde feature enables serde and time/serde, and nothing else.

Which types. The coded enums, the identifiers (MaloId, MeloId, BdewCode, Eic, ObisCode), Resolution and Period, the inputs (MeterInterval, Series, MeterReading, Formula, Zaehlzeitdefinition, the gas and § 14a parameters), and the results of billing, validation, power quality, rollout, community allocation and the forecast (BillingPeriod, Report, En50160Report, …). Not: the error types; the configurations Rules, Policy and LastgangConfig; and the results of substitute, to_lastgang, split_energy, the § 42c assessment, eeg and heat — record the fields you need from those.

The representation is part of the API and covered by semver; a test pins every tag literally.

A quantity is the exact decimal string in every format; a JSON number is a type error, and excess digits are refused rather than rounded. The representation is set per field, not through a rust_decimal feature, which would change every Decimal in your build graph. ObisCode travels as its IEC 62056 string, Resolution as its ISO 8601 duration.

#[non_exhaustive]

Structs with private fields are built through validating constructors — MeterInterval::new, Series::new, Formula::new — so an invalid value cannot be expressed.