metering 0.15.0

//! `IntervalResolution` — typed interval length enum replacing raw `u32` seconds.
//!
//! ## Why typed?
//!
//! Raw `u32` seconds (e.g. `900`) are error-prone and opaque. `IntervalResolution`
//! makes the intended granularity explicit and prevents confusion between hourly
//! and daily data in billing aggregation.
//!
//! ## Fixed vs calendar resolutions
//!
//! | Resolution | Length | Kind |
//! |---|---|---|
//! | `QuarterHour` | 900 s | fixed |
//! | `HalfHour` | 1800 s | fixed |
//! | `Hour` | 3600 s | fixed |
//! | `Custom(n)` | n s | fixed |
//! | `Day` | 23 h, 24 h **or 25 h** | calendar |
//! | `Month` | 28–31 days ±1 h | calendar |
//! | `Year` | 365 or 366 days | calendar |
//!
//! [`IntervalResolution::fixed_seconds`] returns `Some` only for the fixed
//! group. The calendar group has no second count that is right on every date —
//! a German day is 23 hours long each spring and 25 each autumn — so those
//! lengths must be resolved against an actual date via [`crate::calendar`]:
//!
//! ```rust
//! use metering::{IntervalResolution, calendar};
//! use time::macros::date;
//!
//! assert_eq!(IntervalResolution::QuarterHour.fixed_seconds(), Some(900));
//! assert_eq!(IntervalResolution::Day.fixed_seconds(), None);
//!
//! // A day's real length comes from the calendar, not the enum.
//! assert_eq!(calendar::day_length(date!(2026 - 10 - 25)).whole_hours(), 25);
//! ```

use std::fmt;
use std::str::FromStr;

#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

use crate::error::ParseError;

/// Typed interval resolution for meter data time series.
///
/// The canonical string form is an ISO 8601 duration (`PT15M`, `P1D`, …), which
/// [`Display`](fmt::Display) writes and [`FromStr`] reads. [`label`] gives the
/// German name used on invoices and in operator UIs.
///
/// [`label`]: IntervalResolution::label
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub enum IntervalResolution {
    /// 15-minute intervals (900 s) — standard for RLM, iMSys, SMGW.
    QuarterHour,
    /// 30-minute intervals (1800 s) — some legacy MSB systems.
    HalfHour,
    /// Hourly intervals (3600 s) — Gas, some SLP reconstructed profiles.
    Hour,
    /// One Berlin calendar day — 23 h, 24 h or 25 h depending on DST.
    Day,
    /// One Berlin calendar month — billing period granularity.
    Month,
    /// One Berlin calendar year — 365 or 366 days.
    Year,
    /// Custom interval length in seconds (for non-standard cases).
    Custom(u32),
}

impl IntervalResolution {
    /// The interval length in seconds, when it is the same on every date.
    ///
    /// `None` for [`Day`](Self::Day), [`Month`](Self::Month) and
    /// [`Year`](Self::Year), whose lengths depend on the calendar and on DST.
    /// Use [`crate::calendar::day_length`], [`crate::calendar::month_length`] or
    /// [`crate::calendar::year_length`] for those.
    ///
    /// Returning `None` rather than an approximation is deliberate: a caller
    /// computing "how many 15-minute intervals should this day hold" from a flat
    /// 86 400 gets 96 on every day of the year, which raises a false alarm on the
    /// 23-hour spring day and — worse — hides a genuine four-interval gap on the
    /// 25-hour autumn one.
    /// `Custom(0)` is also `None`: a zero-length interval is not a resolution,
    /// and returning `Some(0)` would hand every caller a division by zero.
    #[must_use]
    pub const fn fixed_seconds(self) -> Option<u32> {
        match self {
            Self::QuarterHour => Some(900),
            Self::HalfHour => Some(1800),
            Self::Hour => Some(3600),
            Self::Custom(0) => None,
            Self::Custom(s) => Some(s),
            Self::Day | Self::Month | Self::Year => None,
        }
    }

    /// A nominal length in seconds, for sizing and ordering only.
    ///
    /// `Day` reports 24 h, `Month` 30 days and `Year` 365 days — none of which
    /// is reliably true. **Never use this for interval counts, coverage checks or
    /// billing arithmetic**; it exists to pre-allocate buffers, order resolutions
    /// by coarseness and render rough labels. [`fixed_seconds`] is the arithmetic
    /// accessor and refuses to guess.
    ///
    /// [`fixed_seconds`]: Self::fixed_seconds
    #[must_use]
    pub const fn nominal_seconds(self) -> u32 {
        match self {
            Self::Day => 86_400,
            Self::Month => 30 * 86_400,
            Self::Year => 365 * 86_400,
            other => match other.fixed_seconds() {
                Some(s) => s,
                None => 0,
            },
        }
    }

    /// `true` when the length is the same on every date — everything but
    /// `Day`, `Month`, `Year` and the degenerate `Custom(0)`.
    #[must_use]
    pub const fn is_fixed(self) -> bool {
        self.fixed_seconds().is_some()
    }

    /// `true` when the length depends on the calendar: `Day`, `Month`, `Year`.
    #[must_use]
    pub const fn is_calendar(self) -> bool {
        matches!(self, Self::Day | Self::Month | Self::Year)
    }

    /// Human-readable label (German).
    #[must_use]
    pub const fn label(self) -> &'static str {
        match self {
            Self::QuarterHour => "15-Minuten",
            Self::HalfHour => "30-Minuten",
            Self::Hour => "Stunde",
            Self::Day => "Tag",
            Self::Month => "Monat",
            Self::Year => "Jahr",
            Self::Custom(_) => "Benutzerdefiniert",
        }
    }

    /// The canonical ISO 8601 duration form, as written by [`Display`](fmt::Display)
    /// and read by [`FromStr`].
    ///
    /// `Custom(n)` renders as `PT{n}S`, so every value round-trips.
    #[must_use]
    pub fn to_iso8601(self) -> String {
        match self {
            Self::QuarterHour => "PT15M".to_owned(),
            Self::HalfHour => "PT30M".to_owned(),
            Self::Hour => "PT1H".to_owned(),
            Self::Day => "P1D".to_owned(),
            Self::Month => "P1M".to_owned(),
            Self::Year => "P1Y".to_owned(),
            Self::Custom(s) => format!("PT{s}S"),
        }
    }

    /// Create from raw seconds. Returns `None` for zero.
    ///
    /// Never returns [`Day`](Self::Day), [`Month`](Self::Month) or
    /// [`Year`](Self::Year): those are calendar periods, and 86 400 s is only
    /// *usually* a day. `from_seconds(86_400)` is therefore `Custom(86_400)` —
    /// a fixed 24-hour window, which is a different thing from a German
    /// calendar day and is treated as such by [`crate::calendar`].
    #[must_use]
    pub const fn from_seconds(s: u32) -> Option<Self> {
        match s {
            0 => None,
            900 => Some(Self::QuarterHour),
            1800 => Some(Self::HalfHour),
            3600 => Some(Self::Hour),
            _ => Some(Self::Custom(s)),
        }
    }

    /// `true` when this resolution supports real-time or near-real-time data.
    ///
    /// Quarter-hour and half-hour are the relevant resolutions for iMSys / SMGW.
    #[must_use]
    pub const fn is_subhourly(self) -> bool {
        matches!(self, Self::QuarterHour | Self::HalfHour)
    }
}

impl fmt::Display for IntervalResolution {
    /// Writes the ISO 8601 duration form, which [`FromStr`] reads back.
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(&self.to_iso8601())
    }
}

/// The shape [`IntervalResolution`] accepts, as rendered in a [`ParseError`].
const ISO8601_FORMAT: &str = "an ISO 8601 duration such as PT15M, PT1H, P1D or PT900S";

impl FromStr for IntervalResolution {
    type Err = ParseError;

    /// Parses the ISO 8601 duration forms this crate emits, case-insensitively.
    ///
    /// Accepts the canonical spellings (`PT15M`, `PT30M`, `PT1H`, `P1D`, `P1M`,
    /// `P1Y`) plus any `PT{n}S`, `PT{n}M` or `PT{n}H`, which normalise onto the
    /// named variants where they coincide — `PT900S` is `QuarterHour`, not
    /// `Custom(900)`, so equal durations compare equal.
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let err = || ParseError::format("IntervalResolution", s, ISO8601_FORMAT);
        let upper = s.trim().to_uppercase();
        let seconds = match upper.as_str() {
            "P1D" => return Ok(Self::Day),
            "P1M" => return Ok(Self::Month),
            "P1Y" => return Ok(Self::Year),
            rest => {
                let body = rest.strip_prefix("PT").ok_or_else(err)?;
                let (digits, unit) = body.split_at(body.len().checked_sub(1).ok_or_else(err)?);
                let n: u32 = digits.parse().map_err(|_| err())?;
                match unit {
                    "S" => n,
                    "M" => n.checked_mul(60).ok_or_else(err)?,
                    "H" => n.checked_mul(3600).ok_or_else(err)?,
                    _ => return Err(err()),
                }
            }
        };
        Self::from_seconds(seconds).ok_or_else(err)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn quarter_hour_is_900s() {
        assert_eq!(IntervalResolution::QuarterHour.fixed_seconds(), Some(900));
    }

    /// The bug this API shape exists to prevent: no fixed second count is right
    /// for a calendar period, so none is offered.
    #[test]
    fn calendar_resolutions_have_no_fixed_length() {
        for r in [
            IntervalResolution::Day,
            IntervalResolution::Month,
            IntervalResolution::Year,
        ] {
            assert_eq!(r.fixed_seconds(), None, "{r} must not claim a fixed length");
            assert!(r.is_calendar() && !r.is_fixed());
        }
        for r in [
            IntervalResolution::QuarterHour,
            IntervalResolution::HalfHour,
            IntervalResolution::Hour,
            IntervalResolution::Custom(7200),
        ] {
            assert!(r.is_fixed() && !r.is_calendar(), "{r} must be fixed");
        }
    }

    #[test]
    fn from_seconds_round_trip() {
        for r in [
            IntervalResolution::QuarterHour,
            IntervalResolution::HalfHour,
            IntervalResolution::Hour,
            IntervalResolution::Custom(7200),
        ] {
            let s = r.fixed_seconds().unwrap();
            assert_eq!(IntervalResolution::from_seconds(s), Some(r));
        }
    }

    /// 86 400 s is a fixed 24-hour window, which is not the same thing as a
    /// German calendar day.
    #[test]
    fn a_day_is_not_86400_seconds() {
        assert_eq!(
            IntervalResolution::from_seconds(86_400),
            Some(IntervalResolution::Custom(86_400))
        );
    }

    #[test]
    fn zero_returns_none() {
        assert!(IntervalResolution::from_seconds(0).is_none());
    }

    #[test]
    fn custom_seconds_preserved() {
        let r = IntervalResolution::Custom(7200);
        assert_eq!(r.fixed_seconds(), Some(7200));
        assert!(!r.is_subhourly());
    }

    #[test]
    fn subhourly_detection() {
        assert!(IntervalResolution::QuarterHour.is_subhourly());
        assert!(IntervalResolution::HalfHour.is_subhourly());
        assert!(!IntervalResolution::Hour.is_subhourly());
        assert!(!IntervalResolution::Day.is_subhourly());
    }

    /// `Display` and `FromStr` are inverses over every variant, so a persisted
    /// or logged value always reads back as what was written.
    #[test]
    fn iso8601_round_trips_every_variant() {
        for r in [
            IntervalResolution::QuarterHour,
            IntervalResolution::HalfHour,
            IntervalResolution::Hour,
            IntervalResolution::Day,
            IntervalResolution::Month,
            IntervalResolution::Year,
            IntervalResolution::Custom(300),
            IntervalResolution::Custom(86_400),
        ] {
            let s = r.to_string();
            assert_eq!(s.parse::<IntervalResolution>(), Ok(r), "round trip {s}");
        }
        assert_eq!(IntervalResolution::QuarterHour.to_string(), "PT15M");
        assert_eq!(IntervalResolution::Day.to_string(), "P1D");
        assert_eq!(IntervalResolution::Custom(300).to_string(), "PT300S");
    }

    /// Equal durations parse to the same variant, whichever spelling arrives.
    #[test]
    fn equivalent_spellings_normalise() {
        assert_eq!(
            "PT900S".parse::<IntervalResolution>(),
            Ok(IntervalResolution::QuarterHour)
        );
        assert_eq!(
            "pt15m".parse::<IntervalResolution>(),
            Ok(IntervalResolution::QuarterHour)
        );
        assert_eq!(
            "PT60M".parse::<IntervalResolution>(),
            Ok(IntervalResolution::Hour)
        );
        assert_eq!(
            "PT2H".parse::<IntervalResolution>(),
            Ok(IntervalResolution::Custom(7200))
        );
    }

    #[test]
    fn invalid_resolution_strings_are_rejected() {
        for s in [
            "", "P", "PT", "15M", "PT15X", "PTM", "P1W", "PT-5S", "hourly",
        ] {
            assert!(
                s.parse::<IntervalResolution>().is_err(),
                "{s:?} must not parse"
            );
        }
    }

    /// The nominal length is explicitly an approximation; the test pins that it
    /// is never mistaken for the arithmetic accessor.
    #[test]
    fn nominal_length_is_only_a_hint() {
        assert_eq!(IntervalResolution::Day.nominal_seconds(), 86_400);
        assert_eq!(IntervalResolution::Month.nominal_seconds(), 30 * 86_400);
        assert_eq!(IntervalResolution::Year.nominal_seconds(), 365 * 86_400);
        // ...while the arithmetic accessor refuses all three.
        assert!(IntervalResolution::Month.fixed_seconds().is_none());
    }
}