ridl-ir 0.5.0

//! Whether the value 0 is a legal value of a typl type.
//!
//! 0 is the default a FlatBuffers reader gives an absent scalar or enum
//! field. Two consumers ask the same question and must agree on the answer:
//!
//! - the FlatBuffers codec of `ridl-backend-rust`, which reads an absent
//!   non-optional scalar or enum field as 0 when 0 is legal for its type, and
//!   refuses it as a missing required field when it is not
//!   (driftsys/ridl#472);
//! - `ridl-diff`, which classifies appending such a field as compatible only
//!   when a reader of the new version reads the field absent from an old
//!   payload as 0 (driftsys/ridl#598).
//!
//! The codec reads the lowered codegen model and the diff reads IR v2, so the
//! two call sites hold different message types. The definitions here take
//! what both types carry: a constraint's exact decimal text, and an enum's
//! declared values.

use crate::v2;

/// Whether 0 is a value of a numeric scalar's declared range and step, read
/// from the IR's exact decimal text: `min`, `max` and `step` of a
/// `Constraint`, each absent when unstated.
///
/// 0 must lie within `[min..max]`, and when a `step` is declared it must also
/// be on the grid `min + n·step` for a whole `n` (typl §4.3):
/// `[-1.5..1.5 step 1.0]` holds -1.5, -0.5, 0.5 and 1.5, and not 0. A `step`
/// with no `min` has no anchor, and a bound that is not plain decimal text
/// (`-`, digits, and an optional fractional part) is not read here; in both
/// cases the answer is `false`, so an absent field is refused rather than
/// read as a value that may not be legal. A type with no constraint at all
/// holds 0: the caller passes three `None`s.
///
/// This is decided from the declaration, for a named type and for an inline
/// constraint alike, and not by calling a generated type's `check`: `check`
/// ignores `step` (driftsys/ridl#469), and an inline constraint has no
/// `check`.
pub fn range_holds_zero(min: Option<&str>, max: Option<&str>, step: Option<&str>) -> bool {
    let read = |text: Option<&str>| -> Result<Option<Decimal>, ()> {
        match text {
            None => Ok(None),
            Some(text) => Decimal::parse(text).map(Some).ok_or(()),
        }
    };
    let (Ok(min), Ok(max), Ok(step)) = (read(min), read(max), read(step)) else {
        return false;
    };
    if min.is_some_and(|min| min.units > 0) || max.is_some_and(|max| max.units < 0) {
        return false;
    }
    match (step, min) {
        (None, _) => true,
        (Some(_), None) => false,
        // 0 = min + n·step with a whole n: -min is a whole multiple of step.
        (Some(step), Some(min)) => {
            let scale = min.scale.max(step.scale);
            match (min.rescaled(scale), step.rescaled(scale)) {
                (Some(min), Some(step)) if step > 0 => min % step == 0,
                _ => false,
            }
        }
    }
}

/// The index of the enum value whose number is 0 — the enum's zero member —
/// or `None` when the enum declares none. An enum is legal at 0 exactly when
/// it has a zero member (typl §5.8 names the same member as the enum's
/// init, when it exists).
pub fn enum_zero_member(values: &[v2::EnumValue]) -> Option<usize> {
    values.iter().position(|value| value.value == 0)
}

/// An exact decimal read from the IR's canonical text: `units / 10^scale`.
#[derive(Debug, Clone, Copy)]
struct Decimal {
    units: i128,
    scale: u32,
}

impl Decimal {
    /// Reads `-`, digits, and an optional `.` followed by digits. Anything
    /// else, or more digits than an `i128` holds exactly, is `None`.
    fn parse(text: &str) -> Option<Self> {
        let (negative, unsigned) = match text.strip_prefix('-') {
            Some(rest) => (true, rest),
            None => (false, text),
        };
        let (whole, fraction) = match unsigned.split_once('.') {
            Some((whole, fraction)) => (whole, fraction),
            None => (unsigned, ""),
        };
        let digits_only = |part: &str| part.bytes().all(|byte| byte.is_ascii_digit());
        if whole.is_empty()
            || !digits_only(whole)
            || !digits_only(fraction)
            || (unsigned.contains('.') && fraction.is_empty())
            || whole.len() + fraction.len() > 30
        {
            return None;
        }
        let units: i128 = format!("{whole}{fraction}").parse().ok()?;
        Some(Decimal {
            units: if negative { -units } else { units },
            scale: u32::try_from(fraction.len()).ok()?,
        })
    }

    /// `units` at a larger `scale`, or `None` on overflow.
    fn rescaled(self, scale: u32) -> Option<i128> {
        self.units
            .checked_mul(10i128.checked_pow(scale.checked_sub(self.scale)?)?)
    }
}

#[cfg(test)]
mod tests {
    use super::{enum_zero_member, range_holds_zero, v2};

    #[test]
    fn range_holds_zero_follows_the_range_and_the_step() {
        let cases = [
            (None, None, None, true),
            (Some("0"), Some("10"), None, true),
            (Some("1"), Some("10"), None, false),
            (Some("-10"), Some("-1"), None, false),
            // A bound of exactly 0 holds it, from either side.
            (Some("-10"), Some("0"), None, true),
            (Some("0"), Some("0"), None, true),
            (Some("-1.5"), Some("1.5"), Some("1.0"), false),
            (Some("-1.0"), Some("1.0"), Some("1.0"), true),
            (Some("0.0"), Some("1.0"), Some("0.01"), true),
            (Some("-0.25"), Some("1"), Some("0.125"), true),
            (Some("-0.3"), Some("1"), Some("0.2"), false),
            // A step with no lower bound has no anchor, so it is not decided.
            (None, Some("10"), Some("1"), false),
            // Text this reader does not accept is not decided either.
            (Some("-1e3"), Some("10"), None, false),
            (Some("-1."), Some("10"), None, false),
        ];
        for (min, max, step, legal) in cases {
            assert_eq!(
                range_holds_zero(min, max, step),
                legal,
                "min {min:?}, max {max:?}, step {step:?}"
            );
        }
    }

    #[test]
    fn the_zero_member_is_the_value_numbered_zero_in_any_position() {
        let value = |name: &str, value: i64| v2::EnumValue {
            name: name.to_string(),
            value,
            doc: String::new(),
        };
        assert_eq!(
            enum_zero_member(&[value("ON", 1), value("OFF", 0)]),
            Some(1)
        );
        assert_eq!(
            enum_zero_member(&[value("PARK", 1), value("DRIVE", 2)]),
            None
        );
    }
}