runlimit-core 0.1.0

use thiserror::Error;

use crate::{CounterKey, FixedWindowPolicy, SubjectKey};

/// One proposed quota charge against a policy and opaque subject.
///
/// A newly constructed check has cost 1. Custom costs are validated against
/// the referenced policy so a backend never receives a zero-cost or
/// intrinsically impossible check.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct Check<'a> {
    policy: &'a FixedWindowPolicy,
    subject: SubjectKey,
    cost: u64,
}

impl<'a> Check<'a> {
    /// Constructs a check with the default cost of 1.
    pub const fn new(policy: &'a FixedWindowPolicy, subject: SubjectKey) -> Self {
        Self {
            policy,
            subject,
            cost: 1,
        }
    }

    /// Validates and constructs a check with a custom cost.
    ///
    /// # Errors
    ///
    /// Returns an error when `cost` is zero or exceeds the policy limit.
    pub fn with_cost(
        policy: &'a FixedWindowPolicy,
        subject: SubjectKey,
        cost: u64,
    ) -> Result<Self, CheckError> {
        validate_cost(policy, cost)?;
        Ok(Self {
            policy,
            subject,
            cost,
        })
    }

    /// Returns this check with a validated custom cost.
    ///
    /// # Errors
    ///
    /// Returns an error when `cost` is zero or exceeds the policy limit.
    pub fn try_with_cost(mut self, cost: u64) -> Result<Self, CheckError> {
        validate_cost(self.policy, cost)?;
        self.cost = cost;
        Ok(self)
    }

    /// Returns the policy evaluated by this check.
    pub const fn policy(&self) -> &'a FixedWindowPolicy {
        self.policy
    }

    /// Returns the opaque subject key evaluated by this check.
    pub const fn subject(&self) -> SubjectKey {
        self.subject
    }

    /// Returns the complete logical identity of the stored counter.
    pub const fn counter_key(&self) -> CounterKey {
        CounterKey::new(self.policy.fingerprint(), self.subject)
    }

    /// Returns the nonzero quota cost.
    pub const fn cost(&self) -> u64 {
        self.cost
    }
}

fn validate_cost(policy: &FixedWindowPolicy, cost: u64) -> Result<(), CheckError> {
    if cost == 0 {
        return Err(CheckError::ZeroCost);
    }
    if cost > policy.limit() {
        return Err(CheckError::CostExceedsLimit {
            cost,
            limit: policy.limit(),
        });
    }
    Ok(())
}

/// An invalid check cost.
#[derive(Clone, Copy, Debug, Error, Eq, PartialEq)]
pub enum CheckError {
    /// The requested cost was zero.
    #[error("check cost must be greater than zero")]
    ZeroCost,
    /// The requested cost exceeded the referenced policy's limit.
    #[error("check cost ({cost}) exceeds the policy limit ({limit})")]
    CostExceedsLimit {
        /// Requested cost.
        cost: u64,
        /// Maximum cost accepted by the policy.
        limit: u64,
    },
}

#[cfg(test)]
mod tests {
    use std::time::Duration;

    use super::{Check, CheckError};
    use crate::{FixedWindowPolicy, PolicyId, ScopeId, SubjectKey};

    fn policy() -> FixedWindowPolicy {
        FixedWindowPolicy::new(
            PolicyId::new("auth.login").unwrap(),
            ScopeId::new("client").unwrap(),
            8,
            Duration::from_secs(60),
        )
        .unwrap()
    }

    #[test]
    fn defaults_to_one_unit_of_cost() {
        let policy = policy();
        let subject = SubjectKey::from_digest([1; 32]);
        let check = Check::new(&policy, subject);

        assert_eq!(check.policy(), &policy);
        assert_eq!(check.subject(), subject);
        assert_eq!(check.cost(), 1);
    }

    #[test]
    fn accepts_cost_up_to_and_including_the_limit() {
        let policy = policy();
        let subject = SubjectKey::from_digest([2; 32]);

        assert_eq!(Check::with_cost(&policy, subject, 3).unwrap().cost(), 3);
        assert_eq!(
            Check::new(&policy, subject)
                .try_with_cost(policy.limit())
                .unwrap()
                .cost(),
            policy.limit()
        );
    }

    #[test]
    fn rejects_zero_cost() {
        let policy = policy();

        assert_eq!(
            Check::with_cost(&policy, SubjectKey::from_digest([3; 32]), 0),
            Err(CheckError::ZeroCost)
        );
    }

    #[test]
    fn rejects_cost_above_policy_limit() {
        let policy = policy();

        assert_eq!(
            Check::with_cost(&policy, SubjectKey::from_digest([4; 32]), 9),
            Err(CheckError::CostExceedsLimit { cost: 9, limit: 8 })
        );
    }
}