qubit-budget 0.3.0

Dependency-light resource limit and budget accounting primitives for Qubit Rust crates
Documentation 
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// =============================================================================
//    Copyright (c) 2026 Haixing Hu.
//
//    SPDX-License-Identifier: Apache-2.0
//
//    Licensed under the Apache License, Version 2.0.
// =============================================================================
//! Bounded coefficient and scale checks for `BigDecimal`.

use bigdecimal::BigDecimal;

use super::BigDecimalLimitsBuilder;
use super::BigIntegerLimits;
use crate::resource::MeasuredBudgetError;
use crate::resource::ResourceLimit;
use crate::resource::ResourceQuantity;

/// Composes coefficient limits with an absolute scale limit.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct BigDecimalLimits<R, Q = u64>
where
    Q: ResourceQuantity,
{
    coefficient: BigIntegerLimits<R, Q>,
    max_scale_magnitude: Option<ResourceLimit<R, Q>>,
}

impl<R, Q> BigDecimalLimits<R, Q>
where
    Q: ResourceQuantity,
{
    /// Creates limits with no configured decimal bounds.
    #[inline]
    #[must_use]
    pub const fn new() -> Self {
        Self {
            coefficient: BigIntegerLimits::new(),
            max_scale_magnitude: None,
        }
    }

    /// Creates a builder for decimal limits.
    #[inline]
    #[must_use]
    pub const fn builder() -> BigDecimalLimitsBuilder<R, Q> {
        BigDecimalLimitsBuilder::new()
    }

    /// Converts these limits into a builder for further configuration.
    #[inline]
    #[must_use]
    pub const fn into_builder(self) -> BigDecimalLimitsBuilder<R, Q> {
        BigDecimalLimitsBuilder::from_limits(self)
    }

    /// Returns the coefficient limits.
    #[must_use]
    #[inline(always)]
    pub const fn coefficient_limits(&self) -> &BigIntegerLimits<R, Q> {
        &self.coefficient
    }

    /// Returns the configured scale-magnitude limit, if any.
    #[must_use]
    #[inline(always)]
    pub const fn scale_magnitude_limit(&self) -> Option<&ResourceLimit<R, Q>> {
        self.max_scale_magnitude.as_ref()
    }

    /// Checks scale before checking the borrowed coefficient.
    #[inline]
    pub fn check(&self, value: &BigDecimal) -> Result<(), MeasuredBudgetError<R, Q>>
    where
        R: Clone,
    {
        let (coefficient, scale) = value.as_bigint_and_scale();
        if let Some(limit) = self.max_scale_magnitude.as_ref() {
            let magnitude = Q::try_from_u64(scale.unsigned_abs())
                .map_err(|source| MeasuredBudgetError::quantity(limit.resource().clone(), source))?;
            limit.check(magnitude).map_err(MeasuredBudgetError::from)?;
        }
        self.coefficient.check(coefficient.as_ref())
    }

    /// Replaces coefficient limits during builder composition.
    #[inline(always)]
    pub(super) fn set_coefficient_limits(&mut self, limits: BigIntegerLimits<R, Q>) {
        self.coefficient = limits;
    }

    /// Replaces the scale-magnitude limit during builder composition.
    #[inline(always)]
    pub(super) fn set_scale_magnitude_limit(&mut self, limit: ResourceLimit<R, Q>) {
        self.max_scale_magnitude = Some(limit);
    }
}

impl<R, Q> Default for BigDecimalLimits<R, Q>
where
    Q: ResourceQuantity,
{
    /// Creates unconfigured decimal limits.
    #[inline]
    fn default() -> Self {
        Self::new()
    }
}