physics_in_parallel 3.6.0

/*!
Deterministic displacement kernels for square-lattice pair generation.

A kernel maps indexed random values to a move length or nearest-neighbor
direction code. Sampling is a pure function of an explicit key, sweep, and
sample index, so output does not depend on Rayon worker scheduling.
*/

use std::error::Error;
use std::fmt;

use crate::parallel::parallel_chunk_len;
use rayon::prelude::*;
use serde::{Deserialize, Serialize};

use super::random::{DOMAIN_KERNEL_SAMPLE, IndexedRng, uniform_index, unit_f64};
use crate::rng::{RngConfig, RngConfigError};

/// Serializable description of one square-lattice displacement distribution.
#[derive(Debug, Clone, Copy, Deserialize, PartialEq, Serialize)]
#[serde(tag = "kind", rename_all = "snake_case", deny_unknown_fields)]
pub enum KernelType {
    PowerLaw {
        l: f64,
        c: f64,
        mu: f64,
    },
    #[serde(alias = "uniform")]
    UniformDistance {
        l: f64,
        c: f64,
    },
    NearestNeighbor {
        d: usize,
    },
}

/// Invalid displacement-kernel configuration.
#[derive(Clone, Debug, PartialEq)]
#[non_exhaustive]
pub enum KernelError {
    InvalidPowerLaw { l: f64, c: f64, mu: f64 },
    InvalidUniformDistance { l: f64, c: f64 },
    InvalidNearestNeighborDimension { dimension: usize },
    RngConfig(RngConfigError),
}

impl fmt::Display for KernelError {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::InvalidPowerLaw { l, c, mu } => write!(
                formatter,
                "power-law kernel requires finite l > c > 0 and finite mu > 0; got l={l}, c={c}, mu={mu}"
            ),
            Self::InvalidUniformDistance { l, c } => write!(
                formatter,
                "uniform-distance kernel requires finite l > c; got l={l}, c={c}"
            ),
            Self::InvalidNearestNeighborDimension { dimension } => write!(
                formatter,
                "nearest-neighbor kernel dimension must be positive; got {dimension}"
            ),
            Self::RngConfig(error) => error.fmt(formatter),
        }
    }
}

impl Error for KernelError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        match self {
            Self::RngConfig(error) => Some(error),
            _ => None,
        }
    }
}

/// Indexed displacement distribution used by kernel-configured
/// [`PairGenerator`](super::PairGenerator).
pub trait Kernel: Send + Sync {
    /// Samples one value from `(sweep, sample_index)` using unified RNG configuration.
    fn sample_indexed(
        &self,
        rng: RngConfig,
        sweep: u64,
        sample_index: u64,
    ) -> Result<f64, KernelError>;

    /// Samples a stable batch whose result does not depend on worker count.
    fn sample_batch_indexed(
        &self,
        rng: RngConfig,
        sweep: u64,
        n: usize,
    ) -> Result<Vec<f64>, KernelError>;

    /// Returns the compact configuration for this distribution.
    fn kind(&self) -> KernelType;

    /// Clones this kernel behind a trait object.
    fn boxed_clone(&self) -> Box<dyn Kernel>;
}

impl Clone for Box<dyn Kernel> {
    fn clone(&self) -> Self {
        self.boxed_clone()
    }
}

#[derive(Debug, Clone)]
pub struct PowerLawKernel {
    kind: KernelType,
    l_pow: f64,
    c_pow: f64,
}

impl PowerLawKernel {
    pub fn try_new(l: f64, c: f64, mu: f64) -> Result<Self, KernelError> {
        if !l.is_finite() || !c.is_finite() || !mu.is_finite() || l <= c || c <= 0.0 || mu <= 0.0 {
            return Err(KernelError::InvalidPowerLaw { l, c, mu });
        }
        Ok(Self {
            kind: KernelType::PowerLaw { l, c, mu },
            l_pow: l.powf(-mu),
            c_pow: c.powf(-mu),
        })
    }

    pub fn new(l: f64, c: f64, mu: f64) -> Self {
        Self::try_new(l, c, mu).expect("invalid power-law kernel configuration")
    }
}

impl Kernel for PowerLawKernel {
    fn sample_indexed(
        &self,
        rng: RngConfig,
        sweep: u64,
        sample_index: u64,
    ) -> Result<f64, KernelError> {
        let rng = IndexedRng::new(rng).map_err(KernelError::RngConfig)?;
        Ok(self.sample_resolved(rng, sweep, sample_index))
    }

    fn sample_batch_indexed(
        &self,
        rng: RngConfig,
        sweep: u64,
        n: usize,
    ) -> Result<Vec<f64>, KernelError> {
        sample_batch_resolved(self, rng, sweep, n)
    }

    fn kind(&self) -> KernelType {
        self.kind
    }

    fn boxed_clone(&self) -> Box<dyn Kernel> {
        Box::new(self.clone())
    }
}

impl PowerLawKernel {
    fn sample_resolved(&self, key: IndexedRng, sweep: u64, sample_index: u64) -> f64 {
        let (mu, c) = match self.kind {
            KernelType::PowerLaw { c, mu, .. } => (mu, c),
            _ => unreachable!("PowerLawKernel kind is fixed at construction"),
        };
        let u = unit_f64(key, sweep, DOMAIN_KERNEL_SAMPLE, sample_index, 0, 0);
        (u * (self.l_pow - self.c_pow) + self.c_pow)
            .powf(-1.0 / mu)
            .max(c)
    }
}

#[derive(Debug, Clone)]
pub struct UniformDistanceKernel {
    kind: KernelType,
}

impl UniformDistanceKernel {
    pub fn try_new(l: f64, c: f64) -> Result<Self, KernelError> {
        if !l.is_finite() || !c.is_finite() || l <= c {
            return Err(KernelError::InvalidUniformDistance { l, c });
        }
        Ok(Self {
            kind: KernelType::UniformDistance { l, c },
        })
    }

    pub fn new(l: f64, c: f64) -> Self {
        Self::try_new(l, c).expect("invalid uniform-distance kernel configuration")
    }
}

impl Kernel for UniformDistanceKernel {
    fn sample_indexed(
        &self,
        rng: RngConfig,
        sweep: u64,
        sample_index: u64,
    ) -> Result<f64, KernelError> {
        let rng = IndexedRng::new(rng).map_err(KernelError::RngConfig)?;
        Ok(self.sample_resolved(rng, sweep, sample_index))
    }

    fn sample_batch_indexed(
        &self,
        rng: RngConfig,
        sweep: u64,
        n: usize,
    ) -> Result<Vec<f64>, KernelError> {
        sample_batch_resolved(self, rng, sweep, n)
    }

    fn kind(&self) -> KernelType {
        self.kind
    }

    fn boxed_clone(&self) -> Box<dyn Kernel> {
        Box::new(self.clone())
    }
}

impl UniformDistanceKernel {
    fn sample_resolved(&self, key: IndexedRng, sweep: u64, sample_index: u64) -> f64 {
        let (low, high) = match self.kind {
            KernelType::UniformDistance { l, c } => (c, l),
            _ => unreachable!("UniformDistanceKernel kind is fixed at construction"),
        };
        let u = unit_f64(key, sweep, DOMAIN_KERNEL_SAMPLE, sample_index, 0, 0);
        low + (high - low) * u
    }
}

#[derive(Debug, Clone)]
pub struct NearestNeighborKernel {
    kind: KernelType,
    num_neighbors: usize,
}

impl NearestNeighborKernel {
    pub fn try_new(d: usize) -> Result<Self, KernelError> {
        let num_neighbors = d
            .checked_mul(2)
            .filter(|count| *count > 0)
            .ok_or(KernelError::InvalidNearestNeighborDimension { dimension: d })?;
        Ok(Self {
            kind: KernelType::NearestNeighbor { d },
            num_neighbors,
        })
    }

    pub fn new(d: usize) -> Self {
        Self::try_new(d).expect("invalid nearest-neighbor kernel configuration")
    }
}

impl Kernel for NearestNeighborKernel {
    fn sample_indexed(
        &self,
        rng: RngConfig,
        sweep: u64,
        sample_index: u64,
    ) -> Result<f64, KernelError> {
        let rng = IndexedRng::new(rng).map_err(KernelError::RngConfig)?;
        Ok(self.sample_resolved(rng, sweep, sample_index))
    }

    fn sample_batch_indexed(
        &self,
        rng: RngConfig,
        sweep: u64,
        n: usize,
    ) -> Result<Vec<f64>, KernelError> {
        sample_batch_resolved(self, rng, sweep, n)
    }

    fn kind(&self) -> KernelType {
        self.kind
    }

    fn boxed_clone(&self) -> Box<dyn Kernel> {
        Box::new(self.clone())
    }
}

impl NearestNeighborKernel {
    fn sample_resolved(&self, key: IndexedRng, sweep: u64, sample_index: u64) -> f64 {
        uniform_index(
            key,
            sweep,
            DOMAIN_KERNEL_SAMPLE,
            sample_index,
            0,
            self.num_neighbors,
        ) as f64
    }
}

trait ResolvedKernel {
    fn sample_resolved(&self, rng: IndexedRng, sweep: u64, sample_index: u64) -> f64;
}

impl ResolvedKernel for PowerLawKernel {
    fn sample_resolved(&self, rng: IndexedRng, sweep: u64, sample_index: u64) -> f64 {
        Self::sample_resolved(self, rng, sweep, sample_index)
    }
}

impl ResolvedKernel for UniformDistanceKernel {
    fn sample_resolved(&self, rng: IndexedRng, sweep: u64, sample_index: u64) -> f64 {
        Self::sample_resolved(self, rng, sweep, sample_index)
    }
}

impl ResolvedKernel for NearestNeighborKernel {
    fn sample_resolved(&self, rng: IndexedRng, sweep: u64, sample_index: u64) -> f64 {
        Self::sample_resolved(self, rng, sweep, sample_index)
    }
}

fn sample_batch_resolved<K: ResolvedKernel + Sync>(
    kernel: &K,
    rng: RngConfig,
    sweep: u64,
    n: usize,
) -> Result<Vec<f64>, KernelError> {
    let rng = IndexedRng::new(rng).map_err(KernelError::RngConfig)?;
    let mut output = vec![0.0; n];
    output
        .par_iter_mut()
        .with_min_len(parallel_chunk_len(n).unwrap_or(1))
        .enumerate()
        .for_each(|(index, value)| {
            *value = kernel.sample_resolved(rng, sweep, index as u64);
        });
    Ok(output)
}

#[derive(Clone)]
pub(crate) enum BuiltinKernel {
    PowerLaw(PowerLawKernel),
    UniformDistance(UniformDistanceKernel),
    NearestNeighbor(NearestNeighborKernel),
}

impl BuiltinKernel {
    pub(crate) fn sample_unit(&self, unit: f64) -> f64 {
        match self {
            Self::PowerLaw(kernel) => {
                let (mu, c) = match kernel.kind {
                    KernelType::PowerLaw { c, mu, .. } => (mu, c),
                    _ => unreachable!("power-law kernel kind is fixed at construction"),
                };
                (unit * (kernel.l_pow - kernel.c_pow) + kernel.c_pow)
                    .powf(-1.0 / mu)
                    .max(c)
            }
            Self::UniformDistance(kernel) => {
                let (low, high) = match kernel.kind {
                    KernelType::UniformDistance { l, c } => (c, l),
                    _ => unreachable!("uniform-distance kernel kind is fixed at construction"),
                };
                low + (high - low) * unit
            }
            Self::NearestNeighbor(kernel) => {
                (unit * kernel.num_neighbors as f64).floor() as usize as f64
            }
        }
    }
}

pub(crate) fn try_create_builtin_kernel(
    kernel_type: KernelType,
) -> Result<BuiltinKernel, KernelError> {
    match kernel_type {
        KernelType::PowerLaw { l, c, mu } => {
            Ok(BuiltinKernel::PowerLaw(PowerLawKernel::try_new(l, c, mu)?))
        }
        KernelType::UniformDistance { l, c } => Ok(BuiltinKernel::UniformDistance(
            UniformDistanceKernel::try_new(l, c)?,
        )),
        KernelType::NearestNeighbor { d } => Ok(BuiltinKernel::NearestNeighbor(
            NearestNeighborKernel::try_new(d)?,
        )),
    }
}