rustyml 0.16.0

A high-performance machine learning & deep learning library in pure Rust, offering ML algorithms and neural network support
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//! AdamW optimizer: Adam with decoupled weight decay
//!
//! Holds the [`AdamW`] struct, its constructor, its [`AdamW::with_global_clipnorm`] builder method,
//! and its [`Optimizer`](crate::neural_network::traits::Optimizer) implementation. The moment math
//! and the update loop live in the private `AdamCore` type, shared with
//! [`Adam`](crate::neural_network::optimizers::Adam)

use crate::error::Error;
use crate::neural_network::ctx::Grads;
use crate::neural_network::optimizers::adam_core::AdamCore;
use crate::neural_network::traits::{LayerBase, Optimizer};

/// AdamW (Adam with decoupled weight decay) optimizer
///
/// Identical adaptive moment math to [`Adam`](crate::neural_network::optimizers::Adam), but its
/// `weight_decay` is decoupled. AdamW shrinks the parameter directly by the factor
/// `(1 - learning_rate * weight_decay)` before the gradient step, instead of folding an L2 term
/// into the gradient. The decay therefore does not flow through the moment estimates, and
/// the adaptive denominator does not rescale it. This is the Loshchilov and Hutter formulation,
/// and the better-behaved choice with adaptive optimizers. With `weight_decay == 0.0` it matches
/// `Adam`. Weight decay applies to weight tensors only, never to biases or normalization
/// scale/shift parameters
#[derive(Debug)]
pub struct AdamW {
    /// The shared Adam-family optimizer state
    core: AdamCore,
}

impl AdamW {
    /// Creates a new AdamW optimizer with the specified hyperparameters
    ///
    /// Validates hyperparameters and initializes internal timestep tracking
    ///
    /// # Parameters
    ///
    /// - `learning_rate` - step size for parameter updates
    /// - `beta1` - decay rate for the first moment estimates (typically 0.9)
    /// - `beta2` - decay rate for the second moment estimates (typically 0.999)
    /// - `epsilon` - small constant for numerical stability (typically 1e-8)
    /// - `weight_decay` - decoupled weight-decay coefficient applied directly to the parameters.
    ///   `0.0` disables it. For classic coupled L2 decay use
    ///   [`Adam`](crate::neural_network::optimizers::Adam)
    ///
    /// # Returns
    ///
    /// - `Result<Self, Error>` - a new AdamW optimizer instance or an error
    ///
    /// # Notes
    ///
    /// Gradient clipping is disabled by default. Enable it with [`AdamW::with_global_clipnorm`]
    ///
    /// # Errors
    ///
    /// - `Error::InvalidParameter` - if any hyperparameter is out of range or `weight_decay` is
    ///   negative or not finite
    pub fn new(
        learning_rate: f32,
        beta1: f32,
        beta2: f32,
        epsilon: f32,
        weight_decay: f32,
    ) -> Result<Self, Error> {
        Ok(Self {
            core: AdamCore::new(learning_rate, beta1, beta2, epsilon, weight_decay, true)?,
        })
    }

    /// Enables clip-by-global-norm gradient clipping (disabled by default)
    ///
    /// `global_clipnorm` scales every gradient so the global L2 norm never exceeds it, preserving
    /// the gradient direction
    ///
    /// # Parameters
    ///
    /// - `global_clipnorm` - clip-by-global-norm threshold. Must be positive and finite
    ///
    /// # Returns
    ///
    /// - `Result<Self, Error>` - the updated optimizer, or an error if `global_clipnorm` is not
    ///   positive and finite
    pub fn with_global_clipnorm(self, global_clipnorm: f32) -> Result<Self, Error> {
        Ok(Self {
            core: self.core.with_global_clipnorm(global_clipnorm)?,
        })
    }
}

impl Optimizer for AdamW {
    fn global_clipnorm(&self) -> Option<f32> {
        self.core.global_clipnorm()
    }

    fn learning_rate(&self) -> f32 {
        self.core.learning_rate()
    }

    fn set_learning_rate(&mut self, learning_rate: f32) {
        self.core.set_learning_rate(learning_rate);
    }

    fn step(&mut self) {
        self.core.step();
    }

    fn update(&mut self, scope: usize, layer: &mut dyn LayerBase, grads: &Grads, grad_scale: f32) {
        self.core.update(scope, layer, grads, grad_scale);
    }
}