vikos 0.3.1

//! Implementations of `Model` trait

use crate::{
    array,
    linear_algebra::{FixDimension, Vector},
    Model,
};
use serde_derive::{Deserialize, Serialize};

impl Model for f64 {
    type Features = ();
    type Target = f64;

    fn num_coefficients(&self) -> usize {
        1
    }

    fn coefficient(&mut self, coefficient: usize) -> &mut f64 {
        match coefficient {
            0 => self,
            _ => panic!("coefficient index out of range"),
        }
    }

    fn predict(&self, _: &()) -> f64 {
        *self
    }

    fn gradient(&self, coefficient: usize, _: &()) -> f64 {
        match coefficient {
            0 => 1.0,
            _ => panic!("coefficient index out of range"),
        }
    }
}

/// Models the target as `y = m * x + c`
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Linear<V> {
    /// Slope
    pub m: V,
    /// Offset
    pub c: f64,
}

impl<V: Vector> Linear<V> {
    /// Create a linear model whose features have the specified dimension. If the models internal
    /// Vector type has a fixed dimension known at compile time you can use `default` instead.
    pub fn with_feature_dimension(dimension: usize) -> Self {
        Linear {
            m: V::zero_from_dimension(dimension),
            c: 0f64,
        }
    }
}

impl<V> Default for Linear<V>
where
    V: FixDimension,
{
    fn default() -> Self {
        Linear {
            m: V::zero(),
            c: 0f64,
        }
    }
}

impl<V> Model for Linear<V>
where
    V: Vector,
{
    type Features = V;
    type Target = f64;

    fn num_coefficients(&self) -> usize {
        self.m.dimension() + 1
    }

    fn coefficient(&mut self, coefficient: usize) -> &mut f64 {
        if coefficient == self.m.dimension() {
            &mut self.c
        } else {
            self.m.at_mut(coefficient)
        }
    }

    fn predict(&self, input: &V) -> f64 {
        self.m.dot(input) + self.c
    }

    fn gradient(&self, coefficient: usize, input: &V) -> f64 {
        if coefficient == self.m.dimension() {
            1.0 //derive by c
        } else {
            input.at(coefficient) //derive by m
        }
    }
}

/// Models target as `y = 1/(1+e^(m * x + c))`
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Logistic<V>(Linear<V>);

impl<V: Vector> Logistic<V> {
    /// Create a linear model whose features have the specified dimension. If the models internal
    /// Vector type has a fixed dimension known at compile time you can use `default` instead.
    pub fn with_feature_dimension(dimension: usize) -> Self {
        Logistic(Linear::with_feature_dimension(dimension))
    }
}

impl<V> Default for Logistic<V>
where
    V: FixDimension,
{
    fn default() -> Self {
        Logistic(Linear::default())
    }
}

impl<V> Model for Logistic<V>
where
    Linear<V>: Model<Features = V, Target = f64>,
{
    type Features = V;
    type Target = f64;

    fn num_coefficients(&self) -> usize {
        self.0.num_coefficients()
    }

    fn coefficient(&mut self, coefficient: usize) -> &mut f64 {
        self.0.coefficient(coefficient)
    }

    fn predict(&self, input: &V) -> f64 {
        1.0 / (1.0 + self.0.predict(input).exp())
    }

    fn gradient(&self, coefficient: usize, input: &V) -> f64 {
        let p = self.predict(input);
        -p * (1.0 - p) * self.0.gradient(coefficient, input)
    }
}

/// Models the target as `y = g(m*x + c)`
///
/// # Example
///
/// Logistic regression implemented using a generalized linear model. This is just for
/// demonstration purposes. For this usecase you would usally use `Logistic`.
///
/// ```
/// # use vikos::{model, teacher, cost, learn_history};
/// # let history = [(0.0, true)];
/// let mut model = model::GeneralizedLinearModel::new(|x| 1.0 / (1.0 + x.exp()),
///                                                    |x| -x.exp() / (1.0 + x.exp()).powi(2) );
/// let teacher = teacher::GradientDescent { learning_rate: 0.3 };
/// let cost = cost::MaxLikelihood {};
///
/// learn_history(&teacher,
///               &cost,
///               &mut model,
///               history.iter().cloned());
/// ```
#[derive(Clone)]
pub struct GeneralizedLinearModel<V, G, Dg> {
    /// `Linear` term of the generalized linear `Model`
    pub linear: Linear<V>,
    /// Outer function applied to the result of `linear`
    pub g: G,
    /// Derivation of `g`
    pub g_derivate: Dg,
}

impl<V, G, Dg> GeneralizedLinearModel<V, G, Dg>
where
    G: Fn(f64) -> f64,
    Dg: Fn(f64) -> f64,
{
    /// Creates new model with the coefficients set to zero
    pub fn new(g: G, g_derivate: Dg) -> GeneralizedLinearModel<V, G, Dg>
    where
        V: FixDimension,
    {
        GeneralizedLinearModel {
            linear: Linear::default(),
            g,
            g_derivate,
        }
    }
}

impl<V, F, Df> Model for GeneralizedLinearModel<V, F, Df>
where
    F: Fn(f64) -> f64,
    Df: Fn(f64) -> f64,
    Linear<V>: Model<Features = V, Target = f64>,
{
    type Features = V;
    type Target = f64;

    fn num_coefficients(&self) -> usize {
        self.linear.num_coefficients()
    }

    fn coefficient(&mut self, coefficient: usize) -> &mut f64 {
        self.linear.coefficient(coefficient)
    }

    fn predict(&self, input: &V) -> f64 {
        let f = &self.g;
        f(self.linear.predict(&input))
    }

    fn gradient(&self, coefficient: usize, input: &V) -> f64 {
        let f = &self.g_derivate;
        f(self.linear.predict(&input)) * self.linear.gradient(coefficient, input)
    }
}

/// One vs Rest strategy for multi classification.
///
/// This model combines indivual binary classifactors to a new multi classification model.
///
/// Implementation assumes that the number of coefficients is the same for all models.
#[derive(Debug, Clone, Default, Serialize, Deserialize)]
pub struct OneVsRest<T>(T);

impl<T> OneVsRest<T> {
    /// Create a new One vs Rest model from an array of existing models.
    pub fn new(t: T) -> Self {
        OneVsRest(t)
    }
}

impl<T> Model for OneVsRest<T>
where
    T: array::Array,
    T::Element: Model<Target = f64>,
{
    type Features = <T::Element as Model>::Features;
    type Target = T::Vector;

    fn num_coefficients(&self) -> usize {
        let models = &self.0;
        models.length() * models.at_ref(0).num_coefficients()
    }

    fn coefficient(&mut self, index: usize) -> &mut f64 {
        // If our one vs Rest classifier consists of three models a,b,c with three coefficients 1,2
        // ,3 each we list the coefficients of the combined model as a1,b1,c1,a2,b2,c2,a3,b3,c3.
        let models = &mut self.0;
        let class = index % models.length();
        let n = index / models.length();
        models.at_mut(class).coefficient(n)
    }

    fn predict(&self, input: &Self::Features) -> Self::Target {
        let models = &self.0;
        let mut result = Self::Target::zero_from_dimension(models.length());
        for i in 0..models.length() {
            *result.at_mut(i) = models.at_ref(i).predict(input);
        }
        result
    }

    fn gradient(&self, coefficient: usize, input: &Self::Features) -> Self::Target {
        let models = &self.0;
        let class = coefficient % models.length();
        let mut result = Self::Target::zero_from_dimension(models.length());
        *result.at_mut(class) = models
            .at_ref(class)
            .gradient(coefficient / models.length(), input);
        result
    }
}