nlpodyssey/spago

// Copyright 2019 spaGO Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package gradfn

import (
    "fmt"

    "github.com/nlpodyssey/spago/mat"
)

// SoftShrink function: f(x) = x − λ if x > λ; x + λ if x < −λ; 0 otherwise.
type SoftShrink[O mat.Tensor] struct {
    x      O
    lambda O // scalar
}

// NewSoftShrink returns a new SoftShrink Function.
func NewSoftShrink[O mat.Tensor](x O, lambda O) *SoftShrink[O] {
    return &SoftShrink[O]{
        x:      x,
        lambda: lambda,
    }
}

// Operands returns the list of operands.
func (r *SoftShrink[O]) Operands() []mat.Tensor {
    return []mat.Tensor{r.x, r.lambda}
}

// Forward computes the output of the function.
func (r *SoftShrink[O]) Forward() (mat.Tensor, error) {
    return r.x.Value().(mat.Matrix).ApplyWithAlpha(softShrink, r.lambda.Value().Item().F64()), nil
}

// Backward computes the backward pass.
func (r *SoftShrink[O]) Backward(gy mat.Tensor) error {
    if !mat.SameDims(r.x.Value(), gy) {
        return fmt.Errorf("fn: matrices have incompatible dimensions")
    }
    if r.x.RequiresGrad() {
        gx := r.x.Value().(mat.Matrix).ApplyWithAlpha(softShrinkDeriv, r.lambda.Value().Item().F64())
        gx.ProdInPlace(gy.(mat.Matrix))
        r.x.AccGrad(gx)
    }
    return nil
}