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"
)

// Threshold function: f(x) = x if x > threshold; k otherwise.
type Threshold[O mat.Tensor] struct {
    x         O
    threshold O // scalar
    k         O // scalar
}

// NewThreshold returns a new Threshold Function.
func NewThreshold[O mat.Tensor](x O, threshold, k O) *Threshold[O] {
    return &Threshold[O]{
        x:         x,
        threshold: threshold,
        k:         k,
    }
}

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

// Forward computes the output of the function.
func (r *Threshold[O]) Forward() (mat.Tensor, error) {
    y := r.x.Value().(mat.Matrix).ApplyWithAlpha(
        threshold,
        r.threshold.Value().Item().F64(),
        r.k.Value().Item().F64(),
    )
    return y, nil
}

// Backward computes the backward pass.
func (r *Threshold[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(
            thresholdDeriv,
            r.threshold.Value().Item().F64(),
            r.k.Value().Item().F64(),
        )
        gx.ProdInPlace(gy.(mat.Matrix))
        r.x.AccGrad(gx)
    }
    return nil
}