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"
)

// SubScalar is an element-wise subtraction function with a scalar value.
type SubScalar[O mat.Tensor] struct {
    x1 O
    x2 O // scalar
}

// NewSubScalar returns a new SubScalar Function.
func NewSubScalar[O mat.Tensor](x1 O, x2 O) *SubScalar[O] {
    return &SubScalar[O]{
        x1: x1,
        x2: x2,
    }
}

// Operands returns the list of operands.
func (r *SubScalar[O]) Operands() []mat.Tensor {
    return []mat.Tensor{r.x1, r.x2}
}

// Forward computes the output of the node.
func (r *SubScalar[O]) Forward() (mat.Tensor, error) {
    return r.x1.Value().(mat.Matrix).SubScalar(r.x2.Value().Item().F64()), nil
}

// Backward computes the backward pass.
func (r *SubScalar[O]) Backward(gy mat.Tensor) error {
    if !mat.SameDims(r.x1.Value(), gy) {
        return fmt.Errorf("fn: matrices have incompatible dimensions")
    }
    if r.x1.RequiresGrad() {
        r.x1.AccGrad(gy) // equals to gy.ProdScalar(1.0)
    }
    if r.x2.RequiresGrad() {
        neg := gy.(mat.Matrix).ProdScalar(-1)
        gx := neg.Sum()
        r.x2.AccGrad(gx)
    }
    return nil
}