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"
)

// ReduceMean is an operator to perform reduce-mean function.
type ReduceMean[O mat.Tensor] struct {
    x O
}

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

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

// Forward computes the output of this node.
func (r *ReduceMean[O]) Forward() (mat.Tensor, error) {
    xv := r.x.Value().(mat.Matrix)
    return xv.Sum().ProdScalarInPlace(1 / float64(xv.Size())), nil
}

// Backward computes the backward pass.
func (r *ReduceMean[O]) Backward(gy mat.Tensor) error {
    if !mat.IsScalar(gy) {
        return fmt.Errorf("fn: the gradient had to be a scalar")
    }
    if r.x.RequiresGrad() {
        x := r.x.Value().(mat.Matrix)
        size := x.Size()
        v := gy.Item().F64() / float64(size)
        gx := x.NewMatrix(mat.WithShape(size), mat.WithBacking(mat.CreateInitializedSlice(size, v)))
        r.x.AccGrad(gx)
    }
    return nil
}