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"
)

// Concat is an operator to perform vector concatenation.
type Concat[O mat.Tensor] struct {
    xs    []O
    ySize int
}

// NewConcat returns a new Concat Function.
func NewConcat[O mat.Tensor](xs []O) *Concat[O] {
    return &Concat[O]{
        xs:    xs,
        ySize: 0, // assigned during the Forward()
    }
}

// Operands returns the list of operands.
func (r *Concat[O]) Operands() []O {
    return r.xs
}

// Forward computes the output of the function.
func (r *Concat[O]) Forward() (mat.Tensor, error) {
    if len(r.xs) == 0 {
        return nil, fmt.Errorf("fn: no vectors to concatenate")
    }
    r.ySize = 0 // reset output size
    ms := make([]mat.Matrix, len(r.xs))
    for i, x := range r.xs {
        value := x.Value().(mat.Matrix)
        ms[i] = value
        r.ySize += value.Size()
    }
    return ms[0].NewConcatV(ms...), nil
}

// Backward computes the backward pass.
func (r *Concat[O]) Backward(gy mat.Tensor) error {
    if r.ySize != gy.Size() {
        return fmt.Errorf("fn: vectors with not compatible size: %d != %d", r.ySize, gy.Size())
    }
    sizes := make([]int, len(r.xs))
    for i, x := range r.xs {
        sizes[i] = x.Value().Size()
    }
    xs := r.xs
    for i, gx := range gy.(mat.Matrix).SplitV(sizes...) {
        if xs[i].RequiresGrad() {
            xs[i].AccGrad(gx)
        }
    }
    return nil
}