nlpodyssey/spago

// Copyright 2022 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"
)

// Slice is a function to extract a portion of a matrix.
type Slice[O mat.Tensor] struct {
    x       O
    fromRow int
    fromCol int
    toRow   int
    toCol   int
}

// NewSlice returns a new Slice Function.
func NewSlice[O mat.Tensor](x O, fromRow, fromCol, toRow, toCol int) *Slice[O] {
    return &Slice[O]{
        x:       x,
        fromRow: fromRow,
        fromCol: fromCol,
        toRow:   toRow,
        toCol:   toCol,
    }
}

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

// Forward computes the output of the function.
func (s *Slice[O]) Forward() (mat.Tensor, error) {
    return s.x.Value().(mat.Matrix).Slice(s.fromRow, s.fromCol, s.toRow, s.toCol), nil
}

// Backward computes the backward pass.
func (s *Slice[O]) Backward(gy mat.Tensor) error {
    lx := s.toRow - s.fromRow
    ly := s.toCol - s.fromCol
    if !(gy.Shape()[0] == lx && gy.Shape()[1] == ly) {
        return fmt.Errorf("fn: matrices have incompatible dimensions")
    }
    if s.x.RequiresGrad() {
        gx := s.x.Value().(mat.Matrix).ZerosLike()
        for i := 0; i < lx; i++ {
            for j := 0; j < ly; j++ {
                gx.SetScalar(gy.(mat.Matrix).ScalarAt(i, j), i+s.fromRow, j+s.fromCol)
            }
        }
        s.x.AccGrad(gx)
    }
    return nil
}