Implement forward pass of a maxpool2d layer

Author: milancurcic

src/nf_maxpool2d_layer_submodule.f90

@@ -5,6 +5,7 @@
 contains
 
   pure module function maxpool2d_layer_cons(pool_size, stride) result(res)
+    implicit none
     integer, intent(in) :: pool_size
     integer, intent(in) :: stride
     type(maxpool2d_layer) :: res
@@ -14,6 +15,7 @@ end function maxpool2d_layer_cons
 
 
   module subroutine init(self, input_shape)
+    implicit none
     class(maxpool2d_layer), intent(in out) :: self
     integer, intent(in) :: input_shape(:)
 
@@ -34,13 +36,42 @@ end subroutine init
 
 
   module subroutine forward(self, input)
+    implicit none
     class(maxpool2d_layer), intent(in out) :: self
     real, intent(in) :: input(:,:,:)
-    print *, 'Warning: maxpool2d forward pass not implemented'
+    integer :: input_width, input_height
+    integer :: i, j, n
+    integer :: ii, jj
+    integer :: iend, jend
+
+    input_width = size(input, dim=2)
+    input_height = size(input, dim=2)
+
+    ! Stride along the width and height of the input image
+    do concurrent( &
+      i = 1:input_width:self % stride, &
+      j = 1:input_height:self % stride &
+    )
+
+      ! Indices of the pooling layer
+      ii = i / self % stride + 1
+      jj = j / self % stride + 1
+
+      iend = i + self % pool_size - 1
+      jend = j + self % pool_size - 1
+
+      do concurrent(n = 1:self % channels)
+        !TODO find and store maxloc
+        self % output(n,ii,jj) = maxval(input(n,i:iend,j:jend))
+      end do
+
+    end do
+
   end subroutine forward
 
 
   module subroutine backward(self, input, gradient)
+    implicit none
     class(maxpool2d_layer), intent(in out) :: self
     real, intent(in) :: input(:,:,:)
     real, intent(in) :: gradient(:,:,:)