Equal column spacing in a RecyclerView with GridLayoutManager

ColumnItemDecoration.java

package com.example.columnspacing

import android.graphics.Paint;
import android.graphics.Rect;
import android.support.v4.content.ContextCompat;
import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.View;

/**
 * An ItemDecoration which will apply even horizontal padding to the right and left edges
 * of items in a vertical grid, while keeping the size of each item the same.
 * 
 * It even works when there are different types in each row, as long as each 
 * row only contains one view type and each view type always spans the same number of columns.
 * 
 */ 
public class ColumnItemDecoration extends RecyclerView.ItemDecoration {

    // Horizontal padding
    private final int padding;

    public ColumnItemDecoration() {
        // Set padding (from resources probably)
    }

    @Override public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        GridLayoutManager.LayoutParams layoutParams = (GridLayoutManager.LayoutParams) view.getLayoutParams();
        GridLayoutManager gridLayoutManager = (GridLayoutManager) parent.getLayoutManager();
        float spanSize = gridLayoutManager.getSpanSizeLookup().getSpanSize(position);
        float totalSpanSize = gridLayoutManager.getSpanCount();

        float n = totalSpanSize / spanSize; // num columns
        float c = layoutParams.getSpanIndex() / spanSize; // column index

        float leftPadding = padding * ((n - c) / n);
        float rightPadding = padding * ((c + 1) / n);

        outRect.left = (int) leftPadding;
        outRect.right = (int) rightPadding;
    }
}

@RicardoBelchior
int position = parent.getChildViewHolder(view).getAdapterPosition();