Imbalance

I have a huge castle I'm building with my son, and I thought it would be cool to bind a key to "draw" or fill the currently selected block type in the direction the player is facing, until it hits a wall. Basically, to help quickly build floors and ceilings. So, I manually create a 4 sided building. When inside, I'd like to go to a corner, press a key and have my mod add that block continuously in the direction I'm facing, until it hits a wall. Or, in a similar fashion, fill in an entire floor, so I'd draw/create those blocks any place that doesn't already contain a block in a growing rectangle around the player, again, until another block type is hit. Can anyone point me in the right direction?

I have a huge castle I'm building with my son, and I thought it would be cool to bind a key to "draw" or fill the currently selected block type in the direction the player is facing, until it hits a wall. Basically, to help quickly build floors and ceilings. So, I manually create a 4 sided building. When inside, I'd like to go to a corner, press a key and have my mod add that block continuously in the direction I'm facing, until it hits a wall. Or, in a similar fashion, fill in an entire floor, so I'd draw/create those blocks any place that doesn't already contain a block in a growing rectangle around the player, again, until another block type is hit. Can anyone point me in the right direction?