# PSE Advent Calendar 2022 (Day 18): Light Up the Christmas Tree

This puzzle is part of the Puzzling Stack Exchange Advent Calendar 2022. The accepted answer to this question will be awarded a bounty worth 100 reputation.

< Previous Door Next Door >

The year is 1977. Your family is a little bit behind, so even though it's just one week until Christmas, you're finally getting around to setting up the tree. Your dad is doing most of the work, and he's already gotten the tree up and all of the ornaments on it, but now you've found him rummaging around in an old desk drawer, looking through all sorts of pieces of paper you don't understand.

"I can't find the lighting diagram for the tree," he explained. "I planned out the lights and ornaments in a particular way so that the whole tree would be lit up, and I found the plan for the ornaments, but I can't find the one for the lights!"

"Can I help?" you ask, not knowing where you would look for the diagram, but your dad says, "Don't worry," and leads you back out into the family room.

"I knew this would happen, so I made sure to have a backup. Based on how the ornaments are set up, you should be able to figure out where to put all these lights so that you can light up the whole tree. Remember what you learned in physics class - light can only travel horizontally and vertically."

"Dad, I haven't taken physics yet!" you reply, confused. "I'm only nine years old!"

Ignoring you, he continues. "And, if you do, you might even be able to get a hint as to what's in that big present we got you. As the kids say, it's extremely lit - maybe even too lit."

"Dad, kids won't say 'lit' for another 40 years! How am I supposed to figure any of this out?"

"I have faith in you. I'm gonna go run to the convenience store to grab some cigarettes. Good luck - and don't forget the star at the top!" And laying a finger aside of his nose, he hopped into his Oldsmobile and sped off.

I guess you're on your own now. Can you light up the tree before your dad comes home, and more importantly, can you find out what your Christmas present is?

EDIT: apologies, missed an ornament! The new one is the leftmost one in the third row from the bottom.

• Penpa grid with some setup spoilers: tinyurl.com/2gwl6qd4 Dec 19, 2022 at 3:29

Interpreting

ornaments as black cells in Akari, with blue ornaments containing numbers represented by the number of stripes, we get a valid Akari / Light Up puzzle.