The Doppit Almanac · No. 10

Lightit, a daily Akari puzzle

Place bulbs to light every square

What it is

Lightit is Doppit's take on Akari, the lamp puzzle also sold as Light Up. The board is a dark room broken by walls, some of them numbered. You hang bulbs, and a bulb throws light down its whole row and column until a wall cuts the beam.

The win is total illumination with perfect etiquette: every open cell lit, and no bulb ever shining in another bulb's face. A numbered wall in a corner is the classic foothold, since a 2 with only two neighbours settles both of them at once. Lit corridors glow warm as you work, and a satisfied wall dims under a small check.

How it plays

The lighting code has four clauses, none negotiable.

• A numbered wall wants exactly that many bulbs against its four sides. Diagonals never count.
• A blank wall demands nothing. It just blocks light.
• Two bulbs may never stare at each other down an open row or column.
• Dots are your own no-bulb notes, and a dotted cell still needs light from somewhere.

The difficulties

Four rooms are wired daily. Easy is lit in thirty seconds to a minute. Medium asks one to three minutes of beam-keeping. Hard is three to six, with corners that stay dark until nearly the last bulb. Expert is hard's own twelve by twelve with the numbers cut to the fewest the board can carry, and hard's arguments run out on it. You suppose a bulb in one cell, then follow every mark it forces, the walls that fill and the beams that spread, until a rule breaks. Expert never comes round as a daily; it is yours to practise on. A new board every day, and it costs nothing.

Nearby in the almanac: Campit, Stellit and Wellit