WISPCATCHER

A wisp hides in an old tree. Every night it hops one fork. Catch it in the fewest dawns there are.

You climb a bare tree at night. Somewhere on one of its forks a wisp is hiding, and every night it moves along one limb to the next fork. Before your first dawn you climb freely; after it, every limb you climb takes a night, and every dawn you look at the fork you are standing on.

You never see the wisp until you catch it. What you see is every fork it could be on, glowing. Each look puts some of them out; each night the rest spread to their neighbours again. You catch it only when there is nowhere left for it to be, so luck never enters into it: a catch is a sweep that would have found it wherever it went.

Every tree has a proved par

For each of the forty trees, the least number of dawns that guarantees a catch was worked out exactly, on a computer, before the cartridge was built. That is PAR on screen. Catch it on par for GOLD, two over for SILVER, four over for BRONZE. Going wherever leaves the fewest glows feels right, and on many trees it is still searching when the sun comes up. At twice par the sun is up, the wisp is gone, and the cartridge marks one fork as a clue. After a catch or a full day, press A to watch a proved plan for that tree.

Forty generated trees

Every tree is drawn by the game's own renderer, limbs tapering from the trunk to the twigs, and no two are the same shape. Three are open at the start; every tree you catch opens another. The cartridge keeps your best medal on every tree in its battery save.

A real Game Boy ROM

This is a 128 KB MBC5 cartridge image with a battery save, written in C with GBDK-2020: four shades, a 160x144 screen, the Game Boy's own sound chip for the music and the chimes. The free edition on this page is the first ten trees, running in your browser on the binjgb emulator, and it keeps its medals in your browser. The $5.99 download is the full cartridge: all forty trees and the ending, as a .gb file for the emulator or flash cartridge of your choice, with a manual. We have tested it in an emulator, not on real hardware.

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Controls. D-pad (arrows): hold a direction to climb a limb; small arrows round the climber show which directions climb from where you stand, and a four-way pad reaches every fork. A (X or Space): stay and look again; before the first dawn, begin. START (Enter): start the tree again. SELECT (Shift): back to the tree list, where SELECT turns the sound off and on. B (Z): back. M mutes the browser page. On a phone, an on-screen pad appears.

Paid, from the same studio: WITNESS MARK ($5.99), a Game Boy deduction game: five suspects, a handful of witness marks, and a proof the answer is really there · WARDLOCK ($7.99), forty-eight generated warded locks for the Game Boy, each shipped with its shortest solution · ANATOMIST II: GRAFT ($5.99), a Game Boy roguelike where you graft a part of each monster you kill onto yourself · everything else at csaf.itch.io

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Made by Core Systems Asset Factory. The puzzle is a published search problem (Britnell and Wildon, 2013); the trees, the pars and the plans are generated and computed by our own code.

AI disclosure: the code, art and text were made with generative AI assistance. The cover is composed from the game's own pixel art. The music and sound effects are the Game Boy's own sound channels driven by that code, not the output of a generative audio model.