Sia Sola ($0.99) / Sia Sola Lite (Free), developed by Feuerware;
Ratio - the puzzle ($1.99), developed by Pantorix.
Neither of the apps seems to be an official port, but they both credit Oliver Schaudt for the idea.
During gameplay, Sia Sola looks like this:
The board is a 4x3 grid, containing two large stones and up to 8 smaller stones. As you can see, the large stones are placed inside the grid squares, while the small stones are placed over the grid intersections.
You only move the large stones. At the beginning, they are placed on opposite sides of the board. The goal is to swap their position.
The clever part of the mechanics is the role that the small stones have on the moves you can make. As is clearly explained in the instructions, you can move a stone through a side only if there is exactly one small stone on that side.
The puzzles are challenging; a lot more than it might seem at first sight. There is some sort of parity involved, but due to the two-dimensional movement of the small stones, it's more complicated than that. More often than not, when the solution seems close at hand, you find all your paths blocked and need to trace back.
Ratio has 32 puzzles, split across 4 difficulty levels, so I think it's a verbatim copy of the official game. Sia Sola has 50 puzzles.
Ratio is missing information about the optimal number of moves to solve each puzzle.
In Sia Sola you can move only with drag&drop, in Ratio you can move both with drag&drop and by tapping the destination square. I prefer the latter.
Ratio has a smooth and elegant menu system, which however isn't very intuitive to navigate.
All in all, I prefer Ratio, mostly because it is more polished. YMMV. Regardless of which one you choose, the puzzle is recommended for its elegant mechanics.
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