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In this Chess puzzle, your aim is to switch black and white, with a legal game, and arrive at the final position:

enter image description here

This aim is likely unreachable. So you must try to get the maximum number of pieces switched. Meaning your maximum score is 32 if all pieces are finally switched.

Please note that the pieces are switched, look at the Queens on the wrong squares for instance. In this final (impossible ?) position above, white King was, many moves before on e1 and all 16 pawns are one square away from promotion.

Finally, if two answers get to the same number of pieces correctly switched, I will favor answers with fewest moves. That doubly explains the optimization tag.

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  • $\begingroup$ @Benja You can add spoilers by adding >! in front of your text. You can also just post the answer and someone from the community can edit it for you :) $\endgroup$
    – Prince North Læraðr
    Commented Nov 11, 2020 at 20:03

2 Answers 2

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6 missing pawns (c7,d7,h7;a2,e2,f2), 51 moves.

 1. h4 a5 2. b4 g5 3. hxg5 axb4 4. Rh6 Ra3 5. Ra6 Rh3 6. Ra8 Rh1 7. a4 h5 8. a5
 h4 9. a6 h3 10. a7 h2 11. Ra6 Rh3 12. Rh6 Ra3 13. Rh8 Ra1 14. e4 d5 15. g6 b3
 16. Ba3 Bh6 17. g4 b5 18. c4 f5 19. cxb5 fxg4 20. b6 g3 21. Ke2 Kd7 22. Kf3 Ba6
 23. Bh3+ Kc6 24. Kg4 Kb5 25. Kf5 Kc4 26. Ke6 Kd3 27. Nf3 Nf6 28. Nd4 Nbd7 29.
 Kf7 b2 30. Bxe7 c5 31. b7 Qb6 32. Qa4 Bxd2 33. e5 Ne4 34. Nc6 Ke2 35. Nc3+ Ke1
 36. e6 Ne5+ 37. Ke8 Bf1 38. Bf8 Qb3 39. Qa5 Qd1 40. Qd8 d4 41. Nd5 Nf3 42. e7
 Ng1 43. Nb8 Nc3 44. Nf6 Nb1 45. Ng8 g2 46. Bc8 Bc1 47. g7 c4 48. f4 d3 49. f5
 c3 50. f6 d2 51. f7 c2

Try it online!

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    $\begingroup$ That solution is perfect. I tried other methods of moving the pawns or perhaps promoting a pawn (assuming that all pieces are valued the same as long as they are in their rightful spot). and i ended up just adding together the amount of moves minimum that it would take to move every piece (except for 6 pawns) across the board $\endgroup$
    – Benja
    Commented Nov 12, 2020 at 16:04
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Final result, everything is in place but missing 8 pawns.
enter image description here

First I switched the rooks, mirroring the movements for both on the right side then the left, then the knights, which was trickier because an optimal path took both nights through the same space. Then I switched the bishops, taking care to make sure I didn't accidentally check or checkmate one side or the other, then I moved the pawns in front of the king and queen to the center 4 squares then I moved the king and queen together to avoid check/checkmating, circling counter clockwise around the pawns in the center. Once everything was in place I had to use 4 of the pawns from each side to be able to get them in separate lanes.

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