n = 2 でもいけました。 @swift-5.2.5
let b1 = 17 let w1 = 22 let n = 2 let pb1 = (b1 + w1 - 4 + 1) / 2 let pw1 = (b1 + w1 - 4) / 2 let f1 = b1 - 2 - pb1 assert(f1 == -(w1 - 2 - pw1)) let pb2 = pb1 + n let pw2 = pw1 let b2 = b1 + 2 * n let w2 = w1 - n let f2 = f1 + n assert(f2 == b2 - 2 - pb2) assert(f2 == -(w2 - 2 - pw2)) print("BEFORE: b1 = \(b1), w1 = \(w1), f1 = \(f1)") print("AFTER: b2 = \(b2), w2 = \(w2), f2 = \(f2)") print("b2 - w2 = \(b2 - w2)")