import Foundation
func load(_ path: String) -> (Data, Int, Int, Int, Int) {
    let d = try! Data(contentsOf: URL(fileURLWithPath: path))
    let pixOff = Int(d.withUnsafeBytes { $0.load(fromByteOffset: 10, as: UInt32.self) })
    let w = Int(d.withUnsafeBytes { $0.load(fromByteOffset: 18, as: Int32.self) })
    let hRaw = Int(d.withUnsafeBytes { $0.load(fromByteOffset: 22, as: Int32.self) }); let h = abs(hRaw)
    let B = Int(d.withUnsafeBytes { $0.load(fromByteOffset: 28, as: UInt16.self) })/8
    let rb = (B*w+3) & ~3
    return (d, pixOff, w, h, hRaw > 0 ? 1 : 0) // 1 = bottom-up
}
let (A, poA, w, h, flipA) = load("new.bmp")
let (B2, poB, _, _, flipB) = load("after3.bmp")
let rb = (4*w+3) & ~3
let BS = 45
var blocks: [(Int,Int,Int)] = []
var y = 0
while y < h { var x = 0
  while x < w {
    var diff = 0
    var yy = y; while yy < min(y+BS, h) {
      let ra = poA + (flipA==1 ? (h-1-yy) : yy) * rb
      let rb2 = poB + (flipB==1 ? (h-1-yy) : yy) * rb
      var xx = x; while xx < min(x+BS, w) {
        let ia = ra + xx*4, ib = rb2 + xx*4
        let dr = abs(Int(A[ia+2]) - Int(B2[ib+2]))
        let dg = abs(Int(A[ia+1]) - Int(B2[ib+1]))
        let db = abs(Int(A[ia]) - Int(B2[ib]))
        if dr+dg+db > 60 { diff += 1 }
        xx += 2
      }
      yy += 2
    }
    if diff > 8 { blocks.append((x,y,diff)) }
    x += BS
  }
  y += BS
}
print("changed blocks (x,y,diffPx):")
for b in blocks { print("  x=\(b.0)-\(b.0+BS) y=\(b.1)-\(b.1+BS) px=\(b.2)") }
if blocks.isEmpty { print("  none") }