java - Return the result of swapping the two lower-order bytes of X -

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problem , solution:

/** return result of swapping 2 lower-order bytes of x. * example, if x 0x12345678, swap(x) 0x12347856. */ static int swaplower(int x) {     /* solution */     int lower = x & 0x0000ffff;     int upper = x & 0xffff0000;      return upper | (0xffff & ((lower << 8) | (lower >> 8))); }

i confused how understand solution. tried working through logic did not understand.

also, don't know how come solution in first place!

edit:

properties:

x & 1 = x x & 0 = 0 x | 1 = 1 x | 0 = x

  1. int lower = x & 0x0000ffff = x & 0b00000000000000001111111111111111 = 0b0000000000000000x15 ... x0

  2. int upper = x & 0xffff0000 = x & 0b11111111111111110000000000000000 = 0bx31 ... x160000000000000000

  3. lower << 8 = 0b0000000000000000x15 ... x0 << 8 = 0b00000000x15 ... x000000000

  4. lower >> 8 = 0b0000000000000000x15 ... x0 >> 8 = 0bssssssss00000000x15 ... x8

    • (assuming x signed number, s sign bit; it's 0 if x positive , 1 if x negative)

  5. (lower << 8) | (lower >> 8) = 0b00000000x15 ... x000000000 | 0bssssssss00000000x15 ... x8 = 0bssssssssx15 ... x0x15 ... x8

  6. 0xffff & ((lower << 8) | (lower >> 8)) = 0b000000000000000001111111111111111 & 0bssssssssx15 ... x0x15 ... x8 = 0b00000000000000000x7 ... x0x15 ... x8

  7. upper | (0xffff & ((lower << 8) | (lower >> 8))) = 0bx31 ... x160000000000000000 | 0b00000000000000000x7 ... x0x15 ... x8 = x31 ... x16x7 ... x0x15 ... x8

yes, that's harder understand necessary.

i think easier understand:

int lowest2     = (x & 0x000000ff) << 8; // lowest byte moved 8 bits left int nextlowest2 = (x & 0x0000ff00) >> 8; // next lowest byte move 8 bits right int upper4      = (x & 0xffff0000); return upper4 | lowest2 | nextlowest2;

then when x = 0x12345678:

upper4 | lowest2 | nextlowest2 = 0x12340000 | 0x00007800 | 0x00000056;

but of course if analyse solution gave same thing:

if upper , lower defined as:

int lower = x & 0x0000ffff int upper = x & 0xffff0000;

then:

x                                                = 0x12345678 lower                                            = 0x00005678 (lower << 8)                                     = 0x00567800 (lower >> 8)                                     = 0x00000056 ((lower << 8) | (lower >> 8))                    = 0x00567856 (0xffff & ((lower << 8) | (lower >> 8)))         = 0x00007856 upper                                            = 0x12340000 upper | (0xffff & ((lower << 8) | (lower >> 8))) = 0x12345678

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