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  1. #include <stdio.h>
  2. #include <string.h>
  3.  
  4. void swap(void* pa, void* pb, size_t sz)
  5. {
  6.   char *p1 = pa, *p2 = pb;
  7.   while (sz--)
  8.   {
  9.     char tmp = *p1;
  10.     *p1++ = *p2;
  11.     *p2++ = tmp;
  12.   }
  13. }
  14.  
  15. /*
  16. Want: abcdefgh12345678 -> a1b2c3d4e5f6g7h8
  17.  
  18. a b c d e f g h
  19.   1 2 3 4 5 6 7 8
  20.  
  21.   4 1-sized swaps:
  22.  
  23. a 1 c 3 e 5 g 7
  24.   b 2 d 4 f 6 h 8
  25.  
  26. a1  c3  e5  g7
  27.     b2  d4  f6  h8
  28.  
  29.   2 2-sized swaps:
  30.  
  31. a1  b2  e5  f6
  32.     c3  d4  g7  h8
  33.  
  34. a1b2  e5f6
  35.       c3d4  g7h8
  36.  
  37.   1 4-sized swap:
  38.  
  39. a1b2  c3d4
  40.       e5f6  g7h8
  41.  
  42. a1b2c3d4
  43.         e5f6g7h8
  44. */
  45.  
  46. void interleave(char* s, size_t len)
  47. {
  48.   size_t start, step, i, j;
  49.  
  50.   if (len <= 2)
  51.     return;
  52.  
  53.   if (len & (len - 1))
  54.     return; // only power of 2 lengths are supported
  55.  
  56.   for (start = 1, step = 2;
  57.        step < len;
  58.        start *= 2, step *= 2)
  59.   {
  60.     for (i = start, j = len / 2;
  61.          i < len / 2;
  62.          i += step, j += step)
  63.     {
  64.       swap(s + i,
  65.            s + j,
  66.            step / 2);
  67.     }
  68.   }
  69. }
  70.  
  71. char testData[][64 + 1] =
  72. {
  73.   { "Aa" },
  74.   { "ABab" },
  75.   { "ABCDabcd" },
  76.   { "ABCDEFGHabcdefgh" },
  77.   { "ABCDEFGHIJKLMNOPabcdefghijklmnop" },
  78.   { "ABCDEFGHIJKLMNOPQRSTUVWXYZ0<({[/abcdefghijklmnopqrstuvwxyz1>)}]\\" },
  79. };
  80.  
  81. int main(void)
  82. {
  83.   unsigned i;
  84.  
  85.   for (i = 0; i < sizeof(testData) / sizeof(testData[0]); i++)
  86.   {
  87.     printf("%s -> ", testData[i]);
  88.     interleave(testData[i], strlen(testData[i]));
  89.     printf("%s\n", testData[i]);
  90.   }
  91.  
  92.   return 0;
  93. }
  94.  
Success #stdin #stdout 0s 1832KB
comments ()
stdin
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Standard input is empty
stdout
copy 
Aa -> Aa
ABab -> AaBb
ABCDabcd -> AaBbCcDd
ABCDEFGHabcdefgh -> AaBbCcDdEeFfGgHh
ABCDEFGHIJKLMNOPabcdefghijklmnop -> AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPp
ABCDEFGHIJKLMNOPQRSTUVWXYZ0<({[/abcdefghijklmnopqrstuvwxyz1>)}]\ -> AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz01<>(){}[]/\
https://ideone.com/bYAvRi
language:
C99 (gcc 8.3)
created:
12 years ago
visibility:
public

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