CODE EXAMPLE FOR JAVA

Radix sort

1 #include <assert.h>
   2 #include <limits.h>
   3 #include <string.h>
   4 
   5 #include "radixsort.h"
   6 
   7 /* in-place MSB radix sort for null-terminated strings */
   8 
   9 /* helper routine for swapping */
  10 static void
  11 swapStrings(const char **a, const char **b)
  12 {
  13     const char *temp;
  14 
  15     temp = *a;
  16     *a = *b;
  17     *b = temp;
  18 }
  19 
  20 /* this is the internal routine that assumes all strings are equal for the
  21  * first k characters */
  22 static void
  23 radixSortInternal(int n, const char **a, int k)
  24 {
  25     int i;
  26     int count[UCHAR_MAX+1];  /* number of strings with given character in position k */
  27     int mode;                /* most common position-k character */
  28     const char **bucket[UCHAR_MAX+1]; /* position of character block in output */
  29     const char **top[UCHAR_MAX+1];    /* first unused index in this character block */
  30 
  31     /* loop implements tail recursion on most common character */
  32     while(n > 1) {
  33 
  34         /* count occurrences of each character */
  35         memset(count, 0, sizeof(int)*(UCHAR_MAX+1));
  36 
  37         for(i = 0; i < n; i++) {
  38             count[(unsigned char) a[i][k]]++;
  39         }
  40 
  41         /* find the most common nonzero character */
  42         /* we will handle this specially */
  43         mode = 1;
  44         for(i = 2; i < UCHAR_MAX+1; i++) {
  45             if(count[i] > count[mode]) {
  46                 mode = i;
  47             }
  48         }
  49 
  50         if(count[mode] < n) {
  51 
  52             /* generate bucket and top fields */
  53             bucket[0] = top[0] = a;
  54             for(i = 1; i < UCHAR_MAX+1; i++) {
  55                 top[i] = bucket[i] = bucket[i-1] + count[i-1];
  56             }
  57 
  58             /* reorder elements by k-th character */
  59             /* this is similar to dutch flag algorithm */
  60             /* we start at bottom character and swap values out until everything is in place */
  61             /* invariant is that for all i, bucket[i] <= j < top[i] implies a[j][k] == i */
  62             for(i = 0; i < UCHAR_MAX+1; i++) {
  63                 while(top[i] < bucket[i] + count[i]) {
  64                     if((unsigned char) top[i][0][k] == i) {
  65                         /* leave it in place, advance bucket */
  66                         top[i]++;
  67                     } else {
  68                         /* swap with top of appropriate block */
  69                         swapStrings(top[i], top[(unsigned char) top[i][0][k]]++);
  70                     }
  71                 }
  72             }
  73 
  74             /* we have now reordered everything */
  75             /* recurse on all but 0 and mode */
  76             for(i = 1; i < UCHAR_MAX+1; i++) {
  77                 if(i != mode) {
  78                     radixSortInternal(count[i], bucket[i], k+1);
  79                 }
  80             }
  81 
  82             /* tail recurse on mode */
  83             n = count[mode];
  84             a = bucket[mode];
  85             k = k+1;
  86 
  87         } else {
  88 
  89             /* tail recurse on whole pile */
  90             k = k+1;
  91         }
  92     }
  93 }
  94 
  95 void
  96 radixSort(int n, const char **a)
  97 {
  98     radixSortInternal(n, a, 0);
  99 }