k&R(5.6): Pointer Arrays

/*
   k&r(5.6): Pointer Arrays
   created on Mar 22, 2011
   */
#include    "stdio.h"
#include    "string.h"

#define MAXLINES 5000  /* max #lines to be sorted */

char *lineptr[MAXLINES];  /*pointers to text lines */

int readlines(char *lineptr[], int nlines);
void writelines(char *lineptr[], int nlines);

void qsort(char *lineptr[], int left, int right);

/* sort input lines */
main()
{
    int nlines;  /* number of input lines read */

    if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
        qsort (lineptr, 0, nlines-1);
        writelines(lineptr, nlines);
        return 0;
    }
    else {
        printf("error: input too big to sort\n");
        return 1;
    }
}

#define MAXLEN 1000   /* max length of any input line */
int getline1(char *, int);
char *alloc(int);

/* readlines: read input lines */
int readlines(char *lineptr[], int maxlines)
{
    int len, nlines;
    char *p, line[MAXLEN];

    nlines = 0;
    while ((len = getline1(line, MAXLEN)) > 0)
      if (nlines >= maxlines || (p = alloc(len)) == NULL)
        return -1;
      else {
          line[len-1] = '\0';  /* delete newline */
          strcpy(p, line);
          lineptr[nlines++] = p;
      }
    return nlines;
}

/* writelines: write output lines */
void writelines(char *lineptr[], int nlines)
{
    int i;

    for (i = 0; i < nlines; i++)
      printf("%s\n", lineptr[i]);

}

/* qsort: sort v[left]...v[right] into increasing order */
void qsort(char *v[], int left, int right)
{
    int i, last;
    void swap(char *v[], int i, int j);

    if (left >= right) /* do nothing if array contains */
      return;           /* fewer than two elements */
    swap (v, left, (left+right)/2);
    last = left;
    for (i = left+1; i <= right; i++)
      if (strcmp(v[i], v[left]) < 0)
        swap(v, ++last, i);
    swap (v, left, last);  /* restore partition elem */
    qsort (v, left, last-1);
    qsort (v, last+1, right);
}

/* swap: interchange v[i] and v[j] */
void swap (char *v[], int i, int j)
{
    char *temp;
    temp = v[i];
    v[i] = v[j];
    v[j] = temp;
}

int getline1(char *s, int lim)
{
    int i,c;
   for(i = 0; i < lim-1 && (c=getchar()) != EOF && c!='\n'; ++i)
     s[i] = c;
   if (c == '\n') {
       s[i] = c;
       ++i;
    }
    s[i] = '\0';
    return i;
}


#define     ALLOCSIZE 10000

static char allocbuf[ALLOCSIZE];
static char *allocp = allocbuf;

char *alloc(int n) /* return pointer to n characters */
{
    if (allocbuf + ALLOCSIZE - allocp >= n) {
        allocp += n;
        return allocp - n;
    } else
      return 0;
}