pointer based quick sort

linesort

linesort.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "/home/ishan/Dropbox/code/c/lib/my_getline.h"
#define SIZE 100
#define MAXLINES 20

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

int readline(char**, int);
void quicksort(char **, int, int);
void writelines(char **, int);
void swap(char **, int, int);

int main() {
    int nlines;

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

int readline(char **lineptr, int maxlines) {
    int length, nlines=0;
    char *p, line[SIZE];

    while((length = getl(line, SIZE)) > 0) {
        if(nlines >= MAXLINES) {
            return -1;
        } else {
            p = (char*) malloc(sizeof (char) * length);
            strcpy(p, line);
            lineptr[nlines++] = p;
        }
    }
    
    return nlines;
}

void quicksort(char **lineptr, int left, int right) {
    int i, last;

    if(left >= right) {
        return;
    }

    swap(lineptr, left, (left+right)/2);
    last = left;
    for(i = left; i <= right; i++) {
        if(strcmp(lineptr[i], lineptr[left]) < 0) {
            swap(lineptr, ++last, i);
        }
    }

    swap(lineptr, left, last);
    quicksort(lineptr, left, last-1);
    quicksort(lineptr, last+1, right);
}

void swap(char *v[], int i, int j) {
    char *temp;

    temp = v[i];
    v[i] = v[j];
    v[j] = temp;
}

void writelines(char **lineptr, int nlines) {
    while(nlines) {
        printf("%s\n", *lineptr);
        lineptr++;
        nlines--;
    }
}