Exercise 5.07

Page 110

Rewrite readlines to store lines in an array supplied by main, rather than calling alloc to maintain storage. How much faster is the program?

#include <stdio.h>
#include <string.h>

#define MAXLINES 5000
#define MAXCHARS 500000

char *lineptr[MAXLINES];

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

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

int main()
{
	int nlines;
	char storage[MAXCHARS];

	if ((nlines = readlines(lineptr, MAXLINES, storage, MAXCHARS)) >= 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
int get_line(char *, int);

int readlines(char *lineptr[], int maxlines, char storage[], int maxchars)
{
	int len, nlines;
	char line[MAXLEN];
	int i;

	i = 0;
	nlines = 0;
	while ((len = get_line(line, MAXLEN)) > 0) {
		if (nlines >= maxlines || (i + len) >= maxchars) {
			return -1;
		} else {
			line[len - 1] = '\0';
			strcpy(storage + i, line);
			lineptr[nlines++] = storage + i;
			i += len;
		}
	}
	return nlines;
}

void writelines(char *lineptr[], int nlines)
{
	while (nlines-- > 0) {
		printf("%s\n", *lineptr++);
	}
}

void qsort(char *v[], int left, int right)
{
	int i, last;
	void swap(char *v[], int i, int j);

	if (left >= right) {
		return;
	}
	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);
	qsort(v, left, last - 1);
	qsort(v, last + 1, right);
}

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

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

int get_line(char s[], int lim)
{
	int c, i;

	i = 0;
	while (--lim > 0 && (c = getchar()) != EOF && c != '\n') {
		s[i++] = c;
	}
	if (c == '\n') {
		s[i++] = c;
	}
	s[i] = '\0';
	return i;
}