/*-
 * Copyright (c) 2012 Ed Schouten <ed@FreeBSD.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/cdefs.h>
#include <ctype.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

static char *queue = NULL;
static size_t queuelen = 0, queuesize = 0;
static off_t column = 0;

static void
savebyte(char c)
{

        if (queuelen >= queuesize) {
                queuesize += 128;
                queue = realloc(queue, queuesize);
                if (queue == NULL) {
                        perror("malloc");
                        exit(1);
                }
        }
        queue[queuelen++] = c;

        switch (c) {
        case '\n':
                column = 0;
                break;
        case ' ':
                column++;
                break;
        case '\t':
                column = (column / 8 + 1) * 8;
                break;
        }
}

static bool
peekbyte(size_t back, char c)
{

        return (queuelen >= back && queue[queuelen - back] == c);
}

static void
savewhite(char c, bool leading)
{
        off_t ncolumn;

        switch (c) {
        case '\n':
                if (leading) {
                        /* Remove empty lines before input. */
                        queuelen = 0;
                        column = 0;
                } else {
                        /* Remove trailing whitespace. */
                        while (peekbyte(1, ' ') || peekbyte(1, '\t'))
                                queuelen--;
                        /* Remove redundant empty lines. */
                        if (peekbyte(2, '\n') && peekbyte(1, '\n'))
                                return;
                        savebyte('\n');
                }
                break;
        case ' ':
                savebyte(' ');
                break;
        case '\t':
                /* Convert preceding spaces to tabs. */
                ncolumn = (column / 8 + 1) * 8;
                while (peekbyte(1, ' ')) {
                        queuelen--;
                        column--;
                }
                while (column < ncolumn)
                        savebyte('\t');
                break;
        }
}

static void
printwhite(void)
{
        off_t i;

        /* Merge spaces at the start of a sentence to tabs if possible. */
        if ((column % 8) == 0) {
                for (i = 0; i < column; i++)
                        if (!peekbyte(i + 1, ' '))
                                break;
                if (i == column) {
                        queuelen -= column;
                        for (i = 0; i < column; i += 8)
                                queue[queuelen++] = '\t';
                }
        }

        if (fwrite(queue, 1, queuelen, stdout) != queuelen) {
                perror("write");
                exit(1);
        }
        queuelen = 0;
}

static char
readchar(void)
{
        int c;

        c = getchar();
        if (c == EOF && ferror(stdin)) {
                perror("read");
                exit(1);
        }
        return (c);
}

static void
writechar(char c)
{

        if (putchar(c) == EOF) {
                perror("write");
                exit(1);
        }
        /* XXX: Multi-byte characters. */
        column++;
}

int
main(void)
{
        int c;
        bool leading = true;

        while ((c = readchar()) != EOF) {
                if (isspace(c))
                        /* Save whitespace. */
                        savewhite(c, leading);
                else {
                        /* Reprint whitespace and print regular character. */
                        printwhite();
                        writechar(c);
                        leading = false;
                }
        }
        /* Terminate non-empty files with a newline. */
        if (!leading)
                writechar('\n');
        return (0);
}