#include <stdio.h>
#include <errno.h>
#include <string.h>
#include "fileutil.h"

int ungetc(int c, FILE *fp)
{
	if(c == EOF)
		return c;
	if(!fp->readable) {
		errno = EINVAL;
		return EOF;
	}

	/* If there is no buffer, create a temporary one */
	if(!fp->buf)
		__fp_set_buffer(fp, NULL, 16);

	__fp_buffer_mode_read(fp);

	/* If there is a spot available in the buffer, use it */
	if(fp->bufpos > 0) {
		fp->buf[--fp->bufpos] = c;
		fp->bufungetc++;
		fp->eof = 0;
		return c;
	}

	/* Otherwise, make some room. If the buffer is full of ungetc()
	   characters then we can't preserve the semantics, so we fail. */
	if(fp->bufungetc >= (int)fp->bufsize)
		return EOF;

	/* If the buffer is full, discard read data that isn't ungetc()'d. */
	if(fp->bufread >= fp->bufsize) {
		int discarded = fp->bufread - fp->bufungetc;
		if(fseek(fp, -discarded, SEEK_CUR))
			return EOF;
		fp->bufread -= discarded;
	}

	/* Then move whichever type of read data remains to the very end of the
	   buffer, so we have as much space as possible for more ungetc(). */
	memmove(fp->buf + fp->bufsize - fp->bufread, fp->buf, fp->bufread);
	fp->bufpos = fp->bufsize - fp->bufread;
	fp->bufread = fp->bufsize;

	/* Because we've made space, conclude like the first case */
	fp->buf[--fp->bufpos] = c;
	fp->bufungetc++;
	fp->eof = 0;
	return c;
}