a pretty small c deterministic turing machine

dtm.c

// could probably be much smaller given some work.
// compressed version

#include <stdio.h>
#define w while
#define b break
int i,j;char s='S',d[]="S0a0>S1a0>a0a0>a1a0>a-H--";struct t{t*l;t*r;char v;};
int main(int c, char**v){t*h=0;w(v[1][j]){t*n=new t;n->v=v[1][j];n->l=h;n->r=0;if(h)h->r=n;
h=n;++j;}w(1){if(!h->l)b;h=h->l;}w(1){if(s=='H')b;i=0;w(d[i]){if(s==d[i]&&h->v==d[i+1]){
s=d[i+2];h->v=d[i+3];t*m=new t;m->v='-';if(d[i+4]=='<'){if(!h->l){m->l=0;m->r=h;h->l=m;}
h=h->l;}if(d[i+4]=='>'){if(!h->r){m->l=h;m->r=0;h->r=m;}h=h->r;}b;}i+=5;}}printf("%c:",s);
w(1){if(!h->l)b;h=h->l;}w(h){putchar(h->v);h=h->r;};return 0;}

// readable version

#include <stdio.h>

int i,j;

char state = 'S';
char delta[] = "S0a0>S1a0>a0a0>a1a0>a-H--";

// node for linked list
struct cell
{
	cell		*left;
	cell		*right;
	char	        value;
};

int main(int argc, char **argv)
{
	// create the head
	cell*		head = 0;

	// write input to tape
	while ( argv[1][j] ) {
		// make a new cell for each input character
		cell* newCell = new cell;
		newCell->value = argv[1][j];

		// add it to the right of head
		newCell->left = head;
		newCell->right = 0;

		// if head is a node, add newCell to the left
		if (head) 
			head->right = newCell;

		// move head to the newCell
		head = newCell;

		++j;
	}

	// move head to leftmost character
	while (1) {
		// stop if the next cell is empty
		if(!head->left) 
			break;

		head = head->left;
	}

	while (1) {
		// exit if we reach the halting state
		if ( state == 'H') break;

		i = 0;
		// check the delta function
		while (delta[i]) {
			// does current state and head value match?
			if(state == delta[i] && head->value == delta[i+1]){
				// switch to new state
				state = delta[i+2];
				// write new value
				head->value = delta[i+3];

				// create a new cell (might need this, might not)
				cell* blankCell = new cell;
				blankCell->value = '-';

				// are we moving to the left?
				if (delta[i+4] == '<') {
					// add blankCell if there is no cell here
					if (!head->left) {
						blankCell->left = 0;
						blankCell->right = head;
						head->left = blankCell;
					}

					// set head to left cell
					head = head->left;
				}

				// are we moving to the right?
				if (delta[i+4] == '>') {
					// add blankCell if there is no cell here
					if (!head->right) {
						blankCell->left = head;
						blankCell->right = 0;
						head->right = blankCell;
					}

					// set head to right cell
					head = head->right;
				}

				break;
			}
			i+=5;
		}
	}
	
	// print the state
	printf("%c:",state);

	// move head to far left
	while (1) {
		if ( !head->left )
			break;
		
		head = head->left;
	}

	// move from left to right, printing cell value as we go.
	while (head) {
		putchar( head->value );
		head = head->right;
	};

	// note, we should delete the tape here but we're trying to be small so... ;)

	return 0;
}