2cm.c

/* http://codegolf.stackexchange.com/q/48528/134 */
/* by Robert Clausecker (FUZxxl) */

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <stdint.h>

enum opcode { HALT, PRINTX, INCX, INCY, JMP, DJZX, DJZY };
const char *mnemonics[7] = {
	"HALT", "PRINTX", "INCX", "INCY", "JMP", "DJZX", "DJZY"
};

struct instr {
	unsigned int opcode : 3;
	size_t operand : sizeof (size_t) * CHAR_BIT - 3;
};

static size_t labels[UCHAR_MAX];
static struct instr program[16384 / sizeof (struct instr)];
static int debug;

/*
 * code format:
 * one column label (space if no label)
 * one column opcode: H (HALT), X (INCX / DJZX), Y (INCY / DJZY),
 * J (JMP), P (PRINTX).
 * optional one column operand (distinguishing INCX from DJZX, etc).
 * newline
 */

/* returns program length in instructions */
static size_t
assemble(void)
{
	int label, opchar;
	size_t ip = 0, i;

	/* catch unreferences labels */
	for (i = 0; i < UCHAR_MAX; i++)
		labels[i] = -1;

	for (;; ip++) {
		/* read label */
		if ((label = getchar()) == EOF)
			break;

		if (label != ' ')
			labels[(unsigned char)label] = ip;

		/* read opcode */
		switch (opchar = getchar()) {
		case EOF:
			fprintf(stderr, "syntax error on line %zu: opcode expected\n", ip);
			exit(EXIT_FAILURE);

		case 'H': program[ip].opcode = HALT;	break;
		case 'X': program[ip].opcode = INCX;	break;
		case 'Y': program[ip].opcode = INCY;	break;
		case 'J': program[ip].opcode = JMP;	break;
		case 'P': program[ip].opcode = PRINTX;	break;

		default:
			fprintf(stderr, "syntax error on line %zu: invalid opcode %c\n", ip, opchar);
			exit(EXIT_FAILURE);
		}

		/* read operand if present */
		if ((opchar = getchar()) == EOF) {
			fprintf(stderr, "syntax error on line %zu: unexpected end of file\n", ip);
			exit(EXIT_FAILURE);
		}

		if (opchar == '\n') {
			if (program[ip].opcode <= INCY)
				continue;

			fprintf(stderr, "syntax error on line %zu: operand expected\n", ip);
			exit(EXIT_FAILURE);
		}

		if (program[ip].opcode <= PRINTX) {
			fprintf(stderr, "syntax error on line %zu: unexpected operand\n", ip);
			exit(EXIT_FAILURE);
		}

		/* adjust INCX and INCY */
		if (program[ip].opcode <= INCY)
			program[ip].opcode += 3;

		program[ip].operand = (size_t)opchar;

		/* read newline */
		if ((opchar = getchar()) == EOF) {
			fprintf(stderr, "syntax error on line %zu: unexpected end of file\n", ip);
			exit(EXIT_FAILURE);
		}

		if (opchar != '\n') {
			fprintf(stderr, "syntax error on line %zu: newline expected\n", ip);
		}
	}

	/* adjust labels */
	for (i = 0; i < ip; i++) {
		if (program[i].operand == '\0')
			continue;

		if (labels[program[i].operand] == (size_t)-1) {
			fprintf(stderr, "reference to undefined label %c on line %zu\n", (int)program[i].operand, i);
			exit(EXIT_FAILURE);
		}

		program[i].operand = labels[program[i].operand];
	}

	return ip;
}

/* print out a program listing for debugging */
static void
disassemble(size_t ip, struct instr instr)
{
	fprintf(stderr, "%6zu ", ip);

	if (instr.opcode <= INCY)
		fprintf(stderr, "%s\n", mnemonics[instr.opcode]);
	else
		fprintf(stderr, "%-6s %6zu\n", mnemonics[instr.opcode], (size_t)instr.operand);
}

/* interprete the program */
static void
interpret(uintmax_t x, uintmax_t y)
{
	size_t ip = 0;

	for (;; ip++) {
decode:
		if (debug) {
			fprintf(stderr, "%6jd %6jd ", x, y);
			disassemble(ip, program[ip]);
		}

		switch (program[ip].opcode) {
		case HALT:
			return;

		case PRINTX:
			printf("%ju\n", x);
			break;

		case INCX:
			x++;
			break;

		case INCY:
			y++;
			break;

		case JMP:
			ip = program[ip].operand;
			goto decode;

		case DJZX:
			if (x == 0) {
				ip = program[ip].operand;
				goto decode;
			}

			x--;
			break;

		case DJZY:
			if (y == 0) {
				ip = program[ip].operand;
				goto decode;
			}

			y--;
			break;

		default:
			fputs("decoding error", stderr);
			exit(EXIT_FAILURE);
		}
	}
}

extern int
main(int argc, char *argv[])
{
	uintmax_t x, y = 0;
	size_t plen, i;

	if (argc != 2) {
		fprintf(stderr, "Usage: %s x\n", argv[0]);
		return EXIT_FAILURE;
	}

	debug = getenv("DEBUG") != NULL;
	plen = assemble();
	x = (uintmax_t)atol(argv[1]);

	if (debug) {
		fprintf(stderr, "%zu instructions loaded\n", plen);
		for (i = 0; i < plen; i++)
			disassemble(i, program[i]);
	}

	interpret(x, y);

	return EXIT_SUCCESS;
}