busybox/coreutils/factor.c

/*
 * Copyright (C) 2017 Denys Vlasenko <vda.linux@googlemail.com>
 *
 * Licensed under GPLv2, see file LICENSE in this source tree.
 */
//config:config FACTOR
//config:	bool "factor"
//config:	default y
//config:	help
//config:	  factor factorizes integers

//applet:IF_FACTOR(APPLET(factor, BB_DIR_USR_BIN, BB_SUID_DROP))

//kbuild:lib-$(CONFIG_FACTOR) += factor.o

//usage:#define factor_trivial_usage
//usage:       "NUMBER..."
//usage:#define factor_full_usage "\n\n"
//usage:       "Print prime factors"

#include "libbb.h"

#if 0
# define dbg(...) bb_error_msg(__VA_ARGS__)
#else
# define dbg(...) ((void)0)
#endif

typedef unsigned long long wide_t;
#define WIDE_BITS (unsigned)(sizeof(wide_t)*8)
#define TOPMOST_WIDE_BIT ((wide_t)1 << (WIDE_BITS-1))

#if ULLONG_MAX == (UINT_MAX * UINT_MAX + 2 * UINT_MAX)
/* "unsigned" is half as wide as ullong */
typedef unsigned half_t;
#define HALF_MAX UINT_MAX
#define HALF_FMT ""
#elif ULLONG_MAX == (ULONG_MAX * ULONG_MAX + 2 * ULONG_MAX)
/* long is half as wide as ullong */
typedef unsigned long half_t;
#define HALF_MAX ULONG_MAX
#define HALF_FMT "l"
#else
#error Cant find an integer type which is half as wide as ullong
#endif

/* Returns such x that x+1 > sqrt(N) */
static inline half_t isqrt(wide_t N)
{
	wide_t mask_2bits;
	half_t x;

// Never called with N < 1
//	if (N == 0)
//		return 0;

	/* First approximation x > sqrt(N) - half as many bits:
	 * 1xxxxx -> 111 (six bits to three)
	 * 01xxxx -> 111
	 * 001xxx -> 011
	 * 0001xx -> 011 and so on.
	 */
	x = HALF_MAX;
	mask_2bits = TOPMOST_WIDE_BIT | (TOPMOST_WIDE_BIT >> 1);
	while (!(N & mask_2bits)) {
		x >>= 1;
		mask_2bits >>= 2;
	}
	dbg("x:%"HALF_FMT"x", x);

	for (;;) {
		half_t y = (x + N/x) / 2;
		dbg("y:%x y^2:%llx", y, (wide_t)y * y);
		/*
		 * "real" y may be one bit wider: 0x100000000 and get truncated to 0.
		 * In this case, "real" y is > x. The first check below is for this case:
		 */
		if (y == 0 || y >= x) {
			dbg("isqrt(%llx)=%"HALF_FMT"x", N, x);
			return x;
		}
		x = y;
	}
}

static NOINLINE half_t isqrt_odd(wide_t N)
{
	half_t s = isqrt(N);
	if (s && !(s & 1)) /* even? */
		s--;
	return s;
}

static NOINLINE void factorize(wide_t N)
{
	half_t factor;
	half_t max_factor;
	unsigned count3;
	unsigned count5;
	unsigned count7;

	if (N < 4)
		goto end;

	while (!(N & 1)) {
		printf(" 2");
		N >>= 1;
	}

	max_factor = isqrt_odd(N);
	count3 = 3;
	count5 = 6;
	count7 = 9;
	factor = 3;
	for (;;) {
		/* The division is the most costly part of the loop.
		 * On 64bit CPUs, takes at best 12 cycles, often ~20.
		 */
		while ((N % factor) == 0) { /* not likely */
			N = N / factor;
			printf(" %u"HALF_FMT, factor);
			max_factor = isqrt_odd(N);
		}
 next_factor:
		if (factor >= max_factor)
			break;
		factor += 2;
		/* Rudimentary wheel sieving: skip multiples of 3:
		 * Every third odd number is divisible by three and thus isn't a prime:
		 * 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47...
		 * ^ ^   ^  ^     ^  ^     ^  _     ^  ^     _  ^     ^  ^     ^
		 * (^ = primes, _ = would-be-primes-if-not-divisible-by-5)
		 */
		count7--;
		count5--;
		count3--;
		if (count3 && count5 && count7)
			continue;
		if (count3 == 0)
			count3 = 3;
		if (count5 == 0)
			count5 = 5;
		if (count7 == 0)
			count7 = 7;
		goto next_factor;
	}
 end:
	if (N > 1)
		printf(" %llu", N);
	bb_putchar('\n');
}

int factor_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int factor_main(int argc UNUSED_PARAM, char **argv)
{
	//// coreutils has undocumented option ---debug (three dashes)
	//getopt32(argv, "");
	//argv += optind;
	argv++;

	if (!*argv)
		//TODO: read from stdin
		bb_show_usage();

	do {
		wide_t N;
		const char *numstr;

		/* Coreutils compat */
		numstr = skip_whitespace(*argv);
		if (*numstr == '+')
			numstr++;

		N = bb_strtoull(numstr, NULL, 10);
		if (errno)
			bb_show_usage();
		printf("%llu:", N);
		factorize(N);
	} while (*++argv);

	return EXIT_SUCCESS;
}