C言語 数値を32進数表記にする方法

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

int
encode32(unsigned long n, char *buf, int len)
{
	// _____________________0_________1_________2_________3_
	// _____________________01234567890123456789012345678901
	const char * const d = "0123456789abcdefghijklmnopqrstuv";
	char *limit	= buf + len - 1; // -1 は \0 の分

	unsigned long	base = 32;
	char		*p = buf, *t = buf;
	char tmp;

	do
	{
		*p++ = d [ n % base ];
		if (p > limit) {
			return (-1);
		}
	}
	while (n /= base);

	*p = '\0';
	--p;

	// 文字列を逆に並べ替える
	while (t < p)
	{
		tmp = *t;
		*t++ = *p;
		*p-- = tmp;
	}
	return (0);
}

#define BUF 64

void
encode32test(unsigned long n)
{
	char buf[BUF];
	int	buflen = sizeof(buf);
	
	if (encode32(n, buf, buflen) == 0) {
		printf ("%ld => %s\n", n, buf);
	} else {
		printf ("can not encode32 %ld\n", n);
	}
}

void
encode32test_small_buffer(unsigned long n)
{
	char buf[2];
	int	buflen = sizeof(buf);
	
	if (encode32(n, buf, buflen) == 0) {
		printf ("%ld => %s\n", n, buf);
	} else {
		printf ("can not encode32 %ld\n", n);
	}
}

int
main (int argc, char *argv[])
{
	encode32test (0UL);
	encode32test (10UL);
	encode32test (16UL);
	encode32test (31UL);
	encode32test (32UL);
	encode32test (33UL);
	encode32test (34UL);
	encode32test (64UL);
	encode32test (1023UL);
	encode32test (1024UL);
	encode32test (30000UL);
	encode32test (65545UL);

	encode32test_small_buffer (31UL); // v\0 になる
	encode32test_small_buffer (32UL); // エラーのテスト 10\0 は入らない
	encode32test_small_buffer (33UL); // エラーのテスト


	exit (EXIT_SUCCESS);
}

cc -Wall encode32.c

実行方法

./a.out

実行結果

0 => 0
10 => a
16 => g
31 => v
32 => 10
33 => 11
34 => 12
64 => 20
1023 => vv
1024 => 100
30000 => t9g
65545 => 2009
31 => v
can not encode32 32
can not encode32 33