Encode a buffer into its base16 representation

base16_encode.c

#include <stdint.h>
#include <stddef.h>

static const uint16_t __base16_enc_le[] = {
    0x3030, 0x3130, 0x3230, 0x3330, 0x3430, 0x3530, 0x3630, 0x3730,
    0x3830, 0x3930, 0x4130, 0x4230, 0x4330, 0x4430, 0x4530, 0x4630,
    0x3031, 0x3131, 0x3231, 0x3331, 0x3431, 0x3531, 0x3631, 0x3731,
    0x3831, 0x3931, 0x4131, 0x4231, 0x4331, 0x4431, 0x4531, 0x4631,
    0x3032, 0x3132, 0x3232, 0x3332, 0x3432, 0x3532, 0x3632, 0x3732,
    0x3832, 0x3932, 0x4132, 0x4232, 0x4332, 0x4432, 0x4532, 0x4632,
    0x3033, 0x3133, 0x3233, 0x3333, 0x3433, 0x3533, 0x3633, 0x3733,
    0x3833, 0x3933, 0x4133, 0x4233, 0x4333, 0x4433, 0x4533, 0x4633,
    0x3034, 0x3134, 0x3234, 0x3334, 0x3434, 0x3534, 0x3634, 0x3734,
    0x3834, 0x3934, 0x4134, 0x4234, 0x4334, 0x4434, 0x4534, 0x4634,
    0x3035, 0x3135, 0x3235, 0x3335, 0x3435, 0x3535, 0x3635, 0x3735,
    0x3835, 0x3935, 0x4135, 0x4235, 0x4335, 0x4435, 0x4535, 0x4635,
    0x3036, 0x3136, 0x3236, 0x3336, 0x3436, 0x3536, 0x3636, 0x3736,
    0x3836, 0x3936, 0x4136, 0x4236, 0x4336, 0x4436, 0x4536, 0x4636,
    0x3037, 0x3137, 0x3237, 0x3337, 0x3437, 0x3537, 0x3637, 0x3737,
    0x3837, 0x3937, 0x4137, 0x4237, 0x4337, 0x4437, 0x4537, 0x4637,
    0x3038, 0x3138, 0x3238, 0x3338, 0x3438, 0x3538, 0x3638, 0x3738,
    0x3838, 0x3938, 0x4138, 0x4238, 0x4338, 0x4438, 0x4538, 0x4638,
    0x3039, 0x3139, 0x3239, 0x3339, 0x3439, 0x3539, 0x3639, 0x3739,
    0x3839, 0x3939, 0x4139, 0x4239, 0x4339, 0x4439, 0x4539, 0x4639,
    0x3041, 0x3141, 0x3241, 0x3341, 0x3441, 0x3541, 0x3641, 0x3741,
    0x3841, 0x3941, 0x4141, 0x4241, 0x4341, 0x4441, 0x4541, 0x4641,
    0x3042, 0x3142, 0x3242, 0x3342, 0x3442, 0x3542, 0x3642, 0x3742,
    0x3842, 0x3942, 0x4142, 0x4242, 0x4342, 0x4442, 0x4542, 0x4642,
    0x3043, 0x3143, 0x3243, 0x3343, 0x3443, 0x3543, 0x3643, 0x3743,
    0x3843, 0x3943, 0x4143, 0x4243, 0x4343, 0x4443, 0x4543, 0x4643,
    0x3044, 0x3144, 0x3244, 0x3344, 0x3444, 0x3544, 0x3644, 0x3744,
    0x3844, 0x3944, 0x4144, 0x4244, 0x4344, 0x4444, 0x4544, 0x4644,
    0x3045, 0x3145, 0x3245, 0x3345, 0x3445, 0x3545, 0x3645, 0x3745,
    0x3845, 0x3945, 0x4145, 0x4245, 0x4345, 0x4445, 0x4545, 0x4645,
    0x3046, 0x3146, 0x3246, 0x3346, 0x3446, 0x3546, 0x3646, 0x3746,
    0x3846, 0x3946, 0x4146, 0x4246, 0x4346, 0x4446, 0x4546, 0x4646,
};

static const uint16_t __base16_enc_be[] = {
    0x3030, 0x3031, 0x3032, 0x3033, 0x3034, 0x3035, 0x3036, 0x3037,
    0x3038, 0x3039, 0x3041, 0x3042, 0x3043, 0x3044, 0x3045, 0x3046,
    0x3130, 0x3131, 0x3132, 0x3133, 0x3134, 0x3135, 0x3136, 0x3137,
    0x3138, 0x3139, 0x3141, 0x3142, 0x3143, 0x3144, 0x3145, 0x3146,
    0x3230, 0x3231, 0x3232, 0x3233, 0x3234, 0x3235, 0x3236, 0x3237,
    0x3238, 0x3239, 0x3241, 0x3242, 0x3243, 0x3244, 0x3245, 0x3246,
    0x3330, 0x3331, 0x3332, 0x3333, 0x3334, 0x3335, 0x3336, 0x3337,
    0x3338, 0x3339, 0x3341, 0x3342, 0x3343, 0x3344, 0x3345, 0x3346,
    0x3430, 0x3431, 0x3432, 0x3433, 0x3434, 0x3435, 0x3436, 0x3437,
    0x3438, 0x3439, 0x3441, 0x3442, 0x3443, 0x3444, 0x3445, 0x3446,
    0x3530, 0x3531, 0x3532, 0x3533, 0x3534, 0x3535, 0x3536, 0x3537,
    0x3538, 0x3539, 0x3541, 0x3542, 0x3543, 0x3544, 0x3545, 0x3546,
    0x3630, 0x3631, 0x3632, 0x3633, 0x3634, 0x3635, 0x3636, 0x3637,
    0x3638, 0x3639, 0x3641, 0x3642, 0x3643, 0x3644, 0x3645, 0x3646,
    0x3730, 0x3731, 0x3732, 0x3733, 0x3734, 0x3735, 0x3736, 0x3737,
    0x3738, 0x3739, 0x3741, 0x3742, 0x3743, 0x3744, 0x3745, 0x3746,
    0x3830, 0x3831, 0x3832, 0x3833, 0x3834, 0x3835, 0x3836, 0x3837,
    0x3838, 0x3839, 0x3841, 0x3842, 0x3843, 0x3844, 0x3845, 0x3846,
    0x3930, 0x3931, 0x3932, 0x3933, 0x3934, 0x3935, 0x3936, 0x3937,
    0x3938, 0x3939, 0x3941, 0x3942, 0x3943, 0x3944, 0x3945, 0x3946,
    0x4130, 0x4131, 0x4132, 0x4133, 0x4134, 0x4135, 0x4136, 0x4137,
    0x4138, 0x4139, 0x4141, 0x4142, 0x4143, 0x4144, 0x4145, 0x4146,
    0x4230, 0x4231, 0x4232, 0x4233, 0x4234, 0x4235, 0x4236, 0x4237,
    0x4238, 0x4239, 0x4241, 0x4242, 0x4243, 0x4244, 0x4245, 0x4246,
    0x4330, 0x4331, 0x4332, 0x4333, 0x4334, 0x4335, 0x4336, 0x4337,
    0x4338, 0x4339, 0x4341, 0x4342, 0x4343, 0x4344, 0x4345, 0x4346,
    0x4430, 0x4431, 0x4432, 0x4433, 0x4434, 0x4435, 0x4436, 0x4437,
    0x4438, 0x4439, 0x4441, 0x4442, 0x4443, 0x4444, 0x4445, 0x4446,
    0x4530, 0x4531, 0x4532, 0x4533, 0x4534, 0x4535, 0x4536, 0x4537,
    0x4538, 0x4539, 0x4541, 0x4542, 0x4543, 0x4544, 0x4545, 0x4546,
    0x4630, 0x4631, 0x4632, 0x4633, 0x4634, 0x4635, 0x4636, 0x4637,
    0x4638, 0x4639, 0x4641, 0x4642, 0x4643, 0x4644, 0x4645, 0x4646,
};

static const uint16_t *__base16_enc_tabs[] = {__base16_enc_le, __base16_enc_be};

/**
 * Encode a buffer into its base16 representation
 * @dst     Base16 buffer(at least sized n * 2)
 * @src     Input data buffer
 * @n       Input buffer size
 */
void base16_encode(char * restrict dst, const void * restrict src, size_t n)
{
    static const uint16_t magic = 0x0100;
    static const uint16_t *tab = NULL;

    register size_t i;
    register const uint8_t *s = src;
    register uint16_t *d = (uint16_t *) dst;

    if (tab == NULL) tab = __base16_enc_tabs[* (char *) &magic];

    for (i = 0; i < n; i++) d[i] = tab[*s++];
}