simple-hash.h

#include <assert.h>
#include <limits.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "blake2.h"

/* convert a 32-byte hash into a 44-byte printable string
 * by base64(url)-encoding it */
static inline void
hash_to_base64(unsigned char *restrict dst, const unsigned char *restrict src)
{
	const unsigned char *alph = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
	unsigned v;
	int i, di;

	di = 0;
	for (i=0; i<30; i+=3) {
		v = ((unsigned)src[i] << 16) | ((unsigned)src[i+1] << 8) | (unsigned)src[i+2];
		dst[di+0] = alph[(v>>18)&0x3f];
		dst[di+1] = alph[(v>>12)&0x3f];
		dst[di+2] = alph[(v>>6)&0x3f];
		dst[di+3] = alph[v&0x3f];
		di += 4;
	}
	/* tail: 2 bytes */
	v = ((unsigned)src[30] << 16) | ((unsigned)src[31] << 8);
	dst[40] = alph[(v>>18)&0x3f];
	dst[41] = alph[(v>>12)&0x3f];
	dst[42] = alph[(v>>6)&0x3f];
	dst[43] = '=';
}

/* fast-path for file hashing */
static inline int
fast_hash_file(unsigned char *restrict dst, const char *fname)
{
        unsigned char buf[32];
        unsigned char *mem;
        struct stat stbuf;
	blake2b_state S;
	int err, fd;

        fd = open(fname, O_RDONLY|O_CLOEXEC);
        if (fd < 0)
                return errno;

        if (fstat(fd, &stbuf) < 0)
                goto fail;

        /* use slow fallback */
        if (stbuf.st_rdev || stbuf.st_size > SSIZE_MAX) {
                errno = EINVAL;
                goto fail;
        }

        mem = mmap(NULL, (size_t)stbuf.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
        if (mem == MAP_FAILED)
                goto fail;

	while (close(fd) < 0 && errno == EINTR) ;
        err = blake2b(buf, 32, mem, stbuf.st_size, NULL, 0);
        assert(err == 0);
	hash_to_base64(dst, buf);

        while (munmap(mem, stbuf.st_size) < 0 && errno == EINTR) ;
	return 0;
fail:
        err = errno;
        while (close(fd) < 0 && errno == EINTR) ;
        return err;
}