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

// Unsigned Leading Zero Trim (ULZT) is a variable-length quantity algorithm.
// The first octet advertises the total size and its 'n' bits hold the most
// significant bits. Any following octets hold the least significant bits in
// little-endian order.
//
//  Size Table Including Numeric Range
//
// 1 octet:  n n n n n n n 0 (0, 127)
// 2 octets: n n n n n n 0 1 (128, 16'383)
// 3 octets: n n n n n 0 1 1 (16'384, 2'097'151)
// 4 octets: n n n n 0 1 1 1 (2'097'152, 268'435'455)
// 5 octets: n n n 0 1 1 1 1 (268'435'456, 34'359'738'367)
// 6 octets: n n 0 1 1 1 1 1 (34'359'738'368, 4'398'046'511'103)
// 7 octets: n 0 1 1 1 1 1 1 (4'398'046'511'104, 562'949'953'421'311)
// 8 octets: 0 1 1 1 1 1 1 1 (562'949'953'421'312, 72'057'594'037'927'935)
// 9 octets: 1 1 1 1 1 1 1 1 (72'057'594'037'927'936, 18'446'744'073'709'551'615)


#if __BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__
#error "need little-endian byte order"
#endif

// Encodes up to 9 octets in buf and returns the final position, exclusive.
void* ulzt64enc(void* buf, uint64_t v) {
	uint8_t* first = (uint8_t*) buf;
	uint8_t* p = first + 1;

	unsigned int n = 0;
	uint_fast8_t mask = 0;
	while (v > 127 >> n++) {
		*p++ = v;
		v >>= 8;
		mask = mask << 1 | 1;
	}
	*first = v << n | mask;

	return p;
}

// Decodes up to 9 octets from buf and returns the final position, exclusive.
// The value of address v must be zero and buflen must be grather than zero.
// When EOF, the result is undefined.
void* ulzt64dec(uint64_t* v, void* buf, size_t buflen) {
	uint8_t* p = (uint8_t*) buf;
	uint8_t* vp = (uint8_t*) v;

	uint_fast8_t first = *p++;
	for (; first & 1 && --buflen; first >>= 1)
		*vp++ = *p++;
	if (first) *vp = first >> 1;

	return p;
}