static void do_indexing(heatshrink_encoder *hse) {
#if HEATSHRINK_USE_INDEX
/* Build an index array I that contains flattened linked lists
* for the previous instances of every byte in the buffer.
*
* For example, if buf[200] == 'x', then index[200] will either
* be an offset i such that buf[i] == 'x', or a negative offset
* to indicate end-of-list. This significantly speeds up matching,
* while only using sizeof(uint16_t)*sizeof(buffer) bytes of RAM.
*
* Future optimization options:
* 1. Since any negative value represents end-of-list, the other
* 15 bits could be used to improve the index dynamically.
*
* 2. Likewise, the last lookahead_sz bytes of the index will
* not be usable, so temporary data could be stored there to
* dynamically improve the index.
* */
struct hs_index *hsi = HEATSHRINK_ENCODER_INDEX(hse);
int16_t last[256];
memset(last, 0xFF, sizeof(last));
uint8_t * const data = hse->buffer;
int16_t * const index = hsi->index;
const uint16_t input_offset = get_input_offset(hse);
const uint16_t end = input_offset + hse->input_size;
for (uint16_t i=0; i<end; i++) {
uint8_t v = data[i];
int16_t lv = last[v];
index[i] = lv;
last[v] = i;
}
#else
(void)hse;
#endif
}
/* Return the longest match for the bytes at buf[end:end+maxlen] between
* buf[start] and buf[end-1]. If no match is found, return -1. */
static uint16_t find_longest_match(heatshrink_encoder *hse, uint16_t start,
uint16_t end, const uint16_t maxlen, uint16_t *match_length) {
LOG("-- scanning for match of buf[%u:%u] between buf[%u:%u] (max %u bytes)\n",
end, end + maxlen, start, end + maxlen - 1, maxlen);
uint8_t *buf = hse->buffer;
uint16_t match_maxlen = 0;
uint16_t match_index = MATCH_NOT_FOUND;
uint16_t len = 0;
uint8_t * const needlepoint = &buf[end];
#if HEATSHRINK_USE_INDEX
struct hs_index *hsi = HEATSHRINK_ENCODER_INDEX(hse);
int16_t pos = hsi->index[end];
while (pos - (int16_t)start >= 0) {
uint8_t * const pospoint = &buf[pos];
len = 0;
/* Only check matches that will potentially beat the current maxlen.
* This is redundant with the index if match_maxlen is 0, but the
* added branch overhead to check if it == 0 seems to be worse. */
if (pospoint[match_maxlen] != needlepoint[match_maxlen]) {
pos = hsi->index[pos];
continue;
}
for (len = 1; len < maxlen; len++) {
if (pospoint[len] != needlepoint[len]) break;
}
if (len > match_maxlen) {
match_maxlen = len;
match_index = pos;
if (len == maxlen) { break; } /* won't find better */
}
pos = hsi->index[pos];
}
#else
for (int16_t pos=end - 1; pos - (int16_t)start >= 0; pos--) {
uint8_t * const pospoint = &buf[pos];
if ((pospoint[match_maxlen] == needlepoint[match_maxlen])
&& (*pospoint == *needlepoint)) {
for (len=1; len<maxlen; len++) {
if (0) {
LOG(" --> cmp buf[%d] == 0x%02x against %02x (start %u)\n",
pos + len, pospoint[len], needlepoint[len], start);
}
if (pospoint[len] != needlepoint[len]) { break; }
}
if (len > match_maxlen) {
match_maxlen = len;
match_index = pos;
if (len == maxlen) { break; } /* don't keep searching */
}
}
}
#endif
const size_t break_even_point =
(1 + HEATSHRINK_ENCODER_WINDOW_BITS(hse) +
HEATSHRINK_ENCODER_LOOKAHEAD_BITS(hse));
/* Instead of comparing break_even_point against 8*match_maxlen,
* compare match_maxlen against break_even_point/8 to avoid
* overflow. Since MIN_WINDOW_BITS and MIN_LOOKAHEAD_BITS are 4 and
* 3, respectively, break_even_point/8 will always be at least 1. */
if (match_maxlen > (break_even_point / 8)) {
LOG("-- best match: %u bytes at -%u\n",
match_maxlen, end - match_index);
*match_length = match_maxlen;
return end - match_index;
}
LOG("-- none found\n");
return MATCH_NOT_FOUND;
}
/* Return the longest match for the bytes at buf[end:end+maxlen] between
* buf[start] and buf[end-1]. If no match is found, return -1. */
static uint16_t ICACHE_FLASH_ATTR find_longest_match(heatshrink_encoder *hse, uint16_t start,
uint16_t end, const uint16_t maxlen, uint16_t *match_length) {
LOG("-- scanning for match of buf[%u:%u] between buf[%u:%u] (max %u bytes)\n",
end, end + maxlen, start, end + maxlen - 1, maxlen);
uint8_t *buf = hse->buffer;
uint16_t match_maxlen = 0;
uint16_t match_index = MATCH_NOT_FOUND;
const uint16_t break_even_point = 3;
uint16_t len = 0;
uint8_t * const needlepoint = &buf[end];
#if HEATSHRINK_USE_INDEX
struct hs_index *hsi = HEATSHRINK_ENCODER_INDEX(hse);
int16_t pos = hsi->index[end];
while (pos >= start) {
uint8_t * const pospoint = &buf[pos];
len = 0;
/* Only check matches that will potentially beat the current maxlen.
* This is redundant with the index if match_maxlen is 0, but the
* added branch overhead to check if it == 0 seems to be worse. */
if (pospoint[match_maxlen] != needlepoint[match_maxlen]) {
pos = hsi->index[pos];
continue;
}
for (len = 1; len < maxlen; len++) {
if (pospoint[len] != needlepoint[len]) break;
}
if (len > match_maxlen) {
match_maxlen = len;
match_index = pos;
if (len == maxlen) { break; } /* won't find better */
}
pos = hsi->index[pos];
}
#else
for (int16_t pos=end - 1; pos >= start; pos--) {
uint8_t * const pospoint = &buf[pos];
if ((pospoint[match_maxlen] == needlepoint[match_maxlen])
&& (*pospoint == *needlepoint)) {
for (len=1; len<maxlen; len++) {
if (0) {
LOG(" --> cmp buf[%d] == 0x%02x against %02x (start %u)\n",
pos + len, pospoint[len], needlepoint[len], start);
}
if (pospoint[len] != needlepoint[len]) { break; }
}
if (len > match_maxlen) {
match_maxlen = len;
match_index = pos;
if (len == maxlen) { break; } /* don't keep searching */
}
}
}
#endif
if (match_maxlen >= break_even_point) {
LOG("-- best match: %u bytes at -%u\n",
match_maxlen, end - match_index);
*match_length = match_maxlen;
return end - match_index;
}
LOG("-- none found\n");
return MATCH_NOT_FOUND;
}
