organicmaps/harfbuzz
13
Fork 0
mirror of https://github.com/harfbuzz/harfbuzz.git synced 2025-04-13 08:42:59 +00:00
Code Issues 1 Projects Releases Wiki Activity Actions 7

[decycler] Reduce stack use further

Browse source 
Down to three pointers. Exercise for the reader to prove this is
optimal.
This commit is contained in:
Behdad Esfahbod 2025-02-17 14:57:20 -07:00
parent 646da80c41
commit fb0e181a3e
1 changed files with 20 additions and 6 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

26
src/hb-decycler.hh
View file

@ -55,6 +55,13 @@
* That is, it may not detect a cycle until the cycle is fully traversed,
* even multiple times. See Floyd's algorithm analysis for details.
*
* The implementation saves a pointer storage on the stack by combining
* this->u.decycler and this->u.next into a union. This is possible because
* at any point we only need one of those values. The invariant is that
* after construction, and before destruction, of a node, the u.decycler
* field is always valid. The u.next field is only valid when the node is
* in the traversal path, parent to another node.
*
* There are three method's:
*
* - hb_decycler_node_t() constructor: Creates a new node in the traversal.
@ -89,8 +96,9 @@ struct hb_decycler_t
struct hb_decycler_node_t
{
hb_decycler_node_t (hb_decycler_t &decycler)
: decycler (decycler)
{
u.decycler = &decycler;
if (!decycler.tortoise)
{
// First node.
@ -100,19 +108,21 @@ struct hb_decycler_node_t
tortoise_asleep = !decycler.hare->tortoise_asleep;
this->prev = decycler.hare;
decycler.hare->next = this;
decycler.hare->u.next = this;
decycler.hare = this;
if (!tortoise_asleep)
decycler.tortoise = decycler.tortoise->next; // Time to move.
decycler.tortoise = decycler.tortoise->u.next; // Time to move.
}
~hb_decycler_node_t ()
{
hb_decycler_t &decycler = *u.decycler;
assert (decycler.hare == this);
decycler.hare = prev;
if (prev)
prev->next = nullptr;
prev->u.decycler = &decycler;
if (!tortoise_asleep)
decycler.tortoise = decycler.tortoise->prev;
@ -122,6 +132,8 @@ struct hb_decycler_node_t
{
value = value_;
hb_decycler_t &decycler = *u.decycler;
if (decycler.tortoise == this)
return true; // First node; not a cycle.
@ -132,8 +144,10 @@ struct hb_decycler_node_t
}
private:
hb_decycler_t &decycler;
hb_decycler_node_t *next = nullptr;
union {
hb_decycler_t *decycler;
hb_decycler_node_t *next;
} u = {nullptr};
hb_decycler_node_t *prev = nullptr;
unsigned value = (unsigned) -1;
bool tortoise_asleep = false;