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[SKIPLIST]

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- Add a LookupNodeByIndexSkiplist function and a small test for it in skiplist_test.
- Remove a redundant double-set in LookupElementSkiplist.

Yes, we now have one lookup function that accepts search criteria and returns an element and another one that accepts an index and returns a SKIPLIST_NODE.
It's simply because I have exactly these two cases :)
You're free to implement the two missing functions or refactor this code another way.

svn path=/branches/colins-printing-for-freedom/; revision=68262
This commit is contained in:
Colin Finck 2015-06-25 13:12:01 +00:00
parent 7aef841dea
commit 84416a465a
3 changed files with 51 additions and 2 deletions
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47
reactos/lib/skiplist/skiplist.c
View file

@ -366,8 +366,6 @@ LookupElementSkiplist(PSKIPLIST Skiplist, PVOID Element, PDWORD ElementIndex)
// * Walk through all nodes on this level that come before the node we're looking for.
// * When we have reached such a node, go down a level and continue there.
// * Repeat these steps till we're in level 0, right in front of the node we're looking for.
pNode = &Skiplist->Head;
for (i = Skiplist->MaximumLevel + 1; --i >= 0;)
{
while (pNode->Next[i] && pNode->Next[i] != pLastComparedNode && Skiplist->CompareRoutine(pNode->Next[i]->Element, Element) < 0)
@ -395,3 +393,48 @@ LookupElementSkiplist(PSKIPLIST Skiplist, PVOID Element, PDWORD ElementIndex)
// Return the stored element of the found node.
return pNode->Element;
}
/**
* @name LookupNodeByIndexSkiplist
*
* Looks up a node in the Skiplist at the given position. The efficiency of this operation is O(log N) on average.
*
* @param Skiplist
* Pointer to the SKIPLIST structure to operate on.
*
* @param ElementIndex
* Zero-based position of the node in the Skiplist.
*
* @return
* Returns the found node or NULL if the position is invalid.
*/
PSKIPLIST_NODE
LookupNodeByIndexSkiplist(PSKIPLIST Skiplist, DWORD ElementIndex)
{
CHAR i;
DWORD dwIndex = 0;
PSKIPLIST_NODE pNode = &Skiplist->Head;
// The only way the node can't be found is when the index is out of range.
if (ElementIndex >= Skiplist->NodeCount)
return NULL;
// Do the efficient lookup in Skiplists:
// * Start from the maximum level.
// * Walk through all nodes on this level that come before the node we're looking for.
// * When we have reached such a node, go down a level and continue there.
// * Repeat these steps till we're in level 0, right in front of the node we're looking for.
for (i = Skiplist->MaximumLevel + 1; --i >= 0;)
{
// We compare with <= instead of < here, because the added distances make up a 1-based index while ElementIndex is zero-based,
// so we have to jump one node further.
while (pNode->Next[i] && dwIndex + pNode->Distance[i] <= ElementIndex)
{
dwIndex += pNode->Distance[i];
pNode = pNode->Next[i];
}
}
// We are right in front of the node we're looking for now.
return pNode->Next[0];
}

1
reactos/lib/skiplist/skiplist.h
View file

@ -49,5 +49,6 @@ BOOL InsertElementSkiplist(PSKIPLIST Skiplist, PVOID Element);
BOOL InsertTailElementSkiplist(PSKIPLIST Skiplist, PVOID Element);
PVOID DeleteElementSkiplist(PSKIPLIST Skiplist, PVOID Element);
PVOID LookupElementSkiplist(PSKIPLIST Skiplist, PVOID Element, PDWORD ElementIndex);
PSKIPLIST_NODE LookupNodeByIndexSkiplist(PSKIPLIST Skiplist, DWORD ElementIndex);
#endif

5
reactos/lib/skiplist/skiplist_test.c
View file

@ -67,6 +67,7 @@ main()
DWORD ElementIndex;
DWORD i;
SKIPLIST Skiplist;
PSKIPLIST_NODE pNode;
system("mode con cols=300");
InitializeSkiplist(&Skiplist, MyAlloc, MyCompare, MyFree);
@ -83,6 +84,10 @@ main()
for (i = 0; i < 40; i++)
InsertElementSkiplist(&Skiplist, (PVOID)(rand() % 100));
// Output the third element (with zero-based index 2).
pNode = LookupNodeByIndexSkiplist(&Skiplist, 2);
printf("Element = %lu for index 2\n", (DWORD)pNode->Element);
// Check if an element with number 44 is in the list and output its index.
Element = (DWORD)LookupElementSkiplist(&Skiplist, (PVOID)44, &ElementIndex);
printf("Element = %lu, ElementIndex = %lu\n\n", Element, ElementIndex);