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An abstract mergesort function.

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This commit is contained in:
Bruno Haible
2003-01-16 12:41:41 +00:00
parent c3467c5302
commit 826e4c8ba1
5 changed files with 123 additions and 95 deletions
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100
src/search.cc
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@@ -152,84 +152,6 @@ Search::prepare ()
}
}
/* ----------------------- Sorting the Keyword list ------------------------ */
/* Merges two sorted lists together to form one sorted list.
The sorting criterion depends on which of _occurrence_sort and _hash_sort
is set to true. This is a kludge, but permits nice sharing of almost
identical code without incurring the overhead of a function call for
every comparison. */
KeywordExt_List *
Search::merge (KeywordExt_List *list1, KeywordExt_List *list2) const
{
KeywordExt_List *result;
KeywordExt_List **resultp = &result;
for (;;)
{
if (!list1)
{
*resultp = list2;
break;
}
if (!list2)
{
*resultp = list1;
break;
}
if ((_occurrence_sort
&& list1->first()->_occurrence < list2->first()->_occurrence)
|| (_hash_sort
&& list1->first()->_hash_value > list2->first()->_hash_value))
{
*resultp = list2;
resultp = &list2->rest(); list2 = list1; list1 = *resultp;
}
else
{
*resultp = list1;
resultp = &list1->rest(); list1 = *resultp;
}
}
return result;
}
/* Sorts a list using the recursive merge sort algorithm.
The sorting criterion depends on which of _occurrence_sort and _hash_sort
is set to true. */
KeywordExt_List *
Search::merge_sort (KeywordExt_List *head) const
{
if (!head || !head->rest())
/* List of length 0 or 1. Nothing to do. */
return head;
else
{
/* Determine a list node in the middle. */
KeywordExt_List *middle = head;
for (KeywordExt_List *temp = head->rest();;)
{
temp = temp->rest();
if (temp == NULL)
break;
temp = temp->rest();
middle = middle->rest();
if (temp == NULL)
break;
}
/* Cut the list into two halves.
If the list has n elements, the left half has ceiling(n/2) elements
and the right half has floor(n/2) elements. */
KeywordExt_List *right_half = middle->rest();
middle->rest() = NULL;
/* Sort the two halves, then merge them. */
return merge (merge_sort (head), merge_sort (right_half));
}
}
/* ---------------- Reordering the Keyword list (optional) ----------------- */
/* Computes the sum of occurrences of the _selchars of a keyword.
@@ -251,6 +173,13 @@ Search::compute_occurrence (KeywordExt *ptr) const
return value;
}
/* Comparison function for sorting by decreasing _occurrence valuation. */
static bool
greater_by_occurrence (KeywordExt *keyword1, KeywordExt *keyword2)
{
return keyword1->_occurrence > keyword2->_occurrence;
}
/* Auxiliary function for reorder():
Sets all alphabet characters as undetermined. */
@@ -308,9 +237,7 @@ Search::reorder ()
}
/* Sort the list by decreasing _occurrence valuation. */
_hash_sort = false;
_occurrence_sort = true;
_head = merge_sort (_head);
_head = mergesort_list (_head, greater_by_occurrence);
/* Reorder the list to maximize the efficiency of the search. */
@@ -716,14 +643,19 @@ Search::find_asso_values ()
/* ------------------------------------------------------------------------- */
/* Comparison function for sorting by increasing _hash_value. */
static bool
less_by_hash_value (KeywordExt *keyword1, KeywordExt *keyword2)
{
return keyword1->_hash_value < keyword2->_hash_value;
}
/* Sorts the keyword list by hash value. */
void
Search::sort ()
{
_hash_sort = true;
_occurrence_sort = false;
_head = merge_sort (_head);
_head = mergesort_list (_head, less_by_hash_value);
}
void