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tree234.c: new search234() system.

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This is a thing I've been meaning to set up for a while: it's a
pull-based search system (that is, the caller takes each step of the
search manually, rather than providing a callback), which lets the
caller inspect every step of the search, including the index of each
candidate element in the tree. This allows flexible kinds of query
that play the element and its index off against each other.

I've also rewritten the existing findrelpos234() search function using
the new one as a primitive, because that simplifies it a lot!
This commit is contained in:
Simon Tatham
2018-09-19 12:58:54 +01:00
parent ff7418af73
commit b2d0bd0da4
2 changed files with 246 additions and 89 deletions
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35
tree234.h
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@ -132,6 +132,41 @@ void *findpos234(tree234 * t, void *e, cmpfn234 cmp, int *index);
void *findrelpos234(tree234 * t, void *e, cmpfn234 cmp, int relation,
int *index);
/*
* A more general search type still. Use search234_start() to
* initialise one of these state structures; it will fill in
* state->element with an element of the tree, and state->index with
* the index of that element. If you don't like that element, call
* search234_step, with direction == -1 if you want an element earlier
* in the tree, or +1 if you want a later one.
*
* If either function returns state->element == NULL, then you've
* narrowed the search to a point between two adjacent elements, so
* there are no further elements left to return consistent with the
* constraints you've imposed. In this case, state->index tells you
* how many elements come before the point you narrowed down to. After
* this, you mustn't call search234_step again (unless the state
* structure is first reinitialised).
*
* The use of this search system is that you get both the candidate
* element _and_ its index at every stage, so you can use both of them
* to make your decision. Also, you can remember element pointers from
* earlier in the search.
*
* The fields beginning with underscores are private to the
* implementation, and only exposed so that clients can know how much
* space to allocate for the structure as a whole. Don't modify them.
* (Except that it's safe to copy the whole structure.)
*/
typedef struct search234_state {
void *element;
int index;
int _lo, _hi, _last, _base;
void *_node;
} search234_state;
void search234_start(search234_state *state, tree234 *t);
void search234_step(search234_state *state, int direction);
/*
* Delete an element e in a 2-3-4 tree. Does not free the element,
* merely removes all links to it from the tree nodes.