propose binary heap struct

Signed-off-by: Brian Grenier <grenierb96@gmail.com>

proposals/binary-heap.md

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+# Binary Heap Struct
+
+A binary heap is a data structure useful for tasks like job scheduling,
+graph algorithms, text processing, etc. Python exposes this functionality
+through the `heapq` module, which is a collection of functions operating on lists.
+Given the recent shift towards relaxing our commitment to being a Python "superset",
+I propose we implement this module as a proper struct `BinaryHeap`
+with an interface users have become accustomed to from other Mojo data structures.
+
+## API
+
+The goal of the proposed api is to strike a balance between familiarity for
+python users while also modernizing it. To avoid surprising Python users I have
+made min heap the default, however, users can supply a custom comparator if they
+wish to make it a max heap or sort based on some other key.
+
+Similar to Rusts `std::BinaryHeap` and `std::priority_queue` from C++, we will
+use a heap allocated array (`List` in our case) as the base for our heap implementation.
+
+```mojo
+
+@always_inline
+fn min[T: ComparableCollectionElement](l: T, r: T) -> Bool:
+    ...
+
+@always_inline
+fn max[T: ComparableCollectionElement](l: T, r: T) -> Bool:
+    ...
+
+struct BinaryHeap[
+    T: ComparableCollectionElement,
+    *,
+    compare: fn(l: T, r: T) -> Bool = min_comparator[T]](CollectionElement, Sized, Boolable, ImplicitlyBoolable):
+
+    # ASSUME TRIVIAL CTORs, MOVE AND COPY PLUMBING
+
+    var data: List[T]
+
+    fn __init__(out self, owned item: List[T]):
+        ...
+
+    fn __init__(out self, owned *values: T):
+        ...
+
+    fn push(inout self, owned item: T):
+        # Add an item to the heap
+        ...
+
+    fn pop(inout self) raises -> T
+        # Remove the top item from the heap
+        # Raises if heap is empty
+        ...
+    
+    fn pushpop(inout self, owned item: T) -> T:
+        # Push and item onto the heap, then
+        # return the top item
+        ...
+    
+    fn replace(inout self, owned item: T) raises -> T:
+        # Pops the top item, then pushes a new one
+        # raises if the heap is empty
+        ...
+    
+    fn merge(inout self, owned other: Self):
+        # Consume the elements from another heap
+        ...
+
+    fn peek(self) -> Optional[T]:
+        # Returns the top element without removing it
+        # Would be changed to an optional reference in the future
+        ...
+
+    fn get_n(inout self) -> List[T]:
+        # Returns a list of the top N items in a list.
+        # Analogous to other nsmallest or nlargest depending
+        # on the heap type.
+        ...
+
+    fn reserve(inout self, capacity: Int):
+        ...
+
+    fn clear(inout self):
+        ...
+
+    fn __len__(self) -> Int:
+        ...
+    
+    fn __bool__(self) -> Bool:
+        ...
+```
+
+## Considerations
+
+A small issue with the current implementation is we must require `T` to be comparable,
+even in the case where a custom comparator is used which may only sort based on
+a particular field in a struct, and thus not requiring the struct itself to
+conform to `Comparable`. This issue could be meaningful in the case when `T`
+is a type the user does not control.
+
+There is also the case of how to handle popping an item from an empty heap.
+I have considered three options.
+
+- Raise an Error
+  - This is how I have it currently in this proposal, but I
+am unsure if coloring one of the most frequently used methods is a good idea.
+
+- Return an `Optional[T]`
+  - This would solve the function coloring issue, but introduces overhead to the
+  user that is undesirable in performance critical scenarios
+
+- Treat it as an out of bounds access
+  - Use a debug assert to protect against this during development, and otherwise
+    attempt to access the underlying `List` anyways. Which is probably the least
+    desirable option given its unsafe nature.
+
+We could also implement both a raising and non-raising version of these functions,
+eg. (`pop` and `try_pop`), which is one of my lesser favorite patterns from Rust,
+but would give users flexibility and protect against bad behavior.