[PATCH 3/3] Documentation/core-api: Add min heap API introduction

[Kuan-Wei Chiu]

Introduce an overview of the min heap API, detailing its usage and
functionality. The documentation aims to provide developers with a
clear understanding of how to implement and utilize min heaps within
the Linux kernel, enhancing the overall accessibility of this data
structure.

Signed-off-by: Kuan-Wei Chiu <visitorckw@xxxxxxxxx>
---
 Documentation/core-api/index.rst    |   1 +
 Documentation/core-api/min_heap.rst | 291 ++++++++++++++++++++++++++++
 2 files changed, 292 insertions(+)
 create mode 100644 Documentation/core-api/min_heap.rst

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 6a875743dd4b..563b8fc0002f 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -52,6 +52,7 @@ Library functionality that is used throughout the kernel.
    wrappers/atomic_bitops
    floating-point
    union_find
+   min_heap
 
 Low level entry and exit
 ========================
diff --git a/Documentation/core-api/min_heap.rst b/Documentation/core-api/min_heap.rst
new file mode 100644
index 000000000000..dd2cc5a32fd7
--- /dev/null
+++ b/Documentation/core-api/min_heap.rst
@@ -0,0 +1,291 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============
+Min Heap API
+============
+
+Introduction
+============
+
+The Min Heap API provides a set of functions and macros for managing min-heaps in the Linux kernel.
+A min-heap is a binary tree structure where the value of each node is less than or equal to the
+values of its children, ensuring that the smallest element is always at the root.
+
+This API supports efficient insertion, deletion, and access to the minimum element. It is optimized
+for use in systems with performance constraints and is suitable for scenarios where the minimum
+element needs to be accessed or updated frequently.
+
+This document provides a guide to the Min Heap API, detailing how to define and use min-heaps.
+Please note that users should not directly call functions with **__min_heap_*()** names, but should
+instead use the provided macro wrappers.
+
+In addition to the standard version of the functions, the API also includes a set of inline
+versions for performance-critical scenarios. These inline functions have the same names as their
+non-inline counterparts but include an **_inline** suffix. For example, **__min_heap_init_inline**
+and its corresponding macro wrapper **min_heap_init_inline**. As with the non-inline versions, it
+is important to use the macro wrappers for inline functions instead of directly calling the
+functions themselves.
+
+Data Structures
+===============
+
+Min-Heap Definition
+-------------------
+
+The core data structure for representing a min-heap is defined using the **MIN_HEAP_PREALLOCATED**
+and **DEFINE_MIN_HEAP** macros. These macros allow you to define a min-heap with a preallocated
+buffer or dynamically allocated memory.
+
+Example:
+
+.. code-block:: c
+
+    #define MIN_HEAP_PREALLOCATED(_type, _name, _nr)
+    struct _name {
+        int nr;         /* Number of elements in the heap */
+        int size;       /* Maximum number of elements that can be held */
+        _type *data;    /* Pointer to the heap data */
+        _type preallocated[_nr];  /* Static preallocated array */
+    }
+
+    #define DEFINE_MIN_HEAP(_type, _name) MIN_HEAP_PREALLOCATED(_type, _name, 0)
+
+A typical heap structure will include a counter for the number of elements (`nr`), the maximum
+capacity of the heap (`size`), and a pointer to an array of elements (`data`). Optionally, you can
+specify a static array for preallocated heap storage using **MIN_HEAP_PREALLOCATED**.
+
+Min Heap Callbacks
+------------------
+
+The **struct min_heap_callbacks** provides customization options for ordering
+elements in the heap and swapping them. It contains two function pointers:
+
+.. code-block:: c
+
+    struct min_heap_callbacks {
+        bool (*less)(const void *lhs, const void *rhs, void *args);
+        void (*swp)(void *lhs, void *rhs, void *args);
+    };
+
+- **less** is the comparison function used to establish the order of elements.
+- **swp** is a function for swapping elements in the heap.
+
+Macro Wrappers
+==============
+
+The following macro wrappers are provided for interacting with the heap in a user-friendly manner.
+Each macro corresponds to a function that operates on the heap, and they abstract away direct calls
+to internal functions.
+
+Each macro accepts various parameters that are detailed below.
+
+Heap Initialization
+--------------------
+
+.. code-block:: c
+
+    min_heap_init(heap, data, size);
+
+- **heap**: A pointer to the min-heap structure to be initialized.
+- **data**: A pointer to the buffer where the heap elements will be stored. If `NULL`, the preallocated buffer within the heap structure will be used.
+- **size**: The maximum number of elements the heap can hold.
+
+This macro initializes the heap, setting its initial state. If `data` is `NULL`, the preallocated
+memory inside the heap structure will be used for storage. Otherwise, the user-provided buffer is
+used. The operation is **O(1)**.
+
+**Inline Version:** min_heap_init_inline(heap, data, size)
+
+Accessing the Top Element
+-------------------------
+
+.. code-block:: c
+
+    element = min_heap_peek(heap);
+
+- **heap**: A pointer to the min-heap from which to retrieve the smallest element.
+
+This macro returns a pointer to the smallest element (the root) of the heap, or `NULL` if the heap
+is empty. The operation is **O(1)**.
+
+**Inline Version:** min_heap_peek_inline(heap)
+
+Heap Insertion
+--------------
+
+.. code-block:: c
+
+    success = min_heap_push(heap, element, callbacks, args);
+
+- **heap**: A pointer to the min-heap into which the element should be inserted.
+- **element**: A pointer to the element to be inserted into the heap.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro inserts an element into the heap. It returns `true` if the insertion was successful and
+`false` if the heap is full. The operation is **O(log n)**.
+
+**Inline Version:** min_heap_push_inline(heap, element, callbacks, args)
+
+Heap Removal
+------------
+
+.. code-block:: c
+
+    success = min_heap_pop(heap, callbacks, args);
+
+- **heap**: A pointer to the min-heap from which to remove the smallest element.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro removes the smallest element (the root) from the heap. It returns `true` if the element
+was successfully removed, or `false` if the heap is empty. The operation is **O(log n)**.
+
+**Inline Version:** min_heap_pop_inline(heap, callbacks, args)
+
+Heap Maintenance
+----------------
+
+You can use the following macros to maintain the heap's structure:
+
+.. code-block:: c
+
+    min_heap_sift_down(heap, pos, callbacks, args);
+
+- **heap**: A pointer to the min-heap.
+- **pos**: The index from which to start sifting down.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro restores the heap property by moving the element at the specified index (`pos`) down the
+heap until it is in the correct position. The operation is **O(log n)**.
+
+**Inline Version:** min_heap_sift_down_inline(heap, pos, callbacks, args)
+
+.. code-block:: c
+
+    min_heap_sift_up(heap, idx, callbacks, args);
+
+- **heap**: A pointer to the min-heap.
+- **idx**: The index of the element to sift up.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro restores the heap property by moving the element at the specified index (`idx`) up the
+heap. The operation is **O(log n)**.
+
+**Inline Version:** min_heap_sift_up_inline(heap, idx, callbacks, args)
+
+.. code-block:: c
+
+    min_heapify_all(heap, callbacks, args);
+
+- **heap**: A pointer to the min-heap.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro ensures that the entire heap satisfies the heap property. It is called when the heap is
+built from scratch or after many modifications. The  operation is **O(n)**.
+
+**Inline Version:** min_heapify_all_inline(heap, callbacks, args)
+
+Removing Specific Elements
+--------------------------
+
+.. code-block:: c
+
+    success = min_heap_del(heap, idx, callbacks, args);
+
+- **heap**: A pointer to the min-heap.
+- **idx**: The index of the element to delete.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro removes an element at the specified index (`idx`) from the heap and restores the heap
+property. The operation is **O(log n)**.
+
+**Inline Version:** min_heap_del_inline(heap, idx, callbacks, args)
+
+Other Utilities
+===============
+
+- **min_heap_full(heap)**: Checks whether the heap is full. Complexity: **O(1)**.
+
+.. code-block:: c
+
+    bool full = min_heap_full(heap);
+
+- `heap`: A pointer to the min-heap to check.
+
+This macro returns `true` if the heap is full, otherwise `false`.
+
+**Inline Version:** min_heap_full_inline(heap)
+
+- **min_heap_empty(heap)**: Checks whether the heap is empty. Complexity: **O(1)**.
+
+.. code-block:: c
+
+    bool empty = min_heap_empty(heap);
+
+- `heap`: A pointer to the min-heap to check.
+
+This macro returns `true` if the heap is empty, otherwise `false`.
+
+**Inline Version:** min_heap_empty_inline(heap)
+
+Example Usage
+=============
+
+An example usage of the min-heap API would involve defining a heap structure,
+initializing it, and inserting and removing elements as needed.
+
+.. code-block:: c
+
+    /* Define a preallocated heap for storing up to 10 elements */
+    MIN_HEAP_PREALLOCATED(int, my_heap, 10);
+
+    struct min_heap_callbacks callbacks = {
+        .less = my_less,  /* Custom comparison function */
+        .swp = my_swap,   /* Custom swap function */
+    };
+
+    /* Initialize the heap using the preallocated buffer */
+    min_heap_init(&my_heap, NULL, ARRAY_SIZE(my_heap.preallocated));
+
+    /* If we have an external buffer, we can use it instead */
+    int external_buffer[20];
+    min_heap_init(&my_heap, external_buffer, ARRAY_SIZE(external_buffer));
+
+    /* Insert elements into the heap */
+    int new_element = 5;
+    if (!min_heap_full(&my_heap)) {
+        min_heap_push(&my_heap, &new_element, &callbacks, NULL);
+    }
+
+    /* Peek at the minimum element (without removing it) */
+    int *min_element = min_heap_peek(&my_heap);
+
+    /* Replace the root of the heap with a new element */
+    int replacement_element = 3;
+    min_heap_pop_push(&my_heap, &replacement_element, &callbacks, NULL);
+
+    /* Reorder the heap by sifting down from a given position */
+    min_heap_sift_down(&my_heap, 0, &callbacks, NULL);
+
+    /* Remove the minimum element from the heap */
+    if (!min_heap_empty(&my_heap)) {
+        min_heap_pop(&my_heap, &callbacks, NULL);
+    }
+
+    /* Insert more elements into the heap */
+    new_element = 8;
+    if (!min_heap_full(&my_heap)) {
+        min_heap_push(&my_heap, &new_element, &callbacks, NULL);
+    }
+
+    /* Delete an element from the heap at a specific index */
+    int idx_to_delete = 2;
+    min_heap_del(&my_heap, idx_to_delete, &callbacks, NULL);
+
+    /* Ensure the entire heap maintains heap order */
+    min_heapify_all(&my_heap, &callbacks, NULL);
-- 
2.34.1