// TODO(zig-upgrade): Migrate to the new intrusive list

/// A singly-linked list is headed by a single forward pointer. The elements
/// are singly-linked for minimum space and pointer manipulation overhead at
/// the expense of O(n) removal for arbitrary elements. New elements can be
/// added to the list after an existing element or at the head of the list.
/// A singly-linked list may only be traversed in the forward direction.
/// Singly-linked lists are ideal for applications with large datasets and
/// few or no removals or for implementing a LIFO queue.
pub fn SinglyLinkedList(comptime T: type) type {
    return struct {
        const Self = @This();

        /// Node inside the linked list wrapping the actual data.
        pub const Node = struct {
            next: ?*Node = null,
            data: T,

            pub const Data = T;

            /// Insert a new node after the current one.
            ///
            /// Arguments:
            ///     new_node: Pointer to the new node to insert.
            pub fn insertAfter(node: *Node, new_node: *Node) void {
                new_node.next = node.next;
                node.next = new_node;
            }

            /// Remove a node from the list.
            ///
            /// Arguments:
            ///     node: Pointer to the node to be removed.
            /// Returns:
            ///     node removed
            pub fn removeNext(node: *Node) ?*Node {
                const next_node = node.next orelse return null;
                node.next = next_node.next;
                return next_node;
            }

            /// Iterate over the singly-linked list from this node, until the final node is found.
            /// This operation is O(N).
            pub fn findLast(node: *Node) *Node {
                var it = node;
                while (true) {
                    it = it.next orelse return it;
                }
            }

            /// Iterate over each next node, returning the count of all nodes except the starting one.
            /// This operation is O(N).
            pub fn countChildren(node: *const Node) usize {
                var count: usize = 0;
                var it: ?*const Node = node.next;
                while (it) |n| : (it = n.next) {
                    count += 1;
                }
                return count;
            }

            /// Reverse the list starting from this node in-place.
            /// This operation is O(N).
            pub fn reverse(indirect: *?*Node) void {
                if (indirect.* == null) {
                    return;
                }
                var current: *Node = indirect.*.?;
                while (current.next) |next| {
                    current.next = next.next;
                    next.next = indirect.*;
                    indirect.* = next;
                }
            }
        };

        first: ?*Node = null,

        /// Insert a new node at the head.
        ///
        /// Arguments:
        ///     new_node: Pointer to the new node to insert.
        pub fn prepend(list: *Self, new_node: *Node) void {
            new_node.next = list.first;
            list.first = new_node;
        }

        /// Remove a node from the list.
        ///
        /// Arguments:
        ///     node: Pointer to the node to be removed.
        pub fn remove(list: *Self, node: *Node) void {
            if (list.first == node) {
                list.first = node.next;
            } else {
                var current_elm = list.first.?;
                while (current_elm.next != node) {
                    current_elm = current_elm.next.?;
                }
                current_elm.next = node.next;
            }
        }

        /// Remove and return the first node in the list.
        ///
        /// Returns:
        ///     A pointer to the first node in the list.
        pub fn popFirst(list: *Self) ?*Node {
            const first = list.first orelse return null;
            list.first = first.next;
            return first;
        }

        /// Iterate over all nodes, returning the count.
        /// This operation is O(N).
        pub fn len(list: Self) usize {
            if (list.first) |n| {
                return 1 + n.countChildren();
            } else {
                return 0;
            }
        }
    };
}