const std = @import("std");
const testing = std.testing;

/// An intrusive doubly-linked list. The type T must have a "next" and "prev"
/// field pointing to itself.
///
/// This is an adaptation of the DoublyLinkedList from the Zig standard
/// library, which is MIT licensed. I've removed functionality that I don't
/// need.
pub fn DoublyLinkedList(comptime T: type) type {
    return struct {
        const Self = @This();

        /// The type of the node in the list. This makes it easy to get the
        /// node type from the list type.
        pub const Node = T;

        first: ?*Node = null,
        last: ?*Node = null,

        /// Insert a new node after an existing one.
        ///
        /// Arguments:
        ///     node: Pointer to a node in the list.
        ///     new_node: Pointer to the new node to insert.
        pub fn insertAfter(list: *Self, node: *Node, new_node: *Node) void {
            new_node.prev = node;
            if (node.next) |next_node| {
                // Intermediate node.
                new_node.next = next_node;
                next_node.prev = new_node;
            } else {
                // Last element of the list.
                new_node.next = null;
                list.last = new_node;
            }
            node.next = new_node;
        }

        /// Insert a new node before an existing one.
        ///
        /// Arguments:
        ///     node: Pointer to a node in the list.
        ///     new_node: Pointer to the new node to insert.
        pub fn insertBefore(list: *Self, node: *Node, new_node: *Node) void {
            new_node.next = node;
            if (node.prev) |prev_node| {
                // Intermediate node.
                new_node.prev = prev_node;
                prev_node.next = new_node;
            } else {
                // First element of the list.
                new_node.prev = null;
                list.first = new_node;
            }
            node.prev = new_node;
        }

        /// Insert a new node at the end of the list.
        ///
        /// Arguments:
        ///     new_node: Pointer to the new node to insert.
        pub fn append(list: *Self, new_node: *Node) void {
            if (list.last) |last| {
                // Insert after last.
                list.insertAfter(last, new_node);
            } else {
                // Empty list.
                list.prepend(new_node);
            }
        }

        /// Insert a new node at the beginning of the list.
        ///
        /// Arguments:
        ///     new_node: Pointer to the new node to insert.
        pub fn prepend(list: *Self, new_node: *Node) void {
            if (list.first) |first| {
                // Insert before first.
                list.insertBefore(first, new_node);
            } else {
                // Empty list.
                list.first = new_node;
                list.last = new_node;
                new_node.prev = null;
                new_node.next = null;
            }
        }

        /// Remove a node from the list.
        ///
        /// Arguments:
        ///     node: Pointer to the node to be removed.
        pub fn remove(list: *Self, node: *Node) void {
            if (node.prev) |prev_node| {
                // Intermediate node.
                prev_node.next = node.next;
            } else {
                // First element of the list.
                list.first = node.next;
            }

            if (node.next) |next_node| {
                // Intermediate node.
                next_node.prev = node.prev;
            } else {
                // Last element of the list.
                list.last = node.prev;
            }
        }

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

        /// 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.remove(first);
            return first;
        }
    };
}

test "basic DoublyLinkedList test" {
    const Node = struct {
        data: u32,
        prev: ?*@This() = null,
        next: ?*@This() = null,
    };
    const L = DoublyLinkedList(Node);
    var list: L = .{};

    var one: Node = .{ .data = 1 };
    var two: Node = .{ .data = 2 };
    var three: Node = .{ .data = 3 };
    var four: Node = .{ .data = 4 };
    var five: Node = .{ .data = 5 };

    list.append(&two); // {2}
    list.append(&five); // {2, 5}
    list.prepend(&one); // {1, 2, 5}
    list.insertBefore(&five, &four); // {1, 2, 4, 5}
    list.insertAfter(&two, &three); // {1, 2, 3, 4, 5}

    // Traverse forwards.
    {
        var it = list.first;
        var index: u32 = 1;
        while (it) |node| : (it = node.next) {
            try testing.expect(node.data == index);
            index += 1;
        }
    }

    // Traverse backwards.
    {
        var it = list.last;
        var index: u32 = 1;
        while (it) |node| : (it = node.prev) {
            try testing.expect(node.data == (6 - index));
            index += 1;
        }
    }

    _ = list.popFirst(); // {2, 3, 4, 5}
    _ = list.pop(); // {2, 3, 4}
    list.remove(&three); // {2, 4}

    try testing.expect(list.first.?.data == 2);
    try testing.expect(list.last.?.data == 4);
}