zig/lib/std/linked_list.zig

const std = @import("std.zig");
const debug = std.debug;
const assert = debug.assert;
const testing = std.testing;

/// A doubly-linked list has a pair of pointers to both the head and
/// tail of the list. List elements have pointers to both the previous
/// and next elements in the sequence. The list can be traversed both
/// forward and backward. Some operations that take linear O(n) time
/// with a singly-linked list can be done without traversal in constant
/// O(1) time with a doubly-linked list:
///
/// - Removing an element.
/// - Inserting a new element before an existing element.
/// - Pushing or popping an element from the end of the list.
pub fn DoublyLinkedList(comptime T: type) type {
    return struct {
        const Self = @This();

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

        first: ?*Node = null,
        last: ?*Node = null,
        len: usize = 0,

        /// 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;

            list.len += 1;
        }

        /// 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;

            list.len += 1;
        }

        /// Concatenate list2 onto the end of list1, removing all entries from the former.
        ///
        /// Arguments:
        ///     list1: the list to concatenate onto
        ///     list2: the list to be concatenated
        pub fn concatByMoving(list1: *Self, list2: *Self) void {
            const l2_first = list2.first orelse return;
            if (list1.last) |l1_last| {
                l1_last.next = list2.first;
                l2_first.prev = list1.last;
                list1.len += list2.len;
            } else {
                // list1 was empty
                list1.first = list2.first;
                list1.len = list2.len;
            }
            list1.last = list2.last;
            list2.first = null;
            list2.last = null;
            list2.len = 0;
        }

        /// 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;

                list.len = 1;
            }
        }

        /// 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;
            }

            list.len -= 1;
            assert(list.len == 0 or (list.first != null and list.last != null));
        }

        /// 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 L = DoublyLinkedList(u32);
    var list = L{};

    var one = L.Node{ .data = 1 };
    var two = L.Node{ .data = 2 };
    var three = L.Node{ .data = 3 };
    var four = L.Node{ .data = 4 };
    var five = L.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);
    try testing.expect(list.len == 2);
}

test "DoublyLinkedList concatenation" {
    const L = DoublyLinkedList(u32);
    var list1 = L{};
    var list2 = L{};

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

    list1.append(&one);
    list1.append(&two);
    list2.append(&three);
    list2.append(&four);
    list2.append(&five);

    list1.concatByMoving(&list2);

    try testing.expect(list1.last == &five);
    try testing.expect(list1.len == 5);
    try testing.expect(list2.first == null);
    try testing.expect(list2.last == null);
    try testing.expect(list2.len == 0);

    // Traverse forwards.
    {
        var it = list1.first;
        var index: u32 = 1;
        while (it) |node| : (it = node.next) {
            try testing.expect(node.data == index);
            index += 1;
        }
    }

    // Traverse backwards.
    {
        var it = list1.last;
        var index: u32 = 1;
        while (it) |node| : (it = node.prev) {
            try testing.expect(node.data == (6 - index));
            index += 1;
        }
    }

    // Swap them back, this verifies that concatenating to an empty list works.
    list2.concatByMoving(&list1);

    // Traverse forwards.
    {
        var it = list2.first;
        var index: u32 = 1;
        while (it) |node| : (it = node.next) {
            try testing.expect(node.data == index);
            index += 1;
        }
    }

    // Traverse backwards.
    {
        var it = list2.last;
        var index: u32 = 1;
        while (it) |node| : (it = node.prev) {
            try testing.expect(node.data == (6 - index));
            index += 1;
        }
    }
}