1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160

//! Module for the `Entry` API to easily move a reference in the tree.

use crate::{id::NodeId, tree::Tree};

/// Cursor over the tree elements.
///
/// If a move operation fails we `Return` a result with:
/// - `Ok`: the moved cursor
/// - `Err`: the previous unmodified cursor
#[derive(Debug)]
pub struct Cursor<'a, T> {
    index: usize,
    tree: &'a mut Tree<T>,
}

impl<T> Tree<T> {
    pub fn cursor(&mut self, id: &NodeId) -> Option<Cursor<T>> {
        self.index(id).map(|index| Cursor { index, tree: self })
    }

    pub fn cursor_first(&mut self) -> Option<Cursor<T>> {
        self.first_node.map(|index| Cursor { index, tree: self })
    }

    pub fn cursor_last(&mut self) -> Option<Cursor<T>> {
        self.last_node.map(|index| Cursor { index, tree: self })
    }
}

impl<T> Cursor<'_, T> {
    pub fn get(&self) -> &T {
        &self.tree.nodes[self.index].value
    }

    pub fn get_mut(&mut self) -> &mut T {
        &mut self.tree.nodes[self.index].value
    }

    pub fn parent(&mut self) -> Result<&mut Self, &mut Self> {
        match self.tree.nodes[self.index].parent {
            Some(index) => {
                self.index = index;
                Ok(self)
            }
            None => Err(self),
        }
    }

    pub fn first_child(&mut self) -> Result<&mut Self, &mut Self> {
        match self.tree.nodes[self.index].first_child {
            Some(index) => {
                self.index = index;
                Ok(self)
            }
            None => Err(self),
        }
    }

    pub fn last_child(&mut self) -> Result<&mut Self, &mut Self> {
        match self.tree.nodes[self.index].last_child {
            Some(index) => {
                self.index = index;
                Ok(self)
            }
            None => Err(self),
        }
    }

    pub fn next_sibling(&mut self) -> Result<&mut Self, &mut Self> {
        match self.tree.nodes[self.index].next_sibling {
            Some(index) => {
                self.index = index;
                Ok(self)
            }
            None => Err(self),
        }
    }

    pub fn prev_sibling(&mut self) -> Result<&mut Self, &mut Self> {
        match self.tree.nodes[self.index].prev_sibling {
            Some(index) => {
                self.index = index;
                Ok(self)
            }
            None => Err(self),
        }
    }

    pub fn move_next(&mut self) -> Result<&mut Self, &mut Self> {
        self.first_child()
            .or_else(|cursor| cursor.next_sibling())
            .or_else(|cursor| {
                let mut parent = &cursor.tree.nodes[cursor.index].parent;

                // Iterate to each parent to check if one has a next sibling
                while let Some(parent_index) = parent {
                    let node = &cursor.tree.nodes[*parent_index];

                    if let Some(sibling) = node.next_sibling {
                        cursor.index = sibling;

                        return Ok(cursor);
                    }

                    parent = &node.parent;
                }

                Err(cursor)
            })
    }

    pub fn append_child(&mut self, value: T) -> NodeId {
        NodeId {
            index: self.tree.insert_child_at(self.index, value),
        }
    }

    pub fn append_sibling(&mut self, value: T) -> NodeId {
        NodeId {
            index: self.tree.insert_sibling_at(self.index, value),
        }
    }
}

#[cfg(test)]
mod test {
    use crate::tree::Tree;

    #[test]
    fn should_move_next() {
        let mut tree: Tree<i32> = Tree::new();
        // A
        tree.append_child(0);

        // A -> B
        let b = tree.append_child(1);

        // A -> B -> C
        tree.append_child(2);

        // A -> B -> D
        //   -> C
        tree.insert_sibling_after(&b, 3).unwrap();

        let mut cursor = tree.cursor_first().unwrap();

        assert_eq!(0, *cursor.get());

        cursor.move_next().unwrap();
        assert_eq!(1, *cursor.get());

        cursor.move_next().unwrap();
        assert_eq!(2, *cursor.get());

        cursor.move_next().unwrap();
        assert_eq!(3, *cursor.get());

        assert!(cursor.move_next().is_err());
    }
}