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

use crate::bitmap::Bitmap;

/// Internal state of [`SlicesIterator`]
#[derive(Debug, Clone, PartialEq)]
enum State {
    // normal iteration
    Nominal,
    // nothing more to iterate.
    Finished,
}

/// Iterator over a bitmap that returns slices of set regions
/// This is the most efficient method to extract slices of values from arrays
/// with a validity bitmap.
/// For example, the bitmap `00101111` returns `[(0,4), (6,1)]`
#[derive(Debug, Clone)]
pub struct SlicesIterator<'a> {
    values: std::slice::Iter<'a, u8>,
    count: usize,
    mask: u8,
    max_len: usize,
    current_byte: &'a u8,
    state: State,
    len: usize,
    start: usize,
    on_region: bool,
}

impl<'a> SlicesIterator<'a> {
    /// Creates a new [`SlicesIterator`]
    pub fn new(values: &'a Bitmap) -> Self {
        let (buffer, offset, _) = values.as_slice();
        let mut iter = buffer.iter();

        let (current_byte, state) = match iter.next() {
            Some(b) => (b, State::Nominal),
            None => (&0, State::Finished),
        };

        Self {
            state,
            count: values.len() - values.unset_bits(),
            max_len: values.len(),
            values: iter,
            mask: 1u8.rotate_left(offset as u32),
            current_byte,
            len: 0,
            start: 0,
            on_region: false,
        }
    }

    #[inline]
    fn finish(&mut self) -> Option<(usize, usize)> {
        self.state = State::Finished;
        if self.on_region {
            Some((self.start, self.len))
        } else {
            None
        }
    }

    #[inline]
    fn current_len(&self) -> usize {
        self.start + self.len
    }

    /// Returns the total number of slots.
    /// It corresponds to the sum of all lengths of all slices.
    #[inline]
    pub fn slots(&self) -> usize {
        self.count
    }
}

impl<'a> Iterator for SlicesIterator<'a> {
    type Item = (usize, usize);

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        loop {
            if self.state == State::Finished {
                return None;
            }
            if self.current_len() == self.max_len {
                return self.finish();
            }

            if self.mask == 1 {
                // at the beginning of a byte => try to skip it all together
                match (self.on_region, self.current_byte) {
                    (true, &255u8) => {
                        self.len = std::cmp::min(self.max_len - self.start, self.len + 8);
                        if let Some(v) = self.values.next() {
                            self.current_byte = v;
                        };
                        continue;
                    },
                    (false, &0) => {
                        self.len = std::cmp::min(self.max_len - self.start, self.len + 8);
                        if let Some(v) = self.values.next() {
                            self.current_byte = v;
                        };
                        continue;
                    },
                    _ => (), // we need to run over all bits of this byte
                }
            };

            let value = (self.current_byte & self.mask) != 0;
            self.mask = self.mask.rotate_left(1);

            match (self.on_region, value) {
                (true, true) => self.len += 1,
                (false, false) => self.len += 1,
                (true, false) => {
                    self.on_region = false;
                    let result = (self.start, self.len);
                    self.start += self.len;
                    self.len = 1;
                    if self.mask == 1 {
                        // reached a new byte => try to fetch it from the iterator
                        if let Some(v) = self.values.next() {
                            self.current_byte = v;
                        };
                    }
                    return Some(result);
                },
                (false, true) => {
                    self.start += self.len;
                    self.len = 1;
                    self.on_region = true;
                },
            }

            if self.mask == 1 {
                // reached a new byte => try to fetch it from the iterator
                match self.values.next() {
                    Some(v) => self.current_byte = v,
                    None => return self.finish(),
                };
            }
        }
    }
}