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

use std::cmp;
use std::collections::VecDeque;
use std::io;
use std::io::Read;

/// This is a byte buffer that is built from a vector
/// of byte vectors.  This avoids extra copies when
/// appending a new byte vector, at the expense of
/// more complexity when reading out.
pub struct ChunkVecBuffer {
    chunks: VecDeque<Vec<u8>>,
    limit: usize,
}

impl ChunkVecBuffer {
    pub fn new() -> ChunkVecBuffer {
        ChunkVecBuffer {
            chunks: VecDeque::new(),
            limit: 0,
        }
    }

    /// Sets the upper limit on how many bytes this
    /// object can store.
    ///
    /// Setting a lower limit than the currently stored
    /// data is not an error.
    ///
    /// A zero limit is interpreted as no limit.
    pub fn set_limit(&mut self, new_limit: usize) {
        self.limit = new_limit;
    }

    /// If we're empty
    pub fn is_empty(&self) -> bool {
        self.chunks.is_empty()
    }

    /// How many bytes we're storing
    pub fn len(&self) -> usize {
        let mut len = 0;
        for ch in &self.chunks {
            len += ch.len();
        }
        len
    }

    /// For a proposed append of `len` bytes, how many
    /// bytes should we actually append to adhere to the
    /// currently set `limit`?
    pub fn apply_limit(&self, len: usize) -> usize {
        if self.limit == 0 {
            len
        } else {
            let space = self.limit.saturating_sub(self.len());
            cmp::min(len, space)
        }
    }

    /// Append a copy of `bytes`, perhaps a prefix if
    /// we're near the limit.
    pub fn append_limited_copy(&mut self, bytes: &[u8]) -> usize {
        let take = self.apply_limit(bytes.len());
        self.append(bytes[..take].to_vec());
        take
    }

    /// Take and append the given `bytes`.
    pub fn append(&mut self, bytes: Vec<u8>) -> usize {
        let len = bytes.len();

        if !bytes.is_empty() {
            self.chunks.push_back(bytes);
        }

        len
    }

    /// Take one of the chunks from this object.  This
    /// function panics if the object `is_empty`.
    pub fn take_one(&mut self) -> Vec<u8> {
        self.chunks.pop_front().unwrap()
    }

    /// Read data out of this object, writing it into `buf`
    /// and returning how many bytes were written there.
    pub fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        let mut offs = 0;

        while offs < buf.len() && !self.is_empty() {
            let used = self.chunks[0]
                .as_slice()
                .read(&mut buf[offs..])?;

            self.consume(used);
            offs += used;
        }

        Ok(offs)
    }

    fn consume(&mut self, mut used: usize) {
        while used > 0 && !self.is_empty() {
            if used >= self.chunks[0].len() {
                used -= self.chunks[0].len();
                self.take_one();
            } else {
                self.chunks[0] = self.chunks[0].split_off(used);
                used = 0;
            }
        }
    }

    /// Read data out of this object, passing it `wr`
    pub fn write_to(&mut self, wr: &mut dyn io::Write) -> io::Result<usize> {
        if self.is_empty() {
            return Ok(0);
        }

        let used = wr.write_vectored(
            &self
                .chunks
                .iter()
                .map(|ch| io::IoSlice::new(ch))
                .collect::<Vec<io::IoSlice>>(),
        )?;
        self.consume(used);
        Ok(used)
    }
}

#[cfg(test)]
mod test {
    use super::ChunkVecBuffer;

    #[test]
    fn short_append_copy_with_limit() {
        let mut cvb = ChunkVecBuffer::new();
        cvb.set_limit(12);
        assert_eq!(cvb.append_limited_copy(b"hello"), 5);
        assert_eq!(cvb.append_limited_copy(b"world"), 5);
        assert_eq!(cvb.append_limited_copy(b"hello"), 2);
        assert_eq!(cvb.append_limited_copy(b"world"), 0);

        let mut buf = [0u8; 12];
        assert_eq!(cvb.read(&mut buf).unwrap(), 12);
        assert_eq!(buf.to_vec(), b"helloworldhe".to_vec());
    }
}