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

use std::borrow::Borrow;

use crate::storage::parse;
use crate::ChangeHash;

// These constants correspond to a 1% false positive rate. The values can be changed without
// breaking compatibility of the network protocol, since the parameters used for a particular
// Bloom filter are encoded in the wire format.
const BITS_PER_ENTRY: u32 = 10;
const NUM_PROBES: u32 = 7;

#[derive(Debug, Clone, PartialEq, Eq, Hash, serde::Serialize)]
pub struct BloomFilter {
    num_entries: u32,
    num_bits_per_entry: u32,
    num_probes: u32,
    bits: Vec<u8>,
}

impl Default for BloomFilter {
    fn default() -> Self {
        BloomFilter {
            num_entries: 0,
            num_bits_per_entry: BITS_PER_ENTRY,
            num_probes: NUM_PROBES,
            bits: Vec::new(),
        }
    }
}

#[derive(Debug, thiserror::Error)]
pub(crate) enum ParseError {
    #[error(transparent)]
    Leb128(#[from] parse::leb128::Error),
}

impl BloomFilter {
    pub fn to_bytes(&self) -> Vec<u8> {
        let mut buf = Vec::new();
        if self.num_entries != 0 {
            leb128::write::unsigned(&mut buf, self.num_entries as u64).unwrap();
            leb128::write::unsigned(&mut buf, self.num_bits_per_entry as u64).unwrap();
            leb128::write::unsigned(&mut buf, self.num_probes as u64).unwrap();
            buf.extend(&self.bits);
        }
        buf
    }

    pub(crate) fn parse(input: parse::Input<'_>) -> parse::ParseResult<'_, Self, ParseError> {
        if input.is_empty() {
            Ok((input, Self::default()))
        } else {
            let (i, num_entries) = parse::leb128_u32(input)?;
            let (i, num_bits_per_entry) = parse::leb128_u32(i)?;
            let (i, num_probes) = parse::leb128_u32(i)?;
            let (i, bits) = parse::take_n(bits_capacity(num_entries, num_bits_per_entry), i)?;
            Ok((
                i,
                Self {
                    num_entries,
                    num_bits_per_entry,
                    num_probes,
                    bits: bits.to_vec(),
                },
            ))
        }
    }

    fn get_probes(&self, hash: &ChangeHash) -> Vec<u32> {
        let hash_bytes = hash.0;
        let modulo = 8 * self.bits.len() as u32;

        let mut x =
            u32::from_le_bytes([hash_bytes[0], hash_bytes[1], hash_bytes[2], hash_bytes[3]])
                % modulo;
        let mut y =
            u32::from_le_bytes([hash_bytes[4], hash_bytes[5], hash_bytes[6], hash_bytes[7]])
                % modulo;
        let z = u32::from_le_bytes([hash_bytes[8], hash_bytes[9], hash_bytes[10], hash_bytes[11]])
            % modulo;

        let mut probes = Vec::with_capacity(self.num_probes as usize);
        probes.push(x);
        for _ in 1..self.num_probes {
            x = (x + y) % modulo;
            y = (y + z) % modulo;
            probes.push(x);
        }
        probes
    }

    fn add_hash(&mut self, hash: &ChangeHash) {
        for probe in self.get_probes(hash) {
            self.set_bit(probe as usize);
        }
    }

    fn set_bit(&mut self, probe: usize) {
        if let Some(byte) = self.bits.get_mut(probe >> 3) {
            *byte |= 1 << (probe & 7);
        }
    }

    fn get_bit(&self, probe: usize) -> Option<u8> {
        self.bits
            .get(probe >> 3)
            .map(|byte| byte & (1 << (probe & 7)))
    }

    pub fn contains_hash(&self, hash: &ChangeHash) -> bool {
        if self.num_entries == 0 {
            false
        } else {
            for probe in self.get_probes(hash) {
                if let Some(bit) = self.get_bit(probe as usize) {
                    if bit == 0 {
                        return false;
                    }
                }
            }
            true
        }
    }

    pub fn from_hashes<H: Borrow<ChangeHash>>(hashes: impl ExactSizeIterator<Item = H>) -> Self {
        let num_entries = hashes.len() as u32;
        let num_bits_per_entry = BITS_PER_ENTRY;
        let num_probes = NUM_PROBES;
        let bits = vec![0; bits_capacity(num_entries, num_bits_per_entry)];
        let mut filter = Self {
            num_entries,
            num_bits_per_entry,
            num_probes,
            bits,
        };
        for hash in hashes {
            filter.add_hash(hash.borrow());
        }
        filter
    }
}

fn bits_capacity(num_entries: u32, num_bits_per_entry: u32) -> usize {
    let f = ((f64::from(num_entries) * f64::from(num_bits_per_entry)) / 8_f64).ceil();
    f as usize
}

#[derive(thiserror::Error, Debug)]
#[error("{0}")]
pub struct DecodeError(String);

impl TryFrom<&[u8]> for BloomFilter {
    type Error = DecodeError;

    fn try_from(bytes: &[u8]) -> Result<Self, Self::Error> {
        Self::parse(parse::Input::new(bytes))
            .map(|(_, b)| b)
            .map_err(|e| DecodeError(e.to_string()))
    }
}