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
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
use std::mem::size_of;

/// Most-significant byte, == 0x80
pub const MSB: u8 = 0b1000_0000;
/// All bits except for the most significant. Can be used as bitmask to drop the most-signficant
/// bit using `&` (binary-and).
const DROP_MSB: u8 = 0b0111_1111;

/// How many bytes an integer uses when being encoded as a VarInt.
#[inline]
fn required_encoded_space_unsigned(mut v: u64) -> usize {
    if v == 0 {
        return 1;
    }

    let mut logcounter = 0;
    while v > 0 {
        logcounter += 1;
        v >>= 7;
    }
    logcounter
}

/// How many bytes an integer uses when being encoded as a VarInt.
#[inline]
fn required_encoded_space_signed(v: i64) -> usize {
    required_encoded_space_unsigned(zigzag_encode(v))
}

/// Varint (variable length integer) encoding, as described in
/// https://developers.google.com/protocol-buffers/docs/encoding.
///
/// Uses zigzag encoding (also described there) for signed integer representation.
pub trait VarInt: Sized + Copy {
    /// Returns the number of bytes this number needs in its encoded form. Note: This varies
    /// depending on the actual number you want to encode.
    fn required_space(self) -> usize;
    /// Decode a value from the slice. Returns the value and the number of bytes read from the
    /// slice (can be used to read several consecutive values from a big slice)
    /// return None if all bytes has MSB set.
    fn decode_var(src: &[u8]) -> Option<(Self, usize)>;
    /// Encode a value into the slice. The slice must be at least `required_space()` bytes long.
    /// The number of bytes taken by the encoded integer is returned.
    fn encode_var(self, src: &mut [u8]) -> usize;

    /// Helper: Encode a value and return the encoded form as Vec. The Vec must be at least
    /// `required_space()` bytes long.
    fn encode_var_vec(self) -> Vec<u8> {
        let mut v = vec![0; self.required_space()];
        self.encode_var(&mut v);
        v
    }
}

#[inline]
fn zigzag_encode(from: i64) -> u64 {
    ((from << 1) ^ (from >> 63)) as u64
}

// see: http://stackoverflow.com/a/2211086/56332
// casting required because operations like unary negation
// cannot be performed on unsigned integers
#[inline]
fn zigzag_decode(from: u64) -> i64 {
    ((from >> 1) ^ (-((from & 1) as i64)) as u64) as i64
}

pub(crate) trait VarIntMaxSize {
    fn varint_max_size() -> usize;
}

impl<VI: VarInt> VarIntMaxSize for VI {
    fn varint_max_size() -> usize {
        (size_of::<VI>() * 8 + 7) / 7
    }
}

macro_rules! impl_varint {
    ($t:ty, unsigned) => {
        impl VarInt for $t {
            fn required_space(self) -> usize {
                required_encoded_space_unsigned(self as u64)
            }

            fn decode_var(src: &[u8]) -> Option<(Self, usize)> {
                let (n, s) = u64::decode_var(src)?;
                Some((n as Self, s))
            }

            fn encode_var(self, dst: &mut [u8]) -> usize {
                (self as u64).encode_var(dst)
            }
        }
    };
    ($t:ty, signed) => {
        impl VarInt for $t {
            fn required_space(self) -> usize {
                required_encoded_space_signed(self as i64)
            }

            fn decode_var(src: &[u8]) -> Option<(Self, usize)> {
                let (n, s) = i64::decode_var(src)?;
                Some((n as Self, s))
            }

            fn encode_var(self, dst: &mut [u8]) -> usize {
                (self as i64).encode_var(dst)
            }
        }
    };
}

impl_varint!(usize, unsigned);
impl_varint!(u32, unsigned);
impl_varint!(u16, unsigned);
impl_varint!(u8, unsigned);

impl_varint!(isize, signed);
impl_varint!(i32, signed);
impl_varint!(i16, signed);
impl_varint!(i8, signed);

// Below are the "base implementations" doing the actual encodings; all other integer types are
// first cast to these biggest types before being encoded.

impl VarInt for u64 {
    fn required_space(self) -> usize {
        required_encoded_space_unsigned(self)
    }

    #[inline]
    fn decode_var(src: &[u8]) -> Option<(Self, usize)> {
        let mut result: u64 = 0;
        let mut shift = 0;

        let mut success = false;
        for b in src.iter() {
            let msb_dropped = b & DROP_MSB;
            result |= (msb_dropped as u64) << shift;
            shift += 7;

            if b & MSB == 0 || shift > (9 * 7) {
                success = b & MSB == 0;
                break;
            }
        }

        if success {
            Some((result, shift / 7))
        } else {
            None
        }
    }

    #[inline]
    fn encode_var(self, dst: &mut [u8]) -> usize {
        debug_assert!(dst.len() >= self.required_space());
        let mut n = self;
        let mut i = 0;

        while n >= 0x80 {
            dst[i] = MSB | (n as u8);
            i += 1;
            n >>= 7;
        }

        dst[i] = n as u8;
        i + 1
    }
}

impl VarInt for i64 {
    fn required_space(self) -> usize {
        required_encoded_space_signed(self)
    }

    #[inline]
    fn decode_var(src: &[u8]) -> Option<(Self, usize)> {
        if let Some((result, size)) = u64::decode_var(src) {
            Some((zigzag_decode(result) as Self, size))
        } else {
            None
        }
    }

    #[inline]
    fn encode_var(self, dst: &mut [u8]) -> usize {
        debug_assert!(dst.len() >= self.required_space());
        let mut n: u64 = zigzag_encode(self);
        let mut i = 0;

        while n >= 0x80 {
            dst[i] = MSB | (n as u8);
            i += 1;
            n >>= 7;
        }

        dst[i] = n as u8;
        i + 1
    }
}