ark_ec/models/short_weierstrass/
serialization_flags.rs

1use ark_ff::Field;
2use ark_serialize::Flags;
3
4/// Flags to be encoded into the serialization.
5/// The default flags (empty) should not change the binary representation.
6#[derive(Clone, Copy, PartialEq, Eq, Debug)]
7pub enum SWFlags {
8    /// Represents a point with positive y-coordinate by setting all bits to 0.
9    YIsPositive = 0,
10    /// Represents the point at infinity by setting the setting the last-but-one bit to 1.
11    PointAtInfinity = 1 << 6,
12    /// Represents a point with negative y-coordinate by setting the MSB to 1.
13    YIsNegative = 1 << 7,
14}
15
16impl SWFlags {
17    #[inline]
18    pub fn infinity() -> Self {
19        SWFlags::PointAtInfinity
20    }
21
22    #[inline]
23    pub fn from_y_coordinate(y: impl Field) -> Self {
24        if y <= -y {
25            Self::YIsPositive
26        } else {
27            Self::YIsNegative
28        }
29    }
30
31    #[inline]
32    pub fn is_infinity(&self) -> bool {
33        matches!(self, SWFlags::PointAtInfinity)
34    }
35
36    #[inline]
37    pub fn is_positive(&self) -> Option<bool> {
38        match self {
39            SWFlags::PointAtInfinity => None,
40            SWFlags::YIsPositive => Some(true),
41            SWFlags::YIsNegative => Some(false),
42        }
43    }
44}
45
46impl Default for SWFlags {
47    #[inline]
48    fn default() -> Self {
49        // YIsNegative doesn't change the serialization
50        SWFlags::YIsNegative
51    }
52}
53
54impl Flags for SWFlags {
55    const BIT_SIZE: usize = 2;
56
57    #[inline]
58    fn u8_bitmask(&self) -> u8 {
59        let mut mask = 0;
60        match self {
61            SWFlags::PointAtInfinity => mask |= 1 << 6,
62            SWFlags::YIsNegative => mask |= 1 << 7,
63            _ => (),
64        }
65        mask
66    }
67
68    #[inline]
69    fn from_u8(value: u8) -> Option<Self> {
70        let is_negative = (value >> 7) & 1 == 1;
71        let is_infinity = (value >> 6) & 1 == 1;
72        match (is_negative, is_infinity) {
73            // This is invalid because we only want *one* way to serialize
74            // the point at infinity.
75            (true, true) => None,
76            (false, true) => Some(SWFlags::PointAtInfinity),
77            (true, false) => Some(SWFlags::YIsNegative),
78            (false, false) => Some(SWFlags::YIsPositive),
79        }
80    }
81}
ark_ec/models/short_weierstrass/serialization_flags.rs

ark_ec/models/short_weierstrass/
serialization_flags.rs
