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Affine coordinate are not correctly exposed from the underlying type. #116

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Affine coordinate are not correctly exposed from the underlying type. #116

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Affine coordinate are not correctly exposed from the underlying type.
FilipLaurentiu Jan 29, 2025
b53ca90
Merge branch 'main' into main
FilipLaurentiu Jan 29, 2025
9c0676e
Fix the `Add` implementation
FilipLaurentiu Jan 29, 2025
86e617c
Merge branch 'starknet-io:main' into main
FilipLaurentiu Feb 4, 2025
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6 changes: 3 additions & 3 deletions crates/starknet-types-core/src/curve/affine_point.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -44,12 +44,12 @@ impl AffinePoint {

/// Returns the `x` coordinate of the point.
pub fn x(&self) -> Felt {
Felt(*self.0.x())
Felt(*self.0.to_affine().x())
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here is the to_affine method you are calling

    /// Creates the same point in affine coordinates. That is,
    /// returns [x / z: y / z: 1] where `self` is [x: y: z].
    /// Panics if `self` is the point at infinity.
    pub fn to_affine(&self) -> Self {
        let [x, y, z] = self.coordinates();
        // If it's the point at infinite
        if z == &FieldElement::zero() {
            // We make sure all the points in the infinite have the same values
            return Self::new([
                FieldElement::zero(),
                FieldElement::one(),
                FieldElement::zero(),
            ]);
        };
        let inv_z = z.inv().unwrap();
        ProjectivePoint::new([x * &inv_z, y * inv_z, FieldElement::one()])
    }

From my understanding it can return values that are different form the one x() and y() are currently returning.

So this is a breaking change.
But I'm not sure it is a bug fix.

The methods documentation says it returens the coordinate, but it's not specified if it's in projective or affine coordinate.

You seems to think it should be affine, but currently it's projective

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@FilipLaurentiu FilipLaurentiu Jan 29, 2025 •
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To clarify. The current behavior is to return the value of x from projective representation of a point, unchanged. The equivalent value in affine representation for x is x/z .

The current version works because if you create a point from (x,y) coordinate, the z value will be 1 and converting back to affine you need to do x/1 which is the same x, but if you perform some operations with the point and want to get back the affine coordinate, the z value will not be 1 anymore.

ProjectivePoint::new([x * &inv_z, y * inv_z, FieldElement::one()])

This changes should not break anything, the mul/add operations were added by me recently in e0eff28

}

/// Returns the `y` coordinate of the point.
pub fn y(&self) -> Felt {
Felt(*self.0.y())
Felt(*self.0.to_affine().y())
}

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// Returns the generator point of the StarkCurve
Expand All @@ -70,7 +70,7 @@ impl core::ops::Add<AffinePoint> for AffinePoint {
type Output = AffinePoint;

fn add(self, rhs: Self) -> Self::Output {
AffinePoint(self.0.operate_with_affine(&rhs.0))
AffinePoint(self.0.operate_with(&rhs.0))
}
}

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