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Note atan2 can return -PI #140487

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10 changes: 6 additions & 4 deletions library/std/src/f128.rs
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Expand Up @@ -898,10 +898,12 @@ impl f128 {

/// Computes the four quadrant arctangent of `self` (`y`) and `other` (`x`) in radians.
///
/// * `x = 0`, `y = 0`: `0`
/// * `x >= 0`: `arctan(y/x)` -> `[-pi/2, pi/2]`
/// * `y >= 0`: `arctan(y/x) + pi` -> `(pi/2, pi]`
/// * `y < 0`: `arctan(y/x) - pi` -> `(-pi, -pi/2)`
/// * `x >= +0`, `y >= +0` -> `[+0, pi/2]`
/// * `x <= -0`, `y >= +0` -> `[pi/2, pi]`
/// * `x <= -0`, `y <= -0` -> `[-pi, -pi/2]`
/// * `x >= +0`, `y <= -0` -> `[-pi/2, -0]`
Comment on lines -901 to +904
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@workingjubilee workingjubilee Apr 30, 2025 •
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println!("{}", f32::atan2(-0.0, 0.0)); // -0
println!("{}", f32::atan2(0.0, -0.0)); // 3.1415927
println!("{}", f32::atan2(0.0, 0.0)); // 0
println!("{}", f32::atan2(-0.0, -0.0)); // -3.1415927

...that's kinda fucked up huh.

I think placing the comment about negative versus positive zero after this table-ish-thing makes it harder to read. We should lead with the comment that the sign of 0.0 can affect the result, rather than being treated as equal, even though (0.0 == -0.0) == true.

///
/// Note that positive and negative 0 are distinct floating point values.
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This is always true, what's more important is that this description is not treating them as equal.

///
/// # Unspecified precision
///
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10 changes: 6 additions & 4 deletions library/std/src/f16.rs
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Expand Up @@ -896,10 +896,12 @@ impl f16 {

/// Computes the four quadrant arctangent of `self` (`y`) and `other` (`x`) in radians.
///
/// * `x = 0`, `y = 0`: `0`
/// * `x >= 0`: `arctan(y/x)` -> `[-pi/2, pi/2]`
/// * `y >= 0`: `arctan(y/x) + pi` -> `(pi/2, pi]`
/// * `y < 0`: `arctan(y/x) - pi` -> `(-pi, -pi/2)`
/// * `x >= +0`, `y >= +0` -> `[+0, pi/2]`
/// * `x <= -0`, `y >= +0` -> `[pi/2, pi]`
/// * `x <= -0`, `y <= -0` -> `[-pi, -pi/2]`
/// * `x >= +0`, `y <= -0` -> `[-pi/2, -0]`
///
/// Note that positive and negative 0 are distinct floating point values.
///
/// # Unspecified precision
///
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10 changes: 6 additions & 4 deletions library/std/src/f32.rs
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Expand Up @@ -824,10 +824,12 @@ impl f32 {

/// Computes the four quadrant arctangent of `self` (`y`) and `other` (`x`) in radians.
///
/// * `x = 0`, `y = 0`: `0`
/// * `x >= 0`: `arctan(y/x)` -> `[-pi/2, pi/2]`
/// * `y >= 0`: `arctan(y/x) + pi` -> `(pi/2, pi]`
/// * `y < 0`: `arctan(y/x) - pi` -> `(-pi, -pi/2)`
/// * `x >= +0`, `y >= +0` -> `[+0, pi/2]`
/// * `x <= -0`, `y >= +0` -> `[pi/2, pi]`
/// * `x <= -0`, `y <= -0` -> `[-pi, -pi/2]`
/// * `x >= +0`, `y <= -0` -> `[-pi/2, -0]`
///
/// Note that positive and negative 0 are distinct floating point values.
///
/// # Unspecified precision
///
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10 changes: 6 additions & 4 deletions library/std/src/f64.rs
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Expand Up @@ -824,10 +824,12 @@ impl f64 {

/// Computes the four quadrant arctangent of `self` (`y`) and `other` (`x`) in radians.
///
/// * `x = 0`, `y = 0`: `0`
/// * `x >= 0`: `arctan(y/x)` -> `[-pi/2, pi/2]`
/// * `y >= 0`: `arctan(y/x) + pi` -> `(pi/2, pi]`
/// * `y < 0`: `arctan(y/x) - pi` -> `(-pi, -pi/2)`
/// * `x >= +0`, `y >= +0` -> `[+0, pi/2]`
/// * `x <= -0`, `y >= +0` -> `[pi/2, pi]`
/// * `x <= -0`, `y <= -0` -> `[-pi, -pi/2]`
/// * `x >= +0`, `y <= -0` -> `[-pi/2, -0]`
///
/// Note that positive and negative 0 are distinct floating point values.
///
/// # Unspecified precision
///
Expand Down
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