Add epsilon API

src/common.rs

@@ -5,6 +5,8 @@
 
 #![allow(missing_docs)]
 
+use crate::epsilon::Epsilon;
+
 use arrayvec::ArrayVec;
 
 /// Defines a trait that chooses between libstd or libm implementations of float methods.
@@ -291,6 +293,7 @@ pub fn solve_itp(
     mut ya: f64,
     mut yb: f64,
 ) -> f64 {
+    debug_assert!(epsilon >= Epsilon::default().value);
     let n1_2 = (((b - a) / epsilon).log2().ceil() - 1.0).max(0.0) as usize;
     let nmax = n0 + n1_2;
     let mut scaled_epsilon = epsilon * (1u64 << nmax) as f64;

src/epsilon.rs

@@ -0,0 +1,48 @@
+use core::ops::Mul;
+
+const EPSILON: f64 = f64::EPSILON;
+const MAX_10_EXP: i32 = f64::MAX_10_EXP;
+
+/// `kurbo`'s epsilon (ε).
+#[repr(packed)]
+#[derive(Copy, Clone, Debug)]
+pub struct Epsilon {
+    /// Value of this epsilon. By default, [`f64::EPSILON`].
+    pub value: f64,
+    /// Magnitude of this epsilon. By default, [`f64::MAX_10_EXP`].
+    pub magnitude: i32,
+}
+
+impl Default for Epsilon {
+    #[inline]
+    fn default() -> Epsilon {
+        Self::new()
+    }
+}
+
+impl Epsilon {
+    /// Raises or lowers the magnitude of `kurbo`'s epsilon by `magnitude`, returning a new
+    /// [`Epsilon`].
+    ///
+    /// Returns a new `Epsilon` representing **ε × 10_ᵐ_** where _m_ = magnitude.
+    #[inline]
+    pub const fn ten_pow(magnitude: i32) -> Self {
+        debug_assert!(MAX_10_EXP + magnitude <= MAX_10_EXP);
+
+        Epsilon {
+            magnitude: MAX_10_EXP + magnitude,
+            value: f64::mul(EPSILON, 10.0 * magnitude as f64)
+        }
+    }
+}
+
+impl Epsilon {
+    /// Create `kurbo`'s default epsilon.
+    #[inline]
+    pub const fn new() -> Self {
+        Epsilon {
+            value: EPSILON,
+            magnitude: MAX_10_EXP
+        }
+    }
+}

src/lib.rs

@@ -69,6 +69,7 @@
 //! [`Druid`]: https://docs.rs/druid
 //! [`libm`]: https://docs.rs/libm
 
+#![feature(const_fn_floating_point_arithmetic, const_trait_impl, const_ops)]
 #![forbid(unsafe_code)]
 #![deny(missing_docs, clippy::trivially_copy_pass_by_ref)]
 #![warn(rustdoc::broken_intra_doc_links)]
@@ -92,6 +93,7 @@ mod circle;
 pub mod common;
 mod cubicbez;
 mod ellipse;
+mod epsilon;
 mod insets;
 mod line;
 mod mindist;
@@ -115,6 +117,7 @@ pub use crate::bezpath::*;
 pub use crate::circle::*;
 pub use crate::cubicbez::*;
 pub use crate::ellipse::*;
+pub use crate::epsilon::*;
 pub use crate::insets::*;
 pub use crate::line::*;
 pub use crate::param_curve::*;