dpi: make no_std compatible

dpi/src/lib.rs

@@ -54,13 +54,25 @@
 //!
 //! * `serde`: Enables serialization/deserialization of certain types with [Serde](https://crates.io/crates/serde).
 //! * `mint`: Enables mint (math interoperability standard types) conversions.
+//! * `std` (enabled by default): Uses the standard library mathematical functions (normally through
+//!   your target platform's libm). This feature also changes the library's license from `Apache-2.0
+//!   AND MIT` to `APACHE-2.0` (only). For full details, see the package README.
 //!
+//! To use this library on a target without the standard library available, you should disable
+//! default features (thus disabling the `std` feature, with the license consequences thereof).
 //!
 //! [points]: https://en.wikipedia.org/wiki/Point_(typography)
 //! [picas]: https://en.wikipedia.org/wiki/Pica_(typography)
 
 #![cfg_attr(docsrs, feature(doc_auto_cfg, doc_cfg_hide), doc(cfg_hide(doc, docsrs)))]
-#![forbid(unsafe_code)]
+#![cfg_attr(feature = "std", forbid(unsafe_code))]
+#![no_std]
+
+#[cfg(not(feature = "std"))]
+mod libm;
+
+#[cfg(any(feature = "std", test))]
+extern crate std;
 
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
@@ -74,32 +86,32 @@ pub trait Pixel: Copy + Into<f64> {
 
 impl Pixel for u8 {
     fn from_f64(f: f64) -> Self {
-        f.round() as u8
+        round(f) as u8
     }
 }
 impl Pixel for u16 {
     fn from_f64(f: f64) -> Self {
-        f.round() as u16
+        round(f) as u16
     }
 }
 impl Pixel for u32 {
     fn from_f64(f: f64) -> Self {
-        f.round() as u32
+        round(f) as u32
     }
 }
 impl Pixel for i8 {
     fn from_f64(f: f64) -> Self {
-        f.round() as i8
+        round(f) as i8
     }
 }
 impl Pixel for i16 {
     fn from_f64(f: f64) -> Self {
-        f.round() as i16
+        round(f) as i16
     }
 }
 impl Pixel for i32 {
     fn from_f64(f: f64) -> Self {
-        f.round() as i32
+        round(f) as i32
     }
 }
 impl Pixel for f32 {
@@ -113,6 +125,15 @@ impl Pixel for f64 {
     }
 }
 
+/// Round f to the closest integer, rounding away from `0.0`
+#[inline]
+fn round(f: f64) -> f64 {
+    #[cfg(feature = "std")]
+    return f.round();
+    #[cfg(not(feature = "std"))]
+    return libm::round(f);
+}
+
 /// Checks that the scale factor is a normal positive `f64`.
 ///
 /// All functions that take a scale factor assert that this will return `true`. If you're sourcing
@@ -1270,20 +1291,20 @@ mod tests {
     // Eat coverage for the Debug impls et al
     #[test]
     fn ensure_attrs_do_not_panic() {
-        let _ = format!("{:?}", LogicalPosition::<u32>::default().clone());
+        let _ = std::format!("{:?}", LogicalPosition::<u32>::default().clone());
         HashSet::new().insert(LogicalPosition::<u32>::default());
 
-        let _ = format!("{:?}", PhysicalPosition::<u32>::default().clone());
+        let _ = std::format!("{:?}", PhysicalPosition::<u32>::default().clone());
         HashSet::new().insert(PhysicalPosition::<u32>::default());
 
-        let _ = format!("{:?}", LogicalSize::<u32>::default().clone());
+        let _ = std::format!("{:?}", LogicalSize::<u32>::default().clone());
         HashSet::new().insert(LogicalSize::<u32>::default());
 
-        let _ = format!("{:?}", PhysicalSize::<u32>::default().clone());
+        let _ = std::format!("{:?}", PhysicalSize::<u32>::default().clone());
         HashSet::new().insert(PhysicalSize::<u32>::default());
 
-        let _ = format!("{:?}", Size::Physical((1, 2).into()).clone());
-        let _ = format!("{:?}", Position::Physical((1, 2).into()).clone());
+        let _ = std::format!("{:?}", Size::Physical((1, 2).into()).clone());
+        let _ = std::format!("{:?}", Position::Physical((1, 2).into()).clone());
     }
 
     #[test]

dpi/src/libm.rs

@@ -0,0 +1,56 @@
+// Copyright (c) 2018 Jorge Aparicio
+// Copyright © 2005-2020 Rich Felker, et al.
+// Copyright © 1993,2004 Sun Microsystems or
+// Copyright © 2003-2011 David Schultz or
+// Copyright © 2003-2009 Steven G. Kargl or
+// Copyright © 2003-2009 Bruce D. Evans or
+// Copyright © 2008 Stephen L. Moshier or
+// Copyright © 2017-2018 Arm Limited
+// SPDX-License-Identifier: MIT
+// This file is licensed solely under the terms discussed in LICENSE-LIBM-MIT at the crate root.
+// See the package-level README for full details.
+
+// Taken from https://github.com/rust-lang/libm/blob/master/src/math/mod.rs#L1
+macro_rules! force_eval {
+    ($e:expr) => {
+        unsafe { ::core::ptr::read_volatile(&$e) }
+    };
+}
+
+// Taken from https://github.com/rust-lang/libm/blob/libm-v0.2.11/src/math/round.rs
+pub(crate) fn round(x: f64) -> f64 {
+    trunc(x + copysign(0.5 - 0.25 * f64::EPSILON, x))
+}
+
+// Adapted from: https://github.com/rust-lang/libm/blob/libm-v0.2.11/src/math/trunc.rs#L8-L12
+#[allow(clippy::needless_late_init /*, reason = "The original libm code uses this style" */)]
+fn trunc(x: f64) -> f64 {
+    let x1p120 = f64::from_bits(0x4770000000000000); // 0x1p120f === 2 ^ 120
+
+    let mut i: u64 = x.to_bits();
+    let mut e: i64 = ((i >> 52) & 0x7ff) as i64 - 0x3ff + 12;
+    let m: u64;
+
+    if e >= 52 + 12 {
+        return x;
+    }
+    if e < 12 {
+        e = 1;
+    }
+    m = -1i64 as u64 >> e;
+    if (i & m) == 0 {
+        return x;
+    }
+    force_eval!(x + x1p120);
+    i &= !m;
+    f64::from_bits(i)
+}
+
+// Taken from https://github.com/rust-lang/libm/blob/libm-v0.2.11/src/math/copysign.rs
+fn copysign(x: f64, y: f64) -> f64 {
+    let mut ux = x.to_bits();
+    let uy = y.to_bits();
+    ux &= (!0) >> 1;
+    ux |= uy & (1 << 63);
+    f64::from_bits(ux)
+}