From 640ddc948190fe9a24d6db616cb103d6a94f81f9 Mon Sep 17 00:00:00 2001
From: Dario Nieuwenhuis <dirbaio@dirbaio.net>
Date: Sat, 12 Feb 2022 03:19:46 +0100
Subject: [PATCH] time: optimize math by reducing fractions at compile time.

For example, `as_micros`, `from_micros` now are noops if tick rate is 1MHz.
---
 embassy/src/time/duration.rs | 10 +++++-----
 embassy/src/time/instant.rs  | 10 +++++-----
 embassy/src/time/mod.rs      | 11 +++++++++++
 3 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/embassy/src/time/duration.rs b/embassy/src/time/duration.rs
index e196a00c..47b413f4 100644
--- a/embassy/src/time/duration.rs
+++ b/embassy/src/time/duration.rs
@@ -1,7 +1,7 @@
 use core::fmt;
 use core::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
 
-use super::TICKS_PER_SECOND;
+use super::{GCD_1K, GCD_1M, TICKS_PER_SECOND};
 
 #[derive(Debug, Default, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
@@ -28,12 +28,12 @@ impl Duration {
 
     /// Convert the `Duration` to milliseconds, rounding down.
     pub const fn as_millis(&self) -> u64 {
-        self.ticks * 1000 / TICKS_PER_SECOND
+        self.ticks * (1000 / GCD_1K) / (TICKS_PER_SECOND / GCD_1K)
     }
 
     /// Convert the `Duration` to microseconds, rounding down.
     pub const fn as_micros(&self) -> u64 {
-        self.ticks * 1_000_000 / TICKS_PER_SECOND
+        self.ticks * (1_000_000 / GCD_1M) / (TICKS_PER_SECOND / GCD_1M)
     }
 
     /// Creates a duration from the specified number of clock ticks
@@ -51,7 +51,7 @@ impl Duration {
     /// Creates a duration from the specified number of milliseconds
     pub const fn from_millis(millis: u64) -> Duration {
         Duration {
-            ticks: millis * TICKS_PER_SECOND / 1000,
+            ticks: millis * (TICKS_PER_SECOND / GCD_1K) / (1000 / GCD_1K),
         }
     }
 
@@ -59,7 +59,7 @@ impl Duration {
     /// NOTE: Delays this small may be inaccurate.
     pub const fn from_micros(micros: u64) -> Duration {
         Duration {
-            ticks: micros * TICKS_PER_SECOND / 1_000_000,
+            ticks: micros * (TICKS_PER_SECOND / GCD_1M) / (1_000_000 / GCD_1M),
         }
     }
 
diff --git a/embassy/src/time/instant.rs b/embassy/src/time/instant.rs
index aff83be2..71adbeb3 100644
--- a/embassy/src/time/instant.rs
+++ b/embassy/src/time/instant.rs
@@ -1,7 +1,7 @@
 use core::fmt;
 use core::ops::{Add, AddAssign, Sub, SubAssign};
 
-use super::{driver, Duration, TICKS_PER_SECOND};
+use super::{driver, Duration, GCD_1K, GCD_1M, TICKS_PER_SECOND};
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
@@ -31,14 +31,14 @@ impl Instant {
     /// Create an Instant from a microsecond count since system boot.
     pub const fn from_micros(micros: u64) -> Self {
         Self {
-            ticks: micros * TICKS_PER_SECOND / 1_000_000,
+            ticks: micros * (TICKS_PER_SECOND / GCD_1M) / (1_000_000 / GCD_1M),
         }
     }
 
     /// Create an Instant from a millisecond count since system boot.
     pub const fn from_millis(millis: u64) -> Self {
         Self {
-            ticks: millis * TICKS_PER_SECOND / 1000,
+            ticks: millis * (TICKS_PER_SECOND / GCD_1K) / (1000 / GCD_1K),
         }
     }
 
@@ -61,12 +61,12 @@ impl Instant {
 
     /// Milliseconds since system boot.
     pub const fn as_millis(&self) -> u64 {
-        self.ticks * 1000 / TICKS_PER_SECOND
+        self.ticks * (1000 / GCD_1K) / (TICKS_PER_SECOND / GCD_1K)
     }
 
     /// Microseconds since system boot.
     pub const fn as_micros(&self) -> u64 {
-        self.ticks * 1_000_000 / TICKS_PER_SECOND
+        self.ticks * (1_000_000 / GCD_1M) / (TICKS_PER_SECOND / GCD_1M)
     }
 
     /// Duration between this Instant and another Instant
diff --git a/embassy/src/time/mod.rs b/embassy/src/time/mod.rs
index c8971bd1..ac84b8f8 100644
--- a/embassy/src/time/mod.rs
+++ b/embassy/src/time/mod.rs
@@ -75,3 +75,14 @@ const TPS: u64 = 1_000_000;
 /// allow you to choose a tick rate with Cargo features of their own. You should not
 /// set the `time-tick-*` features for embassy yourself as an end user.
 pub const TICKS_PER_SECOND: u64 = TPS;
+
+const fn gcd(a: u64, b: u64) -> u64 {
+    if b == 0 {
+        a
+    } else {
+        gcd(b, a % b)
+    }
+}
+
+pub(crate) const GCD_1K: u64 = gcd(TICKS_PER_SECOND, 1_000);
+pub(crate) const GCD_1M: u64 = gcd(TICKS_PER_SECOND, 1_000_000);