Fix mapping of Crossplatform Priority to Linux Nice (#32)

src/unix.rs

@@ -413,8 +413,10 @@ impl ThreadPriority {
                     Self::to_allowed_value_for_policy(p as i32, policy).map(|v| v as u32)
                 }
                 ThreadSchedulePolicy::Normal(_) => {
+                    // Mapping a [0..100] priority into niceness [-20..20] needs reversing the ratio,
+                    // as the lowest nice is actually the highest priority.
                     let niceness_values = NICENESS_MAX.abs() + NICENESS_MIN.abs();
-                    let ratio = p as f32 / ThreadPriorityValue::MAX as f32;
+                    let ratio = 1f32 - (p as f32 / ThreadPriorityValue::MAX as f32);
                     let niceness = ((niceness_values as f32 * ratio) as i8 + NICENESS_MAX) as i32;
                     Self::to_allowed_value_for_policy(niceness, policy).map(|v| v as u32)
                 }

tests/unix.rs

@@ -4,6 +4,22 @@ use rstest::rstest;
 use std::convert::TryInto;
 use thread_priority::*;
 
+#[cfg(target_os = "linux")]
+#[test]
+fn get_and_set_priority_with_normal_and_crossplatform() {
+    let nice = unsafe { libc::getpriority(0, 0) };
+    assert_eq!(nice, 0);
+    crate::set_current_thread_priority(ThreadPriority::Crossplatform(30u8.try_into().unwrap()))
+        .unwrap();
+    let nice = unsafe { libc::getpriority(0, 0) };
+    assert!(nice > 0);
+    // Note that increasing priority requires extra permissions (e.g. sudo)
+    crate::set_current_thread_priority(ThreadPriority::Crossplatform(70u8.try_into().unwrap()))
+        .unwrap();
+    let nice = unsafe { libc::getpriority(0, 0) };
+    assert!(nice < 0);
+}
+
 #[cfg(target_os = "linux")]
 #[rstest]
 fn get_and_set_priority_with_normal_policies(