consensus-slots: cleanup SlotDuration config (#10878)

* consensus-slots: cleanup the SlotDuration config

* fix tests

* address review comments

substrate/primitives/consensus/slots/src/lib.rs

@@ -21,6 +21,7 @@
 
 use codec::{Decode, Encode, MaxEncodedLen};
 use scale_info::TypeInfo;
+use sp_timestamp::Timestamp;
 
 /// Unit type wrapper that represents a slot.
 #[derive(Debug, Encode, MaxEncodedLen, Decode, Eq, Clone, Copy, Default, Ord, TypeInfo)]
@@ -64,6 +65,11 @@ impl<T: Into<u64> + Copy> core::cmp::PartialOrd<T> for Slot {
 }
 
 impl Slot {
+	/// Create a new slot by calculating it from the given timestamp and slot duration.
+	pub const fn from_timestamp(timestamp: Timestamp, slot_duration: SlotDuration) -> Self {
+		Slot(timestamp.as_millis() / slot_duration.as_millis())
+	}
+
 	/// Saturating addition.
 	pub fn saturating_add<T: Into<u64>>(self, rhs: T) -> Self {
 		Self(self.0.saturating_add(rhs.into()))
@@ -94,6 +100,32 @@ impl From<Slot> for u64 {
 	}
 }
 
+/// A slot duration defined in milliseconds.
+#[derive(Clone, Copy, Debug, Encode, Decode, Hash, PartialOrd, Ord, PartialEq, Eq, TypeInfo)]
+pub struct SlotDuration(u64);
+
+impl SlotDuration {
+	/// Initialize from the given milliseconds.
+	pub const fn from_millis(millis: u64) -> Self {
+		Self(millis)
+	}
+}
+
+impl SlotDuration {
+	/// Returns `self` as a `u64` representing the duration in milliseconds.
+	pub const fn as_millis(&self) -> u64 {
+		self.0
+	}
+}
+
+#[cfg(feature = "std")]
+impl SlotDuration {
+	/// Returns `self` as [`sp_std::time::Duration`].
+	pub const fn as_duration(&self) -> sp_std::time::Duration {
+		sp_std::time::Duration::from_millis(self.0)
+	}
+}
+
 /// Represents an equivocation proof. An equivocation happens when a validator
 /// produces more than one block on the same slot. The proof of equivocation
 /// are the given distinct headers that were signed by the validator and which