Pruning changes tries (#856)

* changes trie pruning

* add comment

* do not prune changes tries on archive nodes

substrate/core/primitives/src/changes_trie.rs

@@ -28,3 +28,100 @@ pub struct ChangesTrieConfiguration {
 	/// 2 means that every digest_interval^2 there will be a level2-digest, and so on.
 	pub digest_levels: u32,
 }
+
+impl ChangesTrieConfiguration {
+	/// Is digest build enabled?
+	pub fn is_digest_build_enabled(&self) -> bool {
+		self.digest_interval > 1 && self.digest_levels > 0
+	}
+
+	/// Do we need to build digest at given block?
+	pub fn is_digest_build_required_at_block(&self, block: u64) -> bool {
+		block != 0
+			&& self.is_digest_build_enabled()
+			&& block % self.digest_interval == 0
+	}
+
+	/// Returns Some if digest must be built at given block number.
+	/// The tuple is:
+	/// (
+	///  digest level
+	///  digest interval (in blocks)
+	///  step between blocks we're interested in when digest is built
+	/// )
+	pub fn digest_level_at_block(&self, block: u64) -> Option<(u32, u64, u64)> {
+		if !self.is_digest_build_required_at_block(block) {
+			return None;
+		}
+
+		let mut digest_interval = self.digest_interval;
+		let mut current_level = 1u32;
+		let mut digest_step = 1u64;
+		while current_level < self.digest_levels {
+			let new_digest_interval = match digest_interval.checked_mul(self.digest_interval) {
+				Some(new_digest_interval) if block % new_digest_interval == 0 => new_digest_interval,
+				_ => break,
+			};
+
+			digest_step = digest_interval;
+			digest_interval = new_digest_interval;
+			current_level = current_level + 1;
+		}
+
+		Some((
+			current_level,
+			digest_interval,
+			digest_step,
+		))
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::ChangesTrieConfiguration;
+
+	fn config(interval: u64, levels: u32) -> ChangesTrieConfiguration {
+		ChangesTrieConfiguration {
+			digest_interval: interval,
+			digest_levels: levels,
+		}
+	}
+
+	#[test]
+	fn is_digest_build_enabled_works() {
+		assert!(!config(0, 100).is_digest_build_enabled());
+		assert!(!config(1, 100).is_digest_build_enabled());
+		assert!(config(2, 100).is_digest_build_enabled());
+		assert!(!config(100, 0).is_digest_build_enabled());
+		assert!(config(100, 1).is_digest_build_enabled());
+	}
+
+	#[test]
+	fn is_digest_build_required_at_block_works() {
+		assert!(!config(8, 4).is_digest_build_required_at_block(0));
+		assert!(!config(8, 4).is_digest_build_required_at_block(1));
+		assert!(!config(8, 4).is_digest_build_required_at_block(2));
+		assert!(!config(8, 4).is_digest_build_required_at_block(4));
+		assert!(config(8, 4).is_digest_build_required_at_block(8));
+		assert!(!config(8, 4).is_digest_build_required_at_block(9));
+		assert!(config(8, 4).is_digest_build_required_at_block(64));
+		assert!(config(8, 4).is_digest_build_required_at_block(64));
+		assert!(config(8, 4).is_digest_build_required_at_block(512));
+		assert!(config(8, 4).is_digest_build_required_at_block(4096));
+		assert!(!config(8, 4).is_digest_build_required_at_block(4103));
+		assert!(config(8, 4).is_digest_build_required_at_block(4104));
+		assert!(!config(8, 4).is_digest_build_required_at_block(4108));
+	}
+
+	#[test]
+	fn digest_level_at_block_works() {
+		assert_eq!(config(8, 4).digest_level_at_block(0), None);
+		assert_eq!(config(8, 4).digest_level_at_block(7), None);
+		assert_eq!(config(8, 4).digest_level_at_block(63), None);
+		assert_eq!(config(8, 4).digest_level_at_block(8), Some((1, 8, 1)));
+		assert_eq!(config(8, 4).digest_level_at_block(64), Some((2, 64, 8)));
+		assert_eq!(config(8, 4).digest_level_at_block(512), Some((3, 512, 64)));
+		assert_eq!(config(8, 4).digest_level_at_block(4096), Some((4, 4096, 512)));
+		assert_eq!(config(8, 4).digest_level_at_block(4112), Some((1, 8, 1)));
+	}
+}