Add slot-clock

Cargo.toml

@@ -39,6 +39,7 @@ members = [
 	"beacon_chain/utils/boolean-bitfield",
 	"beacon_chain/utils/hashing",
 	"beacon_chain/utils/honey-badger-split",
+	"beacon_chain/utils/slot-clock",
 	"beacon_chain/utils/ssz",
 	"beacon_chain/utils/ssz_helpers",
 	"beacon_chain/utils/vec_shuffle",

beacon_chain/utils/slot-clock/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "slot-clock"
+version = "0.1.0"
+authors = ["Paul Hauner <paul@paulhauner.com>"]
+
+[dependencies]

beacon_chain/utils/slot-clock/src/lib.rs

@@ -0,0 +1,71 @@
+use std::time::{
+    Duration,
+    SystemTime,
+    SystemTimeError,
+};
+
+pub fn slot_now(genesis_seconds: u64, slot_duration_seconds: u64)
+    -> Result<Option<u64>, SystemTimeError>
+{
+    let sys_time = SystemTime::now();
+    let duration_since_epoch = sys_time.duration_since(SystemTime::UNIX_EPOCH)?;
+    let duration_since_genesis = duration_since_epoch
+        .checked_sub(Duration::from_secs(genesis_seconds));
+    match duration_since_genesis {
+        None => Ok(None),
+        Some(d) => Ok(slot_from_duration(slot_duration_seconds, d))
+    }
+}
+
+fn slot_from_duration(slot_duration_seconds: u64, duration: Duration)
+    -> Option<u64>
+{
+    duration.as_secs().checked_div(slot_duration_seconds)
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /*
+     * Note: these tests are using actual system times and could fail if they are executed on a
+     * very slow machine.
+     */
+    #[test]
+    fn test_slot_now() {
+        let s_time = 100;
+
+        let now = SystemTime::now();
+        let since_epoch = now.duration_since(SystemTime::UNIX_EPOCH).unwrap();
+
+        let genesis = since_epoch.as_secs() - s_time * 89;
+        assert_eq!(slot_now(genesis, s_time).unwrap(), Some(89));
+
+        let genesis = since_epoch.as_secs();
+        assert_eq!(slot_now(genesis, s_time).unwrap(), Some(0));
+
+        let genesis = since_epoch.as_secs() - s_time * 42 - 5;
+        assert_eq!(slot_now(genesis, s_time).unwrap(), Some(42));
+    }
+
+    #[test]
+    fn test_slot_from_duration() {
+        let s_time = 100;
+
+        assert_eq!(slot_from_duration(s_time, Duration::from_secs(0)), Some(0));
+        assert_eq!(slot_from_duration(s_time, Duration::from_secs(10)), Some(0));
+        assert_eq!(slot_from_duration(s_time, Duration::from_secs(100)), Some(1));
+        assert_eq!(slot_from_duration(s_time, Duration::from_secs(101)), Some(1));
+        assert_eq!(slot_from_duration(s_time, Duration::from_secs(1000)), Some(10));
+    }
+
+    #[test]
+    fn test_slot_from_duration_slot_time_zero() {
+        let s_time = 0;
+
+        assert_eq!(slot_from_duration(s_time, Duration::from_secs(0)), None);
+        assert_eq!(slot_from_duration(s_time, Duration::from_secs(10)), None);
+        assert_eq!(slot_from_duration(s_time, Duration::from_secs(1000)), None);
+    }
+}