checkpoint: add comments to struct, correct version comment (#115)

* checkpoint: add comments to struct, correct version comment

* remove BTmeta, add snapshot version constant

crown/src/kernel/checkpoint.rs

@@ -13,12 +13,19 @@ use tracing::{debug, error, info, warn};
 
 #[derive(Clone)]
 pub struct Checkpoint {
+    /// Magic bytes to identify checkpoint format
     pub magic_bytes: u64,
+    /// Version of checkpoint
     pub version: u32,
+    /// The buffer that this checkpoint was saved to, either 0 or 1.
     pub buff_index: bool,
+    /// Hash of the boot kernel
     pub ker_hash: Hash,
+    /// Event number
     pub event_num: u64,
+    /// State of the kernel
     pub ker_state: Noun,
+    /// Cold state
     pub cold: Cold,
 }
 
@@ -45,14 +52,21 @@ impl Checkpoint {
 
 #[derive(Encode, Decode, PartialEq, Debug)]
 pub struct JammedCheckpoint {
+    /// Magic bytes to identify checkpoint format
     pub magic_bytes: u64,
+    /// Version of checkpoint
     pub version: u32,
+    /// The buffer this checkpoint was saved to, either 0 or 1
     pub buff_index: bool,
+    /// Hash of the boot kernel
     #[bincode(with_serde)]
     pub ker_hash: Hash,
+    /// Checksum derived from event_num and jam (the entries below)
     #[bincode(with_serde)]
     pub checksum: Hash,
+    /// Event number
     pub event_num: u64,
+    /// Jammed noun of [kernel_state cold_state]
     pub jam: JammedNoun,
 }
 

crown/src/kernel/form.rs

@@ -32,12 +32,7 @@ const LOAD_AXIS: u64 = 4;
 const PEEK_AXIS: u64 = 22;
 const POKE_AXIS: u64 = 23;
 
-/// Enum representing the Sword snapshot metadata fields.
-#[repr(usize)]
-enum BTMetaField {
-    SnapshotVersion = 0,
-    Snapshot = 1,
-}
+const SNAPSHOT_VERSION: u32 = 0;
 
 /// Represents a Sword kernel, containing a Serf and snapshot location.
 pub struct Kernel {
@@ -470,7 +465,7 @@ impl Kernel {
 /// Represents the Serf, which maintains context and provides an interface to
 /// the Sword.
 pub struct Serf {
-    /// Version number of arvo
+    /// Version number of snapshot
     pub version: u32,
     /// Hash of boot kernel
     pub ker_hash: Hash,
@@ -540,7 +535,7 @@ impl Serf {
 
         let version = checkpoint
             .as_ref()
-            .map_or_else(|| 0, |snapshot| snapshot.version);
+            .map_or_else(|| SNAPSHOT_VERSION, |snapshot| snapshot.version);
 
         let mut hasher = Hasher::new();
         hasher.update(kernel_bytes);