1 files changed, 50 insertions, 2 deletions

diff --git a/src/remake/kind.rs b/src/remake/kind.rs
index b36db4c..42bdd88 100644
--- a/src/remake/kind.rs
+++ b/src/remake/kind.rs
@@ -17,6 +17,9 @@
 //! Protocol data under `_continuum` is not a kind's business: the generic part carries
 //! it forward and replays it unchanged.
 
+use std::path::Path;
+
+use anyhow::{Context, anyhow, bail};
 use automerge::AutoCommit;
 use base64::engine::{Engine, general_purpose::STANDARD as BASE64};
 use serde::{Deserialize, Serialize};
@@ -24,6 +27,7 @@ use serde_json::Value as JsonValue;
 
 use super::snapshot::{META_ROOT, canonical};
 use crate::document::store::{self, StoreError};
+use crate::files::{FileKind, Files};
 
 /// Name of the kind used when a request doesn't name one.
 pub(crate) const DEFAULT_KIND: &str = "compact";
@@ -74,8 +78,8 @@ pub(crate) trait RemakeKind: Send + Sync + 'static {
 
     /// Whether the coordinator fixes the base when it commits instead of when it
     /// proposes (no `baseHeads` then). Right for a kind whose content is the old
-    /// document at the base: the later the base, the fewer writes need carrying over,
-    /// and a write made after it is lost if its author restarts before switching.
+    /// document at the base: the later the base, the fewer writes every switching node
+    /// replays as fresh values (whole lists and texts, concurrently on every node).
     fn base_at_commit(&self) -> bool {
         false
     }
@@ -91,6 +95,50 @@ pub(crate) fn by_name(name: &str) -> Option<&'static dyn RemakeKind> {
     }
 }
 
+/// Builds a new swarm's genesis content into `target` (`--genesis --remake SPEC`), the
+/// way a remake of that kind builds a new epoch from an empty one:
+/// * `import:PATH`: an Automerge document, e.g. a backup (its content, without history
+///   and protocol data);
+/// * `replace:PATH`: a JSON document, the store root;
+/// * `compact`: nothing (an empty document compacts to itself).
+pub(crate) fn remake_genesis(
+    spec: &str,
+    files: &Files,
+    target: &mut AutoCommit,
+) -> anyhow::Result<()> {
+    let (name, path) = match spec.split_once(':') {
+        Some((name, path)) => (name, Some(Path::new(path))),
+        None => (spec, None),
+    };
+    let kind = by_name(name).ok_or_else(|| anyhow!("unknown remake kind {name:?}"))?;
+    let request = match (kind.name(), path) {
+        ("compact", None) => return Ok(()),
+        ("compact", Some(_)) => bail!("compact takes no file"),
+        (_, None) => bail!("{name} needs a file: {name}:PATH"),
+        (name, Some(path)) => {
+            let bytes = files
+                .read(FileKind::Remake, path)
+                .with_context(|| format!("Reading {}", path.display()))?
+                .with_context(|| format!("{} doesn't exist", path.display()))?;
+            match name {
+                "import" => serde_json::json!({ "document": BASE64.encode(&bytes) }),
+                "replace" => {
+                    let document: JsonValue = serde_json::from_slice(&bytes)
+                        .with_context(|| format!("{} is not JSON", path.display()))?;
+                    serde_json::json!({ "document": document })
+                }
+                other => bail!("{other} can't make a genesis document"),
+            }
+        }
+    };
+    let prepared = kind
+        .propose(&request)
+        .map_err(|reason| anyhow!("{name}: {reason}"))?;
+    kind.build(&AutoCommit::new(), prepared.content.as_deref(), target)?;
+    target.commit();
+    Ok(())
+}
+
 /// Paths owned by the protocol rather than by any kind.
 pub(crate) fn is_reserved(path: &[String]) -> bool {
     path.first().is_some_and(|root| root == META_ROOT)