Preserves value merge in Rust

implementations/rust/preserves/src/value/merge.rs

@@ -0,0 +1,62 @@
+use super::Embeddable;
+use super::Map;
+use super::NestedValue;
+use super::Record;
+use super::Value;
+
+pub fn merge_seqs<N: NestedValue<D>, D: Embeddable>(mut a: Vec<N>, mut b: Vec<N>) -> Option<Vec<N>> {
+    if a.len() > b.len() {
+        std::mem::swap(&mut a, &mut b);
+    }
+    let mut r = vec![];
+    let mut bi = b.into_iter();
+    for av in a.into_iter() {
+        r.push(merge2(av, bi.next().unwrap())?);
+    }
+    r.extend(bi);
+    Some(r)
+}
+
+pub fn merge2<N: NestedValue<D>, D: Embeddable>(v: N, w: N) -> Option<N> {
+    let (mut v_anns, v_val) = v.pieces();
+    let (w_anns, w_val) = w.pieces();
+    v_anns.modify(|anns| anns.extend(w_anns.to_vec().into_iter()));
+    if v_val == w_val {
+        Some(N::wrap(v_anns, v_val))
+    } else {
+        let maybe_merged = match v_val {
+            Value::Record(rv) =>
+                Some(Value::Record(Record(merge_seqs(rv.0, w_val.into_record()?.0)?))),
+            Value::Sequence(vs) =>
+                Some(Value::Sequence(merge_seqs(vs, w_val.into_sequence()?)?)),
+            Value::Set(_vs) =>
+                None, // unsure how to merge sets
+            Value::Dictionary(vs) => {
+                let mut ws = w_val.into_dictionary()?;
+                let mut rs = Map::new();
+                for (k, vv) in vs.into_iter() {
+                    match ws.remove(&k) {
+                        Some(wv) => { rs.insert(k, merge2(vv, wv)?); }
+                        None => { rs.insert(k, vv); }
+                    }
+                }
+                rs.extend(ws.into_iter());
+                Some(Value::Dictionary(rs))
+            }
+            _ => None,
+        };
+        maybe_merged.map(|vw| N::wrap(v_anns, vw))
+    }
+}
+
+pub fn merge<N: NestedValue<D>, D: Embeddable, I: IntoIterator<Item = N>>(vs: I) -> Option<N> {
+    let mut vs = vs.into_iter();
+    let mut v = vs.next().expect("at least one value in merge()");
+    for w in vs {
+        match merge2(v, w) {
+            Some(merged) => v = merged,
+            None => return None,
+        }
+    }
+    Some(v)
+}

implementations/rust/preserves/src/value/mod.rs

@@ -10,6 +10,7 @@ pub mod signed_integer;
 pub mod suspendable;
 pub mod text;
 pub mod writer;
+pub mod merge;
 
 pub use de::Deserializer;
 pub use de::from_value;
@@ -19,6 +20,7 @@ pub use domain::DomainParse;
 pub use domain::IOValueDomainCodec;
 pub use domain::NoEmbeddedDomainCodec;
 pub use domain::ViaCodec;
+pub use merge::merge;
 pub use packed::PackedReader;
 pub use packed::PackedWriter;
 pub use reader::BinarySource;