MOD-16608 Object memory improvement - pack header, u32 hash table, elide table for small objects

.github/workflows/ci.yml

@@ -26,6 +26,7 @@ jobs:
           - fuzz_json_de
           - fuzz_cbor_decode
           - fuzz_cbor_roundtrip
+          - fuzz_object_ops
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4

fuzz/Cargo.toml

@@ -41,3 +41,10 @@ path = "fuzz_targets/fuzz_cbor_roundtrip.rs"
 test = false
 doc = false
 bench = false
+
+[[bin]]
+name = "fuzz_object_ops"
+path = "fuzz_targets/fuzz_object_ops.rs"
+test = false
+doc = false
+bench = false

fuzz/fuzz_targets/fuzz_object_ops.rs

@@ -0,0 +1,52 @@
+#![no_main]
+
+use arbitrary::Arbitrary;
+use ijson::{IObject, IValue};
+use libfuzzer_sys::fuzz_target;
+use std::collections::HashMap;
+
+// Small key space (u8 -> "k{n}") so op sequences repeatedly hit the same keys,
+// reliably crossing the small-object table threshold in both directions and
+// forcing hash collisions / Robin-Hood displacement on the u32 table.
+#[derive(Arbitrary, Debug)]
+enum Op {
+    Insert(u8, u64),
+    Remove(u8),
+    Get(u8),
+}
+
+fuzz_target!(|ops: Vec<Op>| {
+    let mut obj = IObject::new();
+    let mut oracle: HashMap<u8, u64> = HashMap::new();
+
+    for op in ops {
+        match op {
+            Op::Insert(id, v) => {
+                let k = format!("k{id}");
+                // insert() only errors on OOM / 2^30 overflow; neither is reachable here.
+                let prev = obj
+                    .insert(k.as_str(), IValue::from(v))
+                    .unwrap()
+                    .and_then(|old| old.to_u64());
+                assert_eq!(prev, oracle.insert(id, v));
+            }
+            Op::Remove(id) => {
+                let k = format!("k{id}");
+                let removed = obj.remove(k.as_str()).and_then(|v| v.to_u64());
+                assert_eq!(removed, oracle.remove(&id));
+            }
+            Op::Get(id) => {
+                let k = format!("k{id}");
+                let got = obj.get(k.as_str()).and_then(|v| v.to_u64());
+                assert_eq!(got, oracle.get(&id).copied());
+            }
+        }
+        assert_eq!(obj.len() as usize, oracle.len());
+    }
+
+    // Final agreement: every oracle key present with the right value.
+    for (id, v) in &oracle {
+        let k = format!("k{id}");
+        assert_eq!(obj.get(k.as_str()).and_then(|x| x.to_u64()), Some(*v));
+    }
+});