Account for shared strings proportionally in JSON memory usage

src/array.rs

@@ -1320,17 +1320,17 @@ impl IArray {
         }
     }
 
-    pub(crate) fn mem_allocated(&self) -> usize {
+    pub(crate) fn mem_allocated(&self) -> f64 {
         if self.is_static() {
-            0
+            0.0
         } else {
             let tag = self.header().type_tag();
             let layout_size = Self::layout(self.capacity(), tag).unwrap().size();
             let contained_size = self
                 .as_slice_of::<IValue>()
-                .map(|slice| slice.iter().map(IValue::mem_allocated).sum())
-                .unwrap_or(0);
-            layout_size + contained_size
+                .map(|slice| slice.iter().map(IValue::mem_allocated).sum::<f64>())
+                .unwrap_or(0.0);
+            layout_size as f64 + contained_size
         }
     }
 }

src/object.rs

@@ -918,17 +918,17 @@ impl IObject {
         }
     }
 
-    pub(crate) fn mem_allocated(&self) -> usize {
+    pub(crate) fn mem_allocated(&self) -> f64 {
         if self.is_static() {
-            0
+            0.0
         } else {
             // Layout of a live object's own capacity; it allocated successfully, so
             // recomputing its layout cannot fail.
-            Self::layout(self.capacity()).unwrap().size()
+            Self::layout(self.capacity()).unwrap().size() as f64
                 + self
                     .iter()
                     .map(|(k, v)| k.mem_allocated() + v.mem_allocated())
-                    .sum::<usize>()
+                    .sum::<f64>()
         }
     }
 }

src/string.rs

@@ -291,11 +291,12 @@ impl IString {
         }
     }
 
-    pub(crate) fn mem_allocated(&self) -> usize {
+    pub(crate) fn mem_allocated(&self) -> f64 {
         if self.is_empty() {
-            0
+            0.0
         } else {
-            Self::layout(self.len()).unwrap().size()
+            Self::layout(self.len()).unwrap().size() as f64
+                / self.header().rc.load(AtomicOrdering::Relaxed) as f64
         }
     }
 }

src/unsafe_string.rs

@@ -423,11 +423,14 @@ impl IString {
         self.drop_impl_with_deallocator(|ptr, layout| unsafe { dealloc(ptr, layout) });
     }
 
-    pub(crate) fn mem_allocated(&self) -> usize {
+    pub(crate) fn mem_allocated(&self) -> f64 {
         if self.is_empty() || self.is_inline() {
-            0
+            0.0
         } else {
-            Self::layout(self.len()).unwrap().size()
+            // SAFETY: `self` keeps a counted heap reference alive, so rc is nonzero.
+            // Relaxed suffices for an estimate that may change with concurrent owners.
+            Self::layout(self.len()).unwrap().size() as f64
+                / self.header().rc.load(std::sync::atomic::Ordering::Relaxed) as f64
         }
     }
 }
@@ -630,7 +633,7 @@ mod tests {
                 assert_eq!(istr.as_bytes(), s.as_bytes());
 
                 // Inline strings should have minimal memory overhead
-                assert_eq!(istr.mem_allocated(), 0);
+                assert_eq!(istr.mem_allocated(), 0.0);
             } else {
                 assert!(!istr.is_inline(), "String '{}' should not be inline", s);
             }
@@ -651,7 +654,7 @@ mod tests {
             assert_eq!(istr.as_bytes(), s.as_bytes());
 
             // Heap strings should have memory overhead
-            assert!(istr.mem_allocated() > 0);
+            assert!(istr.mem_allocated() > 0.0);
         }
     }
 

src/value.rs

@@ -351,14 +351,17 @@ impl IValue {
         }
     }
 
-    /// Reports dynamic memory allocated by this value.
-    pub fn mem_allocated(&self) -> usize {
+    /// Reports proportional dynamic memory in bytes, excluding this value itself.
+    /// Shared strings are divided among all live references (including temporary
+    /// owners). Container capacity is fully counted; the intern table is excluded.
+    /// Retain fractional bytes until rounding the final measurement.
+    pub fn mem_allocated(&self) -> f64 {
         use ValueType::*;
         match self.type_() {
             // inline types consume no extra memory
-            Null | Bool => 0,
+            Null | Bool => 0.0,
             // Safety: We checked the type
-            Number => unsafe { self.as_number_unchecked() }.mem_allocated(),
+            Number => unsafe { self.as_number_unchecked() }.mem_allocated() as f64,
             String => unsafe { self.as_string_unchecked() }.mem_allocated(),
             Array => unsafe { self.as_array_unchecked() }.mem_allocated(),
             Object => unsafe { self.as_object_unchecked() }.mem_allocated(),
@@ -1091,7 +1094,7 @@ mod tests {
         assert!(matches!(x.clone().destructure(), Destructured::Null));
         assert!(matches!(x.clone().destructure_ref(), DestructuredRef::Null));
         assert!(matches!(x.clone().destructure_mut(), DestructuredMut::Null));
-        assert_eq!(x.mem_allocated(), 0);
+        assert_eq!(x.mem_allocated(), 0.0);
     }
 
     #[test]
@@ -1112,7 +1115,7 @@ mod tests {
             }
 
             assert_eq!(x.to_bool(), Some(!v));
-            assert_eq!(x.mem_allocated(), 0);
+            assert_eq!(x.mem_allocated(), 0.0);
         }
     }
 
@@ -1147,9 +1150,9 @@ mod tests {
                 assert_eq!(
                     x.mem_allocated(),
                     if (INLINE_LOWER..=INLINE_UPPER).contains(&v) {
-                        0
+                        0.0
                     } else {
-                        mem::size_of::<i64>()
+                        mem::size_of::<i64>() as f64
                     }
                 );
             }
@@ -1181,9 +1184,9 @@ mod tests {
                 assert_eq!(
                     x.mem_allocated(),
                     if v <= i64::MAX as u64 && (INLINE_LOWER..=INLINE_UPPER).contains(&(v as i64)) {
-                        0
+                        0.0
                     } else {
-                        mem::size_of::<u64>()
+                        mem::size_of::<u64>() as f64
                     }
                 );
             }
@@ -1207,7 +1210,7 @@ mod tests {
                 assert!(
                     matches!(x.clone().destructure_mut(), DestructuredMut::Number(u) if *u == INumber::try_from(v).unwrap())
                 );
-                assert_eq!(x.mem_allocated(), mem::size_of::<f64>());
+                assert_eq!(x.mem_allocated(), mem::size_of::<f64>() as f64);
             }
         }
     }
@@ -1224,7 +1227,7 @@ mod tests {
             assert!(matches!(x.clone().destructure(), Destructured::String(u) if u == s));
             assert!(matches!(x.clone().destructure_ref(), DestructuredRef::String(u) if *u == s));
             assert!(matches!(x.clone().destructure_mut(), DestructuredMut::String(u) if *u == s));
-            assert_eq!(x.mem_allocated(), 0);
+            assert_eq!(x.mem_allocated(), 0.0);
         }
 
         let s = String::from("foofoofoo");
@@ -1236,7 +1239,7 @@ mod tests {
         assert!(matches!(x.clone().destructure(), Destructured::String(u) if u == s));
         assert!(matches!(x.clone().destructure_ref(), DestructuredRef::String(u) if *u == s));
         assert!(matches!(x.clone().destructure_mut(), DestructuredMut::String(u) if *u == s));
-        assert_eq!(x.mem_allocated(), 24);
+        assert_eq!(x.mem_allocated(), 24.0);
     }
 
     #[mockalloc::test]
@@ -1253,8 +1256,9 @@ mod tests {
             assert!(matches!(x.clone().destructure_mut(), DestructuredMut::Array(u) if *u == a));
             assert_eq!(
                 x.mem_allocated(),
-                mem::size_of::<usize>()
-                    + ((a.capacity() as usize * mem::size_of::<i32>() + 7) & !7)
+                (mem::size_of::<usize>()
+                    + ((a.capacity() as usize * mem::size_of::<i32>() + 7) & !7))
+                    as f64
             );
         }
     }
@@ -1286,8 +1290,8 @@ mod tests {
                 x.mem_allocated(),
                 o.iter()
                     .map(|(k, v)| k.mem_allocated() + v.mem_allocated())
-                    .sum::<usize>()
-                    + ((raw + 7) & !7)
+                    .sum::<f64>()
+                    + ((raw + 7) & !7) as f64
             );
         }
     }

tests/memory_share.rs

@@ -0,0 +1,31 @@
+use ijson::IValue;
+
+#[test]
+fn proportional_memory_across_documents() {
+    let text = "proportional allocation accounting test string";
+    let first = IValue::from(text);
+    let allocation = first.mem_allocated();
+    let mut docs: Vec<IValue> = (0..99).map(|_| IValue::from(text)).collect();
+    docs.push(first);
+    assert!(docs[0].mem_allocated() < 1.0);
+    assert!((docs.iter().map(IValue::mem_allocated).sum::<f64>() - allocation).abs() < 1e-9);
+    let one = docs.pop().unwrap();
+    drop(docs);
+    assert_eq!(one.mem_allocated(), allocation);
+    drop(one);
+
+    // A field name and nested values share one allocation across documents.
+    let json = format!(r#"{{"{text}":["{text}","{text}"]}}"#);
+    let doc: IValue = serde_json::from_str(&json).unwrap();
+    let original = doc.mem_allocated();
+    let other = IValue::from(text);
+    assert!((doc.mem_allocated() + other.mem_allocated() - original).abs() < 1e-9);
+    assert!((other.mem_allocated() - allocation / 4.0).abs() < 1e-9);
+    drop(other);
+    assert!((doc.mem_allocated() - original).abs() < 1e-9);
+
+    for json in ["null", "true", "[]", "{}", r#""short""#] {
+        let value: IValue = serde_json::from_str(json).unwrap();
+        assert_eq!(value.mem_allocated(), 0.0);
+    }
+}