feat: use HashMap for create_shared_string to fix O(N²) performance (#8958)

* feat: use HashMap for create_shared_string to fix O(N²) performance

* refactor: clean up no_std binary_search_by with direct slice comparison

rust/flatbuffers/src/builder.rs

@@ -24,6 +24,9 @@ use core::marker::PhantomData;
 use core::ops::{Add, AddAssign, Deref, DerefMut, Index, IndexMut, Sub, SubAssign};
 use core::ptr::write_bytes;
 
+#[cfg(feature = "std")]
+use std::collections::HashMap;
+
 use crate::endian_scalar::emplace_scalar;
 use crate::primitives::*;
 use crate::push::{Push, PushAlignment};
@@ -139,6 +142,9 @@ pub struct FlatBufferBuilder<'fbb, A: Allocator = DefaultAllocator> {
 
     min_align: usize,
     force_defaults: bool,
+    #[cfg(feature = "std")]
+    strings_pool: HashMap<String, WIPOffset<&'fbb str>>,
+    #[cfg(not(feature = "std"))]
     strings_pool: Vec<WIPOffset<&'fbb str>>,
 
     _phantom: PhantomData<&'fbb ()>,
@@ -197,6 +203,9 @@ impl<'fbb, A: Allocator> FlatBufferBuilder<'fbb, A> {
 
             min_align: 0,
             force_defaults: false,
+            #[cfg(feature = "std")]
+            strings_pool: HashMap::new(),
+            #[cfg(not(feature = "std"))]
             strings_pool: Vec::new(),
 
             _phantom: PhantomData,
@@ -343,6 +352,31 @@ impl<'fbb, A: Allocator> FlatBufferBuilder<'fbb, A> {
         WIPOffset::new(o.value())
     }
 
+    /// Create a utf8 string, and de-duplicate if already created.
+    ///
+    /// Uses a HashMap to track previously written strings, providing O(1)
+    /// amortized lookup and insertion.
+    #[cfg(feature = "std")]
+    #[inline]
+    pub fn create_shared_string<'a: 'b, 'b>(&'a mut self, s: &'b str) -> WIPOffset<&'fbb str> {
+        self.assert_not_nested(
+            "create_shared_string can not be called when a table or vector is under construction",
+        );
+
+        if let Some(&offset) = self.strings_pool.get(s) {
+            return offset;
+        }
+
+        let address = WIPOffset::new(self.create_byte_string(s.as_bytes()).value());
+        self.strings_pool.insert(s.to_owned(), address);
+        address
+    }
+
+    /// Create a utf8 string, and de-duplicate if already created.
+    ///
+    /// Uses a sorted Vec with binary search to track previously written
+    /// strings when in `no_std` mode.
+    #[cfg(not(feature = "std"))]
     #[inline]
     pub fn create_shared_string<'a: 'b, 'b>(&'a mut self, s: &'b str) -> WIPOffset<&'fbb str> {
         self.assert_not_nested(
@@ -355,19 +389,15 @@ impl<'fbb, A: Allocator> FlatBufferBuilder<'fbb, A> {
 
         let found = self.strings_pool.binary_search_by(|offset| {
             let ptr = offset.value() as usize;
-            // Gets The pointer to the size of the string
             let str_memory = &buf[buf.len() - ptr..];
-            // Gets the size of the written string from buffer
-            let size =
-                u32::from_le_bytes([str_memory[0], str_memory[1], str_memory[2], str_memory[3]])
-                    as usize;
-            // Size of the string size
-            let string_size: usize = 4;
-            // Fetches actual string bytes from index of string after string size
-            // to the size of string plus string size
-            let iter = str_memory[string_size..size + string_size].iter();
-            // Compares bytes of fetched string and current writable string
-            iter.cloned().cmp(s.bytes())
+            let size = u32::from_le_bytes([
+                str_memory[0],
+                str_memory[1],
+                str_memory[2],
+                str_memory[3],
+            ]) as usize;
+            let stored = &str_memory[4..4 + size];
+            stored.cmp(s.as_bytes())
         });
 
         match found {

tests/rust_usage_test/tests/integration_test.rs

@@ -3224,4 +3224,77 @@ fn test_shared_strings() {
     assert_eq!(string_vector.get(1), "foo");
 }
 
+#[test]
+fn test_shared_strings_pool_deduplication() {
+    // Verifies that create_shared_string correctly deduplicates across many
+    // unique strings and that the resulting buffer contains valid data.
+    let mut builder = flatbuffers::FlatBufferBuilder::with_capacity(1024);
+
+    // Insert multiple unique strings and verify each gets a distinct offset.
+    let animals = ["cat", "dog", "bird", "fish", "snake"];
+    let offsets: Vec<_> = animals
+        .iter()
+        .map(|s| builder.create_shared_string(s))
+        .collect();
+    for i in 0..offsets.len() {
+        for j in (i + 1)..offsets.len() {
+            assert_ne!(
+                offsets[i].value(),
+                offsets[j].value(),
+                "unique strings '{}' and '{}' must have different offsets",
+                animals[i],
+                animals[j],
+            );
+        }
+    }
+
+    // Re-insert the same strings and verify they return the original offsets.
+    for (i, s) in animals.iter().enumerate() {
+        let offset = builder.create_shared_string(s);
+        assert_eq!(
+            offset.value(),
+            offsets[i].value(),
+            "duplicate string '{}' must return the same offset",
+            s,
+        );
+    }
+
+    // Verify that reset clears the pool: a previously shared string is no
+    // longer deduplicated against strings from before the reset.
+    builder.reset();
+    let a = builder.create_shared_string("cat");
+    let b = builder.create_shared_string("cat");
+    assert_eq!(a.value(), b.value(), "same string after reset must still deduplicate");
+
+    // Verify that shared strings produce a valid, readable buffer.
+    builder.reset();
+    let shared_name = builder.create_shared_string("goblin");
+    let shared_name_dup = builder.create_shared_string("goblin");
+    assert_eq!(shared_name.value(), shared_name_dup.value());
+
+    let enemy = my_game::example::Monster::create(
+        &mut builder,
+        &my_game::example::MonsterArgs {
+            name: Some(shared_name),
+            ..Default::default()
+        },
+    );
+    let main_name = builder.create_shared_string("goblin");
+    assert_eq!(main_name.value(), shared_name.value());
+
+    let monster = my_game::example::Monster::create(
+        &mut builder,
+        &my_game::example::MonsterArgs {
+            name: Some(main_name),
+            enemy: Some(enemy),
+            ..Default::default()
+        },
+    );
+    builder.finish(monster, None);
+
+    let m = my_game::example::root_as_monster(builder.finished_data()).unwrap();
+    assert_eq!(m.name(), "goblin");
+    assert_eq!(m.enemy().unwrap().name(), "goblin");
+}
+
 }