From 8a14a408a5355c925f178ece715fa176ef14b6b5 Mon Sep 17 00:00:00 2001
From: Acrimon <joel.wejdenstal@gmail.com>
Date: Tue, 4 Aug 2020 19:52:14 +0200
Subject: [PATCH] fix: #100

---
 Cargo.toml |  1 +
 src/lib.rs | 18 ++++++++++++------
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index 657491a..5d693e5 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -18,6 +18,7 @@ raw-api = []
 no_std = ["hashbrown"]
 
 [dependencies]
+num_cpus = "1.13.0"
 ahash = "0.3.8"
 serde = { version = "1.0.114", optional = true, features = ["derive"] }
 cfg-if = "0.1.10"
diff --git a/src/lib.rs b/src/lib.rs
index b832e29..0222dd9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -49,8 +49,13 @@ cfg_if! {
     }
 }
 
-const SHARD_AMOUNT: usize = 8;
-const NCB: usize = SHARD_AMOUNT.trailing_zeros() as usize;
+fn shard_amount() -> usize {
+    (num_cpus::get() * 4).next_power_of_two()
+}
+
+fn ncb(shard_amount: usize) -> usize {
+    shard_amount.trailing_zeros() as usize
+}
 
 /// DashMap is an implementation of a concurrent associative array/hashmap in Rust.
 ///
@@ -167,15 +172,16 @@ impl<'a, K: 'a + Eq + Hash, V: 'a, S: BuildHasher + Clone> DashMap<K, V, S> {
     /// ```
 
     pub fn with_capacity_and_hasher(mut capacity: usize, hasher: S) -> Self {
-        let shift = util::ptr_size_bits() - NCB;
+        let shard_amount = shard_amount();
+        let shift = util::ptr_size_bits() - ncb(shard_amount);
 
         if capacity != 0 {
-            capacity = (capacity + (SHARD_AMOUNT - 1)) & !(SHARD_AMOUNT - 1);
+            capacity = (capacity + (shard_amount - 1)) & !(shard_amount - 1);
         }
 
-        let cps = capacity / SHARD_AMOUNT;
+        let cps = capacity / shard_amount;
 
-        let shards = (0..SHARD_AMOUNT)
+        let shards = (0..shard_amount)
             .map(|_| RwLock::new(HashMap::with_capacity_and_hasher(cps, hasher.clone())))
             .collect();