remove unnecessary copies to reduce memory usage (#11)

tiktoken-node goes from 194mb -> 140mb as shown by `heap -s
<nodejs_pid>` on macOS

src/corebpe.rs

@@ -162,11 +162,11 @@ fn hash_current_thread() -> usize {
     u64::from(x) as usize
 }
 
-const MAX_NUM_THREADS: usize = 128;
+const MAX_NUM_THREADS: usize = 8;
 
 #[derive(Debug)]
 pub struct CoreBPE {
-    encoder: HashMap<Vec<u8>, usize>,
+    encoder: Arc<HashMap<Vec<u8>, usize>>,
     special_tokens_encoder: HashMap<String, usize>,
     decoder: HashMap<usize, Vec<u8>>,
     special_tokens_decoder: HashMap<usize, Vec<u8>>,
@@ -429,7 +429,7 @@ impl CoreBPE {
 
 impl CoreBPE {
     pub fn new(
-        encoder: HashMap<Vec<u8>, usize>,
+        encoder: Arc<HashMap<Vec<u8>, usize>>,
         special_tokens_encoder: HashMap<String, usize>,
         pattern: &str,
     ) -> Result<Self, fancy_regex::Error> {

src/encoding.rs

@@ -14,7 +14,7 @@ pub struct Encoding {
     /// The regular expression pattern used to split text into pieces.
     pat_str: String,
     /// The map from mergeable byte sequences to their ranks.
-    mergeable_ranks: HashMap<Vec<u8>, usize>,
+    mergeable_ranks: Arc<HashMap<Vec<u8>, usize>>,
     /// The maximum length of the keys in `mergeable_ranks`.
     mergeable_ranks_max_key_len: usize,
     /// All prefixes of the mergeable ranks. May or may not be tokens themselves!
@@ -64,7 +64,7 @@ impl Encoding {
     pub fn new(
         name: &str,
         pat_str: &str,
-        mergeable_ranks: HashMap<Vec<u8>, usize>,
+        mergeable_ranks: Arc<HashMap<Vec<u8>, usize>>,
         special_tokens: HashMap<String, usize>,
         explicit_n_vocab: Option<usize>,
     ) -> Result<Self, EncodingError> {

src/load.rs

@@ -5,6 +5,7 @@ use sha2::Sha256;
 // call its methods without adding to the namespace.
 use base64::engine::general_purpose::STANDARD as BASE64;
 use base64::engine::Engine as _;
+use std::sync::Arc;
 
 // define the error
 #[derive(Debug, Clone)]
@@ -16,7 +17,7 @@ pub enum Error {
 pub fn load_tiktoken_bpe(
     tiktoken_bpe_contents: &[u8],
     shasum: &str,
-) -> Result<HashMap<Vec<u8>, usize>, Error> {
+) -> Result<Arc<HashMap<Vec<u8>, usize>>, Error> {
     // check the shasum
     let mut hasher = Sha256::new();
     hasher.update(tiktoken_bpe_contents);
@@ -42,5 +43,5 @@ pub fn load_tiktoken_bpe(
         map.insert(token, rank);
     }
     map.shrink_to_fit();
-    Ok(map)
+    Ok(Arc::new(map))
 }