diff --git a/src/knn.c b/src/knn.c
index 5d120fe..83aca1d 100644
--- a/src/knn.c
+++ b/src/knn.c
@@ -11,26 +11,32 @@
 
 #define K 2
 
+static __thread z_stream defstream = {0};
+static __thread bool defstream_init = false;
+
 // Stolen from https://gist.github.com/arq5x/5315739
 Nob_String_View deflate_sv(Arena *arena, Nob_String_View sv)
 {
-    size_t output_size = sv.count*2;
-    void *output = arena_alloc(arena, output_size);
-
-    z_stream defstream = {0};
     defstream.avail_in = (uInt)sv.count;
     defstream.next_in = (Bytef *)sv.data;
+
+    int ret = defstream_init ? deflateReset(&defstream) : deflateInit(&defstream, Z_BEST_COMPRESSION);
+    defstream_init = true;
+    assert(ret == Z_OK && "Failed to initialize zlib deflate stream");
+    size_t output_size = (size_t)deflateBound(&defstream, defstream.avail_in);
+    void *output = arena_alloc(arena, output_size);
     defstream.avail_out = (uInt)output_size;
     defstream.next_out = (Bytef *)output;
 
-    deflateInit(&defstream, Z_BEST_COMPRESSION);
-    int result = deflate(&defstream, Z_FINISH);
-    assert(result == Z_STREAM_END && "Probably not enough output buffer was allocated");
-    deflateEnd(&defstream);
+    assert(deflate(&defstream, Z_FINISH) == Z_STREAM_END && "Probably `avail_in` is zero");
 
     return nob_sv_from_parts(output, defstream.total_out);
 }
 
+void deflate_end() {
+    deflateEnd(&defstream);
+}
+
 typedef struct {
     size_t klass;
     Nob_String_View text;
@@ -115,6 +121,7 @@ void *klassify_thread(void *params)
             .klass = state->train[i].klass,
         }));
     }
+    deflate_end();
 
     return NULL;
 }