perf(compress): remove Vec allocation in calculate_dynamic_header_size (#418)

Author: 404Setup

Cargo.lock

@@ -0,0 +1,33 @@
+--- src/compress/mod.rs
++++ src/compress/mod.rs
+@@ -1144,14 +1144,15 @@
+             num_offset_syms -= 1;
+         }
+
+-        let mut lens = Vec::with_capacity(num_litlen_syms + num_offset_syms);
+-        lens.extend_from_slice(&self.litlen_lens[..num_litlen_syms]);
+-        lens.extend_from_slice(&self.offset_lens[..num_offset_syms]);
++        let mut lens = [0u8; DEFLATE_NUM_LITLEN_SYMS + DEFLATE_NUM_OFFSET_SYMS];
++        let lens_len = num_litlen_syms + num_offset_syms;
++        lens[..num_litlen_syms].copy_from_slice(&self.litlen_lens[..num_litlen_syms]);
++        lens[num_litlen_syms..lens_len].copy_from_slice(&self.offset_lens[..num_offset_syms]);
+
+         let mut precode_freqs = [0u32; 19];
+         let mut i = 0;
+-        while i < lens.len() {
++        while i < lens_len {
+             let len = lens[i];
+             let mut run = 1;
+-            while i + run < lens.len() && lens[i + run] == len {
++            while i + run < lens_len && lens[i + run] == len {
+                 run += 1;
+             }
+@@ -1172,7 +1173,7 @@
+                 precode_freqs[len as usize] += 1;
+                 run -= 1;
+             }
+-            i += lens[i..].iter().take_while(|&&l| l == len).count();
++            i += lens[i..lens_len].iter().take_while(|&&l| l == len).count();
+         }
+
+         let mut precode_lens = [0u8; 19];

patch.diff

@@ -1144,16 +1144,17 @@ impl Compressor {
             num_offset_syms -= 1;
         }
 
-        let mut lens = Vec::with_capacity(num_litlen_syms + num_offset_syms);
-        lens.extend_from_slice(&self.litlen_lens[..num_litlen_syms]);
-        lens.extend_from_slice(&self.offset_lens[..num_offset_syms]);
+        let mut lens = [0u8; DEFLATE_NUM_LITLEN_SYMS + DEFLATE_NUM_OFFSET_SYMS];
+        let lens_len = num_litlen_syms + num_offset_syms;
+        lens[..num_litlen_syms].copy_from_slice(&self.litlen_lens[..num_litlen_syms]);
+        lens[num_litlen_syms..lens_len].copy_from_slice(&self.offset_lens[..num_offset_syms]);
 
         let mut precode_freqs = [0u32; 19];
         let mut i = 0;
-        while i < lens.len() {
+        while i < lens_len {
             let len = lens[i];
             let mut run = 1;
-            while i + run < lens.len() && lens[i + run] == len {
+            while i + run < lens_len && lens[i + run] == len {
                 run += 1;
             }
             if len == 0 {