Use ArrayVec instead of const cap Vec

Reduces size in memory by a few bytes and entirely avoids heap
allocation

Cargo.lock

@@ -58,6 +58,12 @@ dependencies = [
  "windows-sys 0.60.2",
 ]
 
+[[package]]
+name = "arrayvec"
+version = "0.7.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7c02d123df017efcdfbd739ef81735b36c5ba83ec3c59c80a9d7ecc718f92e50"
+
 [[package]]
 name = "autocfg"
 version = "1.5.0"
@@ -408,6 +414,7 @@ checksum = "a4895175b425cb1f87721b59f0f286c2092bd4af812243672510e1ac53e2e0ad"
 name = "oxipng"
 version = "10.0.0"
 dependencies = [
+ "arrayvec",
  "bitvec",
  "clap",
  "crossbeam-channel",

Cargo.toml

@@ -45,6 +45,7 @@ libdeflater = "1.25.0"
 log = "0.4.28"
 bitvec = "1.0.1"
 rustc-hash = "2.1.1"
+arrayvec = "0.7.6"
 
 [dependencies.env_logger]
 optional = true

src/apng.rs

@@ -1,4 +1,4 @@
-use std::io::Write;
+use arrayvec::ArrayVec;
 
 use crate::{
     PngResult,
@@ -50,17 +50,33 @@ impl Frame {
 
     /// Construct the data for a fcTL chunk using the given sequence number
     #[must_use]
-    pub fn fctl_data(&self, sequence_number: u32) -> Vec<u8> {
+    pub fn fctl_data(&self, sequence_number: u32) -> ArrayVec<u8, 26> {
-        let mut byte_data = Vec::with_capacity(26);
+        let mut byte_data = const { ArrayVec::<_, 26>::new_const() };
-        byte_data.write_all(&sequence_number.to_be_bytes()).unwrap();
+        byte_data
-        byte_data.write_all(&self.width.to_be_bytes()).unwrap();
+            .try_extend_from_slice(&sequence_number.to_be_bytes())
-        byte_data.write_all(&self.height.to_be_bytes()).unwrap();
+            .unwrap();
-        byte_data.write_all(&self.x_offset.to_be_bytes()).unwrap();
+        byte_data
-        byte_data.write_all(&self.y_offset.to_be_bytes()).unwrap();
+            .try_extend_from_slice(&self.width.to_be_bytes())
-        byte_data.write_all(&self.delay_num.to_be_bytes()).unwrap();
+            .unwrap();
-        byte_data.write_all(&self.delay_den.to_be_bytes()).unwrap();
+        byte_data
-        byte_data.push(self.dispose_op);
+            .try_extend_from_slice(&self.height.to_be_bytes())
-        byte_data.push(self.blend_op);
+            .unwrap();
+        byte_data
+            .try_extend_from_slice(&self.x_offset.to_be_bytes())
+            .unwrap();
+        byte_data
+            .try_extend_from_slice(&self.y_offset.to_be_bytes())
+            .unwrap();
+        byte_data
+            .try_extend_from_slice(&self.delay_num.to_be_bytes())
+            .unwrap();
+        byte_data
+            .try_extend_from_slice(&self.delay_den.to_be_bytes())
+            .unwrap();
+        byte_data
+            .try_extend_from_slice(&[self.dispose_op, self.blend_op])
+            .unwrap();
+
         byte_data
     }
 

src/png/mod.rs

@@ -1,5 +1,6 @@
-use std::{fs, io::Write, path::Path, sync::Arc};
+use std::{fs, path::Path, sync::Arc};
 
+use arrayvec::ArrayVec;
 use bitvec::bitarr;
 use libdeflater::{CompressionLvl, Compressor};
 use log::warn;
@@ -164,18 +165,22 @@ impl PngData {
         // PNG header
         let mut output = vec![0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A];
         // IHDR
-        let mut ihdr_data = Vec::with_capacity(13);
+        let mut ihdr_data = const { ArrayVec::<_, 13>::new_const() };
-        ihdr_data.write_all(&self.raw.ihdr.width.to_be_bytes()).ok();
         ihdr_data
-            .write_all(&self.raw.ihdr.height.to_be_bytes())
+            .try_extend_from_slice(&self.raw.ihdr.width.to_be_bytes())
-            .ok();
+            .unwrap();
-        ihdr_data.write_all(&[self.raw.ihdr.bit_depth as u8]).ok();
         ihdr_data
-            .write_all(&[self.raw.ihdr.color_type.png_header_code()])
+            .try_extend_from_slice(&self.raw.ihdr.height.to_be_bytes())
-            .ok();
+            .unwrap();
-        ihdr_data.write_all(&[0]).ok(); // Compression -- deflate
+        ihdr_data
-        ihdr_data.write_all(&[0]).ok(); // Filter method -- 5-way adaptive filtering
+            .try_extend_from_slice(&[
-        ihdr_data.write_all(&[self.raw.ihdr.interlaced as u8]).ok();
+                self.raw.ihdr.bit_depth as u8,
+                self.raw.ihdr.color_type.png_header_code(),
+                0, // Compression -- deflate
+                0, // Filter method -- 5-way adaptive filtering
+                self.raw.ihdr.interlaced as u8,
+            ])
+            .unwrap();
         write_png_block(b"IHDR", &ihdr_data, &mut output);
         // Ancillary chunks - split into those that come before IDAT and those that come after
         let mut aux_split = self.aux_chunks.split(|c| &c.name == b"IDAT");