Use a BufWriter for output instead of creating an intermediary string

This should decrease the number of copies.

src/runner.rs

@@ -7,7 +7,7 @@ use color_eyre::{eyre::eyre, Result};
 use std::convert::TryFrom;
 use std::process::ExitStatus;
 use std::process::Stdio;
-use tokio::io::{self, AsyncBufReadExt, AsyncWriteExt, BufReader};
+use tokio::io::{self, AsyncBufReadExt, AsyncWriteExt, BufReader, BufWriter};
 use tokio::process::Command;
 use tokio_process_stream as tps;
 use tokio_stream::wrappers::LinesStream;
@@ -25,40 +25,48 @@ pub fn buildcmd(cli: &Cli) -> Command {
     cmd
 }
 
-pub fn decorate(prefix: &str, date: bool, line: &str) -> String {
-    let mut string = String::new();
+pub async fn decor_write<T>(
+    prefix: &str,
+    date: bool,
+    line: &str,
+    output: &mut io::BufWriter<T>,
+) -> Result<()>
+where
+    T: AsyncWriteExt + std::marker::Unpin,
+{
     if date {
         let now = chrono::offset::Local::now();
-        string.push_str(&now.format("%Y-%m-%d %H:%M:%S%.6f ").to_string());
+        let date = now.format("%Y-%m-%d %H:%M:%S%.6f ").to_string();
+        output.write_all(date.as_bytes()).await?;
     }
-    string.push_str(prefix);
-    string.push(' ');
-    string.push_str(line);
-    string.push('\n');
-    string
+    output.write_all(prefix.as_bytes()).await?;
+    output.write_all(&[b' ']).await?;
+    output.write_all(line.as_bytes()).await?;
+    output.write_all(&[b'\n']).await?;
+    Ok(())
 }
 
-pub async fn do_write<T>(mut fd: T, prefix: &str, date: bool, line: &str) -> Result<()>
-where
-    T: AsyncWriteExt + std::marker::Unpin,
-{
-    let string = decorate(prefix, date, line);
-    fd.write_all(string.as_bytes()).await.map_err(|e| eyre!(e))
+pub async fn decor_str(prefix: &str, date: bool, line: &str) -> Result<String> {
+    let mut output = Vec::<u8>::new();
+    let mut o = io::BufWriter::new(&mut output);
+    decor_write(prefix, date, line, &mut o).await?;
+    o.flush().await?;
+    Ok(std::str::from_utf8(&output)?.to_owned())
 }
 
 #[tracing::instrument]
 pub async fn run(cli: &Cli) -> Result<ExitStatus> {
     let cmd = buildcmd(cli);
     let mut stream = tps::ProcessStream::try_from(cmd)?;
-    let mut stdout = io::stdout();
-    let mut stderr = io::stderr();
+    let mut stdout = BufWriter::new(io::stdout());
+    let mut stderr = BufWriter::new(io::stderr());
     while let Some(item) = stream.next().await {
         match item {
             tps::Item::Stdout(line) => {
-                do_write(&mut stdout, &cli.prefix, cli.date, &line).await?;
+                decor_write(&cli.prefix, cli.date, &line, &mut stdout).await?;
             }
             tps::Item::Stderr(line) => {
-                do_write(&mut stderr, &cli.prefix, cli.date, &line).await?;
+                decor_write(&cli.prefix, cli.date, &line, &mut stderr).await?;
             }
             tps::Item::Done(s) => {
                 return Ok(s?);
@@ -71,9 +79,9 @@ pub async fn run(cli: &Cli) -> Result<ExitStatus> {
 #[tracing::instrument]
 pub async fn pipe(cli: &Cli) -> Result<()> {
     let mut stdin_lines = LinesStream::new(BufReader::new(io::stdin()).lines());
-    let mut stdout = io::stdout();
+    let mut stdout = BufWriter::new(io::stdout());
     while let Some(line) = stdin_lines.next().await {
-        do_write(&mut stdout, &cli.prefix, cli.date, &line?).await?;
+        decor_write(&cli.prefix, cli.date, &line?, &mut stdout).await?;
     }
     Ok(())
 }

tests/stdecor_tests.rs

@@ -2,9 +2,15 @@
 // This file is subject to the terms and conditions defined in
 // file 'LICENSE', which is part of this source code package.
 
+use color_eyre::Result;
+
 use stdecor::*;
 
-#[test]
-fn test_decorate() {
-    assert_eq!(&runner::decorate("1234", false, "abcd"), "1234 abcd\n");
+#[tokio::test]
+async fn test_decorate() -> Result<()> {
+    assert_eq!(
+        &runner::decor_str("1234", false, "abcd").await?,
+        "1234 abcd\n"
+    );
+    Ok(())
 }