half-I/O cat

src/main.rs

@@ -1,5 +1,8 @@
 use anyhow::{Context as _, Result, bail};
-use std::{fs, io::{Read as _, Write as _}};
+use std::{
+    fs,
+    io::{Read as _, Write as _},
+};
 
 fn main() -> Result<()> {
     let app = App::try_new()?;
@@ -34,7 +37,7 @@ impl App {
             return self.ls(".");
         }
         for filename in &self.filenames {
-            let metadata = fs::metadata(&filename)?;
+            let metadata = fs::metadata(filename)?;
             if metadata.is_dir() {
                 self.ls(filename)?;
             } else {
@@ -45,16 +48,11 @@ impl App {
     }
 
     fn cat(&self, filename: &str) -> Result<()> {
-        let mut f = fs::File::open(filename)?;
-        let mut buf = vec![0u8; 4_096];
-        loop {
-            let bytes_read = f.read(&mut buf)?;
-            let buf = &buf[0..bytes_read];
-            if bytes_read == 0 {
-                break Ok(());
-            }
+        let mut cat = Cat::try_new(filename)?;
+        while let Some(buf) = cat.poll()? {
             std::io::stdout().write_all(buf)?;
         }
+        Ok(())
     }
 
     fn ls(&self, filename: &str) -> Result<()> {
@@ -72,6 +70,28 @@ impl App {
     }
 }
 
+struct Cat {
+    f: fs::File,
+    buf: Vec<u8>,
+}
+
+impl Cat {
+    fn try_new(filename: &str) -> Result<Self> {
+        let f = fs::File::open(filename)?;
+        let buf = vec![0u8; 4_096];
+        Ok(Self { f, buf })
+    }
+
+    fn poll(&mut self) -> Result<Option<&[u8]>> {
+        let bytes_read = self.f.read(&mut self.buf)?;
+        let buf = &self.buf[0..bytes_read];
+        if bytes_read == 0 {
+            return Ok(None);
+        }
+        Ok(Some(buf))
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -80,7 +100,6 @@ mod tests {
     fn test() -> Result<()> {
         let dir = tempfile::tempdir()?;
 
-
         Ok(())
     }
 }