Add detection for zstd compression. Reader must implement Read + Seek

.gitignore

@@ -1,3 +1,4 @@
 /target
 test/
 !test/li.txt
+!test/li.txt.zst

README.md

@@ -38,4 +38,4 @@ to see Haggis implemented in other languages.
 - [x] List archive nodes
 - [x] Override user/group when creating archives
 - [x] Override user/group when extracting archives
-- [ ] Automatically detect zstd compressed archives
+- [x] Automatically detect zstd compressed archives

src/lib.rs

@@ -3,7 +3,7 @@
 use std::{
     collections::HashMap,
     fs,
-    io::{BufWriter, Write},
+    io::{BufWriter, Read, Seek, Write},
     sync::Mutex,
 };
 #[cfg(feature = "parallel")]
@@ -41,6 +41,18 @@ pub use stream::Message as StreamMessage;
 
 pub static MAGIC: [u8; 7] = [0x89, b'h', b'a', b'g', b'g', b'i', b's'];
 
+static ZSTD_MAGIC: [u8; 4] = [0x28, 0xb5, 0x2f, 0xfd];
+
+/// Tries to detect if the bytes in *reader* are zstd compressed
+/// # Errors
+/// Returns `Error` if io fails
+pub fn detect_zstd<R: Read + Seek>(reader: &mut R) -> Result<bool, Error> {
+    let mut buf = [0; 4];
+    reader.read_exact(&mut buf)?;
+    reader.rewind()?;
+    Ok(buf == ZSTD_MAGIC)
+}
+
 /// Creates a haggis archive from a list of files
 /// # Errors
 /// Returns `crate::Error` if io fails or several other error conditions
@@ -176,3 +188,14 @@ pub fn par_stream_archive<W: Write + Send>(
     sender.send(Message::Eof).map_err(|_| Error::SenderError)?;
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn check_zstd() {
+        let mut fd = fs::File::open("test/li.txt.zst").unwrap();
+        assert!(detect_zstd(&mut fd).unwrap());
+    }
+}