bam/async/io/reader/header: Add magic number reader

noodles-bam/src/async/io/reader.rs

@@ -10,9 +10,14 @@ use noodles_bgzf as bgzf;
 use noodles_core::Region;
 use noodles_csi::BinningIndex;
 use noodles_sam::{self as sam, alignment::RecordBuf};
-use tokio::io::{self, AsyncRead, AsyncReadExt, AsyncSeek};
+use tokio::io::{self, AsyncRead, AsyncSeek};
 
-use self::{header::read_header, query::query, record::read_record, record_buf::read_record_buf};
+use self::{
+    header::{magic_number::read_magic_number, read_header},
+    query::query,
+    record::read_record,
+    record_buf::read_record_buf,
+};
 use crate::{io::reader::resolve_region, Record, MAGIC_NUMBER};
 
 /// An async BAM reader.
@@ -427,8 +432,7 @@ async fn read_magic<R>(reader: &mut R) -> io::Result<()>
 where
     R: AsyncRead + Unpin,
 {
-    let mut magic = [0; 4];
-    reader.read_exact(&mut magic).await?;
+    let magic = read_magic_number(reader).await?;
 
     if magic == MAGIC_NUMBER {
         Ok(())

noodles-bam/src/async/io/reader/header.rs

@@ -1,3 +1,4 @@
+pub(super) mod magic_number;
 mod reference_sequences;
 
 use noodles_sam as sam;

noodles-bam/src/async/io/reader/header/magic_number.rs

@@ -0,0 +1,32 @@
+use tokio::io::{self, AsyncRead, AsyncReadExt};
+
+use crate::MAGIC_NUMBER;
+
+pub(crate) async fn read_magic_number<R>(reader: &mut R) -> io::Result<[u8; MAGIC_NUMBER.len()]>
+where
+    R: AsyncRead + Unpin,
+{
+    let mut buf = [0; MAGIC_NUMBER.len()];
+    reader.read_exact(&mut buf).await?;
+    Ok(buf)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn test_read_magic_number() -> io::Result<()> {
+        let src = b"BAM\x01";
+        let mut reader = &src[..];
+        assert_eq!(read_magic_number(&mut reader).await?, *b"BAM\x01");
+
+        let mut src = &[][..];
+        assert!(matches!(
+            read_magic_number(&mut src).await,
+            Err(e) if e.kind() == io::ErrorKind::UnexpectedEof
+        ));
+
+        Ok(())
+    }
+}