implement format guessing

data/common_guess_test/data/misc/example.txt

@@ -0,0 +1 @@
+hello world!

src/guess.rs

@@ -0,0 +1,55 @@
+use crate::cc;
+use core::mem;
+use std::io::Read;
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum FileFormat {
+    TES3,
+    TES4,
+    FO4,
+}
+
+const BSA: u32 = cc::make_four(b"BSA");
+const BTDX: u32 = cc::make_four(b"BTDX");
+
+#[allow(clippy::module_name_repetitions)]
+pub fn guess_format<In>(source: &mut In) -> Option<FileFormat>
+where
+    In: ?Sized + Read,
+{
+    let mut buf = [0u8; mem::size_of::<u32>()];
+    source.read_exact(&mut buf).ok()?;
+    let magic = u32::from_le_bytes(buf);
+    match magic {
+        0x100 => Some(FileFormat::TES3),
+        BSA => Some(FileFormat::TES4),
+        BTDX => Some(FileFormat::FO4),
+        _ => None,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::FileFormat;
+    use anyhow::Context as _;
+    use std::{fs::File, path::Path};
+
+    #[test]
+    fn guess() -> anyhow::Result<()> {
+        let root = Path::new("data/common_guess_test");
+        let tests = [
+            (FileFormat::TES3, "tes3.bsa"),
+            (FileFormat::TES4, "tes4.bsa"),
+            (FileFormat::FO4, "fo4.ba2"),
+        ];
+
+        for (format, file_name) in tests {
+            let mut file = File::open(root.join(file_name))
+                .with_context(|| format!("failed to open file: {file_name}"))?;
+            let guess = crate::guess_format(&mut file);
+            assert_eq!(guess, Some(format));
+        }
+
+        Ok(())
+    }
+}

src/lib.rs

@@ -14,12 +14,15 @@ mod cc;
 mod containers;
 mod derive;
 pub mod fo4;
+mod guess;
 mod hashing;
 mod io;
 mod protocols;
 pub mod tes3;
 pub mod tes4;
 
+pub use guess::{guess_format, FileFormat};
+
 pub struct Borrowed<'borrow>(pub &'borrow [u8]);
 
 pub struct Copied<'copy>(pub &'copy [u8]);