moved low-level code into fat-bits subcrate

fat-bits/src/fs_info.rs

@@ -0,0 +1,43 @@
+use crate::utils::load_u32_le;
+
+#[allow(dead_code)]
+pub struct FsInfo {
+    free_count: u32,
+    next_free: u32,
+}
+
+impl FsInfo {
+    pub fn load(bytes: &[u8]) -> anyhow::Result<FsInfo> {
+        let lead_sig = load_u32_le(&bytes[..4]);
+
+        anyhow::ensure!(
+            lead_sig == 0x41615252,
+            "invalid lead signature: 0x{:#08X} instead of 0x41615252",
+            lead_sig
+        );
+
+        let struct_sig = load_u32_le(&bytes[484..][..4]);
+
+        anyhow::ensure!(
+            struct_sig == 0x61417272,
+            "invalid structural signature: 0x{:#08X} instead of 0x61417272",
+            struct_sig
+        );
+
+        let trail_sig = load_u32_le(&bytes[508..][..4]);
+
+        anyhow::ensure!(
+            trail_sig == 0xAA550000,
+            "invalid trailing signature: 0x{:#08X} instead of 0xAA550000",
+            trail_sig
+        );
+
+        let free_count = load_u32_le(&bytes[488..][..4]);
+        let next_free = load_u32_le(&bytes[492..][..4]);
+
+        Ok(FsInfo {
+            free_count,
+            next_free,
+        })
+    }
+}