add usage

src/main.rs

@@ -1,10 +1,9 @@
 use std::env;
 use std::fs::File;
 use std::path::Path;
+use nbt::{Blob, Value::{self, Compound}, Map};
 
-use nbt::{Blob, Value, Map};
-
-fn read_nbt(path_str: String) -> Option<Blob> {
+fn read_nbt(path_str: &String) -> Option<Blob> {
     let path_to_dat = Path::new(&path_str);
 
     if let Ok(mut file) = File::open(path_to_dat) {
@@ -14,11 +13,24 @@ fn read_nbt(path_str: String) -> Option<Blob> {
     }
 }
 
-fn main() {
-    let nbt_blob = read_nbt(env::args().take(2).nth(1).expect("pls call with .dat")).expect("unable to read nbt");
-
-
-    print_value_map(&nbt_blob.content, &0);
+fn read_key(map: &Map<String,Value>, key: &str) -> Option<Value> {
+    let value = key
+        .split('.')
+        .enumerate()
+        .fold(
+            None, // Option<&V>
+            |last_value, (index, part)| {
+                if index == 0 {
+                    return map.get(part);
+                }
+                return match last_value {
+                    None => None,
+                    Some(Compound(inner)) => inner.get(part),
+                    _ => None
+                };
+            }
+        );
+    value.cloned()
 }
 
 fn print_value_map(map: &Map<String, Value>, level: &usize) {
@@ -36,3 +48,22 @@ fn print_value_map(map: &Map<String, Value>, level: &usize) {
 fn print_formatted_key(key: &str, level: &usize) {
     println!("{space:>count$}{name}", space="", count=*level*4, name=key)
 }
+
+fn main() {
+    let cmd_name = env::args().take(1).next().unwrap_or("nbt_reader".to_string());
+    let usage = format!("Usage: {name} with .dat", name=cmd_name);
+    let file_name = env::args().take(2).nth(1).expect(&usage);
+
+    let nbt_blob = read_nbt(&file_name).unwrap_or_else(|| panic!("no valid nbt found at {path}", path=file_name));
+
+    let key_name = env::args().take(3).nth(2);
+
+    if let Some(nbt_key) = key_name {
+        let value = read_key(&nbt_blob.content, nbt_key.as_str());
+        println!("{}", value.expect("key does not exist"));
+    } else {
+        print_value_map(&nbt_blob.content, &0);
+    }
+
+}
+