Cli: support using truncated hashes

src/main.rs

@@ -1,25 +1,72 @@
 mod object;
 mod cli;
 
+use std::fs::{self, DirEntry};
 use cli::{Cli, Commands};
 use clap::Parser;
 use std::io::Write;
 use hex;
 
 use object::GitObjectStore;
 
+pub const MIN_USER_HASH_LEN: usize = 4;
+
+fn hash_from_str(id_str: &str) -> Option<[u8; 20]> {
+
+    let id_len = id_str.len();
+
+    if id_len < MIN_USER_HASH_LEN || id_len > 40 {
+        eprintln!("Invalid hash length.");
+        return None;
+    }
+
+    let obj_dir = format!(".git/objects/{}/", &id_str[..2]);
+
+    let Ok(contents) = fs::read_dir(obj_dir) else {
+        eprintln!("Invalid hash.");
+        return None;
+    };
+
+    let matches: Vec<DirEntry> = contents
+        .map(|o| o.expect("hash_from_str(): ReadDir"))
+        .filter(|o| o
+                .file_name()
+                .into_string()
+                .expect("hash_from_str(): ReadDir")
+                .starts_with(&id_str[2..]))
+        .collect();
+
+    let matches_len = matches.len();
+
+    if matches_len == 0 {
+        eprintln!("Can't find hash.");
+        return None;
+    } else if matches_len > 1 {
+        eprintln!("Can't disambiguate hash.");
+        return None;
+    }
+
+    let found = &matches[0]
+        .file_name()
+        .into_string()
+        .ok()?;
+
+    let id = format!("{}{}", &id_str[..2], found);
+
+    let id = hex::decode(id).ok()?;
+
+    let id: [u8; 20] = id.try_into().ok()?;
+
+    Some(id)
+}
+
 fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     let cli = Cli::parse();
 
     match cli.command {
         Commands::CatFile(cat_file_args) => {
-            let id = hex::decode(cat_file_args.id)?;
-
-            let id = match id.try_into() {
-                Ok(id) => id,
-                Err(_) => return Err("Invalid id".into())
-            };
+            let id = hash_from_str(&cat_file_args.id).ok_or("Failed.")?;
 
             let obj = match GitObjectStore::get(id) {
                 Some(obj) => obj,