From a7e90fb93dce0eaae02f22c01fef34f11377bb60 Mon Sep 17 00:00:00 2001
From: Lars Wirzenius <liw@sequoia-pgp.org>
Date: Thu, 6 Oct 2022 20:32:26 +0300
Subject: set up hands-on practice

Sponsored-by: author
---
 Cargo.toml  |  1 +
 README.md   | 12 ++++++++++++
 src/lib.rs  | 44 ++++++++++++++++++++++++++++++++++++++++++++
 src/main.rs |  8 +++++++-
 4 files changed, 64 insertions(+), 1 deletion(-)
 create mode 100644 README.md
 create mode 100644 src/lib.rs

diff --git a/Cargo.toml b/Cargo.toml
index 3b67d9a..19abfe0 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -6,3 +6,4 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+sha2 = "0.10.2"
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..35c0b07
--- /dev/null
+++ b/README.md
@@ -0,0 +1,12 @@
+Your mission, should you choose to accept it, is to edit `src/main.rs`
+so that it works like this:
+
+~~~sh
+cargo run -- Cargo.toml README.md
+~~~
+
+The output should be identical to the output of the following command:
+
+~~~sh
+sha256sum Cargo.toml README.md
+~~~
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..93627f1
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,44 @@
+use sha2::{Digest, Sha256};
+use std::fs::File;
+use std::io::{BufReader, Read};
+use std::path::{Path, PathBuf};
+
+const BUF_SIZE: usize = 4096;
+
+#[derive(Debug)]
+pub struct FileChecksum {
+    pub filename: PathBuf,
+    pub checksum: String,
+}
+
+impl FileChecksum {
+    pub fn new(filename: &Path, checksum: String) -> Self {
+        Self {
+            filename: filename.to_path_buf(),
+            checksum,
+        }
+    }
+
+    pub fn print(&self) {
+        println!("{} {}", self.checksum, self.filename.display());
+    }
+}
+
+pub fn checksum(filename: &Path) -> Result<FileChecksum, std::io::Error> {
+    let mut checksum = Sha256::new();
+
+    let file = File::open(filename)?;
+    let mut reader = BufReader::new(file);
+    loop {
+        let mut buf = vec![0; BUF_SIZE];
+        let n = reader.read(&mut buf)?;
+        if n == 0 {
+            break;
+        }
+        checksum.update(&buf[..n]);
+    }
+
+    let checksum = checksum.finalize();
+    let checksum = format!("{checksum:x}");
+    Ok(FileChecksum::new(filename, checksum))
+}
diff --git a/src/main.rs b/src/main.rs
index e7a11a9..155c9e9 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,9 @@
+mod lib;
+use lib::checksum;
+use std::path::Path;
+
 fn main() {
-    println!("Hello, world!");
+    let sum = checksum(Path::new("Cargo.toml")).unwrap();
+    sum.print();
 }
+
-- 
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