Add Rust example (#614)

examples/signature.rs

@@ -0,0 +1,75 @@
+use std::process;
+
+use base64; // https://crates.io/crates/base64
+use hex::{self, FromHexError}; // https://crates.io/crates/hex
+use hmac::{Hmac, Mac, NewMac}; // https://crates.io/crates/hmac
+use sha2::Sha256; // https://crates.io/crates/sha2
+
+type HmacSha256 = Hmac<Sha256>;
+
+const KEY: &'static str = "943b421c9eb07c830af81030552c86009268de4e532ba2ee2eab8247c6da0881";
+const SALT: &'static str = "520f986b998545b4785e0defbc4f3c1203f22de2374a3d53cb7a7fe9fea309c5";
+
+pub struct Image {
+    pub src: &'static str,
+    pub width: usize,
+    pub height: usize,
+    pub dpr: u8,
+    pub ext: &'static str,
+}
+
+#[derive(Debug)]
+pub enum Error {
+    InvalidKey(FromHexError),
+    InvalidSalt(FromHexError),
+}
+
+fn main() {
+    let img = Image {
+        src: "http://img.example.com/pretty/image.jpg",
+        width: 100,
+        height: 80,
+        dpr: 2,
+        ext: "webp",
+    };
+    match sign_url(img) {
+        Ok(url) => {
+            println!("{}", url);
+            process::exit(0);
+        }
+        Err(err) => {
+            eprintln!("{:#?}", err);
+            process::exit(1);
+        }
+    }
+}
+
+pub fn sign_url(img: Image) -> Result<String, Error> {
+    let url = format!(
+        "/rs:{resize_type}:{width}:{height}:{enlarge}:{extend}/dpr:{dpr}/{src}.{ext}",
+        resize_type = "auto",
+        width = img.width,
+        height = img.height,
+        enlarge = 0,
+        extend = 0,
+        dpr = img.dpr,
+        src = base64::encode_config(img.src.as_bytes(), base64::URL_SAFE_NO_PAD),
+        ext = img.ext,
+    );
+    let decoded_key = match hex::decode(KEY) {
+        Ok(key) => key,
+        Err(err) => return Err(Error::InvalidKey(err)),
+    };
+    let decoded_salt = match hex::decode(SALT) {
+        Ok(salt) => salt,
+        Err(err) => return Err(Error::InvalidSalt(err)),
+    };
+    let mut hmac = HmacSha256::new_varkey(&decoded_key).unwrap();
+    hmac.update(&decoded_salt);
+    hmac.update(url.as_bytes());
+    let signature = hmac.finalize().into_bytes();
+    let signature = base64::encode_config(signature.as_slice(), base64::URL_SAFE_NO_PAD);
+    let signed_url = format!("/{signature}{url}", signature = signature, url = url);
+
+    Ok(signed_url)
+}