nostr: fix support for NIP-44 on no_std env

The `f64::log2` function is available only in the `std` library, so replace it to be compatible with no_std env.

Pull-Request: #955
Signed-off-by: Yuki Kishimoto <[email protected]>

Author: yukibtc

CHANGELOG.md

@@ -71,6 +71,7 @@
 
 - keyring: fix keys persistence between OS restarts on Linux (https://github.com/rust-nostr/nostr/pull/942)
 - nostr: fix index out of bounds in `Tags::dedup` (https://github.com/rust-nostr/nostr/pull/949)
+- nostr: fix support for NIP-44 on no_std env (https://github.com/rust-nostr/nostr/pull/955)
 
 ### Removed
 

crates/nostr/src/nips/nip44/v2.rs

@@ -339,8 +339,8 @@ fn log2_round_down(x: usize) -> u32 {
     if x == 0 {
         0
     } else {
-        let x: f64 = x as f64;
-        x.log2().floor() as u32
+        // This is equivalent to floor(log2(x))
+        (usize::BITS - 1) - x.leading_zeros()
     }
 }
 
@@ -390,6 +390,27 @@ mod tests {
         bytes
     }
 
+    // Check if out manual implementation work in the same way as the std one.
+    #[test]
+    fn test_log2_round_down() {
+        let f = |x: usize| -> u32 {
+            let x: f64 = x as f64;
+            x.log2().floor() as u32
+        };
+
+        assert_eq!(log2_round_down(0), f(0));
+        assert_eq!(log2_round_down(1), f(1));
+        assert_eq!(log2_round_down(2), f(2));
+        assert_eq!(log2_round_down(3), f(3));
+        assert_eq!(log2_round_down(4), f(4));
+        assert_eq!(log2_round_down(5), f(5));
+        assert_eq!(log2_round_down(6), f(6));
+        assert_eq!(log2_round_down(7), f(7));
+        assert_eq!(log2_round_down(8), f(8));
+        assert_eq!(log2_round_down(9), f(9));
+        assert_eq!(log2_round_down(10), f(10));
+    }
+
     #[test]
     fn test_valid_get_conversation_key() {
         let json: serde_json::Value = serde_json::from_str(JSON_VECTORS).unwrap();