feat: change difficulty from byte count to bits

pkgs/solver-wasm/src/lib.rs

@@ -7,16 +7,37 @@ use sha2::{Digest, Sha256};
 use wasm_bindgen::prelude::*;
 
 #[wasm_bindgen]
-pub fn solve(nonce: &[u8], target: &[u8]) -> Box<[u8]> {
+pub fn solve(nonce: &[u8], target: &[u8], difficulty_bits: u32) -> Box<[u8]> {
     let mut buf = vec![0u8; 8 + nonce.len()];
     buf[8..].copy_from_slice(nonce);
 
+    let target_whole_bytes = &target[0..(difficulty_bits as usize / 8)];
+
+    let target_rest = {
+        let rest_bits = difficulty_bits % 8;
+        match rest_bits {
+            0 => None,
+            _ => {
+                let mask = 0xffu8.unbounded_shl(8 - rest_bits);
+                let rest = target[target_whole_bytes.len()] & mask;
+                Some((mask, rest))
+            }
+        }
+    };
+
     for i in 0u64.. {
         let i_bytes = u64::to_le_bytes(i);
         buf[0..=7].copy_from_slice(&i_bytes);
         let hash = Sha256::digest(&buf);
-        if &hash[0..target.len()] == target {
-            return i_bytes.into();
+
+        if &hash[0..target_whole_bytes.len()] == target_whole_bytes {
+            let target_rest_ok = match target_rest {
+                None => true,
+                Some((mask, rest)) => (hash[target_whole_bytes.len()] & mask) == rest,
+            };
+            if target_rest_ok {
+                return i_bytes.into();
+            }
         }
     }
 
@@ -28,7 +49,7 @@ mod tests {
     #[test]
     fn solve() {
         assert_eq!(
-            super::solve(&[1, 2], &[3, 4]).as_ref(),
+            super::solve(&[1, 2], &[3, 4, 5], 18).as_ref(),
             [45, 176, 0, 0, 0, 0, 0, 0]
         );
     }