Add const constructor to StarkFelt

- Implement `from<128>` using const logic, so that it can be used as a
  const constructor.
- Also remove `StarkFelt::one()` which in favor of `StarkFelt::from_128(1_u128)`.

Author: giladchase

src/hash.rs

@@ -58,36 +58,27 @@ impl StarkFelt {
     }
 
     /// [StarkFelt] constant that's equal to 0.
-    pub const ZERO: Self = {
-        Self([
-            0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
-            0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
-        ])
-    };
+    pub const ZERO: Self = { Self::from_u128(0_u128) };
 
     /// [StarkFelt] constant that's equal to 1.
-    pub const ONE: Self = {
-        Self([
-            0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
-            0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1,
-        ])
-    };
+    pub const ONE: Self = { Self::from_u128(1_u128) };
 
     /// [StarkFelt] constant that's equal to 2.
-    pub const TWO: Self = {
-        Self([
-            0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
-            0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x2,
-        ])
-    };
+    pub const TWO: Self = { Self::from_u128(2_u128) };
 
     /// [StarkFelt] constant that's equal to 3.
-    pub const THREE: Self = {
-        Self([
-            0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
-            0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x3,
-        ])
-    };
+    pub const THREE: Self = { Self::from_u128(3_u128) };
+
+    pub const fn from_u128(val: u128) -> Self {
+        let mut bytes = [0u8; 32];
+        let val_bytes = val.to_be_bytes();
+        let mut index = 16;
+        while index < 32 {
+            bytes[index] = val_bytes[index - 16];
+            index += 1;
+        }
+        Self(bytes)
+    }
 
     /// Storage efficient serialization for field elements.
     pub fn serialize(&self, res: &mut impl std::io::Write) -> Result<(), Error> {
@@ -179,9 +170,7 @@ impl TryFrom<&str> for StarkFelt {
 
 impl From<u128> for StarkFelt {
     fn from(val: u128) -> Self {
-        let mut bytes = [0u8; 32];
-        bytes[16..32].copy_from_slice(&val.to_be_bytes());
-        Self(bytes)
+        Self::from_u128(val)
     }
 }