Have is_prefix() take in a level rather than a length

poc/idpf.py

@@ -106,13 +106,12 @@ def current_field(Idpf, level):
             else Idpf.FieldLeaf
 
     @classmethod
-    def is_prefix(Idpf, x: int, y: int, L: int) -> bool:
+    def is_prefix(Idpf, x: int, y: int, level: int) -> bool:
         """
-        Returns `True` iff `x` is the prefix of `y` of length `L`.
+        Returns `True` iff `x` is the prefix of `y` at level `level`.
 
         Pre-conditions:
 
-            - `0 < L` and `L <= Idpf.BITS`
+            - `level` in `range(Idpf.BITS)`
         """
-        assert 0 < L and L <= Idpf.BITS
-        return y >> (Idpf.BITS - L) == x
+        return y >> (Idpf.BITS - 1 - level) == x

poc/tests/idpf.py

@@ -25,7 +25,7 @@ def test_idpf(Idpf, alpha, level, prefixes):
             out[i] = vec_add(out[i], out_share[i])
 
     for (got, prefix) in zip(out, prefixes):
-        if Idpf.is_prefix(prefix, alpha, level+1):
+        if Idpf.is_prefix(prefix, alpha, level):
             if level < Idpf.BITS-1:
                 want = beta_inner[level]
             else:
@@ -68,7 +68,7 @@ def test_idpf_exhaustive(Idpf, alpha):
             got = reduce(lambda x, y: vec_add(x, y),
                          map(lambda x: x[prefix], out_shares))
 
-            if Idpf.is_prefix(prefix, alpha, level+1):
+            if Idpf.is_prefix(prefix, alpha, level):
                 if level < Idpf.BITS-1:
                     want = beta_inner[level]
                 else:

poc/tests/test_idpf_poplar.py

@@ -81,3 +81,12 @@ def shard(s):
                 1, public_share, keys[1], level, (alpha,), binder)
             out = vec_add(out_share_0[0], out_share_1[0])[0]
             self.assertEqual(out.as_unsigned(), 1)
+
+    def test_is_prefix(self):
+        cls = IdpfPoplar.with_value_len(1).with_bits(8)
+        self.assertTrue(cls.is_prefix(0b1, 0b11000001, 0))
+        self.assertTrue(cls.is_prefix(0b11, 0b11000001, 1))
+        self.assertTrue(cls.is_prefix(0b110, 0b11000001, 2))
+        self.assertTrue(cls.is_prefix(0b1100, 0b11000001, 3))
+        self.assertFalse(cls.is_prefix(0b111, 0b11000001, 2))
+        self.assertFalse(cls.is_prefix(0b1101, 0b11000001, 3))