utils: backport ceil_log2 and exact_log2.

Author: wanda-phi

amaranth/utils.py

@@ -1,5 +1,28 @@
-__all__ = ["log2_int", "bits_for"]
+import operator
 
+__all__ = ["ceil_log2", "exact_log2", "log2_int", "bits_for"]
+
+
+def ceil_log2(n):
+    """Returns the integer log2 of the smallest power-of-2 greater than or equal to `n`.
+
+    Raises a `ValueError` for negative inputs."""
+    n = operator.index(n)
+    if n < 0:
+        raise ValueError("{n} is negative")
+    if n == 0:
+        return 0
+    return (n - 1).bit_length()
+
+
+def exact_log2(n):
+    """Returns the integer log2 of `n`, which must be an exact power of two.
+
+    Raises a `ValueError` if `n` is not a power of two."""
+    n = operator.index(n)
+    if n <= 0 or (n & (n - 1)):
+        raise ValueError("{n} is not a power of 2")
+    return (n - 1).bit_length()
 
 def log2_int(n, need_pow2=True):
     if n == 0: