From d562d0ea60808d58fccc5584241554132492ec76 Mon Sep 17 00:00:00 2001
From: Wanda <wanda@phinode.net>
Date: Fri, 16 Feb 2024 12:29:31 +0100
Subject: [PATCH] utils: backport `ceil_log2` and `exact_log2`.

---
 amaranth/utils.py | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/amaranth/utils.py b/amaranth/utils.py
index cf98f3d1c..4f25af68b 100644
--- a/amaranth/utils.py
+++ b/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: