`FastFourierTransform::Log2` is recursive

[gabrielfrasantos]

Severity: low
Domain: analysis
Status: VERIFIED — read against a210d34 on 2026-08-10
Suggested labels: policy-violation, analysis

Summary

Log2() calls itself, violating the no-recursion rule in AGENTS.md. It is also a
non-static member function, so it cannot be used in a constant expression without an object.

Location

numerical/analysis/FastFourierTransform.hpp

Evidence

constexpr std::size_t Log2(std::size_t n)
{
    return (n <= 1) ? 0 : 1 + Log2(n >> 1);
}

Why it matters

Deterministic stack usage is a hard requirement for the target platforms. Although the depth here is
bounded by log₂(SIZE_MAX) and the compiler will usually fold it at -O3, the rule exists so that
stack bounds are provable without relying on optimiser behaviour.

Suggested fix

Iterative form, and make it static:

static constexpr std::size_t Log2(std::size_t n) noexcept
{
    std::size_t result{ 0 };
    while (n > 1)
    {
        n >>= 1;
        ++result;
    }
    return result;
}

Or use std::bit_width(n) - 1 from <bit> (C++20) for power-of-two inputs.

Notes

Consider a repository-wide grep for self-recursive functions as part of the same change.