From 8ec337c58930f0e1909341e6320fab2e7b03f5eb Mon Sep 17 00:00:00 2001
From: Roland Conybeare <rconybeare@gmail.com>
Date: Wed, 24 Apr 2024 11:31:10 -0400
Subject: [PATCH] xo-unit: + Quantity.rescale()

---
 include/xo/unit/Quantity.hpp | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/include/xo/unit/Quantity.hpp b/include/xo/unit/Quantity.hpp
index 0a4fc8b6..a70fcdee 100644
--- a/include/xo/unit/Quantity.hpp
+++ b/include/xo/unit/Quantity.hpp
@@ -45,6 +45,21 @@ namespace xo {
             constexpr Quantity unit_qty() const { return Quantity(1, unit_); }
             constexpr Quantity reciprocal() const { return Quantity(1.0 / scale_, unit_.reciprocal()); }
 
+            constexpr
+            auto rescale(const natural_unit<Int> & unit2) const {
+                /* conversion factor from .unit -> unit2*/
+                auto rr = detail::nu_ratio(this->unit_, unit2);
+
+                if (rr.natural_unit_.is_dimensionless()) {
+                    repr_type r_scale = (::sqrt(rr.outer_scale_sq_)
+                                         * rr.outer_scale_exact_.template to<repr_type>()
+                                         * this->scale_);
+                    return Quantity(r_scale, unit2);
+                } else {
+                    return Quantity(std::numeric_limits<repr_type>::quiet_NaN(), unit2);
+                }
+            }
+
             template <typename Quantity2>
             static constexpr
             auto multiply(const Quantity & x, const Quantity2 & y) {