From d6d572df66eb2b5beeb3da7f3b11a328500c33c3 Mon Sep 17 00:00:00 2001
From: Chris Robinson <chris.kcat@gmail.com>
Date: Sat, 18 Nov 2023 22:11:58 -0800
Subject: Handle systems that don't support std::cyl_bessel_i

---
 common/polyphase_resampler.cpp | 47 ++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 45 insertions(+), 2 deletions(-)

(limited to 'common')

diff --git a/common/polyphase_resampler.cpp b/common/polyphase_resampler.cpp
index f1e9eaf3..9c569b3a 100644
--- a/common/polyphase_resampler.cpp
+++ b/common/polyphase_resampler.cpp
@@ -4,6 +4,7 @@
 #include <algorithm>
 #include <cmath>
 #include <numeric>
+#include <stdexcept>
 
 #include "alnumbers.h"
 #include "opthelpers.h"
@@ -15,6 +16,48 @@ constexpr double Epsilon{1e-9};
 
 using uint = unsigned int;
 
+#if __cpp_lib_math_special_functions >= 201603L
+using std::cyl_bessel_i;
+
+#else
+
+/* The zero-order modified Bessel function of the first kind, used for the
+ * Kaiser window.
+ *
+ *   I_0(x) = sum_{k=0}^inf (1 / k!)^2 (x / 2)^(2 k)
+ *          = sum_{k=0}^inf ((x / 2)^k / k!)^2
+ *
+ * This implementation only handles nu = 0, and isn't the most precise (it
+ * starts with the largest value and accumulates successively smaller values,
+ * compounding the rounding and precision error), but it's good enough.
+ */
+template<typename T, typename U>
+U cyl_bessel_i(T nu, U x)
+{
+    if(nu != T{0})
+        throw std::runtime_error{"cyl_bessel_i: nu != 0"};
+
+    /* Start at k=1 since k=0 is trivial. */
+    const double x2{x/2.0};
+    double term{1.0};
+    double sum{1.0};
+    int k{1};
+
+    /* Let the integration converge until the term of the sum is no longer
+     * significant.
+     */
+    double last_sum{};
+    do {
+        const double y{x2 / k};
+        ++k;
+        last_sum = sum;
+        term *= y * y;
+        sum += term;
+    } while(sum != last_sum);
+    return static_cast<U>(sum);
+}
+#endif
+
 /* This is the normalized cardinal sine (sinc) function.
  *
  *   sinc(x) = { 1,                   x = 0
@@ -45,7 +88,7 @@ double Kaiser(const double beta, const double k, const double besseli_0_beta)
 {
     if(!(k >= -1.0 && k <= 1.0))
         return 0.0;
-    return std::cyl_bessel_i(0, beta * std::sqrt(1.0 - k*k)) / besseli_0_beta;
+    return cyl_bessel_i(0, beta * std::sqrt(1.0 - k*k)) / besseli_0_beta;
 }
 
 /* Calculates the size (order) of the Kaiser window.  Rejection is in dB and
@@ -122,7 +165,7 @@ void PPhaseResampler::init(const uint srcRate, const uint dstRate)
     // calculating the left offset to avoid increasing the transition width.
     const uint l{(CalcKaiserOrder(180.0, width)+1) / 2};
     const double beta{CalcKaiserBeta(180.0)};
-    const double besseli_0_beta{std::cyl_bessel_i(0, beta)};
+    const double besseli_0_beta{cyl_bessel_i(0, beta)};
     mM = l*2 + 1;
     mL = l;
     mF.resize(mM);
-- 
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