From 519ee688760c1bc7d76e04acc6239c3b667f272a Mon Sep 17 00:00:00 2001
From: Florian Albrechtskirchinger <falbrechtskirchinger@gmail.com>
Date: Thu, 16 Jun 2022 15:06:31 +0200
Subject: [PATCH] Add _glfw_ffs returning 1-based index of lsb set to one

---
 src/init.c     | 35 +++++++++++++++++++++++++++++++++++
 src/internal.h |  1 +
 2 files changed, 36 insertions(+)

diff --git a/src/init.c b/src/init.c
index d07a492e..d84c13f6 100644
--- a/src/init.c
+++ b/src/init.c
@@ -266,6 +266,41 @@ float _glfw_fmaxf(float a, float b)
         return b;
 }
 
+int _glfw_ffs(int i)
+{
+#if defined(__GNUC__) || defined(__clang__)
+    return __builtin_ffs(i);
+#elif defined(_MSC_VER)
+    unsigned long n, u = i;
+    return _BitScanForward(&n, u) ? n + 1 : 0;
+#else
+    // https://en.wikipedia.org/wiki/Find_first_set#CTZ
+    unsigned int u = i;
+    int n = 1;
+    if (!u) return 0;
+    if (!(u & 0x0000FFFF)) {
+        n += 16;
+        u >>= 16;
+    }
+    if (!(u & 0x000000FF)) {
+        n += 8;
+        u >>= 8;
+    }
+    if (!(u & 0x0000000F)) {
+        n += 4;
+        u >>= 4;
+    }
+    if (!(u & 0x00000003)) {
+        n += 2;
+        u >>= 2;
+    }
+    if (!(u & 0x00000001)) {
+        n += 1;
+    }
+    return n;
+#endif
+}
+
 void* _glfw_calloc(size_t count, size_t size)
 {
     if (count && size)
diff --git a/src/internal.h b/src/internal.h
index 5abf6de2..24be9c53 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -1015,6 +1015,7 @@ int _glfw_min(int a, int b);
 int _glfw_max(int a, int b);
 float _glfw_fminf(float a, float b);
 float _glfw_fmaxf(float a, float b);
+int _glfw_ffs(int i);
 
 void* _glfw_calloc(size_t count, size_t size);
 void* _glfw_realloc(void* pointer, size_t size);