From 930e6752d9f60b2c90e1a729e31bf5ff876a7001 Mon Sep 17 00:00:00 2001
From: bubnikv <bubnikv@gmail.com>
Date: Fri, 3 Mar 2017 12:54:00 +0100
Subject: [PATCH] Reverted unification of positive and negative zeros when
 loaded from an STL file.

---
 xs/src/admesh/stlinit.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/xs/src/admesh/stlinit.c b/xs/src/admesh/stlinit.c
index f797004fb..2e55462df 100644
--- a/xs/src/admesh/stlinit.c
+++ b/xs/src/admesh/stlinit.c
@@ -308,6 +308,29 @@ stl_read(stl_file *stl, int first_facet, int first) {
             printf("stl_read: facet %d.z = %e\r\n", j, facet.vertex[j].z);
       }
 #endif
+
+#if 1
+    {
+      // Positive and negative zeros are possible in the floats, which are considered equal by the FP unit.
+      // When using a memcmp on raw floats, those numbers report to be different.
+      // Unify all +0 and -0 to +0 to make the floats equal under memcmp.
+      uint32_t *f = (uint32_t*)&facet;
+      for (int j = 0; j < 12; ++ j, ++ f) // 3x vertex + normal: 4x3 = 12 floats
+        if (*f == 0x80000000)
+          // Negative zero, switch to positive zero.
+          *f = 0;
+    }
+#else
+    {
+      // Due to the nature of the floating point numbers, close to zero values may be represented with singificantly higher precision 
+      // than the rest of the vertices. Round them to zero.
+      float *f = (float*)&facet;
+      for (int j = 0; j < 12; ++ j, ++ f) // 3x vertex + normal: 4x3 = 12 floats
+        if (*f > -1e-12f && *f < 1e-12f)
+          // Negative zero, switch to positive zero.
+          *f = 0;
+    }
+#endif
     /* Write the facet into memory. */
     memcpy(stl->facet_start+i, &facet, SIZEOF_STL_FACET);
     stl_facet_stats(stl, facet, first);