Implement custom integer->string conversion for hashing

Converts to hexadecimal, reversed order which is faster.

src/libslic3r/SLAPrintSteps.cpp

@@ -145,12 +145,45 @@ void SLAPrint::Steps::hollow_model(SLAPrintObject &po)
 
 struct FaceHash {
 
+    // A 64 bit number's max hex digits
+    static constexpr size_t MAX_NUM_CHARS = 16;
+
     // A hash is created for each triangle to be identifiable. The hash uses
     // only the triangle's geometric traits, not the index in a particular mesh.
     std::unordered_set<std::string> facehash;
 
-    static std::string facekey(const Vec3i &face,
-                               const std::vector<Vec3f> &vertices)
+    // Returns the string in reverse, but that is ok for hashing
+    static std::array<char, MAX_NUM_CHARS + 1> to_chars(int64_t val)
+    {
+        std::array<char, MAX_NUM_CHARS + 1> ret;
+
+        static const constexpr char * Conv = "0123456789abcdef";
+
+        auto ptr = ret.begin();
+        auto uval = static_cast<uint64_t>(std::abs(val));
+        while (uval) {
+            *ptr = Conv[uval & 0xf];
+            ++ptr;
+            uval = uval >> 4;
+        }
+        if (val < 0) { *ptr = '-'; ++ptr; }
+        *ptr = '\0'; // C style string ending
+
+        return ret;
+    }
+
+    static std::string hash(const Vec<3, int64_t> &v)
+    {
+        std::string ret;
+        ret.reserve(3 * MAX_NUM_CHARS);
+
+        for (auto val : v)
+            ret += to_chars(val).data();
+
+        return ret;
+    }
+
+    static std::string facekey(const Vec3i &face, const std::vector<Vec3f> &vertices)
     {
         // Scale to integer to avoid floating points
         std::array<Vec<3, int64_t>, 3> pts = {
@@ -166,15 +199,13 @@ struct FaceHash {
         Vec<3, int64_t> c = a.cross(b) + (pts[0] + pts[1] + pts[2]) / 3;
 
         // Return a concatenated string representation of the coordinates
-        return float_to_string_decimal_point(c(0)) + float_to_string_decimal_point(c(1)) + float_to_string_decimal_point(c(2));
+        return hash(c);
     }
 
-    FaceHash(const indexed_triangle_set &its)
+    FaceHash (const indexed_triangle_set &its): facehash(its.indices.size())
     {
-        for (const Vec3i &face : its.indices) {
-            std::string keystr = facekey(face, its.vertices);
-            facehash.insert(keystr);
-        }
+        for (const Vec3i &face : its.indices)
+            facehash.insert(facekey(face, its.vertices));
     }
 
     bool find(const std::string &key)