add rounding-using-doubles methods on SkScalar and SkRect

include/core/SkScalar.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkScalar_DEFINED
 #define SkScalar_DEFINED
 
@@ skipping 65 matching lines @@
 
 #define SkScalarFloorToScalar(x)    sk_float_floor(x)
 #define SkScalarCeilToScalar(x)     sk_float_ceil(x)
 #define SkScalarRoundToScalar(x)    sk_float_floor((x) + 0.5f)
 
 #define SkScalarFloorToInt(x)       sk_float_floor2int(x)
 #define SkScalarCeilToInt(x)        sk_float_ceil2int(x)
 #define SkScalarRoundToInt(x)       sk_float_round2int(x)
 #define SkScalarTruncToInt(x)       static_cast<int>(x)
 
+/**
+ *  Variant of SkScalarRoundToInt, that performs the rounding step (adding 0.5) explicitly using
+ *  double, to avoid possibly losing the low bit(s) of the answer before calling floor().
+ *
+ *  This routine will likely be slower than SkScalarRoundToInt(), and should only be used when the
+ *  extra precision is known to be valuable.
+ *
+ *  In particular, this catches the following case:
+ *      SkScalar x = 0.49999997;
+ *      int ix = SkScalarRoundToInt(x);
+ *      SkASSERT(0 == ix);    // <--- fails
+ *      ix = SkDScalarRoundToInt(x);
+ *      SkASSERT(0 == ix);    // <--- succeeds
+ */
+static inline int SkDScalarRoundToInt(SkScalar x) {
+    double xx = x;
+    xx += 0.5;
+    return (int)floor(xx);
+}
+
 /** Returns the absolute value of the specified SkScalar
 */
 #define SkScalarAbs(x)          sk_float_abs(x)
 /** Return x with the sign of y
  */
 #define SkScalarCopySign(x, y)  sk_float_copysign(x, y)
 /** Returns the value pinned between 0 and max inclusive
 */
 inline SkScalar SkScalarClampMax(SkScalar x, SkScalar max) {
     return x < 0 ? 0 : x > max ? max : x;
@@ skipping 129 matching lines @@
     SkASSERT(n >= 0);
     for (int i = 0; i < n; ++i) {
         if (a[i] != b[i]) {
             return false;
         }
     }
     return true;
 }
 
 #endif