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);

[bungeman-skia, 2014/05/05 15:39:31]

So this is round to nearest, round half up. C++11 has nearbyint with
FE_TONEAREST, but that is round to nearest, round away from zero. At least
that's what the spec seems to say. libc appears to document that it will instead
round to even. In other words, C++11 and C99 have a round to nearest, but the
tie breaking seems to still be lacking.

[reed1, 2014/05/05 15:56:37]

Agreed, that's why unfortunately we have to write our own.

+}
+
 /** 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