freetype/truetype: fix rounding for negative numbers to match the C

Freetype implementation.

The spec is the "Order of rounding operations" part of
https://developer.apple.com/fonts/TTRefMan/RM02/Chap2.html#rounding
but it wasn't exactly clear to me what "the negative round value
closest to zero" was: if the phase is 1/4, was that value -1/4 or
-3/4? Anyway, the Go code now follows the C code, and code trumps
documentation.

R=bsiegert
CC=golang-dev
https://codereview.appspot.com/14092044

freetype/truetype/hint.go

@@ -1237,27 +1237,21 @@ func dotProduct(x, y f26dot6, q [2]f2dot14) f26dot6 {
 // https://developer.apple.com/fonts/TTRefMan/RM02/Chap2.html#rounding
 func (h *Hinter) round(x f26dot6) f26dot6 {
 	if h.gs.roundPeriod == 0 {
+		// Rounding is off.
 		return x
 	}
-	neg := x < 0
-	x -= h.gs.roundPhase
-	x += h.gs.roundThreshold
 	if x >= 0 {
-		x = (x / h.gs.roundPeriod) * h.gs.roundPeriod
-	} else {
-		x -= h.gs.roundPeriod
-		x += 1
-		x = (x / h.gs.roundPeriod) * h.gs.roundPeriod
-	}
-	x += h.gs.roundPhase
-	if neg {
-		if x >= 0 {
-			x = h.gs.roundPhase - h.gs.roundPeriod
+		ret := (x - h.gs.roundPhase + h.gs.roundThreshold) & -h.gs.roundPeriod
+		if x != 0 && ret < 0 {
+			ret = 0
 		}
-	} else if x < 0 {
-		x = h.gs.roundPhase
+		return ret + h.gs.roundPhase
 	}
-	return x
+	ret := -((-x - h.gs.roundPhase + h.gs.roundThreshold) & -h.gs.roundPeriod)
+	if ret > 0 {
+		ret = 0
+	}
+	return ret - h.gs.roundPhase
 }
 
 func bool2int32(b bool) int32 {

freetype/truetype/hint_test.go

@@ -445,33 +445,33 @@ func TestBytecode(t *testing.T) {
 				opPUSHW000, // [-81]
 				0xff,
 				0xaf,
-				opROUND00,  // [-112]
-				opPUSHW000, // [-112, -80]
+				opROUND00,  // [-80]
+				opPUSHW000, // [-80, -80]
 				0xff,
 				0xb0,
-				opROUND00,  // [-112, -48]
-				opPUSHW000, // [-112, -48, -17]
+				opROUND00,  // [-80, -80]
+				opPUSHW000, // [-80, -80, -17]
 				0xff,
 				0xef,
-				opROUND00,  // [-112, -48, -48]
-				opPUSHW000, // [-112, -48, -48, -16]
+				opROUND00,  // [-80, -80, -16]
+				opPUSHW000, // [-80, -80, -16, -16]
 				0xff,
 				0xf0,
-				opROUND00,  // [-112, -48, -48, -48]
-				opPUSHB000, // [-112, -48, -48, -48, 0]
+				opROUND00,  // [-80, -80, -16, -16]
+				opPUSHB000, // [-80, -80, -16, -16, 0]
 				0,
-				opROUND00,  // [-112, -48, -48, -48, 16]
-				opPUSHB000, // [-112, -48, -48, -48, 16, 16]
+				opROUND00,  // [-80, -80, -16, -16, 16]
+				opPUSHB000, // [-80, -80, -16, -16, 16, 16]
 				16,
-				opROUND00,  // [-112, -48, -48, -48, 16, 16]
-				opPUSHB000, // [-112, -48, -48, -48, 16, 16, 47]
+				opROUND00,  // [-80, -80, -16, -16, 16, 16]
+				opPUSHB000, // [-80, -80, -16, -16, 16, 16, 47]
 				47,
-				opROUND00,  // [-112, -48, -48, -48, 16, 16, 16]
-				opPUSHB000, // [-112, -48, -48, -48, 16, 16, 16, 48]
+				opROUND00,  // [-80, -80, -16, -16, 16, 16, 16]
+				opPUSHB000, // [-80, -80, -16, -16, 16, 16, 16, 48]
 				48,
-				opROUND00, // [-112, -48, -48, -48, 16, 16, 16, 80]
+				opROUND00, // [-80, -80, -16, -16, 16, 16, 16, 80]
 			},
-			[]int32{-112, -48, -48, -48, 16, 16, 16, 80},
+			[]int32{-80, -80, -16, -16, 16, 16, 16, 80},
 			"",
 		},
 		{

freetype/truetype/truetype_test.go

@@ -116,7 +116,7 @@ func testScaling(t *testing.T, filename string, hinter *Hinter) {
 	for i, want := range wants {
 		// TODO: completely implement hinting. For now, only the first N glyphs
 		// of luxisr.ttf are correctly hinted.
-		const N = 8
+		const N = 9
 		if hinter != nil && i == N {
 			break
 		}