From 82460bd368329ebaa61b3f197a368906e66e58fd Mon Sep 17 00:00:00 2001
From: Cosimo Lupo <clupo@google.com>
Date: Tue, 14 Jun 2022 18:06:15 +0100
Subject: [PATCH] otConverters: define a base class for all fixed-size floats
 with toInt/fromInt methods

this will be useful when computing deltas for COLR table fields that use Fixed or F2Dot14 (ItemVariationStore internally treats fixed-value floats as 16 or 32-bit integers)
---
 Lib/fontTools/ttLib/tables/otConverters.py | 60 +++++++++++++---------
 1 file changed, 35 insertions(+), 25 deletions(-)

diff --git a/Lib/fontTools/ttLib/tables/otConverters.py b/Lib/fontTools/ttLib/tables/otConverters.py
index 44fcd0ab3..274cd7d05 100644
--- a/Lib/fontTools/ttLib/tables/otConverters.py
+++ b/Lib/fontTools/ttLib/tables/otConverters.py
@@ -402,40 +402,50 @@ class DeciPoints(FloatValue):
 	def write(self, writer, font, tableDict, value, repeatIndex=None):
 		writer.writeUShort(round(value * 10))
 
-class Fixed(FloatValue):
-	staticSize = 4
+class BaseFixedValue(FloatValue):
+	staticSize = NotImplemented
+	precisionBits = NotImplemented
+	readerMethod = NotImplemented
+	writerMethod = NotImplemented
 	def read(self, reader, font, tableDict):
-		return  fi2fl(reader.readLong(), 16)
+		return  self.fromInt(getattr(reader, self.readerMethod)())
 	def write(self, writer, font, tableDict, value, repeatIndex=None):
-		writer.writeLong(fl2fi(value, 16))
-	@staticmethod
-	def fromString(value):
-		return str2fl(value, 16)
-	@staticmethod
-	def toString(value):
-		return fl2str(value, 16)
+		getattr(writer, self.writerMethod)(self.toInt(value))
+	@classmethod
+	def fromInt(cls, value):
+		return fi2fl(value, cls.precisionBits)
+	@classmethod
+	def toInt(cls, value):
+		return fl2fi(value, cls.precisionBits)
+	@classmethod
+	def fromString(cls, value):
+		return str2fl(value, cls.precisionBits)
+	@classmethod
+	def toString(cls, value):
+		return fl2str(value, cls.precisionBits)
 
-class F2Dot14(FloatValue):
+class Fixed(BaseFixedValue):
+	staticSize = 4
+	precisionBits = 16
+	readerMethod = "readLong"
+	writerMethod = "writeLong"
+
+class F2Dot14(BaseFixedValue):
 	staticSize = 2
-	def read(self, reader, font, tableDict):
-		return  fi2fl(reader.readShort(), 14)
-	def write(self, writer, font, tableDict, value, repeatIndex=None):
-		writer.writeShort(fl2fi(value, 14))
-	@staticmethod
-	def fromString(value):
-		return str2fl(value, 14)
-	@staticmethod
-	def toString(value):
-		return fl2str(value, 14)
+	precisionBits = 14
+	readerMethod = "readShort"
+	writerMethod = "writeShort"
 
 class Angle(F2Dot14):
 	# angles are specified in degrees, and encoded as F2Dot14 fractions of half
 	# circle: e.g. 1.0 => 180, -0.5 => -90, -2.0 => -360, etc.
 	factor = 1.0/(1<<14) * 180  # 0.010986328125
-	def read(self, reader, font, tableDict):
-		return super().read(reader, font, tableDict) * 180
-	def write(self, writer, font, tableDict, value, repeatIndex=None):
-		super().write(writer, font, tableDict, value / 180, repeatIndex=repeatIndex)
+	@classmethod
+	def fromInt(cls, value):
+		return super().fromInt(value) * 180
+	@classmethod
+	def toInt(cls, value):
+		return super().toInt(value / 180)
 	@classmethod
 	def fromString(cls, value):
 		# quantize to nearest multiples of minimum fixed-precision angle