1 files changed, 58 insertions, 0 deletions
diff --git a/Lib/fontTools/misc/roundTools.py b/Lib/fontTools/misc/roundTools.py
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+++ b/Lib/fontTools/misc/roundTools.py
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+"""
+Various round-to-integer helpers.
+"""
+
+import math
+import functools
+import logging
+
+log = logging.getLogger(__name__)
+
+__all__ = [
+	"noRound",
+	"otRound",
+	"maybeRound",
+	"roundFunc",
+]
+
+def noRound(value):
+	return value
+
+def otRound(value):
+	"""Round float value to nearest integer towards ``+Infinity``.
+
+	The OpenType spec (in the section on `"normalization" of OpenType Font Variations <https://docs.microsoft.com/en-us/typography/opentype/spec/otvaroverview#coordinate-scales-and-normalization>`_)
+	defines the required method for converting floating point values to
+	fixed-point. In particular it specifies the following rounding strategy:
+
+		for fractional values of 0.5 and higher, take the next higher integer;
+		for other fractional values, truncate.
+
+	This function rounds the floating-point value according to this strategy
+	in preparation for conversion to fixed-point.
+
+	Args:
+		value (float): The input floating-point value.
+
+	Returns
+		float: The rounded value.
+	"""
+	# See this thread for how we ended up with this implementation:
+	# https://github.com/fonttools/fonttools/issues/1248#issuecomment-383198166
+	return int(math.floor(value + 0.5))
+
+def maybeRound(v, tolerance, round=otRound):
+	rounded = round(v)
+	return rounded if abs(rounded - v) <= tolerance else v
+
+def roundFunc(tolerance, round=otRound):
+    if tolerance < 0:
+        raise ValueError("Rounding tolerance must be positive")
+
+    if tolerance == 0:
+        return noRound
+
+    if tolerance >= .5:
+        return round
+
+    return functools.partial(maybeRound, tolerance=tolerance, round=round)