Merge pull request #1582 from fonttools/max_ctx_recalc

(Re)calculate OS/2.usMaxContext value

Lib/fontTools/feaLib/builder.py

@@ -7,6 +7,7 @@ from fontTools.feaLib.error import FeatureLibError
 from fontTools.feaLib.parser import Parser
 from fontTools.feaLib.ast import FeatureFile
 from fontTools.otlLib import builder as otl
+from fontTools.otlLib.maxContextCalc import maxCtxFont
 from fontTools.ttLib import newTable, getTableModule
 from fontTools.ttLib.tables import otBase, otTables
 from collections import defaultdict, OrderedDict
@@ -137,6 +138,9 @@ class Builder(object):
                 fontTable.table = table
             elif tag in self.font:
                 del self.font[tag]
+        if (any(tag in self.font for tag in ("GPOS", "GSUB")) and
+                "OS/2" in self.font):
+            self.font["OS/2"].usMaxContext = maxCtxFont(self.font)
         if "GDEF" in tables:
             gdef = self.buildGDEF()
             if gdef:

Lib/fontTools/fontBuilder.py

@@ -21,6 +21,7 @@ that works:
     fb.setupHorizontalHeader()
     fb.setupNameTable(...)
     fb.setupOS2()
+    fb.addOpenTypeFeatures(...)
     fb.setupPost()
     fb.save(...)
 
@@ -299,7 +300,7 @@ _OS2Defaults = dict(
     sCapHeight = 0,
     usDefaultChar = 0,  # .notdef
     usBreakChar = 32,   # space
-    usMaxContext = 2,   # just kerning
+    usMaxContext = 0,
     usLowerOpticalPointSize = 0,
     usUpperOpticalPointSize = 0,
 )

Lib/fontTools/otlLib/maxContextCalc.py

@@ -0,0 +1,101 @@
+from __future__ import print_function, division, absolute_import, unicode_literals
+
+__all__ = ['maxCtxFont']
+
+
+def maxCtxFont(font):
+    """Calculate the usMaxContext value for an entire font."""
+
+    maxCtx = 0
+    for tag in ('GSUB', 'GPOS'):
+        if tag not in font:
+            continue
+        table = font[tag].table
+        if not table.LookupList:
+            continue
+        for lookup in table.LookupList.Lookup:
+            for st in lookup.SubTable:
+                maxCtx = maxCtxSubtable(maxCtx, tag, lookup.LookupType, st)
+    return maxCtx
+
+
+def maxCtxSubtable(maxCtx, tag, lookupType, st):
+    """Calculate usMaxContext based on a single lookup table (and an existing
+    max value).
+    """
+
+    # single positioning, single / multiple substitution
+    if (tag == 'GPOS' and lookupType == 1) or (
+        tag == 'GSUB' and lookupType in (1, 2, 3)):
+        maxCtx = max(maxCtx, 1)
+
+    # pair positioning
+    elif tag == 'GPOS' and lookupType == 2:
+        maxCtx = max(maxCtx, 2)
+
+    # ligatures
+    elif tag == 'GSUB' and lookupType == 4:
+        for ligatures in st.ligatures.values():
+            for ligature in ligatures:
+                maxCtx = max(maxCtx, ligature.CompCount)
+
+    # context
+    elif (tag == 'GPOS' and lookupType == 7) or (
+          tag == 'GSUB' and lookupType == 5):
+        maxCtx = maxCtxContextualSubtable(
+            maxCtx, st, 'Pos' if tag == 'GPOS' else 'Sub')
+
+    # chained context
+    elif (tag == 'GPOS' and lookupType == 8) or (
+          tag == 'GSUB' and lookupType == 6):
+        maxCtx = maxCtxContextualSubtable(
+            maxCtx, st, 'Pos' if tag == 'GPOS' else 'Sub', 'Chain')
+
+    # extensions
+    elif (tag == 'GPOS' and lookupType == 9) or (
+          tag == 'GSUB' and lookupType == 7):
+        maxCtx = maxCtxSubtable(
+            maxCtx, tag, st.ExtensionLookupType, st.ExtSubTable)
+
+    # reverse-chained context
+    elif tag == 'GSUB' and lookupType == 8:
+        maxCtx = maxCtxContextualRule(maxCtx, st, 'Reverse')
+
+    return maxCtx
+
+
+def maxCtxContextualSubtable(maxCtx, st, ruleType, chain=''):
+    """Calculate usMaxContext based on a contextual feature subtable."""
+
+    if st.Format == 1:
+        for ruleset in getattr(st, '%s%sRuleSet' % (chain, ruleType)):
+            if ruleset is None:
+                continue
+            for rule in getattr(ruleset, '%s%sRule' % (chain, ruleType)):
+                if rule is None:
+                    continue
+                maxCtx = maxCtxContextualRule(maxCtx, rule, chain)
+
+    elif st.Format == 2:
+        for ruleset in getattr(st, '%s%sClassSet' % (chain, ruleType)):
+            if ruleset is None:
+                continue
+            for rule in getattr(ruleset, '%s%sClassRule' % (chain, ruleType)):
+                if rule is None:
+                    continue
+                maxCtx = maxCtxContextualRule(maxCtx, rule, chain)
+
+    elif st.Format == 3:
+        maxCtx = maxCtxContextualRule(maxCtx, st, chain)
+
+    return maxCtx
+
+
+def maxCtxContextualRule(maxCtx, st, chain):
+    """Calculate usMaxContext based on a contextual feature rule."""
+
+    if not chain:
+        return max(maxCtx, st.GlyphCount)
+    elif chain == 'Reverse':
+        return max(maxCtx, st.GlyphCount + st.LookAheadGlyphCount)
+    return max(maxCtx, st.InputGlyphCount + st.LookAheadGlyphCount)

Lib/fontTools/subset/__init__.py

@@ -7,6 +7,7 @@ from fontTools.misc.py23 import *
 from fontTools.misc.fixedTools import otRound
 from fontTools import ttLib
 from fontTools.ttLib.tables import otTables
+from fontTools.otlLib.maxContextCalc import maxCtxFont
 from fontTools.pens.basePen import NullPen
 from fontTools.misc.loggingTools import Timer
 from fontTools.subset.cff import *
@@ -322,6 +323,10 @@ Other font-specific options:
       Update the 'OS/2 xAvgCharWidth' field after subsetting.
   --no-recalc-average-width
       Don't change the 'OS/2 xAvgCharWidth' field. [default]
+  --recalc-max-context
+      Update the 'OS/2 usMaxContext' field after subsetting.
+  --no-recalc-max-context
+      Don't change the 'OS/2 usMaxContext' field. [default]
   --font-number=<number>
       Select font number for TrueType Collection (.ttc/.otc), starting from 0.
 
@@ -2305,6 +2310,7 @@ class Options(object):
 		self.recalc_timestamp = False # Recalculate font modified timestamp
 		self.prune_unicode_ranges = True # Clear unused 'ulUnicodeRange' bits
 		self.recalc_average_width = False # update 'xAvgCharWidth'
+		self.recalc_max_context = False # update 'usMaxContext'
 		self.canonical_order = None # Order tables as recommended
 		self.flavor = None  # May be 'woff' or 'woff2'
 		self.with_zopfli = False  # use zopfli instead of zlib for WOFF 1.0
@@ -2614,6 +2620,11 @@ class Subsetter(object):
 					if avg_width != font[tag].xAvgCharWidth:
 						font[tag].xAvgCharWidth = avg_width
 						log.info("%s xAvgCharWidth updated: %d", tag, avg_width)
+				if self.options.recalc_max_context:
+					max_context = maxCtxFont(font)
+					if max_context != font[tag].usMaxContext:
+						font[tag].usMaxContext = max_context
+						log.info("%s usMaxContext updated: %d", tag, max_context)
 			clazz = ttLib.getTableClass(tag)
 			if hasattr(clazz, 'prune_post_subset'):
 				with timer("prune '%s'" % tag):

Tests/fontBuilder/data/test.otf.ttx

@@ -233,6 +233,57 @@
     </GlobalSubrs>
   </CFF>
 
+  <GPOS>
+    <Version value="0x00010000"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="DFLT"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="kern"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="0"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=1 -->
+      <Lookup index="0">
+        <LookupType value="2"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <PairPos index="0" Format="1">
+          <Coverage Format="1">
+            <Glyph value="A"/>
+          </Coverage>
+          <ValueFormat1 value="4"/>
+          <ValueFormat2 value="0"/>
+          <!-- PairSetCount=1 -->
+          <PairSet index="0">
+            <!-- PairValueCount=1 -->
+            <PairValueRecord index="0">
+              <SecondGlyph value="a"/>
+              <Value1 XAdvance="-50"/>
+            </PairValueRecord>
+          </PairSet>
+        </PairPos>
+      </Lookup>
+    </LookupList>
+  </GPOS>
+
   <hmtx>
     <mtx name=".notdef" width="600" lsb="100"/>
     <mtx name=".null" width="600" lsb="100"/>

Tests/fontBuilder/data/test.ttf.ttx

@@ -119,7 +119,7 @@
     <sCapHeight value="0"/>
     <usDefaultChar value="0"/>
     <usBreakChar value="32"/>
-    <usMaxContext value="2"/>
+    <usMaxContext value="1"/>
   </OS_2>
 
   <hmtx>
@@ -257,6 +257,45 @@
     </extraNames>
   </post>
 
+  <GSUB>
+    <Version value="0x00010000"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="DFLT"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="salt"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="0"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=1 -->
+      <Lookup index="0">
+        <LookupType value="1"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <SingleSubst index="0" Format="1">
+          <Substitution in="A" out="a"/>
+        </SingleSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
   <DSIG>
     <!-- note that the Digital Signature will be invalid after recompilation! -->
     <tableHeader flag="0x1" numSigs="1" version="1"/>

Tests/fontBuilder/data/test_var.otf.ttx

@@ -105,7 +105,7 @@
     <sCapHeight value="0"/>
     <usDefaultChar value="0"/>
     <usBreakChar value="32"/>
-    <usMaxContext value="2"/>
+    <usMaxContext value="0"/>
   </OS_2>
 
   <hmtx>

Tests/fontBuilder/data/test_var.ttf.ttx

@@ -119,7 +119,7 @@
     <sCapHeight value="0"/>
     <usDefaultChar value="0"/>
     <usBreakChar value="32"/>
-    <usMaxContext value="2"/>
+    <usMaxContext value="0"/>
   </OS_2>
 
   <hmtx>

Tests/fontBuilder/fontBuilder_test.py

@@ -117,6 +117,7 @@ def test_build_ttf(tmpdir):
     fb.setupHorizontalHeader(ascent=824, descent=200)
     fb.setupNameTable(nameStrings)
     fb.setupOS2()
+    fb.addOpenTypeFeatures("feature salt { sub A by a; } salt;")
     fb.setupPost()
     fb.setupDummyDSIG()
 
@@ -145,6 +146,7 @@ def test_build_otf(tmpdir):
     fb.setupHorizontalHeader(ascent=824, descent=200)
     fb.setupNameTable(nameStrings)
     fb.setupOS2()
+    fb.addOpenTypeFeatures("feature kern { pos A a -50; } kern;")
     fb.setupPost()
     fb.setupDummyDSIG()
 

Tests/otlLib/data/gpos_91.ttx

@@ -0,0 +1,99 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont sfntVersion="\x00\x01\x00\x00" ttLibVersion="3.40">
+
+  <GlyphOrder>
+    <!-- The 'id' attribute is only for humans; it is ignored when parsed. -->
+    <GlyphID id="0" name=".notdef"/>
+    <GlyphID id="1" name="A"/>
+  </GlyphOrder>
+
+  <maxp>
+    <!-- Most of this table will be recalculated by the compiler -->
+    <tableVersion value="0x10000"/>
+    <numGlyphs value="2"/>
+    <maxPoints value="0"/>
+    <maxContours value="0"/>
+    <maxCompositePoints value="0"/>
+    <maxCompositeContours value="0"/>
+    <maxZones value="2"/>
+    <maxTwilightPoints value="0"/>
+    <maxStorage value="0"/>
+    <maxFunctionDefs value="0"/>
+    <maxInstructionDefs value="0"/>
+    <maxStackElements value="0"/>
+    <maxSizeOfInstructions value="0"/>
+    <maxComponentElements value="0"/>
+    <maxComponentDepth value="0"/>
+  </maxp>
+
+  <post>
+    <formatType value="2.0"/>
+    <italicAngle value="0.0"/>
+    <underlinePosition value="0"/>
+    <underlineThickness value="0"/>
+    <isFixedPitch value="0"/>
+    <minMemType42 value="0"/>
+    <maxMemType42 value="0"/>
+    <minMemType1 value="0"/>
+    <maxMemType1 value="0"/>
+    <psNames>
+      <!-- This file uses unique glyph names based on the information
+           found in the 'post' table. Since these names might not be unique,
+           we have to invent artificial names in case of clashes. In order to
+           be able to retain the original information, we need a name to
+           ps name mapping for those cases where they differ. That's what
+           you see below.
+            -->
+    </psNames>
+    <extraNames>
+      <!-- following are the name that are not taken from the standard Mac glyph order -->
+    </extraNames>
+  </post>
+
+  <GPOS>
+    <Version value="0x00010000"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="latn"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="0"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=1 -->
+      <Lookup index="0">
+        <LookupType value="9"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ExtensionPos index="0" Format="1">
+          <ExtensionLookupType value="1"/>
+          <SinglePos Format="1">
+            <Coverage Format="1">
+              <Glyph value="A"/>
+            </Coverage>
+            <ValueFormat value="4"/>
+            <Value XAdvance="20"/>
+          </SinglePos>
+        </ExtensionPos>
+      </Lookup>
+    </LookupList>
+  </GPOS>
+
+</ttFont>

Tests/otlLib/data/gsub_51.ttx

@@ -0,0 +1,146 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont sfntVersion="\x00\x01\x00\x00" ttLibVersion="3.40">
+
+  <GlyphOrder>
+    <!-- The 'id' attribute is only for humans; it is ignored when parsed. -->
+    <GlyphID id="0" name=".notdef"/>
+    <GlyphID id="1" name="g20"/>
+    <GlyphID id="2" name="g21"/>
+    <GlyphID id="3" name="g22"/>
+    <GlyphID id="4" name="g60"/>
+    <GlyphID id="5" name="g61"/>
+    <GlyphID id="6" name="g62"/>
+    <GlyphID id="7" name="g63"/>
+  </GlyphOrder>
+
+  <maxp>
+    <!-- Most of this table will be recalculated by the compiler -->
+    <tableVersion value="0x10000"/>
+    <numGlyphs value="8"/>
+    <maxPoints value="0"/>
+    <maxContours value="0"/>
+    <maxCompositePoints value="0"/>
+    <maxCompositeContours value="0"/>
+    <maxZones value="2"/>
+    <maxTwilightPoints value="0"/>
+    <maxStorage value="0"/>
+    <maxFunctionDefs value="0"/>
+    <maxInstructionDefs value="0"/>
+    <maxStackElements value="0"/>
+    <maxSizeOfInstructions value="0"/>
+    <maxComponentElements value="0"/>
+    <maxComponentDepth value="0"/>
+  </maxp>
+
+  <post>
+    <formatType value="2.0"/>
+    <italicAngle value="0.0"/>
+    <underlinePosition value="0"/>
+    <underlineThickness value="0"/>
+    <isFixedPitch value="0"/>
+    <minMemType42 value="0"/>
+    <maxMemType42 value="0"/>
+    <minMemType1 value="0"/>
+    <maxMemType1 value="0"/>
+    <psNames>
+      <!-- This file uses unique glyph names based on the information
+           found in the 'post' table. Since these names might not be unique,
+           we have to invent artificial names in case of clashes. In order to
+           be able to retain the original information, we need a name to
+           ps name mapping for those cases where they differ. That's what
+           you see below.
+            -->
+    </psNames>
+    <extraNames>
+      <!-- following are the name that are not taken from the standard Mac glyph order -->
+    </extraNames>
+  </post>
+
+  <GSUB>
+    <Version value="0x00010000"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="latn"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="4"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=5 -->
+      <Lookup index="0">
+        <LookupType value="1"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <SingleSubst index="0" Format="1">
+          <Substitution in="g20" out="g60"/>
+          <Substitution in="g21" out="g61"/>
+        </SingleSubst>
+      </Lookup>
+      <Lookup index="1">
+        <LookupType value="4"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <LigatureSubst index="0" Format="1">
+          <LigatureSet glyph="g21">
+            <Ligature components="g22" glyph="g61"/>
+          </LigatureSet>
+        </LigatureSubst>
+      </Lookup>
+      <Lookup index="2">
+        <LookupType value="4"/>
+        <LookupFlag value="8"/>
+        <!-- SubTableCount=1 -->
+        <LigatureSubst index="0" Format="1">
+          <LigatureSet glyph="g21">
+            <Ligature components="g22" glyph="g61"/>
+          </LigatureSet>
+        </LigatureSubst>
+      </Lookup>
+      <Lookup index="3">
+        <LookupType value="2"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <MultipleSubst index="0" Format="1">
+          <Substitution in="g21" out="g61,g62,g63"/>
+        </MultipleSubst>
+      </Lookup>
+      <Lookup index="4">
+        <LookupType value="5"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ContextSubst index="0" Format="1">
+          <Coverage Format="1">
+            <Glyph value="g20"/>
+          </Coverage>
+          <!-- SubRuleSetCount=1 -->
+          <SubRuleSet index="0">
+            <!-- SubRuleCount=1 -->
+            <SubRule index="0">
+              <!-- GlyphCount=2 -->
+              <!-- SubstCount=0 -->
+              <Input index="0" value="g20"/>
+            </SubRule>
+          </SubRuleSet>
+        </ContextSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
+</ttFont>

Tests/otlLib/data/gsub_52.ttx

@@ -0,0 +1,150 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont sfntVersion="\x00\x01\x00\x00" ttLibVersion="3.40">
+
+  <GlyphOrder>
+    <!-- The 'id' attribute is only for humans; it is ignored when parsed. -->
+    <GlyphID id="0" name=".notdef"/>
+    <GlyphID id="1" name="g20"/>
+    <GlyphID id="2" name="g21"/>
+    <GlyphID id="3" name="g22"/>
+    <GlyphID id="4" name="g60"/>
+    <GlyphID id="5" name="g61"/>
+    <GlyphID id="6" name="g62"/>
+    <GlyphID id="7" name="g63"/>
+  </GlyphOrder>
+
+  <maxp>
+    <!-- Most of this table will be recalculated by the compiler -->
+    <tableVersion value="0x10000"/>
+    <numGlyphs value="8"/>
+    <maxPoints value="0"/>
+    <maxContours value="0"/>
+    <maxCompositePoints value="0"/>
+    <maxCompositeContours value="0"/>
+    <maxZones value="2"/>
+    <maxTwilightPoints value="0"/>
+    <maxStorage value="0"/>
+    <maxFunctionDefs value="0"/>
+    <maxInstructionDefs value="0"/>
+    <maxStackElements value="0"/>
+    <maxSizeOfInstructions value="0"/>
+    <maxComponentElements value="0"/>
+    <maxComponentDepth value="0"/>
+  </maxp>
+
+  <post>
+    <formatType value="2.0"/>
+    <italicAngle value="0.0"/>
+    <underlinePosition value="0"/>
+    <underlineThickness value="0"/>
+    <isFixedPitch value="0"/>
+    <minMemType42 value="0"/>
+    <maxMemType42 value="0"/>
+    <minMemType1 value="0"/>
+    <maxMemType1 value="0"/>
+    <psNames>
+      <!-- This file uses unique glyph names based on the information
+           found in the 'post' table. Since these names might not be unique,
+           we have to invent artificial names in case of clashes. In order to
+           be able to retain the original information, we need a name to
+           ps name mapping for those cases where they differ. That's what
+           you see below.
+            -->
+    </psNames>
+    <extraNames>
+      <!-- following are the name that are not taken from the standard Mac glyph order -->
+    </extraNames>
+  </post>
+
+  <GSUB>
+    <Version value="0x00010000"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="latn"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="4"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=5 -->
+      <Lookup index="0">
+        <LookupType value="1"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <SingleSubst index="0" Format="1">
+          <Substitution in="g20" out="g60"/>
+          <Substitution in="g21" out="g61"/>
+        </SingleSubst>
+      </Lookup>
+      <Lookup index="1">
+        <LookupType value="4"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <LigatureSubst index="0" Format="1">
+          <LigatureSet glyph="g21">
+            <Ligature components="g22" glyph="g61"/>
+          </LigatureSet>
+        </LigatureSubst>
+      </Lookup>
+      <Lookup index="2">
+        <LookupType value="4"/>
+        <LookupFlag value="8"/>
+        <!-- SubTableCount=1 -->
+        <LigatureSubst index="0" Format="1">
+          <LigatureSet glyph="g21">
+            <Ligature components="g22" glyph="g61"/>
+          </LigatureSet>
+        </LigatureSubst>
+      </Lookup>
+      <Lookup index="3">
+        <LookupType value="2"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <MultipleSubst index="0" Format="1">
+          <Substitution in="g21" out="g61,g62,g63"/>
+        </MultipleSubst>
+      </Lookup>
+      <Lookup index="4">
+        <LookupType value="5"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ContextSubst index="0" Format="2">
+          <Coverage Format="1">
+            <Glyph value="g20"/>
+          </Coverage>
+          <ClassDef Format="2">
+            <ClassDef glyph="g20" class="1"/>
+          </ClassDef>
+          <!-- SubClassSetCount=2 -->
+          <SubClassSet index="0" empty="1"/>
+          <SubClassSet index="1">
+            <!-- SubClassRuleCount=1 -->
+            <SubClassRule index="0">
+              <!-- GlyphCount=2 -->
+              <!-- SubstCount=0 -->
+              <Class index="0" value="1"/>
+            </SubClassRule>
+          </SubClassSet>
+        </ContextSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
+</ttFont>

Tests/otlLib/data/gsub_71.ttx

@@ -0,0 +1,99 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont sfntVersion="\x00\x01\x00\x00" ttLibVersion="3.40">
+
+  <GlyphOrder>
+    <!-- The 'id' attribute is only for humans; it is ignored when parsed. -->
+    <GlyphID id="0" name=".notdef"/>
+    <GlyphID id="1" name="g18"/>
+    <GlyphID id="2" name="g19"/>
+    <GlyphID id="3" name="g23"/>
+    <GlyphID id="4" name="g24"/>
+  </GlyphOrder>
+
+  <maxp>
+    <!-- Most of this table will be recalculated by the compiler -->
+    <tableVersion value="0x10000"/>
+    <numGlyphs value="5"/>
+    <maxPoints value="0"/>
+    <maxContours value="0"/>
+    <maxCompositePoints value="0"/>
+    <maxCompositeContours value="0"/>
+    <maxZones value="2"/>
+    <maxTwilightPoints value="0"/>
+    <maxStorage value="0"/>
+    <maxFunctionDefs value="0"/>
+    <maxInstructionDefs value="0"/>
+    <maxStackElements value="0"/>
+    <maxSizeOfInstructions value="0"/>
+    <maxComponentElements value="0"/>
+    <maxComponentDepth value="0"/>
+  </maxp>
+
+  <post>
+    <formatType value="2.0"/>
+    <italicAngle value="0.0"/>
+    <underlinePosition value="0"/>
+    <underlineThickness value="0"/>
+    <isFixedPitch value="0"/>
+    <minMemType42 value="0"/>
+    <maxMemType42 value="0"/>
+    <minMemType1 value="0"/>
+    <maxMemType1 value="0"/>
+    <psNames>
+      <!-- This file uses unique glyph names based on the information
+           found in the 'post' table. Since these names might not be unique,
+           we have to invent artificial names in case of clashes. In order to
+           be able to retain the original information, we need a name to
+           ps name mapping for those cases where they differ. That's what
+           you see below.
+            -->
+    </psNames>
+    <extraNames>
+      <!-- following are the name that are not taken from the standard Mac glyph order -->
+    </extraNames>
+  </post>
+
+  <GSUB>
+    <Version value="0x00010000"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="latn"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="0"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=1 -->
+      <Lookup index="0">
+        <LookupType value="7"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ExtensionSubst index="0" Format="1">
+          <ExtensionLookupType value="1"/>
+          <SingleSubst Format="1">
+            <Substitution in="g18" out="g23"/>
+            <Substitution in="g19" out="g24"/>
+          </SingleSubst>
+        </ExtensionSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
+</ttFont>

Tests/otlLib/maxContextCalc_test.py

@@ -0,0 +1,76 @@
+from __future__ import print_function, division, absolute_import
+from __future__ import unicode_literals
+
+import os
+import pytest
+from fontTools.ttLib import TTFont
+from fontTools.otlLib.maxContextCalc import maxCtxFont
+from fontTools.feaLib.builder import addOpenTypeFeaturesFromString
+
+
+def test_max_ctx_calc_no_features():
+    font = TTFont()
+    assert maxCtxFont(font) == 0
+    font.setGlyphOrder(['.notdef'])
+    addOpenTypeFeaturesFromString(font, '')
+    assert maxCtxFont(font) == 0
+
+
+def test_max_ctx_calc_features():
+    glyphs = '.notdef space A B C a b c'.split()
+    features = """
+    lookup GSUB_EXT useExtension {
+        sub a by b;
+    } GSUB_EXT;
+
+    lookup GPOS_EXT useExtension {
+        pos a b -10;
+    } GPOS_EXT;
+
+    feature sub1 {
+        sub A by a;
+        sub A B by b;
+        sub A B C by c;
+        sub [A B] C by c;
+        sub [A B] C [A B] by c;
+        sub A by A B;
+        sub A' C by A B;
+        sub a' by b;
+        sub a' b by c;
+        sub a from [A B C];
+        rsub a by b;
+        rsub a' by b;
+        rsub a b' by c;
+        rsub a b' c by A;
+        rsub [a b] c' by A;
+        rsub [a b] c' [a b] by B;
+        lookup GSUB_EXT;
+    } sub1;
+
+    feature pos1 {
+        pos A 20;
+        pos A B -50;
+        pos A B' 10 C;
+        lookup GPOS_EXT;
+    } pos1;
+    """
+    font = TTFont()
+    font.setGlyphOrder(glyphs)
+    addOpenTypeFeaturesFromString(font, features)
+
+    assert maxCtxFont(font) == 3
+
+
+@pytest.mark.parametrize('file_name, max_context', [
+    ('gsub_51', 2),
+    ('gsub_52', 2),
+    ('gsub_71', 1),
+    ('gpos_91', 1),
+])
+def test_max_ctx_calc_features_ttx(file_name, max_context):
+    ttx_path = os.path.join(os.path.dirname(__file__),
+                            'data', '{}.ttx'.format(file_name))
+    font = TTFont()
+    font.importXML(ttx_path)
+
+    assert maxCtxFont(font) == max_context

Tests/subset/subset_test.py

@@ -485,6 +485,24 @@ class SubsetTest(unittest.TestCase):
         subset.main([fontpath, "--recalc-timestamp", "--output-file=%s" % subsetpath, "*"])
         self.assertLess(modified, TTFont(subsetpath)['head'].modified)
 
+    def test_recalc_max_context(self):
+        ttxpath = self.getpath("Lobster.subset.ttx")
+        font = TTFont()
+        font.importXML(ttxpath)
+        max_context = font['OS/2'].usMaxContext
+        _, fontpath = self.compile_font(ttxpath, ".otf")
+        subsetpath = self.temp_path(".otf")
+
+        # by default, the subsetter does not recalculate the usMaxContext
+        subset.main([fontpath, "--drop-tables+=GSUB,GPOS",
+                               "--output-file=%s" % subsetpath])
+        self.assertEqual(max_context, TTFont(subsetpath)['OS/2'].usMaxContext)
+
+        subset.main([fontpath, "--recalc-max-context",
+                               "--drop-tables+=GSUB,GPOS",
+                               "--output-file=%s" % subsetpath])
+        self.assertEqual(0, TTFont(subsetpath)['OS/2'].usMaxContext)
+
     def test_retain_gids_ttf(self):
         _, fontpath = self.compile_font(self.getpath("TestTTF-Regular.ttx"), ".ttf")
         font = TTFont(fontpath)