From d24fd7193411ffaa58a78218bbae98e22dc104ba Mon Sep 17 00:00:00 2001
From: Khaled Hosny <khaledhosny@eglug.org>
Date: Tue, 28 Apr 2020 14:28:16 +0200
Subject: [PATCH] [feaLib] Fix building chained alt substitutions

Support was added in db49f20d6b2629e1ba25c4afd3fb60817387f3d6, but there
was only a parser test and no builder test, fix the build and add a
builder test based on the existing parser one.

Fixes https://github.com/fonttools/fonttools/issues/1901
---
 Lib/fontTools/feaLib/builder.py        |  2 +-
 Tests/feaLib/builder_test.py           |  2 +-
 Tests/feaLib/data/AlternateChained.fea |  3 +
 Tests/feaLib/data/AlternateChained.ttx | 78 ++++++++++++++++++++++++++
 4 files changed, 83 insertions(+), 2 deletions(-)
 create mode 100644 Tests/feaLib/data/AlternateChained.fea
 create mode 100644 Tests/feaLib/data/AlternateChained.ttx

diff --git a/Lib/fontTools/feaLib/builder.py b/Lib/fontTools/feaLib/builder.py
index 72156b8bb..52b23f1b2 100644
--- a/Lib/fontTools/feaLib/builder.py
+++ b/Lib/fontTools/feaLib/builder.py
@@ -828,7 +828,7 @@ class Builder(object):
         if prefix or suffix:
             chain = self.get_lookup_(location, ChainContextSubstBuilder)
             lookup = self.get_chained_lookup_(location, AlternateSubstBuilder)
-            chain.substitutions.append((prefix, [glyph], suffix, [lookup]))
+            chain.substitutions.append((prefix, [{glyph}], suffix, [lookup]))
         else:
             lookup = self.get_lookup_(location, AlternateSubstBuilder)
         if glyph in lookup.alternates:
diff --git a/Tests/feaLib/builder_test.py b/Tests/feaLib/builder_test.py
index 3273ba618..5ce4cc266 100644
--- a/Tests/feaLib/builder_test.py
+++ b/Tests/feaLib/builder_test.py
@@ -71,7 +71,7 @@ class BuilderTest(unittest.TestCase):
         ZeroValue_ChainSinglePos_horizontal ZeroValue_ChainSinglePos_vertical
         PairPosSubtable ChainSubstSubtable ChainPosSubtable LigatureSubtable
         AlternateSubtable MultipleSubstSubtable SingleSubstSubtable
-        aalt_chain_contextual_subst
+        aalt_chain_contextual_subst AlternateChained
     """.split()
 
     def __init__(self, methodName):
diff --git a/Tests/feaLib/data/AlternateChained.fea b/Tests/feaLib/data/AlternateChained.fea
new file mode 100644
index 000000000..45177693e
--- /dev/null
+++ b/Tests/feaLib/data/AlternateChained.fea
@@ -0,0 +1,3 @@
+feature test {
+  sub A B a' [Y y] Z from [a.alt1 a.alt2 a.alt3];
+} test;
diff --git a/Tests/feaLib/data/AlternateChained.ttx b/Tests/feaLib/data/AlternateChained.ttx
new file mode 100644
index 000000000..e02bb7d43
--- /dev/null
+++ b/Tests/feaLib/data/AlternateChained.ttx
@@ -0,0 +1,78 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont>
+
+  <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="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="0"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=2 -->
+      <Lookup index="0">
+        <LookupType value="6"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ChainContextSubst index="0" Format="3">
+          <!-- BacktrackGlyphCount=2 -->
+          <BacktrackCoverage index="0">
+            <Glyph value="B"/>
+          </BacktrackCoverage>
+          <BacktrackCoverage index="1">
+            <Glyph value="A"/>
+          </BacktrackCoverage>
+          <!-- InputGlyphCount=1 -->
+          <InputCoverage index="0">
+            <Glyph value="a"/>
+          </InputCoverage>
+          <!-- LookAheadGlyphCount=2 -->
+          <LookAheadCoverage index="0">
+            <Glyph value="Y"/>
+            <Glyph value="y"/>
+          </LookAheadCoverage>
+          <LookAheadCoverage index="1">
+            <Glyph value="Z"/>
+          </LookAheadCoverage>
+          <!-- SubstCount=1 -->
+          <SubstLookupRecord index="0">
+            <SequenceIndex value="0"/>
+            <LookupListIndex value="1"/>
+          </SubstLookupRecord>
+        </ChainContextSubst>
+      </Lookup>
+      <Lookup index="1">
+        <LookupType value="3"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <AlternateSubst index="0">
+          <AlternateSet glyph="a">
+            <Alternate glyph="a.alt1"/>
+            <Alternate glyph="a.alt2"/>
+            <Alternate glyph="a.alt3"/>
+          </AlternateSet>
+        </AlternateSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
+</ttFont>