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Fix chaining context backtrack matching order #372

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Fix chaining context backtrack matching order #372

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10 changes: 7 additions & 3 deletions src/opentype/OTProcessor.js
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Original file line number Diff line number Diff line change
Expand Up @@ -393,7 +393,9 @@ export default class OTProcessor {

let set = table.chainRuleSets[index];
for (let rule of set) {
if (this.sequenceMatches(-rule.backtrack.length, rule.backtrack)
// Backtrack is stored in reverse order per OpenType spec, so reverse
// it before matching forward from the start position.
if (this.sequenceMatches(-rule.backtrack.length, [...rule.backtrack].reverse())
&& this.sequenceMatches(1, rule.input)
&& this.sequenceMatches(1 + rule.input.length, rule.lookahead)) {
return this.applyLookupList(rule.lookupRecords);
Expand All @@ -414,7 +416,8 @@ export default class OTProcessor {
}

for (let rule of rules) {
if (this.classSequenceMatches(-rule.backtrack.length, rule.backtrack, table.backtrackClassDef) &&
// Backtrack is stored in reverse order per OpenType spec.
if (this.classSequenceMatches(-rule.backtrack.length, [...rule.backtrack].reverse(), table.backtrackClassDef) &&
this.classSequenceMatches(1, rule.input, table.inputClassDef) &&
this.classSequenceMatches(1 + rule.input.length, rule.lookahead, table.lookaheadClassDef)) {
return this.applyLookupList(rule.lookupRecords);
Expand All @@ -424,7 +427,8 @@ export default class OTProcessor {
break;

case 3:
if (this.coverageSequenceMatches(-table.backtrackGlyphCount, table.backtrackCoverage) &&
// Backtrack is stored in reverse order per OpenType spec.
if (this.coverageSequenceMatches(-table.backtrackGlyphCount, [...table.backtrackCoverage].reverse()) &&
this.coverageSequenceMatches(0, table.inputCoverage) &&
this.coverageSequenceMatches(table.inputGlyphCount, table.lookaheadCoverage)) {
return this.applyLookupList(table.lookupRecords);
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9 changes: 9 additions & 0 deletions test/shaping.js
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Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,15 @@ describe('shaping', function () {
'218+545|11+1781|94+1362|26@35,0+1139|34+564|32+1250|3+532|9+1904|96+1088|93+1383|51+569|8+1904|' +
'3+532|33+1225|21+1470|3+532|96+1088|17+1496|96+1088|17+1496|32+1250|3+532|9+1904|95+1104|12+1781|39+1052');

// Exercises chain context backtrack ordering: the Syriac `calt` chain rule
// requires backtrack[0] (mark uni0731) immediately before the input glyph
// (uni0713.Fina) and backtrack[1] (uni0712.Init) one position further
// back. Without reversing the backtrack array, fontkit matches in stored
// order and the rule fails to fire, leaving uni0713.Fina unsubstituted.
test('should match chain context backtrack in OpenType spec order',
'NotoSans/NotoSansSyriacEstrangela-Regular.ttf', 'ܒܱܓ',
'249+2485|141@606,-200+0|17+1496');

test('should shape N\'Ko text', 'NotoSans/NotoSansNKo-Regular.ttf', 'ߞߊ߬ ߞߐߕߐ߮ ߞߎߘߊ ߘߏ߫ ߘߊߦߟߍ߬ ߸ ߏ߬',
'52@10,-300+0|23+1128|3+532|64+985|3+532|52@150,-300+0|84+1268|139+1184|160+1067|76+543|119+1622|3+532|51@10,-300+0|90+1128' +
'|119+1622|3+532|75+543|118+1622|88+1212|137+1114|3+532|54@170,0+0|93+1321|109+1155|94+1321|137+1114|3+532|52@-210,0+0|75+543|137+1114');
Expand Down