Never fuse atoms across a font-style change

iosMath/lib/MTMathList.m

@@ -1707,8 +1707,10 @@ - (MTMathList *)finalized
                 break;
 
             case kMTMathAtomNumber:
-                // combine numbers together
-                if (prevNode && prevNode.type == kMTMathAtomNumber && !prevNode.subScript && !prevNode.superScript) {
+                // combine numbers together, but never across a font-style change:
+                // fuse: keeps the first atom's fontStyle and would corrupt the other's.
+                if (prevNode && prevNode.type == kMTMathAtomNumber && !prevNode.subScript && !prevNode.superScript
+                    && prevNode.fontStyle == newNode.fontStyle) {
                     [prevNode fuse:newNode];
                     // skip the current node, we are done here.
                     continue;

iosMath/render/internal/MTTypesetter.m

@@ -550,8 +550,11 @@ + (NSArray*) preprocessMathList:(MTMathList*) ml
 
         if (atom.type == kMTMathAtomOrdinary) {
             // This is Rule 14 to merge ordinary characters.
-            // combine ordinary atoms together
+            // combine ordinary atoms together, but only within one font style --
+            // the \mathit face is stamped per single-style run (and TeX's own Rule 14
+            // fuses only within a family).
             if (prevNode && prevNode.type == kMTMathAtomOrdinary && !prevNode.subScript && !prevNode.superScript
+                && prevNode.fontStyle == atom.fontStyle
                 && ![prevNode isKindOfClass:[MTLargeDelimiter class]]
                 && ![atom isKindOfClass:[MTLargeDelimiter class]]) {
                 [prevNode fuse:atom];

iosMathTests/MTMathListTest.m

@@ -426,6 +426,48 @@ - (void)testArrayTableFactoryNormalizesVerticalLines
     XCTAssertEqualObjects(table.verticalLines, (@[ @1, @0, @0 ]));
 }
 
+- (void) testFinalizedDoesNotFuseDigitsAcrossFontStyles
+{
+    // Without the guard, fuse: keeps the first atom's style: 1\mathit{2}
+    // silently drops the italic, \mathit{1}2 spreads it onto a plain digit.
+    MTMathList* list = [MTMathListBuilder buildFromString:@"1\\mathit{2}"].finalized;
+    XCTAssertEqual(list.atoms.count, 2);
+    XCTAssertEqualObjects(list.atoms[0].nucleus, @"1");
+    XCTAssertEqual(list.atoms[0].fontStyle, kMTFontStyleDefault);
+    XCTAssertEqualObjects(list.atoms[1].nucleus, @"2");
+    XCTAssertEqual(list.atoms[1].fontStyle, kMTFontStyleItalic);
+
+    list = [MTMathListBuilder buildFromString:@"\\mathit{1}2"].finalized;
+    XCTAssertEqual(list.atoms.count, 2);
+    XCTAssertEqual(list.atoms[0].fontStyle, kMTFontStyleItalic);
+    XCTAssertEqual(list.atoms[1].fontStyle, kMTFontStyleDefault);
+
+    // Style boundaries split the run; the uniform-style middle still fuses.
+    list = [MTMathListBuilder buildFromString:@"1\\mathit{23}4"].finalized;
+    XCTAssertEqual(list.atoms.count, 3);
+    XCTAssertEqualObjects(list.atoms[0].nucleus, @"1");
+    XCTAssertEqualObjects(list.atoms[1].nucleus, @"23");
+    XCTAssertEqualObjects(list.atoms[2].nucleus, @"4");
+
+    // The same guard repairs \mathbf on mixed-style digit runs (LLD §2.6).
+    list = [MTMathListBuilder buildFromString:@"1\\mathbf{2}"].finalized;
+    XCTAssertEqual(list.atoms.count, 2);
+    XCTAssertEqual(list.atoms[0].fontStyle, kMTFontStyleDefault);
+    XCTAssertEqual(list.atoms[1].fontStyle, kMTFontStyleBold);
+}
+
+- (void) testFinalizedStillFusesUniformStyleDigits
+{
+    MTMathList* list = [MTMathListBuilder buildFromString:@"1234"].finalized;
+    XCTAssertEqual(list.atoms.count, 1);
+    XCTAssertEqualObjects(list.atoms[0].nucleus, @"1234");
+
+    list = [MTMathListBuilder buildFromString:@"\\mathit{12}"].finalized;
+    XCTAssertEqual(list.atoms.count, 1);
+    XCTAssertEqualObjects(list.atoms[0].nucleus, @"12");
+    XCTAssertEqual(list.atoms[0].fontStyle, kMTFontStyleItalic);
+}
+
 @end
 
 @interface MTMathAtomTest : XCTestCase

iosMathTests/MTTypesetterTest.m

@@ -17,6 +17,10 @@
 #import "MTMathAtomFactory.h"
 #import "MTMathListBuilder.h"
 
+@interface MTTypesetter (FusionTesting)
++ (NSArray<MTMathAtom*>*) preprocessMathList:(MTMathList*) ml;
+@end
+
 @interface MTTypesetterTest : XCTestCase
 
 @property (nonatomic) MTFont* font;
@@ -3607,4 +3611,54 @@ - (void)testArrayRuleGeometryIsDeterministic
     XCTAssertEqualWithAccuracy(botLine, contentBot - padding, 0.01);
 }
 
+- (void) testPreprocessDoesNotFuseAcrossFontStyles
+{
+    // f\mathit{x}: without the guard both fuse into one Default atom and the
+    // italic on x is lost (LLD §2.6).
+    NSArray<MTMathAtom*>* atoms = [MTTypesetter preprocessMathList:
+        [MTMathListBuilder buildFromString:@"f\\mathit{x}"].finalized];
+    XCTAssertEqual(atoms.count, 2);
+    XCTAssertEqual(atoms[0].fontStyle, kMTFontStyleDefault);
+    XCTAssertEqual(atoms[1].fontStyle, kMTFontStyleItalic);
+
+    atoms = [MTTypesetter preprocessMathList:
+        [MTMathListBuilder buildFromString:@"\\mathit{f}x"].finalized];
+    XCTAssertEqual(atoms.count, 2);
+    XCTAssertEqual(atoms[0].fontStyle, kMTFontStyleItalic);
+    XCTAssertEqual(atoms[1].fontStyle, kMTFontStyleDefault);
+
+    // The bold digit that site A now preserves reaches its bold code point.
+    atoms = [MTTypesetter preprocessMathList:
+        [MTMathListBuilder buildFromString:@"1\\mathbf{2}"].finalized];
+    XCTAssertEqual(atoms.count, 2);
+    XCTAssertEqualObjects(atoms[0].nucleus, @"1");
+    XCTAssertEqualObjects(atoms[1].nucleus, @"\U0001D7D0");
+}
+
+- (void) testPreprocessStillFusesUniformStyleAtoms
+{
+    // Same style still merges...
+    NSArray<MTMathAtom*>* atoms = [MTTypesetter preprocessMathList:
+        [MTMathListBuilder buildFromString:@"\\mathit{fg}"].finalized];
+    XCTAssertEqual(atoms.count, 1);
+
+    // ...including Variable+Number pairs, which meet only at site B because
+    // site A fuses same-type atoms only (LLD §3.3 B).
+    atoms = [MTTypesetter preprocessMathList:
+        [MTMathListBuilder buildFromString:@"\\mathit{a1}"].finalized];
+    XCTAssertEqual(atoms.count, 1);
+    XCTAssertEqual(atoms[0].type, kMTMathAtomOrdinary);
+    XCTAssertEqual(atoms[0].fontStyle, kMTFontStyleItalic);
+}
+
+- (void) testPreprocessUnstyledListUnchanged
+{
+    NSArray<MTMathAtom*>* atoms = [MTTypesetter preprocessMathList:
+        [MTMathListBuilder buildFromString:@"xyz+1"].finalized];
+    XCTAssertEqual(atoms.count, 3);
+    XCTAssertEqualObjects(atoms[0].nucleus, @"\U0001D465\U0001D466\U0001D467");
+    XCTAssertEqualObjects(atoms[1].nucleus, @"+");
+    XCTAssertEqualObjects(atoms[2].nucleus, @"1");
+}
+
 @end