Adds support for stackrel using a subclass of MTLargeOperator.

iosMath/lib/MTMathList.h

@@ -494,4 +494,15 @@ typedef NS_ENUM(NSInteger, MTColumnAlignment) {
 
 @end
 
+/** A subclass of MTLargeOperator, it adds no new functionality,
+  but allows to check if the atom if of said type when building a string
+  out of a math list
+ */
+@interface MTStackRel : MTLargeOperator
+
+/// Convenience init to create the stackrel
+- (instancetype) initWithBottom:(NSString*) bottom andTop:(MTMathList*) top NS_DESIGNATED_INITIALIZER;
+
+@end
+
 NS_ASSUME_NONNULL_END

iosMath/lib/MTMathList.m

@@ -1034,3 +1034,17 @@ - (id)copyWithZone:(NSZone *)zone
 }
 
 @end
+
+#pragma mark - MTStackRel
+
+@implementation MTStackRel : MTLargeOperator
+
+/// Convenience init to create the stackrel
+- (instancetype) initWithBottom:(NSString*) bottom andTop:(MTMathList*) top
+{
+    self = [super initWithValue:bottom limits:YES];
+    self.superScript = top;
+    return self;
+}
+
+@end

iosMath/lib/MTMathListBuilder.m

@@ -506,6 +506,11 @@ - (MTMathAtom*) atomForCommand:(NSString*) command
         mathColorbox.colorString = [self readColor];
         mathColorbox.innerList = [self buildInternal:true];
         return mathColorbox;
+    } else if ([command isEqualToString:@"stackrel"]) {
+        MTMathList* top = [self buildInternal:YES];
+        MTMathList* bottom = [self buildInternal:YES];
+        MTLargeOperator* stack = [[MTStackRel alloc] initWithBottom:bottom.stringValue andTop:top];
+        return stack;
     } else {
         NSString* errorMessage = [NSString stringWithFormat:@"Invalid command \\%@", command];
         [self setError:MTParseErrorInvalidCommand message:errorMessage];
@@ -842,15 +847,20 @@ + (NSString *)mathListToString:(MTMathList *)ml
             MTAccent* accent = (MTAccent*) atom;
             [str appendFormat:@"\\%@{%@}", [MTMathAtomFactory accentName:accent], [self mathListToString:accent.innerList]];
         } else if (atom.type == kMTMathAtomLargeOperator) {
-            MTLargeOperator* op = (MTLargeOperator*) atom;
-            NSString* command = [MTMathAtomFactory latexSymbolNameForAtom:atom];
-            MTLargeOperator* originalOp = (MTLargeOperator*) [MTMathAtomFactory atomForLatexSymbolName:command];
-            [str appendFormat:@"\\%@ ", command];
-            if (originalOp.limits != op.limits) {
-                if (op.limits) {
-                    [str appendString:@"\\limits "];
-                } else {
-                    [str appendString:@"\\nolimits "];
+            if ([atom isKindOfClass:[MTStackRel class]]) {
+                [str appendFormat:@"\\stackrel{%@}{%@}", [self mathListToString:atom.superScript], atom.nucleus];
+                continue;
+            } else {
+                MTLargeOperator* op = (MTLargeOperator*) atom;
+                NSString* command = [MTMathAtomFactory latexSymbolNameForAtom:atom];
+                MTLargeOperator* originalOp = (MTLargeOperator*) [MTMathAtomFactory atomForLatexSymbolName:command];
+                [str appendFormat:@"\\%@ ", command];
+                if (originalOp.limits != op.limits) {
+                    if (op.limits) {
+                        [str appendString:@"\\limits "];
+                    } else {
+                        [str appendString:@"\\nolimits "];
+                    }
                 }
             }
         } else if (atom.type == kMTMathAtomSpace) {

iosMathExample/example/ViewController.m

@@ -82,7 +82,7 @@ - (void)viewDidLoad
     // set the size of the content view
     // Disable horizontal scrolling.
     [self setEqualWidths:contentView andView:self.scrollView];
-    [self setHeight:4350 forView:contentView];
+    [self setHeight:4520 forView:contentView];
 
 
     // Demo formulae
@@ -177,7 +177,7 @@ - (void)viewDidLoad
                            "\\end{cases}" withHeight:60];
 
     self.demoLabels[21] = [self createMathLabel:@"\\color{#ff3333}{c}\\color{#9933ff}{o}\\color{#ff0080}{l}+\\color{#99ff33}{\\frac{\\color{#ff99ff}{o}}{\\color{#990099}{r}}}-\\color{#33ffff}{\\sqrt[\\color{#3399ff}{e}]{\\color{#3333ff}{d}}}" withHeight:60];
-
+  
 
     for (NSUInteger i = 1; i < self.demoLabels.count; i++) {
         self.demoLabels[i].fontSize = 15;
@@ -320,6 +320,10 @@ - (void)viewDidLoad
                        "\\colorbox{#00aaff}{c} & \\colorbox{#f0f0f0}{d}"
                        "\\end{pmatrix}}"
                                  withHeight:70];
+
+    // Test stackrel
+    self.labels[48] = [self createMathLabel:@"\\lim_{x \\to 0} \\frac{\\ln{x}}{x^{2} - 1} \\stackrel{H}{=} \\lim_{x \\to 0} \\frac{\\frac{1}{x}}{2x}" withHeight:90];
+    self.labels[49] = [self createMathLabel:@"\\stackrel{up}{bottom}" withHeight:60];
 
     for (NSUInteger i = 1; i < self.labels.count; i++) {
         [self addLabelWithIndex:i inArray:self.labels toView:contentView];

iosMathTests/MTMathListBuilderTest.m

@@ -1385,4 +1385,23 @@ - (void) testNoLimits
     latex = [MTMathListBuilder mathListToString:list];
     XCTAssertEqualObjects(latex, @"\\sum \\nolimits ", @"%@", desc);
 }
+
+- (void) testStackrel
+{
+  // L'hopital rule
+  NSString *str = @"\\stackrel{H}{=}";
+  MTMathList* list = [MTMathListBuilder buildFromString:str];
+  NSString* desc = [NSString stringWithFormat:@"Error for string:%@", str];
+
+  XCTAssertNotNil(list, @"%@", desc);
+  XCTAssertEqualObjects(@(list.atoms.count), @1, @"%@", desc);
+  MTLargeOperator* op = list.atoms[0];
+  XCTAssertEqual(op.type, kMTMathAtomLargeOperator, @"%@", desc);
+  XCTAssertTrue(op.limits);
+  XCTAssertTrue([op isKindOfClass:[MTStackRel class]]);
+
+  // convert it back to latex
+  NSString* latex = [MTMathListBuilder mathListToString:list];
+  XCTAssertEqualObjects(latex, @"\\stackrel{H}{=}", @"%@", desc);
+}
 @end