Fix crash on lone \sqrt at end of input (#188)

iosMath/lib/MTMathListBuilder.m

@@ -746,15 +746,18 @@ - (MTMathAtom*) atomForCommand:(NSString*) command
     } else if ([command isEqualToString:@"sqrt"]) {
         // A sqrt command with one argument
         MTRadical* rad = [MTRadical new];
-        unichar ch = [self getNextCharacter];
-        if (ch == '[') {
-            // special handling for sqrt[degree]{radicand}
-            rad.degree = [self buildInternal:false stopChar:']'];
-            rad.radicand = [self buildInternal:true];
-        } else {
-            [self unlookCharacter];
-            rad.radicand = [self buildInternal:true];
+        // Guard against a lone "\sqrt" at the end of input: only read a
+        // character if one is available.
+        if ([self hasCharacters]) {
+            unichar ch = [self getNextCharacter];
+            if (ch == '[') {
+                // special handling for sqrt[degree]{radicand}
+                rad.degree = [self buildInternal:false stopChar:']'];
+            } else {
+                [self unlookCharacter];
+            }
         }
+        rad.radicand = [self buildInternal:true];
         return rad;
     } else if ([command isEqualToString:@"left"]) {
         // Save the current inner while a new one gets built.

iosMathTests/MTMathListBuilderTest.m

@@ -378,6 +378,55 @@ - (void) testSqrt
     XCTAssertEqualObjects(latex, @"\\sqrt{2}", @"%@", desc);
 }
 
+- (void) testSqrtAtEnd
+{
+    // A lone \sqrt with no argument at the end of the input must not crash
+    // (it previously asserted in getNextCharacter). It should parse as a
+    // radical with an empty radicand, matching \sqrt{}.
+    NSString *str = @"\\sqrt";
+    MTMathList* list = [MTMathListBuilder buildFromString:str];
+    NSString* desc = [NSString stringWithFormat:@"Error for string:%@", str];
+
+    XCTAssertNotNil(list, @"%@", desc);
+    XCTAssertEqualObjects(@(list.atoms.count), @1, @"%@", desc);
+    MTRadical* rad = list.atoms[0];
+    XCTAssertEqual(rad.type, kMTMathAtomRadical, @"%@", desc);
+    XCTAssertEqualObjects(rad.nucleus, @"", @"%@", desc);
+
+    MTMathList *subList = rad.radicand;
+    XCTAssertNotNil(subList, @"%@", desc);
+    XCTAssertEqualObjects(@(subList.atoms.count), @0, @"%@", desc);
+    XCTAssertNil(rad.degree, @"%@", desc);
+
+    // convert it back to latex
+    NSString* latex = [MTMathListBuilder mathListToString:list];
+    XCTAssertEqualObjects(latex, @"\\sqrt{}", @"%@", desc);
+}
+
+- (void) testSqrtInGroup
+{
+    // A \sqrt with no argument inside a group exercises a different path
+    // (empty radicand via the oneCharOnly stop-char guard) than the
+    // end-of-input case, and must also not crash.
+    NSString *str = @"{\\sqrt}";
+    MTMathList* list = [MTMathListBuilder buildFromString:str];
+    NSString* desc = [NSString stringWithFormat:@"Error for string:%@", str];
+
+    XCTAssertNotNil(list, @"%@", desc);
+    XCTAssertEqualObjects(@(list.atoms.count), @1, @"%@", desc);
+    MTRadical* rad = list.atoms[0];
+    XCTAssertEqual(rad.type, kMTMathAtomRadical, @"%@", desc);
+
+    MTMathList *subList = rad.radicand;
+    XCTAssertNotNil(subList, @"%@", desc);
+    XCTAssertEqualObjects(@(subList.atoms.count), @0, @"%@", desc);
+    XCTAssertNil(rad.degree, @"%@", desc);
+
+    // convert it back to latex
+    NSString* latex = [MTMathListBuilder mathListToString:list];
+    XCTAssertEqualObjects(latex, @"\\sqrt{}", @"%@", desc);
+}
+
 - (void) testSqrtInSqrt
 {
     NSString *str = @"\\sqrt\\sqrt2";