Fixes #459 - Implements the rem CSS unit.

With test proof.

Author: danfickle

openhtmltopdf-core/src/main/java/com/openhtmltopdf/context/StyleReference.java

@@ -39,6 +39,7 @@
 import com.openhtmltopdf.css.sheet.PropertyDeclaration;
 import com.openhtmltopdf.css.sheet.Stylesheet;
 import com.openhtmltopdf.css.sheet.StylesheetInfo;
+import com.openhtmltopdf.css.style.CalculatedStyle;
 import com.openhtmltopdf.extend.NamespaceHandler;
 import com.openhtmltopdf.extend.UserAgentCallback;
 import com.openhtmltopdf.extend.UserInterface;
@@ -72,6 +73,17 @@ public StyleReference(UserAgentCallback userAgent) {
         _stylesheetFactory = new StylesheetFactoryImpl(userAgent);
     }
 
+    /**
+     * Gets the style of the root element, should be html tag.
+     */
+    public CalculatedStyle getRootElementStyle() {
+        if (_context != null && _doc != null) {
+            return _context.getStyle(_doc.getDocumentElement());
+        } else {
+            return null;
+        }
+    }
+
     /**
      * Sets the documentContext attribute of the StyleReference object
      *

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/constants/ValueConstants.java

@@ -151,6 +151,7 @@ public static boolean isAbsoluteUnit(short type) {
                 return false;
                 // refer to values known to the DerivedValue instance (tobe)
             case CSSPrimitiveValue.CSS_EMS:
+            case CSSPrimitiveValue.CSS_REMS:
             case CSSPrimitiveValue.CSS_EXS:
                 // length
             case CSSPrimitiveValue.CSS_IN:

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/parser/CSSParser.java

@@ -1383,6 +1383,7 @@ private List<PropertyValue> expr(boolean literal) throws IOException {
                 case Token.URI:
                 case Token.HASH:
                 case Token.FUNCTION:
+                case Token.DIMENSION:
                     PropertyValue term = term(literal);
                     if (operatorToken != null) {
                         term.setOperator(operatorToken);
@@ -1462,7 +1463,19 @@ private PropertyValue term(boolean literal) throws IOException {
             case Token.TIME:
             case Token.FREQ:
             case Token.DIMENSION:
-                throw new CSSParseException("Unsupported CSS unit " + extractUnit(t), getCurrentLine());
+                String unitType = extractUnit(t);
+
+                if ("rem".equals(unitType)) {
+                    result = new PropertyValue(
+                          CSSPrimitiveValue.CSS_REMS,
+                          sign*Float.parseFloat(extractNumber(t)),
+                          sign(sign) + getTokenValue(t));
+                    next();
+                    skip_whitespace();
+                } else {
+                    throw new CSSParseException("Unsupported CSS unit " + unitType, getCurrentLine());
+                }
+                break;
             case Token.NUMBER:
                 result = new PropertyValue(
                         CSSPrimitiveValue.CSS_NUMBER,

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/parser/CSSPrimitiveValue.java

@@ -27,7 +27,10 @@ public interface CSSPrimitiveValue extends CSSValue {
     public static final short CSS_COUNTER = 23;
     public static final short CSS_RECT = 24;
     public static final short CSS_RGBCOLOR = 25;
-    
+
+    /** rems unit, not official, added by danfickle. */
+    public static final short CSS_REMS = 26;
+
     public short getPrimitiveType();
     public float getFloatValue(short unitType);
     public String getStringValue();

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/parser/property/AbstractPropertyBuilder.java

@@ -173,6 +173,7 @@ public static boolean isLengthHelper(CSSPrimitiveValue value) {
                 || unit == CSSPrimitiveValue.CSS_PX || unit == CSSPrimitiveValue.CSS_IN
                 || unit == CSSPrimitiveValue.CSS_CM || unit == CSSPrimitiveValue.CSS_MM
                 || unit == CSSPrimitiveValue.CSS_PT || unit == CSSPrimitiveValue.CSS_PC
+                || unit == CSSPrimitiveValue.CSS_REMS
                 || (unit == CSSPrimitiveValue.CSS_NUMBER && value.getFloatValue(CSSPrimitiveValue.CSS_IN) == 0.0f);
     }
 

openhtmltopdf-core/src/main/java/com/openhtmltopdf/css/style/derived/LengthValue.java

@@ -144,6 +144,19 @@ public static float calcFloatProportionalValue(CalculatedStyle style,
                 }
 
                 break;
+            case CSSPrimitiveValue.CSS_REMS: {
+                // The rem unit is the same as em except uses the font-size of the html element
+                // rather than the font-size of current/parent element.
+                CalculatedStyle htmlTagStyle = ctx.getCss().getRootElementStyle();
+                if (htmlTagStyle == null) {
+                    // The default font-size for the html tag is 16px.
+                    absVal = relVal * 16f * ctx.getDotsPerPixel();
+                } else {
+                   FontSpecification htmlTagFontSpecification = htmlTagStyle.getFont(ctx);
+                   absVal = relVal * htmlTagFontSpecification.size;
+                }
+                break;
+            }
             case CSSPrimitiveValue.CSS_EXS:
                 // To convert EMS to pixels, we need the height of the lowercase 'Xx' character in the current
                 // element...

openhtmltopdf-examples/src/main/resources/visualtest/expected/issue-459-rem-unit-default.pdf

@@ -0,0 +1,20 @@
+<html>
+<head>
+<style>
+@page {
+  size: 15rem 240px;
+  margin: 0.5rem;
+}
+body {
+  border: 1px solid orange;
+  margin: 1rem;
+  max-width: 12rem;
+}
+</style>
+</head>
+<body>
+  <!-- Should look the same if rem default amount is 16px -->
+  <div style="margin: 16px; padding: 0.1rem 32px; border: 1.6px solid red; background-color: blue; height: 24px;">ONE</div>
+  <div style="margin: 1rem; padding: 1.6px 2rem; border: 0.1rem solid red; background-color: blue; height: 1.5rem;">TWO</div>
+</body>
+</html>

openhtmltopdf-examples/src/main/resources/visualtest/html/issue-459-rem-unit-default.html

@@ -1007,6 +1007,15 @@ public void testIssue446LangSelector() throws IOException {
         assertTrue(vt.runTest("issue-446-lang-selector"));
     }
 
+    /**
+     * Tests that the rem unit work with a document that has no author
+     * font-size set on the html element (defaults to 16px).
+     */
+    @Test
+    public void testIssue459RemUnitDefault() throws IOException {
+        assertTrue(vt.runTest("issue-459-rem-unit-default"));
+    }
+
     /**
      * Tests what the CSS content property is capable of.
      */