Add StreamReadCapability.EXACT_FLOATS to indicate whether parser can expose exact floating-point values or not

src/main/java/com/fasterxml/jackson/core/JsonGenerator.java

@@ -2468,13 +2468,17 @@ public void copyCurrentEvent(JsonParser p) throws IOException
         }
         case ID_NUMBER_FLOAT:
         {
-            NumberType n = p.getNumberType();
-            if (n == NumberType.BIG_DECIMAL) {
-                writeNumber(p.getDecimalValue());
-            } else if (n == NumberType.FLOAT) {
-                writeNumber(p.getFloatValue());
+            if (p.getReadCapabilities().isEnabled(StreamReadCapability.EXACT_FLOATS)) {
+                NumberType n = p.getNumberType();
+                if (n == NumberType.BIG_DECIMAL) {
+                    writeNumber(p.getDecimalValue());
+                } else if (n == NumberType.FLOAT) {
+                    writeNumber(p.getFloatValue());
+                } else {
+                    writeNumber(p.getDoubleValue());
+                }
             } else {
-                writeNumber(p.getDoubleValue());
+                writeNumber(p.getTextCharacters(), p.getTextOffset(), p.getTextLength());
             }
             break;
         }
@@ -2613,13 +2617,17 @@ protected void _copyCurrentContents(JsonParser p) throws IOException
             }
             case ID_NUMBER_FLOAT:
             {
-                NumberType n = p.getNumberType();
-                if (n == NumberType.BIG_DECIMAL) {
-                    writeNumber(p.getDecimalValue());
-                } else if (n == NumberType.FLOAT) {
-                    writeNumber(p.getFloatValue());
+                if (p.getReadCapabilities().isEnabled(StreamReadCapability.EXACT_FLOATS)) {
+                    NumberType n = p.getNumberType();
+                    if (n == NumberType.BIG_DECIMAL) {
+                        writeNumber(p.getDecimalValue());
+                    } else if (n == NumberType.FLOAT) {
+                        writeNumber(p.getFloatValue());
+                    } else {
+                        writeNumber(p.getDoubleValue());
+                    }
                 } else {
-                    writeNumber(p.getDoubleValue());
+                    writeNumber(p.getTextCharacters(), p.getTextOffset(), p.getTextLength());
                 }
                 break;
             }

src/main/java/com/fasterxml/jackson/core/StreamReadCapability.java

@@ -48,6 +48,23 @@ public enum StreamReadCapability
      * This capability is true for many textual formats like CSV, Properties and XML.
      */
     UNTYPED_SCALARS(false),
+
+    /**
+     * Capability that indicates that data format may report floats
+     * as a non-exact {@link com.fasterxml.jackson.core.JsonParser.NumberType},
+     * due to prohibitively expensive parsing costs of determing the precision
+     * upfront. For example, JSON numbers may be reported as
+     * {@link com.fasterxml.jackson.core.JsonParser.NumberType#DOUBLE}
+     * even if they would not fit into a 64-bit double without precision
+     * loss. Methods like {@link JsonParser#getNumberValueExact()} or
+     * {@link JsonParser#getValueAsString()} still report values without
+     * precision loss.
+     *
+     * Capability is false for text formats JSON, but true for binary formats
+     * like Smile, MessagePack, etc., where type is precisely and inexpensively
+     * signaled by a tag.
+     */
+    EXACT_FLOATS(false)
     ;
 
     /**

src/test/java/com/fasterxml/jackson/core/json/JsonParserGeneratorTest.java

@@ -0,0 +1,40 @@
+package com.fasterxml.jackson.core.json;
+
+import com.fasterxml.jackson.core.BaseTest;
+import com.fasterxml.jackson.core.JsonFactory;
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.core.JsonParser;
+
+import java.io.ByteArrayOutputStream;
+import java.io.StringWriter;
+
+public class JsonParserGeneratorTest
+    extends BaseTest {
+    final JsonFactory JSON_F = newStreamFactory();
+
+    public void testCopyCurrentEventBigDecimal() throws Exception {
+        String input = "1e999";
+        JsonParser parser = JSON_F.createParser(input);
+        parser.nextToken();
+        StringWriter stringWriter = new StringWriter();
+        JsonGenerator generator = JSON_F.createGenerator(stringWriter);
+        generator.copyCurrentEvent(parser);
+        parser.close();
+        generator.close();
+        String actual = stringWriter.toString();
+        assertEquals(input, actual);
+    }
+
+    public void testCopyCurrentStructureBigDecimal() throws Exception {
+        String input = "[1e999]";
+        JsonParser parser = JSON_F.createParser(input);
+        parser.nextToken();
+        StringWriter stringWriter = new StringWriter();
+        JsonGenerator generator = JSON_F.createGenerator(stringWriter);
+        generator.copyCurrentStructure(parser);
+        parser.close();
+        generator.close();
+        String actual = stringWriter.toString();
+        assertEquals(input, actual);
+    }
+}