Add method copyCurrentStructureExact() to JsonGenerator

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

@@ -2690,6 +2690,46 @@ public void copyCurrentStructure(JsonParser p) throws IOException
         }
     }
 
+    /**
+     * Same as {@link #copyCurrentStructure} with the exception that copying of numeric
+     * values tries to avoid any conversion losses; in particular for floating-point
+     * numbers. This usually matters when transcoding from textual format like JSON
+     * to a binary format.
+     * See {@link #_copyCurrentFloatValueExact} for details.
+     *
+     * @param p Parser that points to the value to copy
+     *
+     * @throws IOException if there is either an underlying I/O problem or encoding
+     *    issue at format layer
+     *
+     * @since 2.21
+     */
+    public void copyCurrentStructureExact(JsonParser p) throws IOException
+    {
+        JsonToken t = p.currentToken();
+        // Let's handle field-name separately first
+        int id = (t == null) ? ID_NOT_AVAILABLE : t.id();
+        if (id == ID_FIELD_NAME) {
+            writeFieldName(p.currentName());
+            t = p.nextToken();
+            id = (t == null) ? ID_NOT_AVAILABLE : t.id();
+            // fall-through to copy the associated value
+        }
+        switch (id) {
+        case ID_START_OBJECT:
+            writeStartObject();
+            _copyCurrentContentsExact(p);
+            return;
+        case ID_START_ARRAY:
+            writeStartArray();
+            _copyCurrentContentsExact(p);
+            return;
+
+        default:
+            copyCurrentEventExact(p);
+        }
+    }
+
     // @since 2.10
     protected void _copyCurrentContents(JsonParser p) throws IOException
     {
@@ -2753,6 +2793,69 @@ protected void _copyCurrentContents(JsonParser p) throws IOException
         }
     }
 
+    // @since 2.21
+    protected void _copyCurrentContentsExact(JsonParser p) throws IOException
+    {
+        int depth = 1;
+        JsonToken t;
+
+        // Mostly copied from `copyCurrentEventExact()`, but with added nesting counts
+        while ((t = p.nextToken()) != null) {
+            switch (t.id()) {
+            case ID_FIELD_NAME:
+                writeFieldName(p.currentName());
+                break;
+
+            case ID_START_ARRAY:
+                writeStartArray();
+                ++depth;
+                break;
+
+            case ID_START_OBJECT:
+                writeStartObject();
+                ++depth;
+                break;
+
+            case ID_END_ARRAY:
+                writeEndArray();
+                if (--depth == 0) {
+                    return;
+                }
+                break;
+            case ID_END_OBJECT:
+                writeEndObject();
+                if (--depth == 0) {
+                    return;
+                }
+                break;
+
+            case ID_STRING:
+                _copyCurrentStringValue(p);
+                break;
+            case ID_NUMBER_INT:
+                _copyCurrentIntValue(p);
+                break;
+            case ID_NUMBER_FLOAT:
+                _copyCurrentFloatValueExact(p);
+                break;
+            case ID_TRUE:
+                writeBoolean(true);
+                break;
+            case ID_FALSE:
+                writeBoolean(false);
+                break;
+            case ID_NULL:
+                writeNull();
+                break;
+            case ID_EMBEDDED_OBJECT:
+                writeObject(p.getEmbeddedObject());
+                break;
+            default:
+                throw new IllegalStateException("Internal error: unknown current token, "+t);
+            }
+        }
+    }
+
     /**
      * Method for copying current {@link JsonToken#VALUE_NUMBER_FLOAT} value;
      * overridable by format backend implementations.

src/test/java/com/fasterxml/jackson/core/write/GeneratorCopyTest.java

@@ -100,4 +100,39 @@ void copyObjectTokens()
         gen.close();
         assertEquals("{\"a\":1}", sw.toString());
     }
+
+    @Test
+    void copyNumericTokensExactly()
+            throws Exception
+    {
+        JsonFactory jf = JSON_F;
+        final String DOC = a2q("{ 'a':0.123456789123456789123456789, 'b':[" +
+            "{ 'c' : null, 'd' : 0.123456789123456789123456789 }] }");
+        try(JsonParser jp = jf.createParser(new StringReader(DOC))) {
+            StringWriter sw = new StringWriter();
+            try (JsonGenerator gen = jf.createGenerator(sw)) {
+                assertToken(JsonToken.START_OBJECT, jp.nextToken());
+                gen.copyCurrentStructureExact(jp);
+                // which will advance parser to matching end Object
+                assertToken(JsonToken.END_OBJECT, jp.currentToken());
+            }
+
+            assertEquals(
+                a2q("{'a':0.123456789123456789123456789,'b':[" +
+                    "{'c':null,'d':0.123456789123456789123456789}]}"),
+                sw.toString()
+            );
+        }
+
+        try(JsonParser jp = jf.createParser(new StringReader("0.123456789123456789123456789"))) {
+            StringWriter sw = new StringWriter();
+            try (JsonGenerator gen = jf.createGenerator(sw)) {
+                assertToken(JsonToken.VALUE_NUMBER_FLOAT, jp.nextToken());
+                gen.copyCurrentStructureExact(jp);
+                assertToken(JsonToken.VALUE_NUMBER_FLOAT, jp.currentToken());
+            }
+
+            assertEquals("0.123456789123456789123456789", sw.toString());
+        }
+    }
 }