Implement peekJson() for JsonValueReader

The JsonUtf8Reader variant relies on an update to Okio that we're
not quite ready for.

This one became straightforward after I changed out the iterators
to be cloneable. That's necessary to split one iterator into two.
It's too bad the platform's built-in iterators are not cloneable;
that would have been convenient and potentially more efficient.

Related to https://github.com/square/moshi/issues/672

moshi/src/main/java/com/squareup/moshi/JsonReader.java

@@ -180,10 +180,10 @@ public abstract class JsonReader implements Closeable {
   // The nesting stack. Using a manual array rather than an ArrayList saves 20%. This stack will
   // grow itself up to 256 levels of nesting including the top-level document. Deeper nesting is
   // prone to trigger StackOverflowErrors.
-  int stackSize = 0;
-  int[] scopes = new int[32];
-  String[] pathNames = new String[32];
-  int[] pathIndices = new int[32];
+  int stackSize;
+  int[] scopes;
+  String[] pathNames;
+  int[] pathIndices;
 
   /** True to accept non-spec compliant JSON. */
   boolean lenient;
@@ -196,8 +196,21 @@ public abstract class JsonReader implements Closeable {
     return new JsonUtf8Reader(source);
   }
 
+  // Package-private to control subclasses.
   JsonReader() {
-    // Package-private to control subclasses.
+    scopes = new int[32];
+    pathNames = new String[32];
+    pathIndices = new int[32];
+  }
+
+  // Package-private to control subclasses.
+  JsonReader(JsonReader copyFrom) {
+    this.stackSize = copyFrom.stackSize;
+    this.scopes = copyFrom.scopes.clone();
+    this.pathNames = copyFrom.pathNames.clone();
+    this.pathIndices = copyFrom.pathIndices.clone();
+    this.lenient = copyFrom.lenient;
+    this.failOnUnknown = copyFrom.failOnUnknown;
   }
 
   final void pushScope(int newTop) {
@@ -461,6 +474,32 @@ public abstract class JsonReader implements Closeable {
     }
   }
 
+  /**
+   * Returns a new {@code JsonReader} that can read data from this {@code JsonReader} without
+   * consuming it. The returned reader becomes invalid once this one is next read or closed.
+   *
+   * For example, we can use `peek()` to lookahead and read the same data multiple times.
+   *
+   * <pre> {@code
+   *
+   *   Buffer buffer = new Buffer();
+   *   buffer.writeUtf8("[123, 456, 789]")
+   *
+   *   JsonReader jsonReader = JsonReader.of(buffer);
+   *   jsonReader.beginArray();
+   *   jsonReader.nextInt(); // Returns 123, reader contains 456, 789 and ].
+   *
+   *   JsonReader peek = reader.peekReader();
+   *   peek.nextInt() // Returns 456.
+   *   peek.nextInt() // Returns 789.
+   *   peek.endArray()
+   *
+   *   jsonReader.nextInt() // Returns 456, reader contains 789 and ].
+   * }</pre>
+   */
+  // TODO(jwilson): make this public once it's supported in JsonUtf8Reader.
+  abstract JsonReader peekJson();
+
   /**
    * Returns a <a href="http://goessner.net/articles/JsonPath/">JsonPath</a> to
    * the current location in the JSON value.

moshi/src/main/java/com/squareup/moshi/JsonUtf8Reader.java

@@ -1059,6 +1059,10 @@ final class JsonUtf8Reader extends JsonReader {
     return false;
   }
 
+  @Override JsonReader peekJson() {
+    throw new UnsupportedOperationException("TODO");
+  }
+
   @Override public String toString() {
     return "JsonReader(" + source + ")";
   }

moshi/src/main/java/com/squareup/moshi/JsonValueReader.java

@@ -20,10 +20,11 @@ import java.math.BigDecimal;
 import java.util.Arrays;
 import java.util.Iterator;
 import java.util.List;
-import java.util.ListIterator;
 import java.util.Map;
 import javax.annotation.Nullable;
 
+import static com.squareup.moshi.JsonScope.CLOSED;
+
 /**
  * This class reads a JSON document by traversing a Java object comprising maps, lists, and JSON
  * primitives. It does depth-first traversal keeping a stack starting with the root object. During
@@ -32,11 +33,11 @@ import javax.annotation.Nullable;
  * <ul>
  *   <li>The next element to act upon is on the top of the stack.
  *   <li>When the top of the stack is a {@link List}, calling {@link #beginArray()} replaces the
- *       list with a {@link ListIterator}. The first element of the iterator is pushed on top of the
+ *       list with a {@link JsonIterator}. The first element of the iterator is pushed on top of the
  *       iterator.
  *   <li>Similarly, when the top of the stack is a {@link Map}, calling {@link #beginObject()}
- *       replaces the map with an {@link Iterator} of its entries. The first element of the iterator
- *       is pushed on top of the iterator.
+ *       replaces the map with an {@link JsonIterator} of its entries. The first element of the
+ *       iterator is pushed on top of the iterator.
  *   <li>When the top of the stack is a {@link Map.Entry}, calling {@link #nextName()} returns the
  *       entry's key and replaces the entry with its value on the stack.
  *   <li>When an element is consumed it is popped. If the new top of the stack has a non-exhausted
@@ -49,17 +50,31 @@ final class JsonValueReader extends JsonReader {
   /** Sentinel object pushed on {@link #stack} when the reader is closed. */
   private static final Object JSON_READER_CLOSED = new Object();
 
-  private Object[] stack = new Object[32];
+  private Object[] stack;
 
   JsonValueReader(Object root) {
     scopes[stackSize] = JsonScope.NONEMPTY_DOCUMENT;
+    stack = new Object[32];
     stack[stackSize++] = root;
   }
 
+  /** Copy-constructor makes a deep copy for peeking. */
+  JsonValueReader(JsonValueReader copyFrom) {
+    super(copyFrom);
+
+    stack = copyFrom.stack.clone();
+    for (int i = 0; i < stackSize; i++) {
+      if (stack[i] instanceof JsonIterator) {
+        stack[i] = ((JsonIterator) stack[i]).clone();
+      }
+    }
+  }
+
   @Override public void beginArray() throws IOException {
     List<?> peeked = require(List.class, Token.BEGIN_ARRAY);
 
-    ListIterator<?> iterator = peeked.listIterator();
+    JsonIterator iterator = new JsonIterator(
+        Token.END_ARRAY, peeked.toArray(new Object[peeked.size()]), 0);
     stack[stackSize - 1] = iterator;
     scopes[stackSize - 1] = JsonScope.EMPTY_ARRAY;
     pathIndices[stackSize - 1] = 0;
@@ -71,8 +86,8 @@ final class JsonValueReader extends JsonReader {
   }
 
   @Override public void endArray() throws IOException {
-    ListIterator<?> peeked = require(ListIterator.class, Token.END_ARRAY);
-    if (peeked.hasNext()) {
+    JsonIterator peeked = require(JsonIterator.class, Token.END_ARRAY);
+    if (peeked.endToken != Token.END_ARRAY || peeked.hasNext()) {
       throw typeMismatch(peeked, Token.END_ARRAY);
     }
     remove();
@@ -81,7 +96,8 @@ final class JsonValueReader extends JsonReader {
   @Override public void beginObject() throws IOException {
     Map<?, ?> peeked = require(Map.class, Token.BEGIN_OBJECT);
 
-    Iterator<?> iterator = peeked.entrySet().iterator();
+    JsonIterator iterator = new JsonIterator(
+        Token.END_OBJECT, peeked.entrySet().toArray(new Object[peeked.size()]), 0);
     stack[stackSize - 1] = iterator;
     scopes[stackSize - 1] = JsonScope.EMPTY_OBJECT;
 
@@ -92,8 +108,8 @@ final class JsonValueReader extends JsonReader {
   }
 
   @Override public void endObject() throws IOException {
-    Iterator<?> peeked = require(Iterator.class, Token.END_OBJECT);
-    if (peeked instanceof ListIterator || peeked.hasNext()) {
+    JsonIterator peeked = require(JsonIterator.class, Token.END_OBJECT);
+    if (peeked.endToken != Token.END_OBJECT || peeked.hasNext()) {
       throw typeMismatch(peeked, Token.END_OBJECT);
     }
     pathNames[stackSize - 1] = null;
@@ -112,8 +128,7 @@ final class JsonValueReader extends JsonReader {
 
     // If the top of the stack is an iterator, take its first element and push it on the stack.
     Object peeked = stack[stackSize - 1];
-    if (peeked instanceof ListIterator) return Token.END_ARRAY;
-    if (peeked instanceof Iterator) return Token.END_OBJECT;
+    if (peeked instanceof JsonIterator) return ((JsonIterator) peeked).endToken;
     if (peeked instanceof List) return Token.BEGIN_ARRAY;
     if (peeked instanceof Map) return Token.BEGIN_OBJECT;
     if (peeked instanceof Map.Entry) return Token.NAME;
@@ -303,6 +318,10 @@ final class JsonValueReader extends JsonReader {
     }
   }
 
+  @Override JsonReader peekJson() {
+    return new JsonValueReader(this);
+  }
+
   @Override void promoteNameToValue() throws IOException {
     if (hasNext()) {
       String name = nextName();
@@ -313,7 +332,7 @@ final class JsonValueReader extends JsonReader {
   @Override public void close() throws IOException {
     Arrays.fill(stack, 0, stackSize, null);
     stack[0] = JSON_READER_CLOSED;
-    scopes[0] = JsonScope.CLOSED;
+    scopes[0] = CLOSED;
     stackSize = 1;
   }
 
@@ -374,4 +393,33 @@ final class JsonValueReader extends JsonReader {
       }
     }
   }
+
+  static final class JsonIterator implements Iterator<Object>, Cloneable {
+    final Token endToken;
+    final Object[] array;
+    int next;
+
+    JsonIterator(Token endToken, Object[] array, int next) {
+      this.endToken = endToken;
+      this.array = array;
+      this.next = next;
+    }
+
+    @Override public boolean hasNext() {
+      return next < array.length;
+    }
+
+    @Override public Object next() {
+      return array[next++];
+    }
+
+    @Override public void remove() {
+      throw new UnsupportedOperationException();
+    }
+
+    @Override protected JsonIterator clone() {
+      // No need to copy the array; it's read-only.
+      return new JsonIterator(endToken, array, next);
+    }
+  }
 }

moshi/src/test/java/com/squareup/moshi/JsonCodecFactory.java

@@ -96,9 +96,38 @@ abstract class JsonCodecFactory {
       }
     };
 
+    final JsonCodecFactory valuePeek = new JsonCodecFactory() {
+      @Override public JsonReader newReader(String json) throws IOException {
+        return value.newReader(json).peekJson();
+      }
+
+      // TODO(jwilson): fix precision checks and delete his method.
+      @Override boolean implementsStrictPrecision() {
+        return false;
+      }
+
+      @Override JsonWriter newWriter() {
+        return value.newWriter();
+      }
+
+      @Override String json() {
+        return value.json();
+      }
+
+      // TODO(jwilson): support BigDecimal and BigInteger and delete his method.
+      @Override boolean supportsBigNumbers() {
+        return false;
+      }
+
+      @Override public String toString() {
+        return "ValuePeek";
+      }
+    };
+
     return Arrays.asList(
         new Object[] { utf8 },
-        new Object[] { value });
+        new Object[] { value },
+        new Object[] { valuePeek });
   }
 
   abstract JsonReader newReader(String json) throws IOException;

moshi/src/test/java/com/squareup/moshi/JsonReaderTest.java

@@ -985,4 +985,103 @@ public final class JsonReaderTest {
     reader.readJsonValue();
     assertThat(reader.hasNext()).isFalse();
   }
+
+  @Test public void basicPeekJson() throws IOException {
+    JsonReader reader = newReader("{\"a\":12,\"b\":[34,56],\"c\":78}");
+    assumeTrue(reader instanceof JsonValueReader); // Not implemented for JsonUtf8Reader yet!
+    reader.beginObject();
+    assertThat(reader.nextName()).isEqualTo("a");
+    assertThat(reader.nextInt()).isEqualTo(12);
+    assertThat(reader.nextName()).isEqualTo("b");
+    reader.beginArray();
+    assertThat(reader.nextInt()).isEqualTo(34);
+
+    // Peek.
+    JsonReader peekReader = reader.peekJson();
+    assertThat(peekReader.nextInt()).isEqualTo(56);
+    peekReader.endArray();
+    assertThat(peekReader.nextName()).isEqualTo("c");
+    assertThat(peekReader.nextInt()).isEqualTo(78);
+    peekReader.endObject();
+    assertThat(peekReader.peek()).isEqualTo(JsonReader.Token.END_DOCUMENT);
+
+    // Read again.
+    assertThat(reader.nextInt()).isEqualTo(56);
+    reader.endArray();
+    assertThat(reader.nextName()).isEqualTo("c");
+    assertThat(reader.nextInt()).isEqualTo(78);
+    reader.endObject();
+    assertThat(reader.peek()).isEqualTo(JsonReader.Token.END_DOCUMENT);
+  }
+
+  /**
+   * We have a document that requires 12 operations to read. We read it step-by-step with one real
+   * reader. Before each of the real reader’s operations we create a peeking reader and let it read
+   * the rest of the document.
+   */
+  @Test public void peekJsonReader() throws IOException {
+    JsonReader reader = newReader("[12,34,{\"a\":56,\"b\":78},90]");
+    assumeTrue(reader instanceof JsonValueReader); // Not implemented for JsonUtf8Reader yet!
+    for (int i = 0; i < 12; i++) {
+      readPeek12Steps(reader.peekJson(), i, 12);
+      readPeek12Steps(reader, i, i + 1);
+    }
+  }
+
+  /**
+   * Read a fragment of {@code reader}. This assumes the fixed document defined in {@link
+   * #peekJsonReader} and reads a range of it on each call.
+   */
+  private void readPeek12Steps(JsonReader reader, int from, int until) throws IOException {
+    switch (from) {
+      case 0:
+        if (until == 0) break;
+        reader.beginArray();
+        assertThat(reader.getPath()).isEqualTo("$[0]");
+      case 1:
+        if (until == 1) break;
+        assertThat(reader.nextInt()).isEqualTo(12);
+        assertThat(reader.getPath()).isEqualTo("$[1]");
+      case 2:
+        if (until == 2) break;
+        assertThat(reader.nextInt()).isEqualTo(34);
+        assertThat(reader.getPath()).isEqualTo("$[2]");
+      case 3:
+        if (until == 3) break;
+        reader.beginObject();
+        assertThat(reader.getPath()).isEqualTo("$[2].");
+      case 4:
+        if (until == 4) break;
+        assertThat(reader.nextName()).isEqualTo("a");
+        assertThat(reader.getPath()).isEqualTo("$[2].a");
+      case 5:
+        if (until == 5) break;
+        assertThat(reader.nextInt()).isEqualTo(56);
+        assertThat(reader.getPath()).isEqualTo("$[2].a");
+      case 6:
+        if (until == 6) break;
+        assertThat(reader.nextName()).isEqualTo("b");
+        assertThat(reader.getPath()).isEqualTo("$[2].b");
+      case 7:
+        if (until == 7) break;
+        assertThat(reader.nextInt()).isEqualTo(78);
+        assertThat(reader.getPath()).isEqualTo("$[2].b");
+      case 8:
+        if (until == 8) break;
+        reader.endObject();
+        assertThat(reader.getPath()).isEqualTo("$[3]");
+      case 9:
+        if (until == 9) break;
+        assertThat(reader.nextInt()).isEqualTo(90);
+        assertThat(reader.getPath()).isEqualTo("$[4]");
+      case 10:
+        if (until == 10) break;
+        reader.endArray();
+        assertThat(reader.getPath()).isEqualTo("$");
+      case 11:
+        if (until == 11) break;
+        assertThat(reader.peek()).isEqualTo(JsonReader.Token.END_DOCUMENT);
+        assertThat(reader.getPath()).isEqualTo("$");
+    }
+  }
 }