/*
 * Copyright 2014 Google Inc. All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.flatbuffers;

import static com.google.flatbuffers.Constants.*;
import java.util.Arrays;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.charset.Charset;

/**
 * Class that helps you build a FlatBuffer.  See the section
 * <a href="http://google.github.io/flatbuffers/md__java_usage.html">"Use in Java"</a> in the
 * main FlatBuffers documentation.
 */
public class FlatBufferBuilder {
    ByteBuffer bb;          // Where we construct the FlatBuffer.
    int space;              // Remaining space in the ByteBuffer.
    static final Charset utf8charset = Charset.forName("UTF-8");
    int minalign = 1;       // Minimum alignment encountered so far.
    int[] vtable = null;    // The vtable for the current table.
    int vtable_in_use = 0;  // The amount of fields we're actually using.
    boolean nested = false; // Whether we are currently serializing a table.
    int object_start;       // Starting offset of the current struct/table.
    int[] vtables = new int[16];  // List of offsets of all vtables.
    int num_vtables = 0;          // Number of entries in `vtables` in use.
    int vector_num_elems = 0;     // For the current vector being built.
    boolean force_defaults = false; // False omits default values from the serialized data

   /**
    * Start with a buffer of size {@code initial_size}, then grow as required.
    *
    * @param initial_size The initial size of the internal buffer to use
    */
    public FlatBufferBuilder(int initial_size) {
        if (initial_size <= 0) initial_size = 1;
        space = initial_size;
        bb = newByteBuffer(initial_size);
    }

    /**
     * Alternative constructor allowing reuse of {@link ByteBuffer}s.  The builder
     * can still grow the buffer as necessary.  User classes should make sure
     * to call {@link #dataBuffer()} to obtain the resulting encoded message
     *
     * @param existing_bb The byte buffer to reuse
     */
    public FlatBufferBuilder(ByteBuffer existing_bb) {
        init(existing_bb);
    }

    /**
     * Alternative initializer that allows reusing this object on an existing
     * ByteBuffer. This method resets the builder's internal state, but keeps
     * objects that have been allocated for temporary storage.
     *
     * @param existing_bb The byte buffer to reuse
     * @return this
     */
    public FlatBufferBuilder init(ByteBuffer existing_bb){
        bb = existing_bb;
        bb.clear();
        bb.order(ByteOrder.LITTLE_ENDIAN);
        minalign = 1;
        space = bb.capacity();
        vtable_in_use = 0;
        nested = false;
        object_start = 0;
        num_vtables = 0;
        vector_num_elems = 0;
        return this;
    }

    static ByteBuffer newByteBuffer(int capacity) {
        ByteBuffer newbb = ByteBuffer.allocate(capacity);
        newbb.order(ByteOrder.LITTLE_ENDIAN);
        return newbb;
    }

    /**
     * Doubles the size of the backing {link ByteBuffer} and copies the old data towards the
     * end of the new buffer (since we build the buffer backwards).
     *
     * @param bb The current buffer with the existing data
     * @return A new byte buffer with the old data copied copied to it.  The data is
     * located at the end of the buffer.
     */
    static ByteBuffer growByteBuffer(ByteBuffer bb) {
        int old_buf_size = bb.capacity();
        if ((old_buf_size & 0xC0000000) != 0)  // Ensure we don't grow beyond what fits in an int.
            throw new AssertionError("FlatBuffers: cannot grow buffer beyond 2 gigabytes.");
        int new_buf_size = old_buf_size << 1;
        bb.position(0);
        ByteBuffer nbb = newByteBuffer(new_buf_size);
        nbb.position(new_buf_size - old_buf_size);
        nbb.put(bb);
        return nbb;
    }

   /**
    * Offset relative to the end of the buffer.
    *
    * @return Offset relative to the end of the buffer.
    */
    public int offset() {
        return bb.capacity() - space;
    }

   /**
    * Add zero valued bytes to prepare a new entry to be added
    *
    * @param byte_size Number of bytes to add.
    */
    public void pad(int byte_size) {
        for (int i = 0; i < byte_size; i++) bb.put(--space, (byte)0);
    }

   /**
    * Prepare to write an element of {@code size} after {@code additional_bytes}
    * have been written, e.g. if you write a string, you need to align such
    * the int length field is aligned to {@link com.google.flatbuffers.Constants#SIZEOF_INT}, and
    * the string data follows it directly.  If all you need to do is alignment, {@code additional_bytes}
    * will be 0.
    *
    * @param size This is the of the new element to write
    * @param additional_bytes The padding size
    */
    public void prep(int size, int additional_bytes) {
        // Track the biggest thing we've ever aligned to.
        if (size > minalign) minalign = size;
        // Find the amount of alignment needed such that `size` is properly
        // aligned after `additional_bytes`
        int align_size = ((~(bb.capacity() - space + additional_bytes)) + 1) & (size - 1);
        // Reallocate the buffer if needed.
        while (space < align_size + size + additional_bytes) {
            int old_buf_size = bb.capacity();
            bb = growByteBuffer(bb);
            space += bb.capacity() - old_buf_size;
        }
        pad(align_size);
    }

    // Add a scalar to the buffer, backwards from the current location.
    // Doesn't align nor check for space.
    public void putBoolean(boolean x) { bb.put      (space -= 1, (byte)(x ? 1 : 0)); }
    public void putByte   (byte    x) { bb.put      (space -= 1, x); }
    public void putShort  (short   x) { bb.putShort (space -= 2, x); }
    public void putInt    (int     x) { bb.putInt   (space -= 4, x); }
    public void putLong   (long    x) { bb.putLong  (space -= 8, x); }
    public void putFloat  (float   x) { bb.putFloat (space -= 4, x); }
    public void putDouble (double  x) { bb.putDouble(space -= 8, x); }

    // Adds a scalar to the buffer, properly aligned, and the buffer grown
    // if needed.
    public void addBoolean(boolean x) { prep(1, 0); putBoolean(x); }
    public void addByte   (byte    x) { prep(1, 0); putByte   (x); }
    public void addShort  (short   x) { prep(2, 0); putShort  (x); }
    public void addInt    (int     x) { prep(4, 0); putInt    (x); }
    public void addLong   (long    x) { prep(8, 0); putLong   (x); }
    public void addFloat  (float   x) { prep(4, 0); putFloat  (x); }
    public void addDouble (double  x) { prep(8, 0); putDouble (x); }

   /**
    * Adds on offset, relative to where it will be written.
    *
    * @param off The offset to add
    */
    public void addOffset(int off) {
        prep(SIZEOF_INT, 0);  // Ensure alignment is already done.
        assert off <= offset();
        off = offset() - off + SIZEOF_INT;
        putInt(off);
    }

   /**
    * Start a new array/vector of objects.  Users usually will not call
    * this directly.  The {@code FlatBuffers} compiler will create a start/end
    * method for vector types in generated code.
    * <p>
    * The expected sequence of calls is:
    * <ol>
    * <li>Start the array using this method.</li>
    * <li>Call {@link #addOffset(int)} {@code num_elems} number of times to set
    * the offset of each element in the array.</li>
    * <li>Call {@link #endVector()} to retrieve the offset of the array.</li>
    * </ol>
    * <p>
    * For example, to create an array of strings, do:
    * <pre>{@code
    * // Need 10 strings
    * FlatBufferBuilder builder = new FlatBufferBuilder(existingBuffer);
    * int[] offsets = new int[10];
    *
    * for (int i = 0; i < 10; i++) {
    *   offsets[i] = fbb.createString(" " + i);
    * }
    *
    * // Have the strings in the buffer, but don't have a vector.
    * // Add a vector that references the newly created strings:
    * builder.startVector(4, offsets.length, 4);
    *
    * // Add each string to the newly created vector
    * // The strings are added in reverse order since the buffer
    * // is filled in back to front
    * for (int i = offsets.length - 1; i >= 0; i--) {
    *   builder.addOffset(offsets[i]);
    * }
    *
    * // Finish off the vector
    * int offsetOfTheVector = fbb.endVector();
    * }</pre>
    *
    * @param elem_size The size of each element in the array
    * @param num_elems The number of elements in the array
    * @param alignment The alignment of the array
    */
    public void startVector(int elem_size, int num_elems, int alignment) {
        notNested();
        vector_num_elems = num_elems;
        prep(SIZEOF_INT, elem_size * num_elems);
        prep(alignment, elem_size * num_elems); // Just in case alignment > int.
    }

   /**
    * Finish off the creation of an array and all its elements.  The array
    * must be created with {@link #startVector(int, int, int)}.
    *
    * @return The offset at which the newly created array starts.
    * @see #startVector(int, int, int)
    */
    public int endVector() {
        putInt(vector_num_elems);
        return offset();
    }

   /**
    * Encode the string {@code s} in the buffer using UTF-8.
    *
    * @param s The string to encode
    * @return The offset in the buffer where the encoded string starts
    */
    public int createString(String s) {
        byte[] utf8 = s.getBytes(utf8charset);
        addByte((byte)0);
        startVector(1, utf8.length, 1);
        bb.position(space -= utf8.length);
        bb.put(utf8, 0, utf8.length);
        return endVector();
    }

   /**
    * Should not be creating any other object, string or vector
    * while an object is being constructed
    */
    public void notNested() {
        if (nested)
            throw new AssertionError("FlatBuffers: object serialization must not be nested.");
    }

   /**
    * Structures are always stored inline, they need to be created right
    * where they're used.  You'll get this assertion failure if you
    * created it elsewhere.
    *
    * @param obj The offset of the created object
    */
    public void Nested(int obj) {
        if (obj != offset())
            throw new AssertionError("FlatBuffers: struct must be serialized inline.");
    }

   /**
    * Start encoding a new object in the buffer.  Users will not usually need to
    * call this directly. The {@code FlatBuffers} compiler will generate helper methods
    * that call this method internally.
    * <p>
    * For example, using the "Monster" code found on the
    * <a href="http://google.github.io/flatbuffers/md__java_usage.html">landing page</a>. An
    * object of type {@code Monster} can be created using the following code:
    *
    * <pre>{@code
    * int testArrayOfString = Monster.createTestarrayofstringVector(fbb, new int[] {
    *   fbb.createString("test1"),
    *   fbb.createString("test2")
    * });
    *
    * Monster.startMonster(fbb);
    * Monster.addPos(fbb, Vec3.createVec3(fbb, 1.0f, 2.0f, 3.0f, 3.0,
    *   Color.Green, (short)5, (byte)6));
    * Monster.addHp(fbb, (short)80);
    * Monster.addName(fbb, str);
    * Monster.addInventory(fbb, inv);
    * Monster.addTestType(fbb, (byte)Any.Monster);
    * Monster.addTest(fbb, mon2);
    * Monster.addTest4(fbb, test4);
    * Monster.addTestarrayofstring(fbb, testArrayOfString);
    * int mon = Monster.endMonster(fbb);
    * }</pre>
    * <p>
    * Here:
    * <ul>
    * <li>The call to {@code Monster#startMonster(FlatBufferBuilder)} will call this
    * method with the right number of fields set.</li>
    * <li>{@code Monster#endMonster(FlatBufferBuilder)} will ensure {@link #endObject()} is called.</li>
    * </ul>
    * <p>
    * It's not recommended to call this method directly.  If it's called manually, you must ensure
    * to audit all calls to it whenever fields are added or removed from your schema.  This is
    * automatically done by the code generated by the {@code FlatBuffers} compiler.
    *
    * @param numfields The number of fields found in this object.
    */
    public void startObject(int numfields) {
        notNested();
        if (vtable == null || vtable.length < numfields) vtable = new int[numfields];
        vtable_in_use = numfields;
        Arrays.fill(vtable, 0, vtable_in_use, 0);
        nested = true;
        object_start = offset();
    }

    // Add a scalar to a table at `o` into its vtable, with value `x` and default `d`
    public void addBoolean(int o, boolean x, boolean d) { if(force_defaults || x != d) { addBoolean(x); slot(o); } }
    public void addByte   (int o, byte    x, int     d) { if(force_defaults || x != d) { addByte   (x); slot(o); } }
    public void addShort  (int o, short   x, int     d) { if(force_defaults || x != d) { addShort  (x); slot(o); } }
    public void addInt    (int o, int     x, int     d) { if(force_defaults || x != d) { addInt    (x); slot(o); } }
    public void addLong   (int o, long    x, long    d) { if(force_defaults || x != d) { addLong   (x); slot(o); } }
    public void addFloat  (int o, float   x, double  d) { if(force_defaults || x != d) { addFloat  (x); slot(o); } }
    public void addDouble (int o, double  x, double  d) { if(force_defaults || x != d) { addDouble (x); slot(o); } }
    public void addOffset (int o, int     x, int     d) { if(force_defaults || x != d) { addOffset (x); slot(o); } }

    // Structs are stored inline, so nothing additional is being added. `d` is always 0.
    public void addStruct(int voffset, int x, int d) {
        if(x != d) {
            Nested(x);
            slot(voffset);
        }
    }

    // Set the current vtable at `voffset` to the current location in the buffer.
    public void slot(int voffset) {
        vtable[voffset] = offset();
    }

   /**
    * Finish off writing the object that is under construction.
    *
    * @return The offset to the object inside {@link #dataBuffer()}
    * @see #startObject(int)
    */
    public int endObject() {
        if (vtable == null || !nested)
            throw new AssertionError("FlatBuffers: endObject called without startObject");
        addInt(0);
        int vtableloc = offset();
        // Write out the current vtable.
        for (int i = vtable_in_use - 1; i >= 0 ; i--) {
            // Offset relative to the start of the table.
            short off = (short)(vtable[i] != 0 ? vtableloc - vtable[i] : 0);
            addShort(off);
        }

        final int standard_fields = 2; // The fields below:
        addShort((short)(vtableloc - object_start));
        addShort((short)((vtable_in_use + standard_fields) * SIZEOF_SHORT));

        // Search for an existing vtable that matches the current one.
        int existing_vtable = 0;
        outer_loop:
        for (int i = 0; i < num_vtables; i++) {
            int vt1 = bb.capacity() - vtables[i];
            int vt2 = space;
            short len = bb.getShort(vt1);
            if (len == bb.getShort(vt2)) {
                for (int j = SIZEOF_SHORT; j < len; j += SIZEOF_SHORT) {
                    if (bb.getShort(vt1 + j) != bb.getShort(vt2 + j)) {
                        continue outer_loop;
                    }
                }
                existing_vtable = vtables[i];
                break outer_loop;
            }
        }

        if (existing_vtable != 0) {
            // Found a match:
            // Remove the current vtable.
            space = bb.capacity() - vtableloc;
            // Point table to existing vtable.
            bb.putInt(space, existing_vtable - vtableloc);
        } else {
            // No match:
            // Add the location of the current vtable to the list of vtables.
            if (num_vtables == vtables.length) vtables = Arrays.copyOf(vtables, num_vtables * 2);
            vtables[num_vtables++] = offset();
            // Point table to current vtable.
            bb.putInt(bb.capacity() - vtableloc, offset() - vtableloc);
        }

        nested = false;
        return vtableloc;
    }

    // This checks a required field has been set in a given table that has
    // just been constructed.
    public void required(int table, int field) {
        int table_start = bb.capacity() - table;
        int vtable_start = table_start - bb.getInt(table_start);
        boolean ok = bb.getShort(vtable_start + field) != 0;
        // If this fails, the caller will show what field needs to be set.
        if (!ok)
            throw new AssertionError("FlatBuffers: field " + field + " must be set");
    }

    public void finish(int root_table) {
        prep(minalign, SIZEOF_INT);
        addOffset(root_table);
        bb.position(space);
    }

    public void finish(int root_table, String file_identifier) {
        prep(minalign, SIZEOF_INT + FILE_IDENTIFIER_LENGTH);
        if (file_identifier.length() != FILE_IDENTIFIER_LENGTH)
            throw new AssertionError("FlatBuffers: file identifier must be length " +
                                     FILE_IDENTIFIER_LENGTH);
        for (int i = FILE_IDENTIFIER_LENGTH - 1; i >= 0; i--) {
            addByte((byte)file_identifier.charAt(i));
        }
        finish(root_table);
    }

    /**
     * In order to save space, fields that are set to their default value
     * don't get serialized into the buffer. Forcing defaults provides a
     * way to manually disable this optimization.
     *
     * @param forceDefaults true always serializes default values
     * @return this
     */
    public FlatBufferBuilder forceDefaults(boolean forceDefaults){
        this.force_defaults = forceDefaults;
        return this;
    }

    // Get the ByteBuffer representing the FlatBuffer. Only call this after you've
    // called finish(). The actual data starts at the ByteBuffer's current position,
    // not necessarily at 0.
    public ByteBuffer dataBuffer() { return bb; }

   /**
    * The FlatBuffer data doesn't start at offset 0 in the {@link ByteBuffer}, but
    * now the {@code ByteBuffer}'s position is set to that location upon {@link #finish(int)}.
    *
    * @return The {@link ByteBuffer#position() position} the data starts in {@link #dataBuffer()}
    * @deprecated This method should not be needed anymore, but is left
    * here for the moment to document this API change. It will be removed in the future.
    */
    @Deprecated
    private int dataStart() {
        return space;
    }

   /**
    * Utility function for copying a byte array from {@code start} to
    * {@code start} + {@code length}
    *
    * @param start Start copying at this offset
    * @param length How many bytes to copy
    * @return A range copy of the {@link #dataBuffer() data buffer}
    * @throws IndexOutOfBoundsException If the range of bytes is ouf of bound
    */
    public byte[] sizedByteArray(int start, int length){
        byte[] array = new byte[length];
        bb.position(start);
        bb.get(array);
        return array;
    }

   /**
    * Utility function for copying a byte array that starts at 0.
    *
    * @return A full copy of the {@link #dataBuffer() data buffer}
    */
    public byte[] sizedByteArray() {
        return sizedByteArray(space, bb.capacity() - space);
    }
}