[Swift] Migrates struct write APIS to write directly to the buffer (#6093)

* Migrates struct write APIS to in place APIS

* Fixes indentation in grpc swift

2 files changed, 74 insertions, 13 deletions

diff --git a/swift/Sources/FlatBuffers/ByteBuffer.swift b/swift/Sources/FlatBuffers/ByteBuffer.swift
index 924a0d8f..855d5ff6 100644
--- a/swift/Sources/FlatBuffers/ByteBuffer.swift
+++ b/swift/Sources/FlatBuffers/ByteBuffer.swift
@@ -166,7 +166,7 @@ public struct ByteBuffer {
     @usableFromInline mutating func push<T: Scalar>(elements: [T]) {
         let size = elements.count &* MemoryLayout<T>.size
         ensureSpace(size: size)
-        elements.lazy.reversed().forEach { (s) in
+        elements.reversed().forEach { (s) in
             push(value: s, len: MemoryLayout.size(ofValue: s))
         }
     }
@@ -175,12 +175,32 @@ public struct ByteBuffer {
     /// - Parameters:
     ///   - value: Pointer to the object in memory
     ///   - size: Size of Value being written to the buffer
+    @available(*, deprecated, message: "0.9.0 will be removing the following method. Regenerate the code")
     @usableFromInline mutating func push(struct value: UnsafeMutableRawPointer, size: Int) {
         ensureSpace(size: size)
         memcpy(_storage.memory.advanced(by: writerIndex &- size), value, size)
         defer { value.deallocate() }
         _writerSize = _writerSize &+ size
     }
+    
+    /// Prepares the buffer to receive a struct of certian size.
+    /// The alignment of the memory is already handled since we already called preAlign
+    /// - Parameter size: size of the struct
+    @usableFromInline mutating func prepareBufferToReceiveStruct(of size: Int) {
+        ensureSpace(size: size)
+        _writerSize = _writerSize &+ size
+    }
+    
+    /// Reverse the input direction to the buffer, since `FlatBuffers` uses a back to front, following method will take current `writerIndex`
+    /// and writes front to back into the buffer, respecting the padding & the alignment
+    /// - Parameters:
+    ///   - value: value of type Scalar
+    ///   - position: position relative to the `writerIndex`
+    ///   - len: length of the value in terms of bytes
+    @usableFromInline mutating func reversePush<T: Scalar>(value: T, position: Int, len: Int) {
+        var v = value
+        memcpy(_storage.memory.advanced(by: writerIndex &+ position), &v, len)
+    }
 
     /// Adds an object of type Scalar into the buffer
     /// - Parameters:
@@ -201,7 +221,7 @@ public struct ByteBuffer {
         if str.utf8.withContiguousStorageIfAvailable({ self.push(bytes: $0, len: len) }) != nil {
         } else {
             let utf8View = str.utf8
-            for c in utf8View.lazy.reversed() {
+            for c in utf8View.reversed() {
                 push(value: c, len: 1)
             }
         }
@@ -246,14 +266,11 @@ public struct ByteBuffer {
         return size
     }
     
-    /// Resizes the buffer size
-    /// - Parameter size: new size for the buffer
-    @usableFromInline mutating internal func resize(_ size: Int) {
+    /// pops the written VTable if it's already written into the buffer
+    /// - Parameter size: size of the `VTable`
+    @usableFromInline mutating internal func pop(_ size: Int) {
         assert((_writerSize &- size) > 0, "New size should NOT be a negative number")
-        var zero: UInt8 = 0
-        for i in 0..<(_writerSize &- size) {
-            memcpy(_storage.memory.advanced(by: writerIndex &+ i), &zero, MemoryLayout<UInt8>.size)
-        }
+        memset(_storage.memory.advanced(by: writerIndex), 0, _writerSize &- size)
         _writerSize = size
     }
     
diff --git a/swift/Sources/FlatBuffers/FlatBufferBuilder.swift b/swift/Sources/FlatBuffers/FlatBufferBuilder.swift
index de6fa848..cfdb20ce 100644
--- a/swift/Sources/FlatBuffers/FlatBufferBuilder.swift
+++ b/swift/Sources/FlatBuffers/FlatBufferBuilder.swift
@@ -187,7 +187,7 @@ public struct FlatBufferBuilder {
             let vTableOff = Int(vTableOffset)
             let space = _bb.capacity &- vTableOff
             _bb.write(value: Int32(offset &- vTableOff), index: space, direct: true)
-            _bb.resize(_bb.capacity &- space)
+            _bb.pop(_bb.capacity &- space)
         } else {
             _bb.write(value: Int32(vt_use &- vTableOffset), index: Int(vTableOffset))
             _vtables.append(_bb.size)
@@ -304,7 +304,7 @@ public struct FlatBufferBuilder {
     mutating public func createVector<T: Enum>(_ elements: [T], size: Int) -> Offset<UOffset> {
         let size = size
         startVector(size, elementSize: T.byteSize)
-        for e in elements.lazy.reversed() {
+        for e in elements.reversed() {
             _bb.push(value: e.value, len: T.byteSize)
         }
         return Offset(offset: endVector(len: size))
@@ -323,7 +323,7 @@ public struct FlatBufferBuilder {
     /// - returns: Offset of the vector
     mutating public func createVector<T>(ofOffsets offsets: [Offset<T>], len: Int) -> Offset<UOffset> {
         startVector(len, elementSize: MemoryLayout<Offset<T>>.size)
-        for o in offsets.lazy.reversed() {
+        for o in offsets.reversed() {
             push(element: o)
         }
         return Offset(offset: endVector(len: len))
@@ -347,14 +347,31 @@ public struct FlatBufferBuilder {
     ///   - structs: An array of UnsafeMutableRawPointer
     ///   - type: Type of the struct being written
     /// - returns: Offset of the vector
+    @available(*, deprecated, message: "0.9.0 will be removing the following method. Regenerate the code")
     mutating public func createVector<T: Readable>(structs: [UnsafeMutableRawPointer],
                                           type: T.Type) -> Offset<UOffset> {
         startVector(structs.count &* T.size, elementSize: T.alignment)
-        for i in structs.lazy.reversed() {
+        for i in structs.reversed() {
             create(struct: i, type: T.self)
         }
         return Offset(offset: endVector(len: structs.count))
     }
+    
+    /// Starts a vector of struct that considers the size and alignment of the struct
+    /// - Parameters:
+    ///   - count: number of elements to be written
+    ///   - size: size of struct
+    ///   - alignment: alignment of the struct
+    mutating public func startVectorOfStructs(count: Int, size: Int, alignment: Int) {
+        startVector(count &* size, elementSize: alignment)
+    }
+    
+    /// Ends the vector of structs and writtens the current offset
+    /// - Parameter count: number of written elements
+    /// - Returns: Offset of type UOffset
+    mutating public func endVectorOfStructs(count: Int) -> Offset<UOffset> {
+        return Offset<UOffset>(offset: endVector(len: count))
+    }
 
     // MARK: - Inserting Structs
     
@@ -363,6 +380,7 @@ public struct FlatBufferBuilder {
     ///   - s: Flatbuffer struct
     ///   - type: Type of the element to be serialized
     /// - returns: Offset of the Object
+    @available(*, deprecated, message: "0.9.0 will be removing the following method. Regenerate the code")
     @discardableResult
     mutating public func create<T: Readable>(struct s: UnsafeMutableRawPointer,
                                     type: T.Type) -> Offset<UOffset> {
@@ -372,6 +390,32 @@ public struct FlatBufferBuilder {
         return Offset(offset: _bb.size)
     }
     
+    /// prepares the ByteBuffer to receive a struct of size and alignment
+    /// - Parameters:
+    ///   - size: size of written struct
+    ///   - alignment: alignment of written struct
+    mutating public func createStructOf(size: Int, alignment: Int) {
+        preAlign(len: size, alignment: alignment)
+        _bb.prepareBufferToReceiveStruct(of: size)
+    }
+    
+    /// Adds scalars front to back instead of the default behavior of the normal add
+    /// - Parameters:
+    ///   - v: element of type Scalar
+    ///   - postion: position relative to the `writerIndex`
+    mutating public func reverseAdd<T: Scalar>(v: T, postion: Int) {
+        _bb.reversePush(value: v,
+                        position: postion,
+                        len: MemoryLayout<T>.size)
+    }
+    
+    /// Ends the struct and returns the current buffer size
+    /// - Returns: Offset of type UOffset
+    @discardableResult
+    public func endStruct() -> Offset<UOffset> {
+        return Offset(offset: _bb.size)
+    }
+    
     /// Adds the offset of a struct into the vTable
     ///
     /// The function fatalErrors if we pass an offset that is out of range