refactor and cleanup [skip ci]

submodules/Postbox/Sources/MappedFile.swift

@@ -0,0 +1,51 @@
+import Foundation
+
+public final class MappedFile {
+    private var handle: Int32
+    private var currentSize: Int
+    private var memory: UnsafeMutableRawPointer
+    
+    public init(path: String) {
+        self.handle = open(path, O_RDWR | O_CREAT | O_APPEND, S_IRUSR | S_IWUSR)
+        
+        var value = stat()
+        stat(path, &value)
+        self.currentSize = Int(value.st_size)
+        
+        self.memory = mmap(nil, self.currentSize, PROT_READ | PROT_WRITE, MAP_SHARED, self.handle, 0)
+    }
+    
+    deinit {
+        munmap(self.memory, self.currentSize)
+        close(self.handle)
+    }
+    
+    public var size: Int {
+        get {
+            return self.currentSize
+        } set(value) {
+            if value != self.currentSize {
+                munmap(self.memory, self.currentSize)
+                ftruncate(self.handle, off_t(value))
+                self.currentSize = value
+                self.memory = mmap(nil, self.currentSize, PROT_READ | PROT_WRITE, MAP_SHARED, self.handle, 0)
+            }
+        }
+    }
+    
+    public func synchronize() {
+        msync(self.memory, self.currentSize, MS_ASYNC)
+    }
+    
+    public func write(at range: Range<Int>, from data: UnsafeRawPointer) {
+        memcpy(self.memory.advanced(by: range.lowerBound), data, range.count)
+    }
+    
+    public func read(at range: Range<Int>, to data: UnsafeMutableRawPointer) {
+        memcpy(data, self.memory.advanced(by: range.lowerBound), range.count)
+    }
+    
+    public func clear() {
+        memset(self.memory, 0, self.currentSize)
+    }
+}