Move LLVM Bitstream Utils from swift-driver and add a Swift/Clang Serialized Diags Reader

Sources/TSCUtility/Bits.swift

@@ -0,0 +1,96 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2020 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See http://swift.org/LICENSE.txt for license information
+ See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+import Foundation
+
+struct Bits: RandomAccessCollection {
+  var buffer: Data
+
+  var startIndex: Int { return 0 }
+  var endIndex: Int { return buffer.count * 8 }
+
+  subscript(index: Int) -> UInt8 {
+    let byte = buffer[index / 8]
+    return (byte >> UInt8(index % 8)) & 1
+  }
+
+  func readBits(atOffset offset: Int, count: Int) -> UInt64 {
+    precondition(count >= 0 && count <= 64)
+    precondition(offset >= 0)
+    precondition(offset &+ count >= offset)
+    precondition(offset &+ count <= self.endIndex)
+
+    return buffer.withUnsafeBytes { (bytes: UnsafeRawBufferPointer) in
+      let upperBound = offset &+ count
+      let topByteIndex = upperBound >> 3
+      var result: UInt64 = 0
+      if upperBound & 7 != 0 {
+        let mask: UInt8 = (1 << UInt8(upperBound & 7)) &- 1
+        result = UInt64(bytes[topByteIndex] & mask)
+      }
+      for i in ((offset >> 3)..<(upperBound >> 3)).reversed() {
+        result <<= 8
+        result |= UInt64(bytes[i])
+      }
+      if offset & 7 != 0 {
+        result >>= UInt64(offset & 7)
+      }
+      return result
+    }
+  }
+
+  struct Cursor {
+    enum Error: Swift.Error { case bufferOverflow }
+
+    let buffer: Bits
+    private var offset: Int = 0
+
+    init(buffer: Bits) {
+      self.buffer = buffer
+    }
+
+    init(buffer: Data) {
+      self.init(buffer: Bits(buffer: buffer))
+    }
+
+    var isAtEnd: Bool {
+      return offset == buffer.count
+    }
+
+    func peek(_ count: Int) throws -> UInt64 {
+      if buffer.count - offset < count { throw Error.bufferOverflow }
+      return buffer.readBits(atOffset: offset, count: count)
+    }
+
+    mutating func read(_ count: Int) throws -> UInt64 {
+      defer { offset += count }
+      return try peek(count)
+    }
+
+    mutating func read(bytes count: Int) throws -> Data {
+      precondition(count >= 0)
+      precondition(offset & 0b111 == 0)
+      let newOffset = offset &+ (count << 3)
+      precondition(newOffset >= offset)
+      if newOffset > buffer.count { throw Error.bufferOverflow }
+      defer { offset = newOffset }
+      return buffer.buffer.dropFirst(offset >> 3).prefix((newOffset - offset) >> 3)
+    }
+
+    mutating func advance(toBitAlignment align: Int) throws {
+      precondition(align > 0)
+      precondition(offset &+ (align&-1) >= offset)
+      precondition(align & (align &- 1) == 0)
+      if offset % align == 0 { return }
+      offset = (offset &+ align) & ~(align &- 1)
+      if offset > buffer.count { throw Error.bufferOverflow }
+    }
+  }
+}