Shuffle batch mode partitions if a seed is provided

Sources/SwiftDriver/Driver/CompilerMode.swift

@@ -31,7 +31,7 @@ public enum CompilerMode: Equatable {
 /// Information about batch mode, which is used to determine how to form
 /// the batches of jobs.
 public struct BatchModeInfo: Equatable {
-  let seed: Int
+  let seed: Int?
   let count: Int?
   let sizeLimit: Int?
 }

Sources/SwiftDriver/Driver/Driver.swift

@@ -689,7 +689,7 @@ extension Driver {
 
     // For batch mode, collect information
     if wantBatchMode {
-      let batchSeed = parseIntOption(&parsedOptions, option: .driverBatchSeed, diagnosticsEngine: diagnosticsEngine) ?? 0
+      let batchSeed = parseIntOption(&parsedOptions, option: .driverBatchSeed, diagnosticsEngine: diagnosticsEngine)
       let batchCount = parseIntOption(&parsedOptions, option: .driverBatchCount, diagnosticsEngine: diagnosticsEngine)
       let batchSizeLimit = parseIntOption(&parsedOptions, option: .driverBatchSizeLimit, diagnosticsEngine: diagnosticsEngine)
       return .batchCompile(BatchModeInfo(seed: batchSeed, count: batchCount, sizeLimit: batchSizeLimit))

Sources/SwiftDriver/Jobs/Planning.swift

@@ -304,7 +304,11 @@ extension Driver {
       partitionIndices.append(contentsOf: Array(repeating: partitionIdx, count: fillCount))
     }
     assert(partitionIndices.count == numInputFiles)
-    // FIXME: If info.seed is non-null, shuffle.
+
+    if let seed = info.seed {
+      var generator = PredictableRandomNumberGenerator(seed: UInt64(seed))
+      partitionIndices.shuffle(using: &generator)
+    }
 
     // Form the actual partitions.
     var assignment: [TypedVirtualPath : Int] = [:]

Sources/SwiftDriver/Utilities/PredictableRandomNumberGenerator.swift

@@ -0,0 +1,55 @@
+//===------------ PredictableRandomNumberGenerator.swift ------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+/// An _insecure_ random number generator which given an initial seed will generate a _predictable_
+/// sequence of pseudo-random numbers. This generator is not thread safe.
+///
+/// The generator uses the [xoshiro256**](http://prng.di.unimi.it/xoshiro256starstar.c)
+/// algorithm to produce its output. It is initialized using the
+/// [splitmix64](http://prng.di.unimi.it/splitmix64.c) algorithm.
+public struct PredictableRandomNumberGenerator: RandomNumberGenerator {
+
+  var state: (UInt64, UInt64, UInt64, UInt64)
+
+  public init(seed: UInt64) {
+    func initNext(_ state: inout UInt64) -> UInt64 {
+      state = state &+ 0x9e3779b97f4a7c15
+      var z = state
+      z = (z ^ (z &>> 30)) &* 0xbf58476d1ce4e5b9
+      z = (z ^ (z &>> 27)) &* 0x94d049bb133111eb
+      return z ^ (z &>> 31)
+    }
+
+    var initState = seed
+    state = (initNext(&initState), initNext(&initState),
+             initNext(&initState), initNext(&initState))
+  }
+
+  mutating public func next() -> UInt64 {
+    defer {
+      let t = state.1 &<< 17
+      state.2 ^= state.0
+      state.3 ^= state.1;
+      state.1 ^= state.2;
+      state.0 ^= state.3;
+      state.2 ^= t;
+      state.3 = state.3.rotateLeft(45)
+    }
+    return (state.1 &* 5).rotateLeft(7) &* 9
+  }
+}
+
+fileprivate extension UInt64 {
+  func rotateLeft(_ numBits: UInt8) -> UInt64 {
+    return (self &<< numBits) | (self &>> (64 - numBits))
+  }
+}

Tests/SwiftDriverTests/PredictableRandomNumberGeneratorTests.swift

@@ -0,0 +1,49 @@
+//===--------------- PredictableRandomNumberGeneratorTests.swift ----------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import XCTest
+import SwiftDriver
+
+/// This generator is deterministic and platform independent, so the sequence for each seed should remain constant.
+final class PredictableRandomNumberGeneratorTests: XCTestCase {
+  func testPredictability() {
+    var generator = PredictableRandomNumberGenerator(seed: 42)
+    XCTAssertEqual([generator.next(), generator.next(), generator.next(),
+                    generator.next(), generator.next()],
+                   [1546998764402558742, 6990951692964543102,
+                    12544586762248559009, 17057574109182124193,
+                    18295552978065317476])
+
+    var generator2 = PredictableRandomNumberGenerator(seed: 42)
+    XCTAssertEqual([generator2.next(), generator2.next(), generator2.next(),
+                    generator2.next(), generator2.next()],
+                   [1546998764402558742, 6990951692964543102,
+                    12544586762248559009, 17057574109182124193,
+                    18295552978065317476])
+  }
+
+  func testUnusualSeeds() {
+    var generator = PredictableRandomNumberGenerator(seed: 0)
+    XCTAssertEqual([generator.next(), generator.next(), generator.next(),
+                    generator.next(), generator.next()],
+                   [11091344671253066420, 13793997310169335082,
+                    1900383378846508768, 7684712102626143532,
+                    13521403990117723737])
+
+    var generator2 = PredictableRandomNumberGenerator(seed: 0xFFFFFFFFFFFFFFFF)
+    XCTAssertEqual([generator2.next(), generator2.next(), generator2.next(),
+                    generator2.next(), generator2.next()],
+                   [10328197420357168392, 14156678507024973869,
+                    9357971779955476126, 13791585006304312367,
+                    10463432026814718762])
+  }
+}