Merge pull request swiftlang#51 from kastiglione/dl/adjust-batch-partition-count-calculation

Adjust batch partition count calculation

Author: DougGregor

Sources/SwiftDriver/Jobs/Planning.swift

@@ -302,12 +302,23 @@ extension Driver {
     // pre-batch-mode 1000. I.e. it's still running 96% fewer
     // subprocesses than before. And significantly: it's doing so while
     // not exceeding the RAM of a typical 2-core laptop.
+
+    // An explanation of why the partition calculation isn't integer
+    // division. Using an example, a module of 26 files exceeds the
+    // limit of 25 and must be compiled in 2 batches. Integer division
+    // yields 26/25 = 1 batch, but a single batch of 26 exceeds the
+    // limit. The calculation must round up, which can be calculated
+    // using: `(x + y - 1) / y`
+    let divideRoundingUp = { num, div in
+        return (num + div - 1) / div
+    }
+
     let defaultSizeLimit = 25
     let numInputFiles = swiftInputFiles.count
     let sizeLimit = info.sizeLimit ?? defaultSizeLimit
 
     let numTasks = numParallelJobs ?? 1
-    return max(numTasks, numInputFiles / sizeLimit)
+    return max(numTasks, divideRoundingUp(numInputFiles, sizeLimit))
   }
 
   /// Describes the partitions used when batching.