Performance improvements for cli json report and minor refactoring (#724)

Major change:
* Identified hotspot in `JsonElem.hashCode` because it hashes children recursively. Hence simplified hashing scheme, reducing CPU time on `JsonReport.apply()` from 64.7 sec to 2.5 sec

Other minor changes:
* Make `JsonRequirement` use immutable classes
* Simplify `JsonRequirement` composition

cli/src/main/scala-2.11/coursier/cli/Helper.scala

@@ -716,12 +716,12 @@ class Helper(
           a.isOptional && notFound
       }
 
-    val artifactToFile: collection.mutable.Map[String, File] = collection.mutable.Map()
-    val files0 = results.collect {
+    val artifactToFile = results.collect {
       case (artifact: Artifact, \/-(f)) =>
-        artifactToFile.put(artifact.url, f)
-        f
-    }
+        (artifact.url, f)
+    }.toMap
+
+    val files0 = artifactToFile.values.toSeq
 
     logger.foreach(_.stop())
 
@@ -769,9 +769,9 @@ class Helper(
 
       val artifacts: Seq[(Dependency, Artifact)] = res.dependencyArtifacts
 
-      val jsonReq = JsonPrintRequirement(artifactToFile, depToArtifacts, conflictResolutionForRoots)
+      val jsonReq = JsonPrintRequirement(artifactToFile, depToArtifacts)
       val roots = deps.toVector.map(JsonElem(_, artifacts, Option(jsonReq), res, printExclusions = verbosityLevel >= 1, excluded = false, colors = false))
-      val jsonStr = JsonReport(roots, jsonReq.conflictResolutionForRoots)(_.children, _.reconciledVersionStr, _.requestedVersionStr, _.downloadedFiles)
+      val jsonStr = JsonReport(roots, conflictResolutionForRoots)(_.children, _.reconciledVersionStr, _.requestedVersionStr, _.downloadedFiles)
 
       val pw = new PrintWriter(new File(jsonOutputFile))
       pw.write(jsonStr)

cli/src/main/scala-2.11/coursier/cli/util/JsonReport.scala

@@ -1,6 +1,7 @@
 package coursier.cli.util
 
 import java.io.File
+import java.util.Objects
 
 import coursier.Artifact
 import coursier.core.{Attributes, Dependency, Resolution}
@@ -8,10 +9,10 @@ import coursier.util.Print
 
 import scala.collection.mutable
 import scala.collection.parallel.ParSeq
+import argonaut._
+import Argonaut._
 
-import argonaut._, Argonaut._
-
-final case class JsonPrintRequirement(fileByArtifact: collection.mutable.Map[String, File], depToArtifacts: Map[Dependency, Vector[Artifact]], conflictResolutionForRoots: Map[String, String])
+final case class JsonPrintRequirement(fileByArtifact: Map[String, File], depToArtifacts: Map[Dependency, Vector[Artifact]])
 
 final case class DepNode(coord: String, files: Vector[(String, String)], dependencies: Set[String])
 
@@ -163,4 +164,11 @@ final case class JsonElem(dep: Dependency,
       dependencies.map(JsonElem(_, artifacts, jsonPrintRequirement, resolution, colors, printExclusions, excluded = false)) ++
         (if (printExclusions) excluded else Nil)
     }
+
+    /**
+      * Override the hashcode to explicitly exclude `children`, because children will result in recursive hash on
+      * children's children, causing performance issue. Hash collision should be rare, but when that happens, the
+      * default equality check should take of the recursive aspect of `children`.
+      */
+    override def hashCode(): Int = Objects.hash(dep, requestedVersionStr, reconciledVersion, downloadedFiles)
 }