Adds support for the signature directive to the paradox configuration

(copied from the akka-http configuration)

project/ParadoxSupport.scala

@@ -4,6 +4,8 @@
 
 package akka
 
+import java.io.{File, FileNotFoundException}
+
 import _root_.io.github.lukehutch.fastclasspathscanner.FastClasspathScanner
 import com.lightbend.paradox.markdown._
 import com.lightbend.paradox.sbt.ParadoxPlugin.autoImport._
@@ -12,16 +14,22 @@ import org.pegdown.ast._
 import sbt.Keys._
 import sbt._
 
+import scala.io.{Codec, Source}
+
 import scala.collection.JavaConverters._
 
 object ParadoxSupport {
   val paradoxWithCustomDirectives = Seq(
     paradoxDirectives ++= Def.taskDyn {
+      val log = streams.value.log
       val classpath = (fullClasspath in Compile).value.files.map(_.toURI.toURL).toArray
       val classloader = new java.net.URLClassLoader(classpath, this.getClass().getClassLoader())
       lazy val scanner = new FastClasspathScanner("akka").addClassLoader(classloader).scan()
       val allClasses = scanner.getNamesOfAllClasses.asScala.toVector
       Def.task { Seq(
+        { context: Writer.Context ⇒
+                    new SignatureDirective(context.location.tree.label, context.properties, msg ⇒ log.warn(msg))
+        },
         { _: Writer.Context ⇒ new UnidocDirective(allClasses) }
       )}
     }.value
@@ -84,4 +92,47 @@ object ParadoxSupport {
       }
     }
   }
+
+  class SignatureDirective(page: Page, variables: Map[String, String], logWarn: String => Unit) extends LeafBlockDirective("signature") {
+    def render(node: DirectiveNode, visitor: Visitor, printer: Printer): Unit =
+      try {
+        val labels = node.attributes.values("identifier").asScala.map(_.toLowerCase())
+        val source = node.source match {
+          case direct: DirectiveNode.Source.Direct => direct.value
+          case _                                   => sys.error("Source references are not supported")
+        }
+        val file =
+          if (source startsWith "$") {
+            val baseKey = source.drop(1).takeWhile(_ != '$')
+            val base = new File(PropertyUrl(s"signature.$baseKey.base_dir", variables.get).base.trim)
+            val effectiveBase = if (base.isAbsolute) base else new File(page.file.getParentFile, base.toString)
+            new File(effectiveBase, source.drop(baseKey.length + 2))
+          } else new File(page.file.getParentFile, source)
+
+        val Signature = """\s*((def|val|type) (\w+)(?=[:(\[]).*)(\s+\=.*)""".r // stupid approximation to match a signature
+        //println(s"Looking for signature regex '$Signature'")
+        val text =
+          Source.fromFile(file)(Codec.UTF8).getLines.collect {
+            case line@Signature(signature, kind, l, definition) if labels contains l.toLowerCase() =>
+              //println(s"Found label '$l' with sig '$full' in line $line")
+              if (kind == "type") signature + definition
+              else signature
+          }.mkString("\n")
+
+        if (text.trim.isEmpty) {
+          logWarn(
+            s"Did not find any signatures with one of those names [${labels.mkString(", ")}] in ${node.source} " +
+            s"(was referenced from [${page.path}])")
+
+          new HtmlBlockNode(s"""<div style="color: red;">[Broken signature inclusion [${labels.mkString(", ")}] to [${node.source}]</div>""").accept(visitor)
+        } else {
+          val lang = Option(node.attributes.value("type")).getOrElse(Snippet.language(file))
+          new VerbatimNode(text, lang).accept(visitor)
+        }
+      } catch {
+        case e: FileNotFoundException =>
+          throw new SnipDirective.LinkException(s"Unknown snippet [${e.getMessage}] referenced from [${page.path}]")
+      }
+  }
+
 }