Add support for pretty printing XML

xml/src/main/scala/fs2/data/xml/Attr.scala

@@ -18,4 +18,15 @@ package fs2
 package data
 package xml
 
+import cats.Show
+import cats.syntax.all._
+
 case class Attr(name: QName, value: List[XmlEvent.XmlTexty])
+
+object Attr {
+
+  implicit val show: Show[Attr] = Show.show { case Attr(name, value) =>
+    show"""$name="${value.foldMap(_.render)}""""
+  }
+
+}

xml/src/main/scala/fs2/data/xml/internals/Renderer.scala

@@ -0,0 +1,140 @@
+/*
+ * Copyright 2024 fs2-data Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package fs2
+package data
+package xml
+package internals
+
+import cats.syntax.all._
+
+private[xml] class Renderer(collapseEmpty: Boolean, resetOnChunk: Boolean, indent: String, attributeThreshold: Int)
+    extends Collector.Builder[XmlEvent, String] {
+
+  private val builder = new StringBuilder
+
+  private var level = 0
+
+  private var newline = false
+
+  private var skipClose = false
+
+  private def indentation(): Unit =
+    if (newline) {
+      builder.append('\n')
+      builder.append(indent * level)
+    }
+
+  override def +=(chunk: Chunk[XmlEvent]): Unit = {
+    if (resetOnChunk)
+      builder.setLength(0)
+    chunk.foreach {
+      case e @ (XmlEvent.XmlDecl(_, _, _) | XmlEvent.XmlPI(_, _)) =>
+        indentation()
+        builder ++= e.show
+        newline = true
+
+      case XmlEvent.Comment(content) =>
+        newline = true
+        indentation()
+        builder ++= "<!--"
+        content.linesIterator.foreach { line =>
+          indentation()
+          builder ++= line.trim()
+        }
+        indentation()
+        builder ++= "-->"
+
+      case XmlEvent.StartTag(name, attributes, isEmpty) =>
+        indentation()
+        val renderedName = name.show
+        builder ++= show"<$renderedName"
+
+        attributes match {
+          case a :: as =>
+            val exceedThreshold = as.size > attributeThreshold - 1
+            builder ++= show"<$renderedName $a"
+            as.foreach { a =>
+              if (exceedThreshold) {
+                builder += '\n'
+                builder ++= " " * (renderedName.length() + 2)
+              } else {
+                builder += ' '
+              }
+              builder ++= a.show
+            }
+          case Nil => // do nothing
+        }
+
+        if (isEmpty && collapseEmpty) {
+          builder ++= " />"
+          skipClose = true
+        } else {
+          builder += '>'
+          level += 1
+        }
+        newline = true
+
+      case XmlEvent.EndTag(name) =>
+        level -= 1
+        if (!skipClose) {
+          indentation()
+          builder ++= show"</$name>"
+        }
+        skipClose = false
+        newline = true
+
+      case XmlEvent.XmlString(content, true) =>
+        indentation()
+        show"<![CDATA[$content]]>"
+        newline = true
+
+      case XmlEvent.XmlString(content, false) =>
+        content.linesWithSeparators
+        content.linesIterator.foreach { line =>
+          indentation()
+          if (newline)
+            builder ++= line.stripLeading()
+          else
+            builder ++= line
+          newline = true
+        }
+        newline = content.matches("^.*\n\\s*$")
+
+      case e =>
+        indentation()
+        builder ++ e.show
+        newline = false
+    }
+  }
+
+  override def result: String = builder.result()
+
+}
+
+private[xml] object Renderer {
+
+  def pipe[F[_]](collapseEmpty: Boolean, indent: String, attributeThreshold: Int): Pipe[F, XmlEvent, String] =
+    in =>
+      Stream.suspend(Stream.emit(new Renderer(collapseEmpty, true, indent, attributeThreshold))).flatMap { builder =>
+        in.mapChunks { chunk =>
+          builder += chunk
+          Chunk.singleton(builder.result)
+        }
+
+      }
+
+}

xml/src/main/scala/fs2/data/xml/package.scala

@@ -80,12 +80,39 @@ package object xml {
     * without additional (or original) whitespace and with empty tags being collapsed to the short self-closed form <x/>
     * if collapseEmpty is true. Preserves chunking, each String in the output will correspond to one event in the input.
     */
+  @deprecated(message = "Use `fs2.data.xml.render.raw() instead.`", since = "fs2-data 1.11.0")
   def render[F[_]](collapseEmpty: Boolean = true): Pipe[F, XmlEvent, String] =
-    _.zipWithPrevious.map {
-      case (_, st: XmlEvent.StartTag)                                                 => st.render(collapseEmpty)
-      case (Some(XmlEvent.StartTag(_, _, true)), XmlEvent.EndTag(_)) if collapseEmpty => ""
-      case (_, event)                                                                 => event.show
-    }
+    render.raw(collapseEmpty)
+
+  object render {
+
+    /**
+    * Render the incoming xml events to their string representation. The output will be concise,
+    * without additional (or original) whitespace and with empty tags being collapsed to the short self-closed form <x/>
+    * if collapseEmpty is true. Preserves chunking, each String in the output will correspond to one event in the input.
+    */
+    def raw[F[_]](collapseEmpty: Boolean = true): Pipe[F, XmlEvent, String] =
+      _.zipWithPrevious.map {
+        case (_, st: XmlEvent.StartTag)                                                 => st.render(collapseEmpty)
+        case (Some(XmlEvent.StartTag(_, _, true)), XmlEvent.EndTag(_)) if collapseEmpty => ""
+        case (_, event)                                                                 => event.show
+      }
+
+    /**
+      * Render the incoming xml events intot a prettified string representation.
+      * _Prettified_ means that nested tags will be indented as per `indent` parameter
+      * and text data (except for `CDATA`, which remains untouched) is indented to the current
+      * indentation level after each new line.
+      *
+      * This pipe can be used when whitespace characters are not relevant to the application
+      * and to make it more readable to human beings.
+      */
+    def pretty[F[_]](collapseEmpty: Boolean = true,
+                     indent: String = "  ",
+                     attributeThreshold: Int = 3): Pipe[F, XmlEvent, String] =
+      Renderer.pipe(collapseEmpty, indent, attributeThreshold)
+
+  }
 
   val ncNameStart = CharRanges.fromRanges(
     ('A', 'Z'),