Experiment with path mapping

okio/src/commonMain/kotlin/okio/FileSystemExtension.kt

@@ -18,5 +18,35 @@ package okio
 /**
  * Marks an object that can be attached to a [FileSystem], and that supplements the file system's
  * capabilities.
+ *
+ * Implementations must support transforms to input and output paths with [PathMapper]. To simplify
+ * implementation, use [PathMapper.NONE] by default and use [chain] to combine mappers.
+ *
+ * ```kotlin
+ * class DiskUsageExtension private constructor(
+ *   private val pathMapper: PathMapper,
+ * ) : FileSystemExtension {
+ *   constructor() : this(PathMapper.NONE)
+ *
+ *   override fun map(pathMapper: PathMapper): FileSystemExtension {
+ *     return DiskUsageExtension(chain(pathMapper, this.pathMapper))
+ *   }
+ *
+ *   fun sizeOnDisk(path: Path): Long {
+ *     val mappedPath = pathMapper.onPathParameter(path, "sizeOnDisk", "path")
+ *     return lookUpSizeOnDisk(mappedPath)
+ *   }
+ *
+ *   fun largestFiles(): Sequence<Path> {
+ *     val largestFiles: Sequence<Path> = lookUpLargestFiles()
+ *     return largestFiles.map {
+ *       pathMapper.onPathResult(it, "largestFiles")
+ *     }
+ *   }
+ * }
+ * ```
  */
-interface FileSystemExtension
+interface FileSystemExtension {
+  /** Returns a file system of the same type, that applies [pathMapper] to all paths. */
+  fun map(pathMapper: PathMapper) : FileSystemExtension
+}

okio/src/commonMain/kotlin/okio/Okio.kt

@@ -85,3 +85,30 @@ inline fun <reified E : FileSystemExtension> FileSystem.extend(extension: E): Fi
 
 /** Returns the extension for [E] if it exists, and null otherwise. */
 inline fun <reified E : FileSystemExtension> FileSystem.extension(): E? = extension(E::class)
+
+
+fun chain(outer: PathMapper, inner: PathMapper): PathMapper {
+  return object : PathMapper {
+    override fun onPathParameter(path: Path, functionName: String, parameterName: String): Path {
+      return inner.onPathParameter(
+        outer.onPathParameter(
+          path,
+          functionName,
+          parameterName,
+        ),
+        functionName,
+        parameterName,
+      )
+    }
+
+    override fun onPathResult(path: Path, functionName: String): Path {
+      return outer.onPathResult(
+        inner.onPathResult(
+          path,
+          functionName,
+        ),
+        functionName,
+      )
+    }
+  }
+}

okio/src/commonMain/kotlin/okio/PathMapper.kt

@@ -0,0 +1,13 @@
+package okio
+
+interface PathMapper {
+  fun onPathParameter(path: Path, functionName: String, parameterName: String): Path
+  fun onPathResult(path: Path, functionName: String): Path
+
+  companion object {
+    val NONE = object : PathMapper {
+      override fun onPathParameter(path: Path, functionName: String, parameterName: String) = path
+      override fun onPathResult(path: Path, functionName: String) = path
+    }
+  }
+}