From eb96a4281a4b869b55598f364b8bf2077a13bd6b Mon Sep 17 00:00:00 2001
From: John Sundell <john@sundell.co>
Date: Fri, 5 Apr 2019 18:14:47 +0200
Subject: [PATCH] Add API for computing a path relative to a parent folder

This enables an easy way to work with relative paths, for example when
dealing with multiple files that are within the same folder structure.
---
 Sources/Files.swift               | 24 ++++++++++++++++++++++++
 Tests/FilesTests/FilesTests.swift | 23 +++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/Sources/Files.swift b/Sources/Files.swift
index 23b87ab..83ed08c 100644
--- a/Sources/Files.swift
+++ b/Sources/Files.swift
@@ -237,6 +237,30 @@ public class FileSystem {
                 self.name = pathComponents[pathComponents.count - 2]
             }
         }
+
+        /**
+         *  Return this item's path relative to a given parent folder
+         *
+         *  - parameter folder: The parent folder to return a relative path to
+         *
+         *  - returns: Either a relative path, if the passed folder is indeed
+         *  a parent (even if it's not a direct one) for this item. Otherwise
+         *  the item's full path is returned.
+         */
+        public func path(relativeTo folder: Folder) -> String {
+            guard path.hasPrefix(folder.path) else {
+                return path
+            }
+
+            let index = path.index(path.startIndex, offsetBy: folder.path.count)
+            var subpath = path[index...]
+
+            if subpath.hasSuffix("/") {
+                subpath.removeLast()
+            }
+
+            return String(subpath)
+        }
         
         /**
          *  Rename the item
diff --git a/Tests/FilesTests/FilesTests.swift b/Tests/FilesTests/FilesTests.swift
index 6492a4a..e56b621 100644
--- a/Tests/FilesTests/FilesTests.swift
+++ b/Tests/FilesTests/FilesTests.swift
@@ -589,6 +589,27 @@ class FilesTests: XCTestCase {
         }
     }
 
+    func testRelativePaths() {
+        performTest {
+            let file = try folder.createFile(named: "FileA")
+            let subfolder = try folder.createSubfolder(named: "Folder")
+            let fileInSubfolder = try subfolder.createFile(named: "FileB")
+
+            XCTAssertEqual(file.path(relativeTo: folder), "FileA")
+            XCTAssertEqual(subfolder.path(relativeTo: folder), "Folder")
+            XCTAssertEqual(fileInSubfolder.path(relativeTo: folder), "Folder/FileB")
+        }
+    }
+
+    func testRelativePathIsAbsolutePathForNonParent() {
+        performTest {
+            let file = try folder.createFile(named: "FileA")
+            let subfolder = try folder.createSubfolder(named: "Folder")
+
+            XCTAssertEqual(file.path(relativeTo: subfolder), file.path)
+        }
+    }
+
     func testCreatingFileFromFileSystem() {
         performTest {
             let fileName = "three"
@@ -789,6 +810,8 @@ class FilesTests: XCTestCase {
         ("testAccessingHomeFolder", testAccessingHomeFolder),
         ("testAccessingCurrentWorkingDirectory", testAccessingCurrentWorkingDirectory),
         ("testNameExcludingExtensionWithLongFileName", testNameExcludingExtensionWithLongFileName),
+        ("testRelativePaths", testRelativePaths),
+        ("testRelativePathIsAbsolutePathForNonParent", testRelativePathIsAbsolutePathForNonParent),
         ("testCreatingFileFromFileSystem", testCreatingFileFromFileSystem),
         ("testCreateFileFromFileSystemIfNeeded", testCreateFileFromFileSystemIfNeeded),
         ("testCreatingFolderFromFileSystem", testCreatingFolderFromFileSystem),