✨ Build revertable preset (#175)

For any given preset, we need to be able to determine what we'd need to
do in order to revert the preset. That means figuring out what the diff
is between the current PHP setup and what the preset would change.

We'll persist that to its own preset, which can be reapplied if needed.
The "revertable" preset is persisted to the following file:

    ~/.config/phpmon/preset_revert.json

If that file is present and valid, the app should enable the 'revert'
option. (That still needs to be implemented.)

phpmon/Domain/Presets/Preset.swift

@@ -8,7 +8,7 @@
 
 import Foundation
 
-struct Preset: Decodable {
+struct Preset: Codable {
     let name: String
     let version: String?
     let extensions: [String: Bool]
@@ -21,33 +21,16 @@ struct Preset: Decodable {
              configuration = "configuration"
     }
 
-    public func getMenuItemText() -> String {
-        var info = extensions.count == 1
-        ? "preset.extension".localized(extensions.count)
-        : "preset.extensions".localized(extensions.count)
-        info += ", "
-        info += configuration.count == 1
-        ? "preset.preference".localized(configuration.count)
-        : "preset.preferences".localized(configuration.count)
-
-        if self.version == nil {
-            return "<span style=\"font-family: '-apple-system'; font-size: 12px;\">"
-            + "<b>\(name.stripped)</b><br/>"
-            + "<i style=\"font-size: 11px;\">"
-            + info + "</i>"
-            + "</span>"
-        }
-
-        return "<span style=\"font-family: '-apple-system'; font-size: 12px;\">"
-        + "<b>\(name.stripped)</b><br/>"
-        + "<i style=\"font-size: 11px;\">"
-        + "Switches to PHP \(version!)<br/>"
-        + info + "</i>"
-        + "</span>"
-    }
+    // MARK: Applying
 
+    /**
+     Applies a given preset.
+     */
     public func apply() {
         Task {
+            // Save the preset that would revert this preset
+            self.persistRevert()
+
             // Apply the PHP version if is considered a valid version
             if self.version != nil {
                 await switchToPhpVersionIfValid()
@@ -72,6 +55,8 @@ struct Preset: Decodable {
         }
     }
 
+    // MARK: - Apply Functionality
+
     private func switchToPhpVersionIfValid() async {
         if PhpEnv.shared.currentInstall.version.short == self.version! {
             Log.info("The version we are supposed to switch to is already active.")
@@ -113,4 +98,108 @@ struct Preset: Decodable {
             Log.err("Setting \(key) to \(value) failed.")
         }
     }
+
+    // MARK: - Menu Items
+
+    public func getMenuItemText() -> String {
+        var info = extensions.count == 1
+        ? "preset.extension".localized(extensions.count)
+        : "preset.extensions".localized(extensions.count)
+        info += ", "
+        info += configuration.count == 1
+        ? "preset.preference".localized(configuration.count)
+        : "preset.preferences".localized(configuration.count)
+
+        if self.version == nil {
+            return "<span style=\"font-family: '-apple-system'; font-size: 12px;\">"
+            + "<b>\(name.stripped)</b><br/>"
+            + "<i style=\"font-size: 11px;\">"
+            + info + "</i>"
+            + "</span>"
+        }
+
+        return "<span style=\"font-family: '-apple-system'; font-size: 12px;\">"
+        + "<b>\(name.stripped)</b><br/>"
+        + "<i style=\"font-size: 11px;\">"
+        + "Switches to PHP \(version!)<br/>"
+        + info + "</i>"
+        + "</span>"
+    }
+
+    // MARK: - Reverting
+
+    public var revertSnapshot: Preset {
+        return Preset(
+            name: "Revert",
+            version: diffVersion(),
+            extensions: diffExtensions(),
+            configuration: diffConfiguration()
+        )
+    }
+
+    /**
+     Returns the version that was previously active, which would revert this preset's version.
+     Returns nil if the version is not specified or the same.
+     */
+    private func diffVersion() -> String? {
+        guard let version = self.version else {
+            return nil
+        }
+
+        if PhpEnv.shared.currentInstall.version.short != version {
+            return PhpEnv.shared.currentInstall.version.short
+        } else {
+            return nil
+        }
+    }
+
+    /**
+     Returns a list of extensions which would revert this presets's setup.
+     */
+    private func diffExtensions() -> [String: Bool] {
+        var items: [String: Bool] = [:]
+
+        for (key, value) in self.extensions {
+            for foundExt in PhpEnv.phpInstall.extensions
+            where foundExt.name == key && foundExt.enabled != value {
+                // Save the original value of the extension
+                items[foundExt.name] = foundExt.enabled
+            }
+        }
+
+        return items
+    }
+
+    /**
+     Returns a list of configuration items which would revert this presets's setup.
+     */
+    private func diffConfiguration() -> [String: String?] {
+        var items: [String: String?] = [:]
+
+        for (key, _) in self.configuration {
+            guard let file = PhpEnv.shared.getConfigFile(forKey: key) else {
+                break
+            }
+
+            items[key] = file.get(for: key)
+        }
+
+        return items
+    }
+
+    /**
+     Persists the revert as a JSON file, so it can be read from a file after restarting PHP Monitor.
+     */
+    private func persistRevert() {
+        let data = try! JSONEncoder().encode(self.revertSnapshot)
+
+        Shell.run("mkdir -p ~/.config/phpmon")
+
+        try! String(data: data, encoding: .utf8)!
+            .write(
+                toFile: "/Users/\(Paths.whoami)/.config/phpmon/preset_revert.json",
+                atomically: true,
+                encoding: .utf8
+            )
+    }
 }