Simplify theme overrides (#1652)

* Simplify theme overrides

Theme overrides are now loaded the same way key overrides are loaded.
The config file, `theme.ron`, does not have to contain a complete theme
anymore. Instead, it is possible to specify only the values that are
supposed to override their corresponding default values.

* Document breaking change in changelog
* Test that override differs from default
* Convert existing theme to patch

4 files changed, 90 insertions, 48 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 75e6b322..489b2993 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Breaking Change
 * `focus_XYZ` key bindings are merged into the `move_XYZ` set, so only one way to bind arrow-like keys from now on ([#1539](https://github.com/extrawurst/gitui/issues/1539))
+* Do you use a custom theme?
+
+  The way themes work got changed and simplified ([see docs](https://github.com/extrawurst/gitui/blob/master/THEMES.md) for more info):
+
+  * The format of `theme.ron` has changed: you only specify the colors etc. that should differ from their default value
+  * Future additions of colors etc. will not break existing themes anymore
 
 ### Added
 * allow reset (soft,mixed,hard) from commit log ([#1500](https://github.com/extrawurst/gitui/issues/1500))
@@ -41,6 +47,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * allow `copy` file path on revision files and status tree [[@yanganto]](https://github.com/yanganto)  ([#1516](https://github.com/extrawurst/gitui/pull/1516))
 * print message of where log will be written if `-l` is set ([#1472](https://github.com/extrawurst/gitui/pull/1472))
 * show remote branches in log [[@cruessler](https://github.com/cruessler)] ([#1501](https://github.com/extrawurst/gitui/issues/1501))
+* simplify theme overrides [[@cruessler](https://github.com/cruessler)] ([#1367](https://github.com/extrawurst/gitui/issues/1367))
 
 ### Fixes
 * fixed side effect of crossterm 0.26 on windows that caused double input of all keys [[@pm100]](https://github/pm100) ([#1686](https://github.com/extrawurst/gitui/pull/1686))
diff --git a/THEMES.md b/THEMES.md
index b3b8b1a3..b1d36f90 100644
--- a/THEMES.md
+++ b/THEMES.md
@@ -3,15 +3,27 @@
 default on light terminal:
 ![](assets/light-theme.png)
 
-to change the colors of the default theme you have to modify `theme.ron` file
-[Ron format](https://github.com/ron-rs/ron) located at config path. The path differs depending on the operating system:
+To change the colors of the default theme you need to add a `theme.ron` file that contains the colors you want to override. Note that you don’t have to specify the full theme anymore (as of 0.23). Instead, it is sufficient to override just the values that you want to differ from their default values.
+
+The file uses the [Ron format](https://github.com/ron-rs/ron) and is located at one of the following paths, depending on your operating system:
 
 * `$HOME/.config/gitui/theme.ron` (mac)
 * `$XDG_CONFIG_HOME/gitui/theme.ron` (linux using XDG)
 * `$HOME/.config/gitui/theme.ron` (linux)
 * `%APPDATA%/gitui/theme.ron` (Windows)
 
-Alternatively you may make a theme in the same directory mentioned above with and select with the `-t` flag followed by the name of the file in the directory. E.g. If you are on linux calling `gitui -t arc.ron` wil use `$XDG_CONFIG_HOME/gitui/arc.ron` or `$HOME/.config/gitui/arc.ron`
+Alternatively, you can create a theme in the same directory mentioned above and use it with the `-t` flag followed by the name of the file in the directory. E.g. If you are on linux calling `gitui -t arc.ron`, this will load the theme in `$XDG_CONFIG_HOME/gitui/arc.ron` or `$HOME/.config/gitui/arc.ron`.
+
+Example theme override:
+
+```
+(
+    selection_bg: Some(Blue),
+    selection_fg: Some(White),
+)
+```
+
+Note that you need to wrap values in `Some` due to the way the overrides work (as of 0.23).
 
 Notes:
 
diff --git a/src/main.rs b/src/main.rs
index b1c0ed1f..6cab1fa0 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -138,9 +138,7 @@ fn main() -> Result<()> {
 	let key_config = KeyConfig::init()
 		.map_err(|e| eprintln!("KeyConfig loading error: {e}"))
 		.unwrap_or_default();
-	let theme = Theme::init(&cliargs.theme)
-		.map_err(|e| eprintln!("Theme loading error: {e}"))
-		.unwrap_or_default();
+	let theme = Theme::init(&cliargs.theme);
 
 	setup_terminal()?;
 	defer! {
diff --git a/src/ui/style.rs b/src/ui/style.rs
index fe6c3cd8..1139cfe7 100644
--- a/src/ui/style.rs
+++ b/src/ui/style.rs
@@ -1,21 +1,15 @@
 use anyhow::Result;
 use asyncgit::{DiffLineType, StatusItemType};
 use ratatui::style::{Color, Modifier, Style};
-use ron::{
-	de::from_bytes,
-	ser::{to_string_pretty, PrettyConfig},
-};
+use ron::ser::{to_string_pretty, PrettyConfig};
 use serde::{Deserialize, Serialize};
-use std::{
-	fs::{self, File},
-	io::{Read, Write},
-	path::PathBuf,
-	rc::Rc,
-};
+use std::{fs::File, io::Write, path::PathBuf, rc::Rc};
+use struct_patch::Patch;
 
 pub type SharedTheme = Rc<Theme>;
 
-#[derive(Serialize, Deserialize, Debug, Copy, Clone)]
+#[derive(Serialize, Deserialize, Debug, Copy, Clone, Patch)]
+#[patch_derive(Serialize, Deserialize)]
 pub struct Theme {
 	selected_tab: Color,
 	command_fg: Color,
@@ -261,44 +255,46 @@ impl Theme {
 			.bg(self.push_gauge_bg)
 	}
 
-	// This will only be called when theme.ron doesn't already exists
-	fn save(&self, theme_file: &PathBuf) -> Result<()> {
-		let mut file = File::create(theme_file)?;
-		let data = to_string_pretty(self, PrettyConfig::default())?;
+	fn load_patch(theme_path: &PathBuf) -> Result<ThemePatch> {
+		let file = File::open(theme_path)?;
+
+		Ok(ron::de::from_reader(file)?)
+	}
+
+	fn load_old_theme(theme_path: &PathBuf) -> Result<Self> {
+		let old_file = File::open(theme_path)?;
+
+		Ok(ron::de::from_reader::<File, Self>(old_file)?)
+	}
+
+	// This is supposed to be called when theme.ron doesn't already exists.
+	fn save_patch(&self, theme_path: &PathBuf) -> Result<()> {
+		let mut file = File::create(theme_path)?;
+		let patch = self.into_patch_by_diff(Self::default());
+		let data = to_string_pretty(&patch, PrettyConfig::default())?;
+
 		file.write_all(data.as_bytes())?;
+
 		Ok(())
 	}
 
-	fn read_file(theme_file: PathBuf) -> Result<Self> {
-		let mut f = File::open(theme_file)?;
-		let mut buffer = Vec::new();
-		f.read_to_end(&mut buffer)?;
-		Ok(from_bytes(&buffer)?)
-	}
+	pub fn init(theme_path: &PathBuf) -> Self {
+		let mut theme = Self::default();
+
+		if let Ok(patch) = Self::load_patch(theme_path) {
+			theme.apply(patch);
+		} else if let Ok(old_theme) = Self::load_old_theme(theme_path)
+		{
+			theme = old_theme;
 
-	pub fn init(file: &PathBuf) -> Result<Self> {
-		if file.exists() {
-			match Self::read_file(file.clone()) {
-				Err(e) => {
-					let config_path = file.clone();
-					let config_path_old =
-						format!("{}.old", file.to_string_lossy());
-					fs::rename(
-						config_path.clone(),
-						config_path_old.clone(),
-					)?;
-
-					Self::default().save(file)?;
-
-					Err(anyhow::anyhow!("{}\n Old file was renamed to {:?}.\n Defaults loaded and saved as {:?}",
-                        e,config_path_old,config_path.to_string_lossy()))
-				}
-				Ok(res) => Ok(res),
+			if theme.save_patch(theme_path).is_ok() {
+				log::info!("Converted old theme to new format.");
+			} else {
+				log::warn!("Failed to save theme in new format.");
 			}
-		} else {
-			Self::default().save(file)?;
-			Ok(Self::default())
 		}
+
+		theme
 	}
 }
 
@@ -329,3 +325,32 @@ impl Default for Theme {
 		}
 	}
 }
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use pretty_assertions::assert_eq;
+	use std::io::Write;
+	use tempfile::NamedTempFile;
+
+	#[test]
+	fn test_smoke() {
+		let mut file = NamedTempFile::new().unwrap();
+
+		writeln!(
+			file,
+			r"
+(
+	selection_bg: Some(White),
+)
+"
+		)
+		.unwrap();
+
+		let theme = Theme::init(&file.path().to_path_buf());
+
+		assert_eq!(theme.selection_fg, Theme::default().selection_fg);
+		assert_eq!(theme.selection_bg, Color::White);
+		assert_ne!(theme.selection_bg, Theme::default().selection_bg);
+	}
+}