Add support for config file (#9)

## Summary

Allows users to use a .github/action-format.toml

## Test Plan

Added tests.

Author: MatthewMckee4

.github/action-format.toml

@@ -0,0 +1,5 @@
+# Configuration for action-format
+# See: https://github.com/MatthewMckee4/action-format
+
+# Files to ignore (release.yml uses auto-generated changelog formatting)
+ignore = ["release.yml"]

Cargo.lock

@@ -23,10 +23,12 @@ fs-err = "3.2"
 insta = { version = "1.44", features = ["filters"] }
 owo-colors = "4.2"
 regex = "1.12"
+serde = { version = "1.0", features = ["derive"] }
 similar = { version = "2.7", features = ["inline"] }
 tempfile = "3.23"
 terminal_size = "0.4"
 thiserror = "2.0"
+toml = "0.8"
 walkdir = "2.5"
 
 [workspace.lints.clippy]

Cargo.toml

@@ -9,7 +9,9 @@ authors.workspace = true
 license.workspace = true
 
 [dependencies]
+serde = { workspace = true }
 thiserror = { workspace = true }
+toml = { workspace = true }
 
 [lints]
 workspace = true

crates/action-format-core/Cargo.toml

@@ -1,12 +1,19 @@
+use std::path::Path;
+
+use serde::Deserialize;
+
 /// Configuration for the formatter.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Deserialize)]
+#[serde(default)]
 pub struct FormatterConfig {
     /// Number of spaces for indentation (default: 2)
     pub indent_size: usize,
     /// Whether to add blank lines between steps (default: true)
     pub separate_steps: bool,
     /// Whether to add blank lines between jobs (default: true)
     pub separate_jobs: bool,
+    /// Files to ignore (can be full paths like `.github/workflows/ci.yml` or just filenames like `ci.yml`)
+    pub ignore: Vec<String>,
 }
 
 impl Default for FormatterConfig {
@@ -15,6 +22,56 @@ impl Default for FormatterConfig {
             indent_size: 2,
             separate_steps: true,
             separate_jobs: true,
+            ignore: Vec::new(),
         }
     }
 }
+
+impl FormatterConfig {
+    /// Load configuration from a TOML file, falling back to defaults if file doesn't exist.
+    pub fn from_file(path: &Path) -> Result<Self, ConfigError> {
+        if !path.exists() {
+            return Ok(Self::default());
+        }
+
+        let content = std::fs::read_to_string(path).map_err(|e| ConfigError::Read {
+            path: path.to_path_buf(),
+            source: e,
+        })?;
+
+        toml::from_str(&content).map_err(|e| ConfigError::Parse {
+            path: path.to_path_buf(),
+            source: e,
+        })
+    }
+
+    /// Check if a file should be ignored based on the ignore patterns.
+    pub fn should_ignore(&self, path: &Path) -> bool {
+        let path_str = path.to_string_lossy();
+        let file_name = path.file_name().and_then(|n| n.to_str());
+
+        self.ignore.iter().any(|pattern| {
+            // Match full path
+            path_str == *pattern
+                || path_str.ends_with(pattern)
+                // Match just the filename
+                || file_name.is_some_and(|name| name == pattern)
+        })
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum ConfigError {
+    #[error("failed to read config file '{path}'")]
+    Read {
+        path: std::path::PathBuf,
+        #[source]
+        source: std::io::Error,
+    },
+    #[error("failed to parse config file '{path}'")]
+    Parse {
+        path: std::path::PathBuf,
+        #[source]
+        source: toml::de::Error,
+    },
+}

crates/action-format-core/src/config.rs

@@ -2,6 +2,6 @@ mod config;
 mod formatter;
 mod parser;
 
-pub use config::FormatterConfig;
+pub use config::{ConfigError, FormatterConfig};
 pub use formatter::{format_file, format_string};
 pub use parser::FormatError;

crates/action-format-core/src/lib.rs

@@ -14,6 +14,7 @@ mod printer;
 use printer::Printer;
 
 const WORKFLOWS_DIR: &str = ".github/workflows";
+const CONFIG_FILE: &str = ".github/action-format.toml";
 
 #[derive(Copy, Clone)]
 pub enum ExitStatus {
@@ -71,15 +72,18 @@ fn run(cli: &Cli, printer: Printer) -> Result<ExitStatus> {
         anyhow::bail!("No {WORKFLOWS_DIR} directory found");
     }
 
-    let config = FormatterConfig::default();
+    let config_path = Path::new(CONFIG_FILE);
+    let config = FormatterConfig::from_file(config_path)?;
+
     let mut any_changed = false;
     let mut any_error = false;
 
     let walker = walkdir::WalkDir::new(workflows_path)
         .sort_by_file_name()
         .into_iter()
         .filter_map(Result::ok)
-        .filter(|e| is_workflow_file(e.path()));
+        .filter(|e| is_workflow_file(e.path()))
+        .filter(|e| !config.should_ignore(e.path()));
 
     for entry in walker {
         match process_file(entry.path(), &config, cli, printer) {

crates/action-format/src/main.rs

@@ -76,6 +76,15 @@ impl TestContext {
         self
     }
 
+    /// Create a config file at .github/action-format.toml with the given content.
+    pub fn config(&self, content: &str) -> &Self {
+        self.root
+            .child(".github/action-format.toml")
+            .write_str(content.strip_prefix('\n').unwrap_or(content))
+            .expect("Failed to write config file");
+        self
+    }
+
     /// Read a file from .github/workflows and return its contents.
     pub fn read_workflow(&self, name: &str) -> String {
         std::fs::read_to_string(self.root.join(format!(".github/workflows/{name}")))