refactor: Extract types and error handling to separate modules

- Move type definitions to src/types.rs (105 lines)
  - SquashedMergeHandling, ReportLevel, MergeResult enums
  - MergeOptions, MergeCommitInfo, MergeStats structs
  - BranchSearchResult, SortBranch enums
- Move ErrorExt trait to src/error.rs (32 lines)
- Update src/main.rs with module declarations
- Reduce main.rs from 3,602 to ~3,466 lines
- No functional changes, all tests passing

Phase 1 of simple file split plan (SIMPLE_SPLIT_PLAN.md)

Author: dashed

src/error.rs

@@ -0,0 +1,32 @@
+use git2::Error;
+
+// For API consistency, we create our own Error variants
+pub trait ErrorExt {
+    #[allow(dead_code)]
+    fn from_str(message: &str) -> Self;
+    fn merge_conflict(branch: String, upstream: String, message: Option<String>) -> Self;
+    fn git_command_failed(command: String, status: i32, stdout: String, stderr: String) -> Self;
+}
+
+impl ErrorExt for Error {
+    fn from_str(message: &str) -> Self {
+        Error::from_str(message)
+    }
+
+    fn merge_conflict(branch: String, upstream: String, message: Option<String>) -> Self {
+        let mut error_msg = format!("Merge conflict between {} and {}", upstream, branch);
+        if let Some(details) = message {
+            error_msg.push('\n');
+            error_msg.push_str(&details);
+        }
+        Error::from_str(&error_msg)
+    }
+
+    fn git_command_failed(command: String, status: i32, stdout: String, stderr: String) -> Self {
+        let error_msg = format!(
+            "Git command failed: {}\nStatus: {}\nStdout: {}\nStderr: {}",
+            command, status, stdout, stderr
+        );
+        Error::from_str(&error_msg)
+    }
+}

src/main.rs

@@ -14,113 +14,11 @@ use git2::{
 use rand::Rng;
 use regex::Regex;
 
-// Merge options types
-#[derive(Debug, PartialEq, Clone, Copy)]
-enum SquashedMergeHandling {
-    // Reset the branch to the parent branch
-    Reset,
+mod error;
+mod types;
 
-    // Skip merging the branch
-    Skip,
-
-    // Force a merge despite the squashed merge detection
-    Merge,
-}
-
-#[derive(Debug, PartialEq, Clone, Copy)]
-enum ReportLevel {
-    // Minimal reporting (just success/failure)
-    Minimal,
-
-    // Standard reporting (summary with counts)
-    Standard,
-
-    // Detailed reporting (all actions and their results)
-    Detailed,
-}
-
-enum MergeResult {
-    // Successfully merged with changes
-    Success(String), // Contains the merge output message
-
-    // Already up-to-date, no changes needed
-    AlreadyUpToDate,
-
-    // Merge conflict occurred
-    Conflict(String), // Contains the conflict message
-}
-
-// For API consistency, we create our own Error variants
-trait ErrorExt {
-    #[allow(dead_code)]
-    fn from_str(message: &str) -> Self;
-    fn merge_conflict(branch: String, upstream: String, message: Option<String>) -> Self;
-    fn git_command_failed(command: String, status: i32, stdout: String, stderr: String) -> Self;
-}
-
-impl ErrorExt for Error {
-    fn from_str(message: &str) -> Self {
-        Error::from_str(message)
-    }
-
-    fn merge_conflict(branch: String, upstream: String, message: Option<String>) -> Self {
-        let mut error_msg = format!("Merge conflict between {} and {}", upstream, branch);
-        if let Some(details) = message {
-            error_msg.push('\n');
-            error_msg.push_str(&details);
-        }
-        Error::from_str(&error_msg)
-    }
-
-    fn git_command_failed(command: String, status: i32, stdout: String, stderr: String) -> Self {
-        let error_msg = format!(
-            "Git command failed: {}\nStatus: {}\nStdout: {}\nStderr: {}",
-            command, status, stdout, stderr
-        );
-        Error::from_str(&error_msg)
-    }
-}
-
-struct MergeOptions {
-    // Skip the merge of the root branch into the first branch
-    ignore_root: bool,
-
-    // Git merge options passed to all merge operations
-    merge_flags: Vec<String>,
-
-    // Whether to use fork point detection (more accurate but slower)
-    use_fork_point: bool,
-
-    // How to handle squashed merges (reset, skip, merge)
-    squashed_merge_handling: SquashedMergeHandling,
-
-    // Print verbose output
-    verbose: bool,
-
-    // Return to original branch after merging
-    return_to_original: bool,
-
-    // Use simple merge mode
-    simple_mode: bool,
-
-    // Level of detail in the final report
-    report_level: ReportLevel,
-}
-
-impl Default for MergeOptions {
-    fn default() -> Self {
-        MergeOptions {
-            ignore_root: false,
-            merge_flags: vec![],
-            use_fork_point: true,
-            squashed_merge_handling: SquashedMergeHandling::Reset,
-            verbose: false,
-            return_to_original: true,
-            simple_mode: false,
-            report_level: ReportLevel::Standard,
-        }
-    }
-}
+use error::ErrorExt;
+use types::*;
 
 fn executable_name() -> String {
     let name = std::env::current_exe()
@@ -201,18 +99,6 @@ fn print_rebase_error(executable_name: &str, branch: &str, upstream_branch: &str
     );
 }
 
-enum BranchSearchResult {
-    NotPartOfAnyChain,
-    Branch(Branch),
-}
-
-enum SortBranch {
-    First,
-    Last,
-    Before(Branch),
-    After(Branch),
-}
-
 #[derive(Clone, PartialEq)]
 struct Branch {
     branch_name: String,
@@ -818,20 +704,6 @@ struct GitChain {
     repo: Repository,
 }
 
-// Structure to hold merge commit information
-#[derive(Debug)]
-struct MergeCommitInfo {
-    message: Option<String>,
-    stats: Option<MergeStats>,
-}
-
-#[derive(Debug)]
-struct MergeStats {
-    files_changed: usize,
-    insertions: usize,
-    deletions: usize,
-}
-
 impl GitChain {
     fn init() -> Result<Self, Error> {
         let name_of_current_executable = executable_name();

src/types.rs

@@ -0,0 +1,102 @@
+// Merge options types
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum SquashedMergeHandling {
+    // Reset the branch to the parent branch
+    Reset,
+
+    // Skip merging the branch
+    Skip,
+
+    // Force a merge despite the squashed merge detection
+    Merge,
+}
+
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum ReportLevel {
+    // Minimal reporting (just success/failure)
+    Minimal,
+
+    // Standard reporting (summary with counts)
+    Standard,
+
+    // Detailed reporting (all actions and their results)
+    Detailed,
+}
+
+pub enum MergeResult {
+    // Successfully merged with changes
+    Success(String), // Contains the merge output message
+
+    // Already up-to-date, no changes needed
+    AlreadyUpToDate,
+
+    // Merge conflict occurred
+    Conflict(String), // Contains the conflict message
+}
+
+pub struct MergeOptions {
+    // Skip the merge of the root branch into the first branch
+    pub ignore_root: bool,
+
+    // Git merge options passed to all merge operations
+    pub merge_flags: Vec<String>,
+
+    // Whether to use fork point detection (more accurate but slower)
+    pub use_fork_point: bool,
+
+    // How to handle squashed merges (reset, skip, merge)
+    pub squashed_merge_handling: SquashedMergeHandling,
+
+    // Print verbose output
+    pub verbose: bool,
+
+    // Return to original branch after merging
+    pub return_to_original: bool,
+
+    // Use simple merge mode
+    pub simple_mode: bool,
+
+    // Level of detail in the final report
+    pub report_level: ReportLevel,
+}
+
+impl Default for MergeOptions {
+    fn default() -> Self {
+        MergeOptions {
+            ignore_root: false,
+            merge_flags: vec![],
+            use_fork_point: true,
+            squashed_merge_handling: SquashedMergeHandling::Reset,
+            verbose: false,
+            return_to_original: true,
+            simple_mode: false,
+            report_level: ReportLevel::Standard,
+        }
+    }
+}
+
+pub enum BranchSearchResult {
+    NotPartOfAnyChain,
+    Branch(crate::Branch),
+}
+
+pub enum SortBranch {
+    First,
+    Last,
+    Before(crate::Branch),
+    After(crate::Branch),
+}
+
+// Structure to hold merge commit information
+#[derive(Debug)]
+pub struct MergeCommitInfo {
+    pub message: Option<String>,
+    pub stats: Option<MergeStats>,
+}
+
+#[derive(Debug)]
+pub struct MergeStats {
+    pub files_changed: usize,
+    pub insertions: usize,
+    pub deletions: usize,
+}