//! Git patch creation and application for checkpoint recovery.

use flate2::read::GzDecoder;
use flate2::write::GzEncoder;
use flate2::Compression;
use std::io::{Read, Write};
use std::path::Path;
use thiserror::Error;
use tokio::process::Command;

/// Errors that can occur during patch operations.
#[derive(Error, Debug)]
pub enum PatchError {
    #[error("Git command failed: {0}")]
    GitCommand(String),

    #[error("Compression error: {0}")]
    Compression(#[from] std::io::Error),

    #[error("Patch too large: {size} bytes (max: {max} bytes)")]
    TooLarge { size: usize, max: usize },

    #[error("Empty patch (no changes)")]
    EmptyPatch,

    #[error("Failed to apply patch: {0}")]
    ApplyFailed(String),
}

/// Create a compressed git diff from worktree changes.
///
/// Generates a diff between `base_sha` and HEAD, then compresses it with gzip.
/// Returns the compressed patch bytes and the number of files changed.
pub async fn create_patch(
    worktree_path: &Path,
    base_sha: &str,
) -> Result<(Vec<u8>, usize), PatchError> {
    // Get the diff between base commit and HEAD
    let output = Command::new("git")
        .current_dir(worktree_path)
        .args(["diff", base_sha, "HEAD", "--binary"])
        .output()
        .await
        .map_err(|e| PatchError::GitCommand(format!("Failed to run git diff: {}", e)))?;

    if !output.status.success() {
        let stderr = String::from_utf8_lossy(&output.stderr);
        return Err(PatchError::GitCommand(format!("git diff failed: {}", stderr)));
    }

    let diff_data = output.stdout;
    if diff_data.is_empty() {
        return Err(PatchError::EmptyPatch);
    }

    // Count files changed
    let files_output = Command::new("git")
        .current_dir(worktree_path)
        .args(["diff", base_sha, "HEAD", "--name-only"])
        .output()
        .await
        .map_err(|e| PatchError::GitCommand(format!("Failed to count files: {}", e)))?;

    let files_count = if files_output.status.success() {
        String::from_utf8_lossy(&files_output.stdout)
            .lines()
            .filter(|l| !l.is_empty())
            .count()
    } else {
        0
    };

    // Compress with gzip
    let mut encoder = GzEncoder::new(Vec::new(), Compression::default());
    encoder.write_all(&diff_data)?;
    let compressed = encoder.finish()?;

    Ok((compressed, files_count))
}

/// Apply a compressed patch to restore worktree state.
///
/// The worktree should already be checked out at `base_sha` before calling this.
pub async fn apply_patch(worktree_path: &Path, patch_data: &[u8]) -> Result<(), PatchError> {
    // Decompress gzip
    let mut decoder = GzDecoder::new(patch_data);
    let mut decompressed = Vec::new();
    decoder.read_to_end(&mut decompressed)?;

    if decompressed.is_empty() {
        return Err(PatchError::EmptyPatch);
    }

    // Apply the patch using git apply
    let mut child = Command::new("git")
        .current_dir(worktree_path)
        .args(["apply", "--binary", "-"])
        .stdin(std::process::Stdio::piped())
        .stdout(std::process::Stdio::piped())
        .stderr(std::process::Stdio::piped())
        .spawn()
        .map_err(|e| PatchError::GitCommand(format!("Failed to spawn git apply: {}", e)))?;

    // Write patch to stdin
    if let Some(mut stdin) = child.stdin.take() {
        use tokio::io::AsyncWriteExt;
        stdin.write_all(&decompressed).await?;
        drop(stdin); // Close stdin to signal EOF
    }

    let output = child
        .wait_with_output()
        .await
        .map_err(|e| PatchError::GitCommand(format!("Failed to wait for git apply: {}", e)))?;

    if !output.status.success() {
        let stderr = String::from_utf8_lossy(&output.stderr);
        return Err(PatchError::ApplyFailed(stderr.to_string()));
    }

    Ok(())
}

/// Get the parent commit SHA (HEAD~1) from a worktree.
pub async fn get_parent_sha(worktree_path: &Path) -> Result<String, PatchError> {
    let output = Command::new("git")
        .current_dir(worktree_path)
        .args(["rev-parse", "HEAD~1"])
        .output()
        .await
        .map_err(|e| PatchError::GitCommand(format!("Failed to get parent SHA: {}", e)))?;

    if !output.status.success() {
        let stderr = String::from_utf8_lossy(&output.stderr);
        return Err(PatchError::GitCommand(format!(
            "git rev-parse HEAD~1 failed: {}",
            stderr
        )));
    }

    Ok(String::from_utf8_lossy(&output.stdout).trim().to_string())
}

/// Get the current HEAD commit SHA from a worktree.
pub async fn get_head_sha(worktree_path: &Path) -> Result<String, PatchError> {
    let output = Command::new("git")
        .current_dir(worktree_path)
        .args(["rev-parse", "HEAD"])
        .output()
        .await
        .map_err(|e| PatchError::GitCommand(format!("Failed to get HEAD SHA: {}", e)))?;

    if !output.status.success() {
        let stderr = String::from_utf8_lossy(&output.stderr);
        return Err(PatchError::GitCommand(format!(
            "git rev-parse HEAD failed: {}",
            stderr
        )));
    }

    Ok(String::from_utf8_lossy(&output.stdout).trim().to_string())
}

/// Resolve the merge-base SHA for diffing against the main/master branch.
///
/// Tries in order:
/// 1. Upstream tracking branch merge-base
/// 2. Common branches: origin/main, origin/master, main, master
/// 3. Fallback: HEAD~1
///
/// Returns `Err(PatchError::EmptyPatch)` if the merge-base equals HEAD (no diff).
pub async fn get_merge_base_sha(worktree_path: &Path) -> Result<String, PatchError> {
    // Try to get the upstream tracking branch
    let upstream_result = Command::new("git")
        .current_dir(worktree_path)
        .args(["rev-parse", "--abbrev-ref", "--symbolic-full-name", "@{u}"])
        .output()
        .await;

    let base = if let Ok(output) = upstream_result {
        if output.status.success() {
            let upstream = String::from_utf8_lossy(&output.stdout).trim().to_string();
            // Get merge-base with upstream
            let merge_base = Command::new("git")
                .current_dir(worktree_path)
                .args(["merge-base", "HEAD", &upstream])
                .output()
                .await;

            if let Ok(mb_output) = merge_base {
                if mb_output.status.success() {
                    Some(
                        String::from_utf8_lossy(&mb_output.stdout)
                            .trim()
                            .to_string(),
                    )
                } else {
                    None
                }
            } else {
                None
            }
        } else {
            None
        }
    } else {
        None
    };

    // Get current HEAD SHA for comparison
    let head_sha = Command::new("git")
        .current_dir(worktree_path)
        .args(["rev-parse", "HEAD"])
        .output()
        .await
        .ok()
        .filter(|o| o.status.success())
        .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string());

    // If we couldn't find upstream, try common default branches
    let base = if base.is_none() {
        let default_branches = ["origin/main", "origin/master", "main", "master"];
        let mut found_base = None;

        for branch in default_branches {
            let merge_base = Command::new("git")
                .current_dir(worktree_path)
                .args(["merge-base", "HEAD", branch])
                .output()
                .await;

            if let Ok(output) = merge_base {
                if output.status.success() {
                    let mb_sha = String::from_utf8_lossy(&output.stdout).trim().to_string();
                    // Skip if merge-base equals HEAD (would result in empty diff)
                    if head_sha.as_ref() != Some(&mb_sha) {
                        found_base = Some(mb_sha);
                        break;
                    }
                }
            }
        }
        found_base
    } else {
        // Also check upstream base
        if base.as_ref() == head_sha.as_ref() {
            None
        } else {
            base
        }
    };

    // If still nothing, fall back to HEAD~1
    Ok(base.unwrap_or_else(|| "HEAD~1".to_string()))
}

/// Result of creating an export patch.
#[derive(Debug, Clone)]
pub struct ExportPatchResult {
    /// The uncompressed, human-readable patch content.
    pub patch_content: String,
    /// Number of files changed in the patch.
    pub files_count: usize,
    /// Number of lines added.
    pub lines_added: usize,
    /// Number of lines removed.
    pub lines_removed: usize,
    /// The base commit SHA that the patch is diffed against.
    pub base_commit_sha: String,
}

/// Create an uncompressed git diff patch for export.
///
/// This creates a human-readable patch that can be applied manually or
/// shared as a file. Unlike `create_patch`, this version is not compressed
/// and is suitable for display or export.
///
/// If `base_sha` is provided, the diff is between that commit and HEAD.
/// If `base_sha` is None, it attempts to find the merge-base with the default branch
/// or falls back to diffing uncommitted changes against HEAD.
pub async fn create_export_patch(
    worktree_path: &Path,
    base_sha: Option<&str>,
) -> Result<ExportPatchResult, PatchError> {
    // Determine the base SHA to diff against
    let resolved_base_sha = match base_sha {
        Some(sha) => sha.to_string(),
        None => get_merge_base_sha(worktree_path).await?,
    };

    // Get diff stats using --stat
    let stat_output = Command::new("git")
        .current_dir(worktree_path)
        .args(["diff", "--stat", &resolved_base_sha, "HEAD"])
        .output()
        .await
        .map_err(|e| PatchError::GitCommand(format!("Failed to run git diff --stat: {}", e)))?;

    // Parse the stat output to get line counts
    let (lines_added, lines_removed) = if stat_output.status.success() {
        parse_diff_stat(&String::from_utf8_lossy(&stat_output.stdout))
    } else {
        (0, 0)
    };

    // Get the actual diff content
    let diff_output = Command::new("git")
        .current_dir(worktree_path)
        .args(["diff", &resolved_base_sha, "HEAD"])
        .output()
        .await
        .map_err(|e| PatchError::GitCommand(format!("Failed to run git diff: {}", e)))?;

    if !diff_output.status.success() {
        let stderr = String::from_utf8_lossy(&diff_output.stderr);
        return Err(PatchError::GitCommand(format!("git diff failed: {}", stderr)));
    }

    let patch_content = String::from_utf8_lossy(&diff_output.stdout).to_string();

    // Check for empty patch
    if patch_content.trim().is_empty() {
        return Err(PatchError::EmptyPatch);
    }

    // Count files changed
    let files_output = Command::new("git")
        .current_dir(worktree_path)
        .args(["diff", "--name-only", &resolved_base_sha, "HEAD"])
        .output()
        .await
        .map_err(|e| PatchError::GitCommand(format!("Failed to count files: {}", e)))?;

    let files_count = if files_output.status.success() {
        String::from_utf8_lossy(&files_output.stdout)
            .lines()
            .filter(|l| !l.is_empty())
            .count()
    } else {
        0
    };

    Ok(ExportPatchResult {
        patch_content,
        files_count,
        lines_added,
        lines_removed,
        base_commit_sha: resolved_base_sha,
    })
}

/// Parse git diff --stat output to extract lines added and removed.
/// The last line typically looks like: " 3 files changed, 45 insertions(+), 12 deletions(-)"
fn parse_diff_stat(stat_output: &str) -> (usize, usize) {
    let mut lines_added = 0;
    let mut lines_removed = 0;

    // Look for the summary line at the end
    for line in stat_output.lines().rev() {
        let line = line.trim();
        if line.contains("changed") || line.contains("insertion") || line.contains("deletion") {
            // Parse insertions
            if let Some(idx) = line.find("insertion") {
                let before = &line[..idx];
                if let Some(num_str) = before.split_whitespace().last() {
                    if let Ok(num) = num_str.parse::<usize>() {
                        lines_added = num;
                    }
                }
            }
            // Parse deletions
            if let Some(idx) = line.find("deletion") {
                let before = &line[..idx];
                if let Some(num_str) = before.split(',').last() {
                    if let Some(num_str) = num_str.trim().split_whitespace().next() {
                        if let Ok(num) = num_str.parse::<usize>() {
                            lines_removed = num;
                        }
                    }
                }
            }
            break;
        }
    }

    (lines_added, lines_removed)
}

/// Checkout a specific commit in the worktree.
pub async fn checkout_commit(worktree_path: &Path, sha: &str) -> Result<(), PatchError> {
    let output = Command::new("git")
        .current_dir(worktree_path)
        .args(["checkout", sha])
        .output()
        .await
        .map_err(|e| PatchError::GitCommand(format!("Failed to checkout: {}", e)))?;

    if !output.status.success() {
        let stderr = String::from_utf8_lossy(&output.stderr);
        return Err(PatchError::GitCommand(format!(
            "git checkout {} failed: {}",
            sha, stderr
        )));
    }

    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::fs;
    use tempfile::TempDir;

    async fn setup_test_repo() -> TempDir {
        let dir = TempDir::new().unwrap();
        let path = dir.path();

        // Initialize git repo
        Command::new("git")
            .current_dir(path)
            .args(["init"])
            .output()
            .await
            .unwrap();

        // Configure git user
        Command::new("git")
            .current_dir(path)
            .args(["config", "user.email", "test@test.com"])
            .output()
            .await
            .unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["config", "user.name", "Test"])
            .output()
            .await
            .unwrap();

        // Create initial commit
        fs::write(path.join("file.txt"), "initial").unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["add", "."])
            .output()
            .await
            .unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["commit", "-m", "initial"])
            .output()
            .await
            .unwrap();

        dir
    }

    #[tokio::test]
    async fn test_create_and_apply_patch() {
        let dir = setup_test_repo().await;
        let path = dir.path();

        // Get base SHA
        let base_sha = get_parent_sha(path).await;
        // This will fail since there's only one commit
        assert!(base_sha.is_err());

        // Make another commit first
        fs::write(path.join("file.txt"), "modified").unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["add", "."])
            .output()
            .await
            .unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["commit", "-m", "modified"])
            .output()
            .await
            .unwrap();

        // Now get the base SHA
        let base_sha = get_parent_sha(path).await.unwrap();

        // Create patch
        let (patch_data, files_count) = create_patch(path, &base_sha).await.unwrap();
        assert!(!patch_data.is_empty());
        assert_eq!(files_count, 1);

        // Reset to base and apply patch
        checkout_commit(path, &base_sha).await.unwrap();
        assert_eq!(fs::read_to_string(path.join("file.txt")).unwrap(), "initial");

        apply_patch(path, &patch_data).await.unwrap();
        assert_eq!(
            fs::read_to_string(path.join("file.txt")).unwrap(),
            "modified"
        );
    }

    #[tokio::test]
    async fn test_empty_patch() {
        let dir = setup_test_repo().await;
        let path = dir.path();

        // Make another commit
        fs::write(path.join("file.txt"), "modified").unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["add", "."])
            .output()
            .await
            .unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["commit", "-m", "modified"])
            .output()
            .await
            .unwrap();

        // Get current HEAD
        let head_output = Command::new("git")
            .current_dir(path)
            .args(["rev-parse", "HEAD"])
            .output()
            .await
            .unwrap();
        let head_sha = String::from_utf8_lossy(&head_output.stdout)
            .trim()
            .to_string();

        // Try to create patch from HEAD to HEAD (no changes)
        let result = create_patch(path, &head_sha).await;
        assert!(matches!(result, Err(PatchError::EmptyPatch)));
    }

    #[tokio::test]
    async fn test_create_export_patch() {
        let dir = setup_test_repo().await;
        let path = dir.path();

        // Get the initial commit SHA before making changes
        let initial_sha = get_head_sha(path).await.unwrap();

        // Make some changes and commit
        fs::write(path.join("file.txt"), "modified content").unwrap();
        fs::write(path.join("new_file.txt"), "new file content").unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["add", "."])
            .output()
            .await
            .unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["commit", "-m", "changes for export"])
            .output()
            .await
            .unwrap();

        // Create export patch with explicit base
        let result = create_export_patch(path, Some(&initial_sha)).await.unwrap();

        // Verify the result
        assert!(!result.patch_content.is_empty());
        assert_eq!(result.files_count, 2); // file.txt and new_file.txt
        assert!(result.lines_added > 0);
        assert_eq!(result.base_commit_sha, initial_sha);

        // The patch should contain diff headers
        assert!(result.patch_content.contains("diff --git"));
        assert!(result.patch_content.contains("new_file.txt"));
    }

    #[tokio::test]
    async fn test_create_export_patch_no_base() {
        let dir = setup_test_repo().await;
        let path = dir.path();

        // Make a second commit so we have something to diff
        fs::write(path.join("file.txt"), "modified").unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["add", "."])
            .output()
            .await
            .unwrap();
        Command::new("git")
            .current_dir(path)
            .args(["commit", "-m", "second commit"])
            .output()
            .await
            .unwrap();

        // Create export patch without explicit base (will use HEAD~1)
        let result = create_export_patch(path, None).await.unwrap();

        // Verify the result
        assert!(!result.patch_content.is_empty());
        assert_eq!(result.files_count, 1);
        assert!(result.patch_content.contains("diff --git"));
    }

    #[tokio::test]
    async fn test_parse_diff_stat() {
        // Test the parse_diff_stat function with various formats
        let stat1 = " 3 files changed, 45 insertions(+), 12 deletions(-)";
        let (added, removed) = parse_diff_stat(stat1);
        assert_eq!(added, 45);
        assert_eq!(removed, 12);

        let stat2 = " 1 file changed, 10 insertions(+)";
        let (added, removed) = parse_diff_stat(stat2);
        assert_eq!(added, 10);
        assert_eq!(removed, 0);

        let stat3 = " 2 files changed, 5 deletions(-)";
        let (added, removed) = parse_diff_stat(stat3);
        assert_eq!(added, 0);
        assert_eq!(removed, 5);
    }
}