use std::cmp;
use std::path::Path;

use anyhow::{Context, Result};
use bstr::ByteSlice;
use forge_domain::FileInfo;

use crate::error::Error;

impl crate::ForgeFS {
    /// Reads a specific range of lines from a file.
    ///
    /// # Arguments
    /// * `path` - Path to the file to read
    /// * `start_line` - Starting line number (1-based, inclusive)
    /// * `end_line` - Ending line number (1-based, inclusive)
    ///
    /// Returns a tuple containing:
    /// - The range content as a UTF-8 string.
    /// - FileInfo containing metadata about the read operation including line
    ///   positions and a `content_hash` computed from the **full** file
    ///   content, so callers can store a hash that matches what a subsequent
    ///   whole-file read would produce (used by the external-change detector).
    pub async fn read_range_utf8<T: AsRef<Path>>(
        path: T,
        start_line: u64,
        end_line: u64,
    ) -> Result<(String, FileInfo)> {
        let path_ref = path.as_ref();

        // Basic validation
        if start_line > end_line {
            return Err(Error::StartGreaterThanEnd { start: start_line, end: end_line }.into());
        }

        // Open and check if file is binary
        let mut file = tokio::fs::File::open(path_ref)
            .await
            .with_context(|| format!("Failed to open file {}", path_ref.display()))?;

        if start_line == 0 || end_line == 0 {
            return Err(Error::IndexStartingWithZero { start: start_line, end: end_line }.into());
        }

        let (is_text, file_type) = Self::is_binary(&mut file).await?;
        if !is_text {
            return Err(Error::BinaryFileNotSupported(file_type).into());
        }

        // Read file content
        let content = tokio::fs::read(path_ref)
            .await
            .with_context(|| format!("Failed to read file content from {}", path_ref.display()))?;
        let content = content.to_str_lossy();

        // Hash the full file content so callers get a stable, whole-file hash
        // that matches what the external-change detector reads back from disk.
        let content_hash = crate::ForgeFS::compute_hash(content.as_ref());

        if start_line < 2 && content.is_empty() {
            // If the file is empty, return empty content
            return Ok((
                String::new(),
                FileInfo::new(start_line, end_line, 0, content_hash),
            ));
        }
        // Split into lines
        let lines: Vec<&str> = content.lines().collect();
        let total_lines = lines.len() as u64;

        // Convert to 0-based indexing
        let start_pos = start_line.saturating_sub(1);
        let mut end_pos = end_line.saturating_sub(1);

        // Validate start position
        if start_pos >= total_lines {
            return Err(
                Error::StartBeyondFileSize { start: start_line, total: total_lines }.into(),
            );
        }

        // Cap end position at last line
        end_pos = cmp::min(end_pos, total_lines - 1);

        // Calculate actual end line (1-based) that was used
        let actual_end_line = cmp::min(end_line, total_lines);

        let info = FileInfo::new(start_line, actual_end_line, total_lines, content_hash);

        // Extract requested lines
        let result_content = if start_pos == 0 && end_pos == total_lines - 1 {
            content.to_string() // Return full content if requesting entire file
        } else {
            lines
                .get(start_pos as usize..=end_pos as usize)
                .map(|slice| slice.join("\n"))
                .unwrap_or_default()
        };

        Ok((result_content, info))
    }
}

#[cfg(test)]
mod test {
    use anyhow::Result;
    use pretty_assertions::assert_eq;
    use tokio::fs;

    // Helper to create a temporary file with test content
    async fn create_test_file(content: &str) -> Result<tempfile::NamedTempFile> {
        let file = tempfile::NamedTempFile::new()?;
        fs::write(file.path(), content).await?;
        Ok(file)
    }

    #[tokio::test]
    async fn test_read_range_utf8() -> Result<()> {
        let content =
            "Line 1\nLine 2\nLine 3\nLine 4\nLine 5\nLine 6\nLine 7\nLine 8\nLine 9\nLine 10";
        let file = create_test_file(content).await?;
        let full_hash = crate::ForgeFS::compute_hash(content);

        // Test reading a range of lines
        let (result, info) = crate::ForgeFS::read_range_utf8(file.path(), 2, 5).await?;
        assert_eq!(result, "Line 2\nLine 3\nLine 4\nLine 5");
        assert_eq!(info.start_line, 2);
        assert_eq!(info.end_line, 5);
        assert_eq!(info.total_lines, 10);
        assert_eq!(info.content_hash, full_hash);

        // Test reading from start
        let (result, info) = crate::ForgeFS::read_range_utf8(file.path(), 1, 3).await?;
        assert_eq!(result, "Line 1\nLine 2\nLine 3");
        assert_eq!(info.start_line, 1);
        assert_eq!(info.end_line, 3);
        assert_eq!(info.content_hash, full_hash);

        // Test reading to end
        let (result, info) = crate::ForgeFS::read_range_utf8(file.path(), 8, 10).await?;
        assert_eq!(result, "Line 8\nLine 9\nLine 10");
        assert_eq!(info.start_line, 8);
        assert_eq!(info.end_line, 10);
        assert_eq!(info.content_hash, full_hash);

        // Test reading entire file
        let (result, info) = crate::ForgeFS::read_range_utf8(file.path(), 1, 10).await?;
        assert_eq!(result, content);
        assert_eq!(info.start_line, 1);
        assert_eq!(info.end_line, 10);
        assert_eq!(info.content_hash, full_hash);

        // Test single line
        let (result, info) = crate::ForgeFS::read_range_utf8(file.path(), 5, 5).await?;
        assert_eq!(result, "Line 5");
        assert_eq!(info.start_line, 5);
        assert_eq!(info.end_line, 5);
        assert_eq!(info.content_hash, full_hash);

        // Test first line specifically
        let (result, info) = crate::ForgeFS::read_range_utf8(file.path(), 1, 1).await?;
        assert_eq!(result, "Line 1");
        assert_eq!(info.start_line, 1);
        assert_eq!(info.end_line, 1);
        assert_eq!(info.total_lines, 10);
        assert_eq!(info.content_hash, full_hash);

        // Test invalid ranges
        assert!(
            crate::ForgeFS::read_range_utf8(file.path(), 8, 5)
                .await
                .is_err()
        );
        assert!(
            crate::ForgeFS::read_range_utf8(file.path(), 15, 10)
                .await
                .is_err()
        );
        assert!(
            crate::ForgeFS::read_range_utf8(file.path(), 0, 5)
                .await
                .is_err()
        );

        Ok(())
    }

    #[tokio::test]
    async fn test_utf8_multi_line_handling() -> Result<()> {
        let content = "Hello world!\nこんにちは 世界!\nПривет мир!\nBonjour le monde!";
        let file = create_test_file(content).await?;

        // Test reading a range that includes multi-byte characters
        let (result, info) = crate::ForgeFS::read_range_utf8(file.path(), 2, 3).await?;
        assert_eq!(result, "こんにちは 世界!\nПривет мир!");
        assert_eq!(info.start_line, 2);
        assert_eq!(info.end_line, 3);

        Ok(())
    }

    #[tokio::test]
    async fn test_invalid_utf8_handling() -> Result<()> {
        let content = b"Hello world!\nValid line\n\xFF\xFE\xFD Invalid UTF-8\nAnother valid line";
        let file = tempfile::NamedTempFile::new()?;
        fs::write(file.path(), content).await?;

        // Attempt to read the file shouldn't fail with invalid UTF-8 error
        let result = crate::ForgeFS::read_range_utf8(&file.path(), 1, 4).await;
        assert!(result.is_ok());
        Ok(())
    }

    #[tokio::test]
    async fn test_end_line_capping() -> Result<()> {
        let content = "Line 1\nLine 2\nLine 3\nLine 4\nLine 5";
        let file = create_test_file(content).await?;

        // Test: end_line = total_lines (exact match)
        let (actual_content, actual_info) =
            crate::ForgeFS::read_range_utf8(file.path(), 1, 5).await?;
        assert_eq!(actual_content, content);
        assert_eq!(
            (
                actual_info.start_line,
                actual_info.end_line,
                actual_info.total_lines
            ),
            (1, 5, 5)
        );

        // Test: end_line = total_lines + 1 (one beyond)
        let (actual_content, actual_info) =
            crate::ForgeFS::read_range_utf8(file.path(), 1, 6).await?;
        assert_eq!(actual_content, content);
        assert_eq!(
            (
                actual_info.start_line,
                actual_info.end_line,
                actual_info.total_lines
            ),
            (1, 5, 5)
        );

        // Test: end_line >> total_lines (far beyond)
        let (actual_content, actual_info) =
            crate::ForgeFS::read_range_utf8(file.path(), 1, 10000).await?;
        assert_eq!(actual_content, content);
        assert_eq!(
            (
                actual_info.start_line,
                actual_info.end_line,
                actual_info.total_lines
            ),
            (1, 5, 5)
        );

        // Test: range starting in the middle with excessive end_line
        let (actual_content, actual_info) =
            crate::ForgeFS::read_range_utf8(file.path(), 3, 100).await?;
        assert_eq!(actual_content, "Line 3\nLine 4\nLine 5");
        assert_eq!(
            (
                actual_info.start_line,
                actual_info.end_line,
                actual_info.total_lines
            ),
            (3, 5, 5)
        );

        // Test: reading last line with excessive end_line
        let (actual_content, actual_info) =
            crate::ForgeFS::read_range_utf8(file.path(), 5, 100).await?;
        assert_eq!(actual_content, "Line 5");
        assert_eq!(
            (
                actual_info.start_line,
                actual_info.end_line,
                actual_info.total_lines
            ),
            (5, 5, 5)
        );

        Ok(())
    }

    #[tokio::test]
    async fn test_large_file_ranges() -> Result<()> {
        // Create a 5000-line file to test various range scenarios
        let lines: Vec<String> = (1..=5000).map(|i| format!("Line {}", i)).collect();
        let content = lines.join("\n");
        let file = create_test_file(&content).await?;

        // Test: range within file bounds
        let (actual_content, actual_info) =
            crate::ForgeFS::read_range_utf8(file.path(), 1, 50).await?;
        assert_eq!(actual_content.lines().count(), 50);
        assert_eq!(
            (
                actual_info.start_line,
                actual_info.end_line,
                actual_info.total_lines
            ),
            (1, 50, 5000)
        );

        // Test: range in the middle
        let (actual_content, actual_info) =
            crate::ForgeFS::read_range_utf8(file.path(), 2500, 3500).await?;
        assert_eq!(actual_content.lines().count(), 1001); // Lines 2500-3500 inclusive
        assert_eq!(
            (
                actual_info.start_line,
                actual_info.end_line,
                actual_info.total_lines
            ),
            (2500, 3500, 5000)
        );

        // Test: end beyond file bounds (simulates max_read_size limiting)
        let (actual_content, actual_info) =
            crate::ForgeFS::read_range_utf8(file.path(), 4990, 6000).await?;
        assert_eq!(actual_content.lines().count(), 11); // Lines 4990-5000
        assert_eq!(
            (
                actual_info.start_line,
                actual_info.end_line,
                actual_info.total_lines
            ),
            (4990, 5000, 5000)
        );

        Ok(())
    }
}