//! File reader helpers for plain-text, BGZF, and Tabix-indexed genomic files.
//!
//! **Important:** `.gz` files are assumed to be **BGZF** (block gzip), not standard gzip.
//! BGZF is the format produced by `bgzip` and used by BAM, VCF, BED.gz, etc.
//! Plain `gzip` output will not decompress correctly here.
//!
//! # Tabix region fetch
//!
//! [`fetch_tabix_lines`] performs random-access region queries against any
//! Tabix-indexed BGZF file, returning the raw data lines that overlap the
//! requested [`GenomeRange`]. Format-specific wrappers in `bed.rs` and `vcf.rs`
//! parse those lines into typed structures.

use std::{
    fs::{self, File},
    io::{BufRead, BufReader, Read, Write},
    path::Path,
};

use anyhow::Context;
use log::debug;
use noodles_bgzf as bgzf;
use noodles_core::{region::Interval, Position};
use noodles_csi::BinningIndex;
use noodles_tabix as tabix;

use crate::{
    helpers::TempFileGuard,
    io::writers::{finalize_bgzf_file, get_gz_writer},
    positions::GenomeRange,
};

/// Type alias for a BGZF reader wrapping any `Read` source.
pub type BGZFReader<R> = bgzf::io::Reader<R>;

/// Open a plain-text or BGZF-compressed file for reading.
///
/// The file format is detected from the extension. Supported extensions:
/// `gz` (BGZF), `vcf`, `bed`, `tsv`, `json`, `chain`.
///
/// `.gz` files are opened with [`BGZFReader`] — they must be BGZF-encoded,
/// not plain gzip.
///
/// # Arguments
///
/// * `path` - Path to the file
///
/// # Returns
///
/// A boxed `Read` impl, buffered internally.
///
/// # Errors
///
/// Returns an error if the extension is missing, unrecognised, or the file
/// cannot be opened.
pub fn get_reader(path: &str) -> anyhow::Result<Box<dyn Read>> {
    debug!("Reading: {path}");

    let file_type = Path::new(path)
        .extension()
        .and_then(|s| s.to_str())
        .with_context(|| format!("can't parse extension from {path}"))?;

    anyhow::ensure!(
        matches!(file_type, "gz" | "vcf" | "bed" | "tsv" | "json" | "chain"),
        "unsupported file extension: {file_type}"
    );

    let raw_reader = File::open(path).with_context(|| format!("failed to open {path}"))?;

    match file_type {
        "gz" => Ok(Box::new(BufReader::new(BGZFReader::new(raw_reader)))),
        _ => Ok(Box::new(BufReader::new(raw_reader))),
    }
}

/// Open a file as a BGZF reader, compressing it first if it is not already `.gz`.
///
/// If `path` does not end in `.gz`, [`compress_to_bgzip`] is called to produce
/// `<path>.gz` on disk before opening it. This is a **side-effecting** operation:
/// the compressed file is created and cached for subsequent calls.
///
/// # Arguments
///
/// * `path` - Path to the input file (plain text or BGZF)
///
/// # Returns
///
/// A [`BGZFReader`] over the (possibly newly created) `.gz` file.
///
/// # Errors
///
/// Returns an error if the extension cannot be determined, compression fails,
/// or the resulting `.gz` file cannot be opened.
pub fn get_gz_reader(path: &str) -> anyhow::Result<BGZFReader<File>> {
    debug!("Reading: {path}");

    let file_type = Path::new(path)
        .extension()
        .and_then(|s| s.to_str())
        .with_context(|| format!("can't parse extension from {path}"))?;

    let path = if file_type != "gz" {
        compress_to_bgzip(path)?
    } else {
        path.to_string()
    };

    let file = File::open(&path).with_context(|| format!("failed to open BGZF file: {path}"))?;

    Ok(BGZFReader::new(file))
}

/// Compress a plain-text file to BGZF, returning the path of the `.gz` output.
///
/// If `<input_path>.gz` already exists it is returned immediately (cached).
/// Otherwise the file is written atomically: a UUID-named temp file is created
/// in the same directory as the output (same filesystem, guaranteeing an atomic
/// rename), then renamed to `<input_path>.gz` on success. A [`TempFileGuard`]
/// ensures the temp file is removed automatically if the function fails or panics.
///
/// # Arguments
///
/// * `input_path` - Path to the plain-text input file
///
/// # Returns
///
/// The path of the BGZF-compressed output file (`<input_path>.gz`).
///
/// # Errors
///
/// Returns an error if the input cannot be read, the output cannot be written,
/// or the atomic rename fails.
pub fn compress_to_bgzip(input_path: &str) -> anyhow::Result<String> {
    let output_path = format!("{input_path}.gz");

    if Path::new(&output_path).exists() {
        return Ok(output_path);
    }

    debug!("Compressing {input_path} → {output_path}");

    let tmp_dir = Path::new(input_path)
        .parent()
        .unwrap_or(Path::new("."));

    let mut guard = TempFileGuard::new();
    let tmp_path = guard.tmp_path(".gz", tmp_dir);
    let tmp_str = tmp_path.to_string_lossy();

    let input_file = File::open(input_path)
        .with_context(|| format!("failed to open input file: {input_path}"))?;
    let mut reader = BufReader::new(input_file);
    let mut writer = get_gz_writer(&tmp_str, false)?;

    let mut buffer = [0u8; 8192];
    loop {
        let n = reader
            .read(&mut buffer)
            .with_context(|| format!("failed reading {input_path}"))?;
        if n == 0 { break; }
        writer
            .write_all(&buffer[..n])
            .with_context(|| format!("failed writing {tmp_str}"))?;
    }

    finalize_bgzf_file(writer, &tmp_str)?;

    fs::rename(&tmp_path, &output_path)
        .with_context(|| format!("failed to rename {tmp_str} → {output_path}"))?;

    guard.disarm();
    Ok(output_path)
}

/// Visit every data line in a Tabix-indexed BGZF file that overlaps `region`,
/// calling `on_line` for each one.
///
/// This is the zero-allocation core of the tabix fetch family. A single `String`
/// buffer is reused across all lines; `on_line` receives a `&str` reference into
/// that buffer — no intermediate `Vec<String>` is ever built. Callers that need
/// a collected result (e.g. [`fetch_tabix_lines`], [`fetch_bed`](super::bed::fetch_bed))
/// simply push into their own accumulator inside the closure.
///
/// Header lines (`#`) and blank lines are skipped before `on_line` is called.
/// Lines are newline-stripped before being passed to the callback.
///
/// # Coordinate system
///
/// `region` is **0-based half-open** `[start, end)`. The conversion to 1-based
/// inclusive Tabix positions is performed internally:
/// - `tabix_start = region.range.start + 1`
/// - `tabix_end   = region.range.end`
///
/// # Arguments
///
/// * `bgz_path` - Path to the BGZF file; index expected at `<bgz_path>.tbi`
/// * `region` - Genomic interval to query (0-based half-open)
/// * `on_line` - Callback invoked for each matching data line; returning `Err`
///   aborts the fetch immediately and propagates the error
///
/// # Errors
///
/// Returns an error if the index or file cannot be read, a seek fails, or
/// `on_line` returns an error.
pub fn fetch_tabix_lines_with<F>(
    bgz_path: &str,
    region: &GenomeRange,
    mut on_line: F,
) -> anyhow::Result<()>
where
    F: FnMut(&str) -> anyhow::Result<()>,
{
    let tbi_path = format!("{bgz_path}.tbi");

    let index = tabix::fs::read(&tbi_path)
        .with_context(|| format!("cannot read tabix index: {tbi_path}"))?;

    let rname = region.contig();

    let ref_seq_id = match index
        .header()
        .ok_or_else(|| anyhow::anyhow!("tabix index has no header: {tbi_path}"))?
        .reference_sequence_names()
        .get_index_of(rname.as_bytes())
    {
        Some(id) => id,
        None => return Ok(()),
    };

    // 0-based half-open [start, end) → 1-based inclusive [start+1, end]
    let interval_start = Position::try_from(region.range.start as usize + 1)
        .context("region start overflow")?;
    let interval_end = Position::try_from(region.range.end as usize)
        .context("region end overflow")?;

    let interval = Interval::from(interval_start..=interval_end);
    let chunks = index
        .query(ref_seq_id, interval)
        .context("tabix region query failed")?;

    if chunks.is_empty() {
        return Ok(());
    }

    let mut reader = bgzf::io::Reader::new(
        File::open(bgz_path).with_context(|| format!("cannot open {bgz_path}"))?,
    );

    // Single buffer reused across all lines — no per-line allocation.
    let mut buf = String::new();

    for chunk in chunks {
        reader.seek(chunk.start()).context("BGZF seek failed")?;

        loop {
            if reader.virtual_position() >= chunk.end() {
                break;
            }
            buf.clear();
            let n = reader.read_line(&mut buf).context("BGZF read_line failed")?;
            if n == 0 {
                break;
            }
            let line = buf.trim_end_matches(|c| c == '\n' || c == '\r');
            if !line.is_empty() && !line.starts_with('#') {
                on_line(line)?;
            }
        }
    }

    Ok(())
}

/// Convenience wrapper over [`fetch_tabix_lines_with`] that collects results
/// into a `Vec<String>`.
///
/// Prefer [`fetch_tabix_lines_with`] directly when you need to parse or filter
/// lines, to avoid the intermediate string allocation.
///
/// # Errors
///
/// Returns an error if the index or file cannot be read.
pub fn fetch_tabix_lines(bgz_path: &str, region: &GenomeRange) -> anyhow::Result<Vec<String>> {
    let mut lines = Vec::new();
    fetch_tabix_lines_with(bgz_path, region, |line| {
        lines.push(line.to_owned());
        Ok(())
    })?;
    Ok(lines)
}