fastx-io 0.1.0

//! `samtools faidx`-compatible FASTA indexing and random access.
//!
//! A `.fai` index is five tab-separated columns per record: name, sequence
//! length, byte offset of the first base, bases per line and bytes per line.
//! With it, any region of a multi-gigabyte reference can be fetched with a
//! single seek — which is what makes variant callers and primer designers
//! practical.
//!
//! ```no_run
//! use fastx::index::IndexedFasta;
//!
//! let mut fasta = IndexedFasta::open("hg38.fa")?;
//! let region = fasta.fetch_region("chr1", 1_000_000, 1_000_060)?;
//! assert_eq!(region.len(), 60);
//! # Ok::<(), fastx::Error>(())
//! ```

use std::collections::HashMap;
use std::fs::File;
use std::io::{self, BufRead, BufReader, Read, Seek, SeekFrom, Write};
use std::path::{Path, PathBuf};

use crate::error::{Error, Result};
use crate::record::Sequence;

/// One line of a `.fai` index.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct FaiRecord {
    /// Sequence name (the header up to the first whitespace).
    pub name: String,
    /// Number of bases in the sequence.
    pub length: u64,
    /// Byte offset of the first base in the FASTA file.
    pub offset: u64,
    /// Bases per sequence line.
    pub line_bases: u64,
    /// Bytes per sequence line, including the line terminator.
    pub line_width: u64,
}

impl FaiRecord {
    /// Byte offset of base `base` (0-based) within the FASTA file.
    fn byte_offset(&self, base: u64) -> u64 {
        let full_lines = base / self.line_bases;
        let within = base % self.line_bases;
        self.offset + full_lines * self.line_width + within
    }
}

/// A parsed or freshly built FASTA index.
#[derive(Debug, Clone, Default)]
pub struct FastaIndex {
    records: Vec<FaiRecord>,
    by_name: HashMap<String, usize>,
}

impl FastaIndex {
    /// Build an index by scanning an uncompressed FASTA stream.
    ///
    /// Fails with [`Error::Index`] when a record has ragged line lengths, since
    /// such a file cannot be indexed by arithmetic — exactly as `samtools faidx`
    /// reports it.
    pub fn build<R: Read>(reader: R) -> Result<FastaIndex> {
        let mut reader = BufReader::with_capacity(128 * 1024, reader);
        let mut index = FastaIndex::default();
        let mut line = Vec::new();
        let mut offset: u64 = 0;
        // Partially built record plus a flag for "we already saw a short line".
        let mut current: Option<(FaiRecord, bool)> = None;

        loop {
            line.clear();
            let read = reader.read_until(b'\n', &mut line)?;
            if read == 0 {
                break;
            }
            let content = trim_newline(&line);
            if line.first() == Some(&b'>') {
                if let Some((record, _)) = current.take() {
                    index.push(record)?;
                }
                // Must use the same rule as the reader, or the index will not
                // contain the names the reader hands back.
                let name =
                    String::from_utf8_lossy(crate::record::header_id(&content[1..])).into_owned();
                if name.is_empty() {
                    return Err(Error::Index(format!(
                        "record header at byte {offset} has no name"
                    )));
                }
                current = Some((
                    FaiRecord {
                        name,
                        length: 0,
                        offset: offset + read as u64,
                        line_bases: 0,
                        line_width: 0,
                    },
                    false,
                ));
            } else if let Some((record, saw_short_line)) = current.as_mut() {
                let bases = content.len() as u64;
                if record.line_bases == 0 && bases == 0 {
                    // A blank line before any sequence line: the record has not
                    // really started, so move its offset past the blank rather
                    // than pointing at it and reading the wrong bases later.
                    record.offset = offset + read as u64;
                } else if record.line_bases == 0 {
                    record.line_bases = bases;
                    record.line_width = read as u64;
                } else if *saw_short_line || bases > record.line_bases {
                    // Only the final line of a record may be shorter than the
                    // rest, and no line may be longer.
                    return Err(Error::Index(format!(
                        "sequence {:?} has different line lengths",
                        record.name
                    )));
                } else if bases < record.line_bases {
                    *saw_short_line = true;
                }
                record.length += bases;
            } else if !content.is_empty() {
                return Err(Error::Index(format!(
                    "sequence data at byte {offset} precedes any header"
                )));
            }
            offset += read as u64;
        }
        if let Some((record, _)) = current.take() {
            index.push(record)?;
        }
        Ok(index)
    }

    /// Build an index for a FASTA file on disk.
    ///
    /// The offsets are always positions in the *uncompressed* data, so the same
    /// `.fai` describes a file whether or not it is BGZF-compressed — which is
    /// exactly how `samtools faidx` behaves. A BGZF file additionally needs a
    /// `.gzi` to be seekable; [`IndexedFasta::open`] takes care of that.
    ///
    /// Plain gzip (a single deflate stream, as `gzip` produces) cannot be
    /// randomly accessed at all, and is rejected.
    pub fn build_from_path<P: AsRef<Path>>(path: P) -> Result<FastaIndex> {
        let path = path.as_ref();
        FastaIndex::build(open_for_scanning(path)?)
    }

    /// Parse an existing `.fai` file.
    pub fn parse<R: Read>(reader: R) -> Result<FastaIndex> {
        let reader = BufReader::new(reader);
        let mut index = FastaIndex::default();
        for (lineno, line) in reader.lines().enumerate() {
            let line = line?;
            if line.trim().is_empty() {
                continue;
            }
            let mut fields = line.split('\t');
            let mut next = |what: &str| -> Result<String> {
                fields.next().map(str::to_string).ok_or_else(|| {
                    Error::Index(format!("line {}: missing {what} column", lineno + 1))
                })
            };
            let name = next("name")?;
            let numbers: Vec<u64> = ["length", "offset", "line_bases", "line_width"]
                .into_iter()
                .map(|what| -> Result<u64> {
                    next(what)?.trim().parse::<u64>().map_err(|e| {
                        Error::Index(format!("line {}: bad {what} column: {e}", lineno + 1))
                    })
                })
                .collect::<Result<Vec<u64>>>()?;
            index.push(FaiRecord {
                name,
                length: numbers[0],
                offset: numbers[1],
                line_bases: numbers[2],
                line_width: numbers[3],
            })?;
        }
        Ok(index)
    }

    /// Parse a `.fai` file from disk.
    pub fn from_path<P: AsRef<Path>>(path: P) -> Result<FastaIndex> {
        let path = path.as_ref();
        FastaIndex::parse(
            File::open(path).map_err(|e| {
                Error::Io(io::Error::new(e.kind(), format!("{}: {e}", path.display())))
            })?,
        )
    }

    /// Serialise the index in `.fai` format.
    pub fn write<W: Write>(&self, out: &mut W) -> Result<()> {
        for r in &self.records {
            writeln!(
                out,
                "{}\t{}\t{}\t{}\t{}",
                r.name, r.length, r.offset, r.line_bases, r.line_width
            )?;
        }
        Ok(())
    }

    /// Write the index next to the FASTA file as `<path>.fai`.
    pub fn write_to_path<P: AsRef<Path>>(&self, fasta_path: P) -> Result<PathBuf> {
        let target = fai_path(fasta_path.as_ref());
        let mut file = File::create(&target)?;
        self.write(&mut file)?;
        file.flush()?;
        Ok(target)
    }

    fn push(&mut self, record: FaiRecord) -> Result<()> {
        if self.by_name.contains_key(&record.name) {
            return Err(Error::Index(format!(
                "duplicate sequence name {:?}",
                record.name
            )));
        }
        if record.length > 0 && record.line_bases == 0 {
            return Err(Error::Index(format!(
                "sequence {:?} has a zero line length",
                record.name
            )));
        }
        self.by_name.insert(record.name.clone(), self.records.len());
        self.records.push(record);
        Ok(())
    }

    /// Look up a sequence by name.
    pub fn get(&self, name: &str) -> Option<&FaiRecord> {
        self.by_name.get(name).map(|&i| &self.records[i])
    }

    /// All records, in file order.
    pub fn records(&self) -> &[FaiRecord] {
        &self.records
    }

    /// Number of indexed sequences.
    pub fn len(&self) -> usize {
        self.records.len()
    }

    /// True when the index holds no sequences.
    pub fn is_empty(&self) -> bool {
        self.records.is_empty()
    }

    /// Sum of all sequence lengths.
    pub fn total_length(&self) -> u64 {
        self.records.iter().map(|r| r.length).sum()
    }
}

/// The conventional index path for a FASTA file: `<path>.fai`.
pub fn fai_path(fasta: &Path) -> PathBuf {
    let mut name = fasta.as_os_str().to_os_string();
    name.push(".fai");
    PathBuf::from(name)
}

/// Anything [`IndexedFasta`] can read from: a plain file, or a decompressing
/// reader that can seek in uncompressed coordinates.
pub trait ReadSeek: Read + Seek {}

impl<T: Read + Seek> ReadSeek for T {}

/// The source type [`IndexedFasta::open`] produces.
pub type BoxedSource = Box<dyn ReadSeek + Send>;

/// Random access to an indexed FASTA file.
///
/// The source only has to be [`Read`] and [`Seek`], so the same code serves a
/// plain file and a BGZF-compressed one — [`crate::bgzf::BgzfReader`] seeks in
/// uncompressed coordinates, which is the coordinate system a `.fai` uses.
pub struct IndexedFasta<S: ReadSeek = BoxedSource> {
    source: S,
    index: FastaIndex,
    path: PathBuf,
}

impl IndexedFasta<BoxedSource> {
    /// Open a FASTA file, loading `<path>.fai` if it exists or building the
    /// index in memory if it does not.
    ///
    /// Handles plain and BGZF-compressed FASTA. For BGZF it uses `<path>.gzi`
    /// when present and otherwise builds one in memory, which is a header-only
    /// scan.
    pub fn open<P: AsRef<Path>>(path: P) -> Result<IndexedFasta> {
        let path = path.as_ref();
        let index = match FastaIndex::from_path(fai_path(path)) {
            Ok(index) => index,
            Err(Error::Io(e)) if e.kind() == io::ErrorKind::NotFound => {
                FastaIndex::build_from_path(path)?
            }
            Err(e) => return Err(e),
        };
        IndexedFasta::with_index(path, index)
    }

    /// Open a FASTA file with an index that is already in hand.
    pub fn with_index<P: AsRef<Path>>(path: P, index: FastaIndex) -> Result<IndexedFasta> {
        let path = path.as_ref().to_path_buf();
        let source = open_for_seeking(&path)?;
        Ok(IndexedFasta {
            source,
            index,
            path,
        })
    }
}

impl<S: ReadSeek> IndexedFasta<S> {
    /// Use an arbitrary seekable source, for data that is not in a file.
    pub fn from_source(source: S, index: FastaIndex) -> IndexedFasta<S> {
        IndexedFasta {
            source,
            index,
            path: PathBuf::from("<memory>"),
        }
    }

    /// The index in use.
    pub fn index(&self) -> &FastaIndex {
        &self.index
    }

    /// The path of the FASTA file.
    pub fn path(&self) -> &Path {
        &self.path
    }

    /// Fetch a whole sequence by name.
    pub fn fetch(&mut self, name: &str) -> Result<Sequence> {
        let length = self
            .index
            .get(name)
            .ok_or_else(|| Error::UnknownSequence(name.to_string()))?
            .length;
        self.fetch_region(name, 0, length)
    }

    /// Fetch the half-open, 0-based region `start..end`.
    ///
    /// Note that command line tools such as `samtools faidx` use 1-based
    /// inclusive coordinates: `chr1:11-20` is `fetch_region("chr1", 10, 20)`.
    pub fn fetch_region(&mut self, name: &str, start: u64, end: u64) -> Result<Sequence> {
        let record = self
            .index
            .get(name)
            .ok_or_else(|| Error::UnknownSequence(name.to_string()))?
            .clone();
        if start > end || end > record.length {
            return Err(Error::OutOfBounds {
                id: name.to_string(),
                start,
                end,
                length: record.length,
            });
        }
        let bases = (end - start) as usize;
        let mut seq = Vec::with_capacity(bases);
        if bases > 0 {
            let from = record.byte_offset(start);
            let to = record.byte_offset(end - 1) + 1;
            self.source.seek(SeekFrom::Start(from))?;
            let mut raw = vec![0u8; (to - from) as usize];
            self.source.read_exact(&mut raw).map_err(|e| {
                if e.kind() == io::ErrorKind::UnexpectedEof {
                    Error::Index(format!(
                        "{}: index does not match the FASTA file (truncated at {name})",
                        self.path.display()
                    ))
                } else {
                    Error::Io(e)
                }
            })?;
            seq.extend(raw.into_iter().filter(|b| *b != b'\n' && *b != b'\r'));
        }
        if seq.len() != bases {
            return Err(Error::Index(format!(
                "{}: index does not match the FASTA file (got {} of {bases} bases for {name})",
                self.path.display(),
                seq.len()
            )));
        }
        let id = if start == 0 && end == record.length {
            name.to_string()
        } else {
            format!("{name}:{}-{end}", start + 1)
        };
        Ok(Sequence::fasta(id, seq))
    }

    /// Fetch a region given as `name`, `name:start-end` or `name:start..end`
    /// using 1-based inclusive coordinates, as command line tools do.
    ///
    /// ```no_run
    /// # use fastx::index::IndexedFasta;
    /// let mut fasta = IndexedFasta::open("ref.fa")?;
    /// let first_ten = fasta.fetch_locus("chr1:1-10")?;
    /// assert_eq!(first_ten.len(), 10);
    /// # Ok::<(), fastx::Error>(())
    /// ```
    pub fn fetch_locus(&mut self, locus: &str) -> Result<Sequence> {
        let (name, range) = match locus.rsplit_once(':') {
            None => return self.fetch(locus),
            Some((name, range)) => (name, range),
        };
        let range = range.replace("..", "-").replace(',', "");
        let (start, end) = match range.split_once('-') {
            Some((s, e)) => (s, e),
            None => (range.as_str(), range.as_str()),
        };
        let parse = |text: &str, what: &str| -> Result<u64> {
            text.trim()
                .parse::<u64>()
                .map_err(|e| Error::Index(format!("bad {what} coordinate in {locus:?}: {e}")))
        };
        let start = parse(start, "start")?;
        let end = parse(end, "end")?;
        if start == 0 {
            return Err(Error::Index(format!("{locus:?}: coordinates are 1-based")));
        }
        self.fetch_region(name, start - 1, end)
    }
}

/// Open a FASTA file for a sequential scan, decompressing BGZF on the way.
fn open_for_scanning(path: &Path) -> Result<Box<dyn Read>> {
    let file = open_file(path)?;
    match probe(path)? {
        Container::Plain => Ok(Box::new(file)),
        #[cfg(feature = "gzip")]
        Container::Bgzf => Ok(Box::new(crate::bgzf::BgzfReader::new(file)?)),
        #[cfg(feature = "gzip")]
        Container::PlainGzip => Err(plain_gzip_error(path)),
    }
}

/// Open a FASTA file for random access in uncompressed coordinates.
fn open_for_seeking(path: &Path) -> Result<BoxedSource> {
    let file = open_file(path)?;
    match probe(path)? {
        Container::Plain => Ok(Box::new(file)),
        #[cfg(feature = "gzip")]
        Container::Bgzf => {
            let index = match crate::bgzf::GziIndex::from_path(crate::bgzf::gzi_path(path)) {
                Ok(index) => index,
                // No `.gzi` on disk: a header-only scan is cheap enough to do now.
                Err(Error::Io(e)) if e.kind() == io::ErrorKind::NotFound => {
                    crate::bgzf::GziIndex::build(open_file(path)?)?
                }
                Err(e) => return Err(e),
            };
            Ok(Box::new(
                crate::bgzf::BgzfReader::new(file)?.with_index(index),
            ))
        }
        #[cfg(feature = "gzip")]
        Container::PlainGzip => Err(plain_gzip_error(path)),
    }
}

/// How a FASTA file is packaged.
///
/// Only `Plain` exists without the `gzip` feature, where a compressed file is
/// rejected before it ever gets classified.
enum Container {
    /// Uncompressed.
    Plain,
    /// Block-compressed gzip: seekable.
    #[cfg(feature = "gzip")]
    Bgzf,
    /// One deflate stream: readable start to finish, but not seekable.
    #[cfg(feature = "gzip")]
    PlainGzip,
}

fn probe(path: &Path) -> Result<Container> {
    let mut file = open_file(path)?;
    let mut head = [0u8; 128];
    let mut filled = 0;
    while filled < head.len() {
        match file.read(&mut head[filled..]) {
            Ok(0) => break,
            Ok(n) => filled += n,
            Err(e) if e.kind() == io::ErrorKind::Interrupted => continue,
            Err(e) => return Err(Error::Io(e)),
        }
    }
    let head = &head[..filled];
    if crate::format::Compression::from_magic(head) == crate::format::Compression::None {
        return Ok(Container::Plain);
    }
    #[cfg(feature = "gzip")]
    {
        if crate::bgzf::is_bgzf(head) {
            Ok(Container::Bgzf)
        } else {
            Ok(Container::PlainGzip)
        }
    }
    #[cfg(not(feature = "gzip"))]
    Err(Error::FeatureDisabled("gzip"))
}

#[cfg(feature = "gzip")]
fn plain_gzip_error(path: &Path) -> Error {
    Error::Index(format!(
        "{}: this is plain gzip, which cannot be randomly accessed. \
         Recompress it as BGZF (`bgzip`, or fastx's own gzip output) to index it.",
        path.display()
    ))
}

fn open_file(path: &Path) -> Result<File> {
    File::open(path)
        .map_err(|e| Error::Io(io::Error::new(e.kind(), format!("{}: {e}", path.display()))))
}

fn trim_newline(line: &[u8]) -> &[u8] {
    let mut end = line.len();
    if end > 0 && line[end - 1] == b'\n' {
        end -= 1;
    }
    if end > 0 && line[end - 1] == b'\r' {
        end -= 1;
    }
    &line[..end]
}

#[cfg(test)]
mod tests {
    use super::*;

    const FASTA: &[u8] = b">chr1 first\nACGTACGTAC\nGGGG\n>chr2\nTTTTTTTTTT\nTTTTTTTTTT\n";

    #[test]
    fn builds_index_matching_samtools_layout() {
        let index = FastaIndex::build(FASTA).unwrap();
        assert_eq!(index.len(), 2);
        let chr1 = index.get("chr1").unwrap();
        assert_eq!(chr1.name, "chr1");
        assert_eq!(chr1.length, 14);
        assert_eq!(chr1.offset, 12); // ">chr1 first\n" is 12 bytes
        assert_eq!(chr1.line_bases, 10);
        assert_eq!(chr1.line_width, 11);

        let chr2 = index.get("chr2").unwrap();
        assert_eq!(chr2.length, 20);
        assert_eq!(chr2.offset, 12 + 11 + 5 + 6);
        assert_eq!(index.total_length(), 34);
    }

    #[test]
    fn round_trips_fai_text() {
        let index = FastaIndex::build(FASTA).unwrap();
        let mut text = Vec::new();
        index.write(&mut text).unwrap();
        assert_eq!(text, b"chr1\t14\t12\t10\t11\nchr2\t20\t34\t10\t11\n");
        let reparsed = FastaIndex::parse(&text[..]).unwrap();
        assert_eq!(reparsed.records(), index.records());
    }

    #[test]
    fn rejects_ragged_lines() {
        let ragged = b">a\nACGT\nAC\nACGT\n";
        assert!(matches!(
            FastaIndex::build(&ragged[..]),
            Err(Error::Index(_))
        ));
        // A short final line is fine — that is how every FASTA file ends.
        assert!(FastaIndex::build(&b">a\nACGT\nAC\n"[..]).is_ok());
    }

    #[test]
    fn blank_line_before_the_sequence_does_not_shift_the_offset() {
        // ">a\n" is 3 bytes, then a blank line, so the bases start at byte 4.
        let index = FastaIndex::build(&b">a\n\nACGT\n>b\nAC\n"[..]).unwrap();
        let a = index.get("a").unwrap();
        assert_eq!((a.offset, a.length, a.line_bases), (4, 4, 4));

        // A record made only of blank lines has no bases to point at.
        let index = FastaIndex::build(&b">a\n\n\n>b\nAC\n"[..]).unwrap();
        assert_eq!(index.get("a").unwrap().length, 0);
        assert_eq!(index.get("b").unwrap().length, 2);
    }

    #[test]
    fn rejects_duplicate_names() {
        assert!(matches!(
            FastaIndex::build(&b">a\nAC\n>a\nGT\n"[..]),
            Err(Error::Index(_))
        ));
    }

    #[test]
    fn names_match_what_the_reader_produces() {
        // A non-breaking space is not a field separator, so the whole thing is
        // the name. Splitting on Unicode whitespace here would index a name the
        // reader never produces, and `fetch` would fail on a valid record.
        let fasta = ">chr\u{a0}1 description\nACGT\n";
        let index = FastaIndex::build(fasta.as_bytes()).unwrap();
        let records = crate::read_all_from(fasta.as_bytes()).unwrap();
        assert_eq!(records[0].id, "chr\u{a0}1");
        assert!(index.get(&records[0].id).is_some());

        // A header whose first byte is a space has no name, exactly as the
        // reader reports an empty id there.
        assert!(matches!(
            FastaIndex::build(&b"> a\nACGT\n"[..]),
            Err(Error::Index(_))
        ));
        assert!(crate::read_all_from(&b"> a\nACGT\n"[..]).is_err());
    }

    #[test]
    fn rejects_data_before_header() {
        assert!(matches!(
            FastaIndex::build(&b"ACGT\n>a\nAC\n"[..]),
            Err(Error::Index(_))
        ));
    }

    #[test]
    fn fetches_regions() {
        let dir = std::env::temp_dir().join(format!("fastx-index-{}", std::process::id()));
        std::fs::create_dir_all(&dir).unwrap();
        let path = dir.join("ref.fa");
        std::fs::write(&path, FASTA).unwrap();

        let index = FastaIndex::build_from_path(&path).unwrap();
        index.write_to_path(&path).unwrap();
        assert!(fai_path(&path).exists());

        let mut fasta = IndexedFasta::open(&path).unwrap();
        assert_eq!(fasta.fetch("chr1").unwrap().seq, b"ACGTACGTACGGGG");
        assert_eq!(fasta.fetch("chr2").unwrap().seq, b"T".repeat(20));

        // Region entirely inside the first line.
        assert_eq!(fasta.fetch_region("chr1", 0, 4).unwrap().seq, b"ACGT");
        // Region spanning a line break.
        assert_eq!(fasta.fetch_region("chr1", 8, 12).unwrap().seq, b"ACGG");
        // Region on the final short line.
        assert_eq!(fasta.fetch_region("chr1", 10, 14).unwrap().seq, b"GGGG");
        // Empty region.
        assert!(fasta.fetch_region("chr1", 5, 5).unwrap().seq.is_empty());
        // 1-based loci.
        assert_eq!(fasta.fetch_locus("chr1:1-4").unwrap().seq, b"ACGT");
        assert_eq!(fasta.fetch_locus("chr1:9..12").unwrap().seq, b"ACGG");
        assert_eq!(fasta.fetch_locus("chr2").unwrap().seq.len(), 20);
        assert_eq!(fasta.fetch_locus("chr1:1-4").unwrap().id, "chr1:1-4");

        // Errors.
        assert!(matches!(
            fasta.fetch("nope"),
            Err(Error::UnknownSequence(_))
        ));
        assert!(matches!(
            fasta.fetch_region("chr1", 0, 99),
            Err(Error::OutOfBounds { .. })
        ));
        assert!(fasta.fetch_locus("chr1:0-4").is_err());
        assert!(fasta.fetch_locus("chr1:x-4").is_err());

        std::fs::remove_dir_all(&dir).ok();
    }

    #[cfg(feature = "gzip")]
    #[test]
    fn indexes_and_fetches_from_bgzf() {
        use std::io::Write;

        let dir = std::env::temp_dir().join(format!("fastx-bgzf-idx-{}", std::process::id()));
        std::fs::create_dir_all(&dir).unwrap();

        // A reference big enough to span several BGZF blocks.
        let mut fasta = Vec::new();
        for chromosome in 0..3 {
            fasta.extend_from_slice(format!(">chr{chromosome} test\n").as_bytes());
            for line in 0..1_500 {
                let base = b"ACGT"[(chromosome + line) % 4];
                fasta.extend(std::iter::repeat(base).take(60));
                fasta.push(b'\n');
            }
        }

        let bgzf_path = dir.join("ref.fa.gz");
        let mut writer = crate::bgzf::BgzfWriter::create(&bgzf_path).unwrap();
        writer.write_all(&fasta).unwrap();
        let (_, gzi) = writer.finish_with_index().unwrap();
        gzi.write_to_path(&bgzf_path).unwrap();

        // The .fai is identical to the one for the uncompressed file, because
        // its offsets are uncompressed positions.
        let index = FastaIndex::build_from_path(&bgzf_path).unwrap();
        let plain_index = FastaIndex::build(&fasta[..]).unwrap();
        assert_eq!(index.records(), plain_index.records());
        index.write_to_path(&bgzf_path).unwrap();

        // Fetching must agree with the uncompressed file, including regions that
        // straddle both line breaks and BGZF block boundaries.
        let plain_path = dir.join("ref.fa");
        std::fs::write(&plain_path, &fasta).unwrap();
        let mut compressed = IndexedFasta::open(&bgzf_path).unwrap();
        let mut plain = IndexedFasta::open(&plain_path).unwrap();

        for name in ["chr0", "chr1", "chr2"] {
            assert_eq!(compressed.fetch(name).unwrap(), plain.fetch(name).unwrap());
            for (start, end) in [(0, 1), (59, 61), (1_000, 1_100), (89_000, 90_000)] {
                assert_eq!(
                    compressed.fetch_region(name, start, end).unwrap(),
                    plain.fetch_region(name, start, end).unwrap(),
                    "{name}:{start}-{end}"
                );
            }
        }

        // Without a .gzi it still works, by scanning the block headers.
        std::fs::remove_file(crate::bgzf::gzi_path(&bgzf_path)).unwrap();
        let mut rescanned = IndexedFasta::open(&bgzf_path).unwrap();
        assert_eq!(
            rescanned.fetch_region("chr1", 1_000, 1_100).unwrap(),
            plain.fetch_region("chr1", 1_000, 1_100).unwrap()
        );

        std::fs::remove_dir_all(&dir).ok();
    }

    #[cfg(feature = "gzip")]
    #[test]
    fn refuses_to_index_plain_gzip() {
        use std::io::Write;

        let dir = std::env::temp_dir().join(format!("fastx-plain-gz-{}", std::process::id()));
        std::fs::create_dir_all(&dir).unwrap();
        let path = dir.join("ref.fa.gz");

        // A single deflate stream, as `gzip` writes: no blocks to seek to.
        let file = std::fs::File::create(&path).unwrap();
        let mut encoder = flate2::write::GzEncoder::new(file, flate2::Compression::fast());
        encoder.write_all(FASTA).unwrap();
        encoder.finish().unwrap();

        let error = FastaIndex::build_from_path(&path).unwrap_err();
        assert!(matches!(error, Error::Index(_)), "{error}");
        assert!(error.to_string().contains("BGZF"), "{error}");
        // The streaming reader is perfectly happy with it, though.
        assert_eq!(crate::read_all(&path).unwrap().len(), 2);

        std::fs::remove_dir_all(&dir).ok();
    }
}