readcon_core/

writer.rs

use crate::types::{
    ConFrame, SECTION_CHARGES, SECTION_DISPLACEMENTS, SECTION_ENERGIES, SECTION_FORCES,
    SECTION_MAGMOMS, SECTION_SPINS, SECTION_SPREADS, SECTION_VELOCITIES, encode_fixed_bitmask,
    meta,
};
use serde_json::json;
use std::fs::File;
use std::io::{self, BufWriter, Write};
use std::path::Path;

/// Default floating-point precision used for writing coordinates, cell dimensions, and masses.
const DEFAULT_FLOAT_PRECISION: usize = 6;

/// Numeric text representation for CON frame fields.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum FloatFormat {
    /// A fixed number of digits after the decimal point.
    DecimalPlaces(usize),
    /// The shortest scientific representation preserving each finite f64.
    /// Signed zero is preserved; decimal precision does not limit this mode.
    RoundTrip,
}

impl Default for FloatFormat {
    fn default() -> Self {
        Self::DecimalPlaces(DEFAULT_FLOAT_PRECISION)
    }
}

/// A writer that can serialize and write `ConFrame` objects to any output stream.
///
/// This struct encapsulates a writer (like a file) and provides a high-level API
/// for writing simulation frames in the `.con` format.
///
/// # Example
/// ```no_run
/// # use std::fs::File;
/// # use readcon_core::types::ConFrame;
/// # use readcon_core::writer::ConFrameWriter;
/// # let frames: Vec<ConFrame> = Vec::new();
/// let mut writer = ConFrameWriter::from_path("output.con").unwrap();
/// writer.extend(frames.iter()).unwrap();
/// ```
pub struct ConFrameWriter<W: Write> {
    writer: BufWriter<W>,
    float_format: FloatFormat,
    /// When true: sort metadata keys in JSON, emit sections in canonical
    /// order (velocities, forces, energies), fixed precision suitable for
    /// content-stable corpus writes / semantic-ish dedup. Opt-in so default
    /// writes keep historical float formatting.
    canonical: bool,
    /// Cache for the JSON metadata line: when consecutive frames share
    /// the same (spec_version, sections-set, metadata) triple the
    /// serialized JSON object is identical, so reusing the cached
    /// string skips the per-frame `serde_json::Map::insert` rebuild
    /// and re-serialisation. Hot for trajectory writes where every
    /// frame has the same `units` / `potential` / `validate` keys.
    metadata_cache: Option<MetadataCacheEntry>,
    /// One frame of CON text. Filled then flushed with a single `write_all`
    /// so the sink is not entered once per atom line.
    scratch: Vec<u8>,
}

#[derive(Debug)]
struct MetadataCacheEntry {
    /// Snapshot of the inputs that fully determine the serialized JSON
    /// metadata line. Cheap to clone; cheaper than re-serialising the
    /// whole map on every frame.
    spec_version: u32,
    has_velocities: bool,
    has_forces: bool,
    has_energies: bool,
    has_charges: bool,
    has_spins: bool,
    has_magmoms: bool,
    has_displacements: bool,
    has_spreads: bool,
    metadata: std::collections::BTreeMap<String, serde_json::Value>,
    /// Cached serialised metadata line (without trailing newline).
    serialized: String,
}

impl MetadataCacheEntry {
    fn matches(
        &self,
        spec_version: u32,
        has_velocities: bool,
        has_forces: bool,
        has_energies: bool,
        has_charges: bool,
        has_spins: bool,
        has_magmoms: bool,
        has_displacements: bool,
        has_spreads: bool,
        metadata: &std::collections::BTreeMap<String, serde_json::Value>,
    ) -> bool {
        self.spec_version == spec_version
            && self.has_velocities == has_velocities
            && self.has_forces == has_forces
            && self.has_energies == has_energies
            && self.has_charges == has_charges
            && self.has_spins == has_spins
            && self.has_magmoms == has_magmoms
            && self.has_displacements == has_displacements
            && self.has_spreads == has_spreads
            && self.metadata.len() == metadata.len()
            && self
                .metadata
                .iter()
                .zip(metadata)
                .all(|((ka, a), (kb, b))| ka == kb && metadata_value_eq(a, b))
    }
}

// Cache equality preserves the numeric representation, including signed zero,
// throughout nested JSON values.
fn metadata_value_eq(a: &serde_json::Value, b: &serde_json::Value) -> bool {
    use serde_json::Value;
    match (a, b) {
        (Value::Number(a), Value::Number(b)) if a.is_f64() && b.is_f64() => {
            a.as_f64().map(f64::to_bits) == b.as_f64().map(f64::to_bits)
        }
        (Value::Array(a), Value::Array(b)) => {
            a.len() == b.len() && a.iter().zip(b).all(|(a, b)| metadata_value_eq(a, b))
        }
        (Value::Object(a), Value::Object(b)) => {
            a.len() == b.len()
                && a.iter()
                    .all(|(key, a)| b.get(key).is_some_and(|b| metadata_value_eq(a, b)))
        }
        _ => a == b,
    }
}

// General implementation for any type that implements `Write`.
impl<W: Write> ConFrameWriter<W> {
    /// Creates a new `ConFrameWriter` that wraps a given writer.
    ///
    /// # Arguments
    ///
    /// * `writer` - Any type that implements `std::io::Write`, e.g., a `File`.
    pub fn new(writer: W) -> Self {
        Self::with_float_format(writer, FloatFormat::default())
    }

    /// Creates a new `ConFrameWriter` with a custom floating-point precision.
    ///
    /// # Arguments
    ///
    /// * `writer` - Any type that implements `std::io::Write`.
    /// * `precision` - Number of decimal places for floating-point output.
    pub fn with_precision(writer: W, precision: usize) -> Self {
        Self::with_float_format(writer, FloatFormat::DecimalPlaces(precision))
    }

    /// Creates a writer with an explicit numeric text representation.
    ///
    /// `FloatFormat::RoundTrip` retains small forces and coordinates exactly.
    /// Metadata floats use the JSON serializer independently of this setting.
    pub fn with_float_format(writer: W, float_format: FloatFormat) -> Self {
        Self {
            writer: BufWriter::new(writer),
            float_format,
            canonical: false,
            metadata_cache: None,
            scratch: Vec::with_capacity(16 * 1024),
        }
    }

    /// Opt-in **canonical** serialization: BTree-ordered metadata keys in JSON,
    /// fixed section order, stable float precision (default 6). Use for corpus
    /// materialization and content-stable hashes; not required for on-disk fidelity
    /// of spans preserved from `next_with_raw_span`.
    pub fn canonical(mut self, on: bool) -> Self {
        self.set_canonical(on);
        self
    }

    /// Set or clear canonical mode on an existing writer (C ABI / FFI).
    pub fn set_canonical(&mut self, on: bool) {
        self.canonical = on;
        if on {
            self.metadata_cache = None;
        }
    }

    /// Whether canonical serialization is enabled.
    pub fn is_canonical(&self) -> bool {
        self.canonical
    }

    fn refresh_metadata_cache(&mut self, frame: &ConFrame) {
        let spec_version = frame.header.spec_version;
        let has_vel = frame.has_velocities();
        let has_frc = frame.has_forces();
        let has_eng = frame.has_energies();
        let has_chg = frame.has_charges();
        let has_spn = frame.has_spins();
        let has_mm = frame.has_magmoms();
        let has_dsp = frame.has_displacements();
        let has_spr = frame.has_spreads();

        let cache_hit = !self.canonical
            && self.metadata_cache.as_ref().is_some_and(|c| {
                c.matches(
                    spec_version,
                    has_vel,
                    has_frc,
                    has_eng,
                    has_chg,
                    has_spn,
                    has_mm,
                    has_dsp,
                    has_spr,
                    &frame.header.metadata,
                )
            });
        if cache_hit {
            return;
        }

        let mut meta_obj = serde_json::Map::new();
        meta_obj.insert(meta::CON_SPEC_VERSION.into(), json!(spec_version));
        let mut sections = Vec::new();
        if has_vel {
            sections.push(json!(SECTION_VELOCITIES));
        }
        if has_frc {
            sections.push(json!(SECTION_FORCES));
        }
        if has_eng {
            sections.push(json!(SECTION_ENERGIES));
        }
        if has_chg {
            sections.push(json!(SECTION_CHARGES));
        }
        if has_spn {
            sections.push(json!(SECTION_SPINS));
        }
        if has_mm {
            sections.push(json!(SECTION_MAGMOMS));
        }
        if has_dsp {
            sections.push(json!(SECTION_DISPLACEMENTS));
        }
        if has_spr {
            sections.push(json!(SECTION_SPREADS));
        }
        let validate = frame
            .header
            .metadata
            .get(meta::VALIDATE)
            .and_then(|value| value.as_bool())
            .unwrap_or(false);
        if !sections.is_empty() || validate {
            meta_obj.insert(meta::SECTIONS.into(), json!(sections));
        }
        for (k, v) in &frame.header.metadata {
            if k == meta::CON_SPEC_VERSION || k == meta::SECTIONS {
                continue;
            }
            meta_obj.insert(k.clone(), v.clone());
        }
        if let Some(u) = meta_obj.get(meta::UNITS).cloned() {
            if let Ok(c) = crate::units::canonicalize_units_object(&u) {
                meta_obj.insert(meta::UNITS.into(), c);
            }
        }
        if spec_version >= 3 {
            let need_default = match meta_obj.get(meta::UNITS) {
                None => true,
                Some(u) => crate::units::validate_v3_units_metadata(u).is_err(),
            };
            if need_default {
                meta_obj.insert(meta::UNITS.into(), crate::units::default_v3_units_json());
            }
        }
        let serialized = serde_json::Value::Object(meta_obj).to_string();
        self.metadata_cache = Some(MetadataCacheEntry {
            spec_version,
            has_velocities: has_vel,
            has_forces: has_frc,
            has_energies: has_eng,
            has_charges: has_chg,
            has_spins: has_spn,
            has_magmoms: has_mm,
            has_displacements: has_dsp,
            has_spreads: has_spr,
            metadata: frame.header.metadata.clone(),
            serialized,
        });
    }

    /// Writes a single `ConFrame` to the output stream.
    pub fn write_frame(&mut self, frame: &ConFrame) -> io::Result<()> {
        let prec = self.float_format;
        self.refresh_metadata_cache(frame);
        let meta_line = self
            .metadata_cache
            .as_ref()
            .expect("metadata_cache populated above")
            .serialized
            .clone();
        self.scratch.clear();
        {
            let buf = &mut self.scratch;

            // --- Write the 9-line Header ---
            let _ = writeln!(buf, "{}", frame.header.prebox_header.user);
            let _ = writeln!(buf, "{meta_line}");
            push_f64(buf, frame.header.boxl[0], prec);
            buf.push(b' ');
            push_f64(buf, frame.header.boxl[1], prec);
            buf.push(b' ');
            push_f64(buf, frame.header.boxl[2], prec);
            buf.push(b'\n');
            push_f64(buf, frame.header.angles[0], prec);
            buf.push(b' ');
            push_f64(buf, frame.header.angles[1], prec);
            buf.push(b' ');
            push_f64(buf, frame.header.angles[2], prec);
            buf.push(b'\n');
            let _ = writeln!(buf, "{}", frame.header.postbox_header[0]);
            let _ = writeln!(buf, "{}", frame.header.postbox_header[1]);
            let _ = writeln!(buf, "{}", frame.header.natm_types);

            for (i, n) in frame.header.natms_per_type.iter().enumerate() {
                if i > 0 {
                    buf.push(b' ');
                }
                push_u64(buf, *n as u64);
            }
            buf.push(b'\n');

            for (i, m) in frame.header.masses_per_type.iter().enumerate() {
                if i > 0 {
                    buf.push(b' ');
                }
                push_f64(buf, *m, prec);
            }
            buf.push(b'\n');

            // --- Write the Atom Data ---
            let mut atom_idx_offset = 0;
            for (type_idx, &num_atoms_in_type) in frame.header.natms_per_type.iter().enumerate() {
                let symbol = &frame.atom_data[atom_idx_offset].symbol;
                let _ = writeln!(buf, "{symbol}");
                let _ = writeln!(buf, "Coordinates of Component {}", type_idx + 1);

                for i in 0..num_atoms_in_type {
                    let atom = &frame.atom_data[atom_idx_offset + i];
                    push_xyz_line(
                        buf,
                        atom.x,
                        atom.y,
                        atom.z,
                        prec,
                        encode_fixed_bitmask(atom.fixed),
                        atom.atom_id,
                    );
                }
                atom_idx_offset += num_atoms_in_type;
            }

            // --- Write optional velocity section ---
            if frame.has_velocities() {
                buf.push(b'\n');

                let mut vel_idx_offset = 0;
                for (type_idx, &num_atoms_in_type) in frame.header.natms_per_type.iter().enumerate()
                {
                    let symbol = &frame.atom_data[vel_idx_offset].symbol;
                    let _ = writeln!(buf, "{symbol}");
                    let _ = writeln!(buf, "Velocities of Component {}", type_idx + 1);

                    for i in 0..num_atoms_in_type {
                        let atom = &frame.atom_data[vel_idx_offset + i];
                        let [vx, vy, vz] = atom.velocity.unwrap_or([0.0; 3]);
                        push_xyz_line(
                            buf,
                            vx,
                            vy,
                            vz,
                            prec,
                            encode_fixed_bitmask(atom.fixed),
                            atom.atom_id,
                        );
                    }
                    vel_idx_offset += num_atoms_in_type;
                }
            }

            // --- Write optional force section ---
            if frame.has_forces() {
                buf.push(b'\n');

                let mut force_idx_offset = 0;
                for (type_idx, &num_atoms_in_type) in frame.header.natms_per_type.iter().enumerate()
                {
                    let symbol = &frame.atom_data[force_idx_offset].symbol;
                    let _ = writeln!(buf, "{symbol}");
                    let _ = writeln!(buf, "Forces of Component {}", type_idx + 1);

                    for i in 0..num_atoms_in_type {
                        let atom = &frame.atom_data[force_idx_offset + i];
                        let [fx, fy, fz] = atom.force.unwrap_or([0.0; 3]);
                        push_xyz_line(
                            buf,
                            fx,
                            fy,
                            fz,
                            prec,
                            encode_fixed_bitmask(atom.fixed),
                            atom.atom_id,
                        );
                    }
                    force_idx_offset += num_atoms_in_type;
                }
            }

            // --- Write optional energies section ---
            if frame.has_energies() {
                buf.push(b'\n');

                let mut energy_idx_offset = 0;
                for (type_idx, &num_atoms_in_type) in frame.header.natms_per_type.iter().enumerate()
                {
                    let symbol = &frame.atom_data[energy_idx_offset].symbol;
                    let _ = writeln!(buf, "{symbol}");
                    let _ = writeln!(buf, "Energies of Component {}", type_idx + 1);

                    for i in 0..num_atoms_in_type {
                        let atom = &frame.atom_data[energy_idx_offset + i];
                        let e = atom.energy.unwrap_or(0.0);
                        push_scalar_line(
                            buf,
                            e,
                            prec,
                            encode_fixed_bitmask(atom.fixed),
                            atom.atom_id,
                        );
                    }
                    energy_idx_offset += num_atoms_in_type;
                }
            }

            if frame.has_charges() {
                buf.push(b'\n');
                let mut off = 0;
                for (type_idx, &num_atoms_in_type) in frame.header.natms_per_type.iter().enumerate()
                {
                    let symbol = &frame.atom_data[off].symbol;
                    let _ = writeln!(buf, "{symbol}");
                    let _ = writeln!(buf, "Charges of Component {}", type_idx + 1);
                    for i in 0..num_atoms_in_type {
                        let atom = &frame.atom_data[off + i];
                        let q = atom.charge.unwrap_or(0.0);
                        push_scalar_line(
                            buf,
                            q,
                            prec,
                            encode_fixed_bitmask(atom.fixed),
                            atom.atom_id,
                        );
                    }
                    off += num_atoms_in_type;
                }
            }

            if frame.has_spins() {
                buf.push(b'\n');
                let mut off = 0;
                for (type_idx, &num_atoms_in_type) in frame.header.natms_per_type.iter().enumerate()
                {
                    let symbol = &frame.atom_data[off].symbol;
                    let _ = writeln!(buf, "{symbol}");
                    let _ = writeln!(buf, "Spins of Component {}", type_idx + 1);
                    for i in 0..num_atoms_in_type {
                        let atom = &frame.atom_data[off + i];
                        let s = atom.spin.unwrap_or(0.0);
                        push_scalar_line(
                            buf,
                            s,
                            prec,
                            encode_fixed_bitmask(atom.fixed),
                            atom.atom_id,
                        );
                    }
                    off += num_atoms_in_type;
                }
            }

            if frame.has_magmoms() {
                buf.push(b'\n');
                let mut off = 0;
                for (type_idx, &num_atoms_in_type) in frame.header.natms_per_type.iter().enumerate()
                {
                    let symbol = &frame.atom_data[off].symbol;
                    let _ = writeln!(buf, "{symbol}");
                    let _ = writeln!(buf, "Magmoms of Component {}", type_idx + 1);
                    for i in 0..num_atoms_in_type {
                        let atom = &frame.atom_data[off + i];
                        let [mx, my, mz] = atom.magmom.unwrap_or([0.0; 3]);
                        push_xyz_line(
                            buf,
                            mx,
                            my,
                            mz,
                            prec,
                            encode_fixed_bitmask(atom.fixed),
                            atom.atom_id,
                        );
                    }
                    off += num_atoms_in_type;
                }
            }

            if frame.has_displacements() {
                buf.push(b'\n');
                let mut off = 0;
                for (type_idx, &num_atoms_in_type) in frame.header.natms_per_type.iter().enumerate()
                {
                    let symbol = &frame.atom_data[off].symbol;
                    let _ = writeln!(buf, "{symbol}");
                    let _ = writeln!(buf, "Displacements of Component {}", type_idx + 1);
                    for i in 0..num_atoms_in_type {
                        let atom = &frame.atom_data[off + i];
                        let [dx, dy, dz] = atom.displacement.unwrap_or([0.0; 3]);
                        push_xyz_line(
                            buf,
                            dx,
                            dy,
                            dz,
                            prec,
                            encode_fixed_bitmask(atom.fixed),
                            atom.atom_id,
                        );
                    }
                    off += num_atoms_in_type;
                }
            }

            if frame.has_spreads() {
                buf.push(b'\n');
                let mut off = 0;
                for (type_idx, &num_atoms_in_type) in frame.header.natms_per_type.iter().enumerate()
                {
                    let symbol = &frame.atom_data[off].symbol;
                    let _ = writeln!(buf, "{symbol}");
                    let _ = writeln!(buf, "Spreads of Component {}", type_idx + 1);
                    for i in 0..num_atoms_in_type {
                        let atom = &frame.atom_data[off + i];
                        let [sx, sy, sz] = atom.spread.unwrap_or([0.0; 3]);
                        if !crate::types::spread_row_ok([sx, sy, sz]) {
                            return Err(std::io::Error::new(
                                std::io::ErrorKind::InvalidInput,
                                format!(
                                    "spreads: atom {} has a negative or non-finite spread",
                                    off + i
                                ),
                            ));
                        }
                        push_xyz_line(
                            buf,
                            sx,
                            sy,
                            sz,
                            prec,
                            encode_fixed_bitmask(atom.fixed),
                            atom.atom_id,
                        );
                    }
                    off += num_atoms_in_type;
                }
            }
        }

        self.writer.write_all(&self.scratch)
    }

    /// Flushes buffered data to the underlying writer and reports errors.
    pub fn flush(&mut self) -> io::Result<()> {
        self.writer.flush()
    }

    /// Writes all frames from an iterator to the output stream.
    ///
    /// This is the most convenient way to write a multi-frame file.
    pub fn extend<'a>(&mut self, frames: impl Iterator<Item = &'a ConFrame>) -> io::Result<()> {
        for frame in frames {
            self.write_frame(frame)?;
        }
        Ok(())
    }
}

fn push_u64(buf: &mut Vec<u8>, n: u64) {
    push_u128(buf, u128::from(n));
}

fn push_u128(buf: &mut Vec<u8>, mut n: u128) {
    let mut tmp = [0u8; 40];
    let mut i = 40;
    if n == 0 {
        buf.push(b'0');
        return;
    }
    while n > 0 {
        i -= 1;
        tmp[i] = b'0' + (n % 10) as u8;
        n /= 10;
    }
    buf.extend_from_slice(&tmp[i..]);
}

fn push_f64(buf: &mut Vec<u8>, value: f64, format: FloatFormat) {
    match format {
        FloatFormat::DecimalPlaces(precision) => push_f64_prec(buf, value, precision),
        FloatFormat::RoundTrip => {
            let _ = write!(buf, "{value:e}");
        }
    }
}

/// Fixed-point `f64` with `prec` digits after the decimal, matching `{:.prec$}`.
/// Non-finite values and `prec > 17` fall back to `std::fmt`.
fn push_f64_prec(buf: &mut Vec<u8>, v: f64, prec: usize) {
    // The six-decimal path uses fixed-point arithmetic; other precisions
    // use the standard formatter.
    if !v.is_finite() || prec != 6 {
        let _ = write!(buf, "{v:.prec$}");
        return;
    }
    if v.is_sign_negative() {
        buf.push(b'-');
    }
    let ax = v.abs();
    if prec == 0 {
        push_u128(buf, ax.round() as u128);
        return;
    }
    let scale = 10u128.pow(prec as u32);
    let n = (ax * scale as f64).round() as u128;
    let int_part = n / scale;
    let frac = n % scale;
    push_u128(buf, int_part);
    buf.push(b'.');
    let mut tmp = [b'0'; 20];
    let mut x = frac;
    let mut i = prec;
    while i > 0 {
        i -= 1;
        tmp[i] = b'0' + (x % 10) as u8;
        x /= 10;
    }
    buf.extend_from_slice(&tmp[..prec]);
}

fn push_xyz_line(
    buf: &mut Vec<u8>,
    x: f64,
    y: f64,
    z: f64,
    prec: FloatFormat,
    fixed: u8,
    atom_id: u64,
) {
    push_f64(buf, x, prec);
    buf.push(b' ');
    push_f64(buf, y, prec);
    buf.push(b' ');
    push_f64(buf, z, prec);
    buf.push(b' ');
    push_u64(buf, u64::from(fixed));
    buf.push(b' ');
    push_u64(buf, atom_id);
    buf.push(b'\n');
}

fn push_scalar_line(buf: &mut Vec<u8>, v: f64, prec: FloatFormat, fixed: u8, atom_id: u64) {
    push_f64(buf, v, prec);
    buf.push(b' ');
    push_u64(buf, u64::from(fixed));
    buf.push(b' ');
    push_u64(buf, atom_id);
    buf.push(b'\n');
}

// Implementation block specifically for when the writer is a `File`.
impl ConFrameWriter<File> {
    /// Creates a new `ConFrameWriter` that writes to a file at the given path.
    ///
    /// This is a convenience function that creates the file and wraps it.
    pub fn from_path<P: AsRef<Path>>(path: P) -> io::Result<Self> {
        let file = File::create(path)?;
        Ok(Self::new(file))
    }

    /// Creates a new `ConFrameWriter` that writes to a file with a custom precision.
    pub fn from_path_with_precision<P: AsRef<Path>>(path: P, precision: usize) -> io::Result<Self> {
        let file = File::create(path)?;
        Ok(Self::with_precision(file, precision))
    }
}

// Gzip-compressed writer constructors.
impl ConFrameWriter<flate2::write::GzEncoder<File>> {
    /// Creates a gzip-compressed writer for the given path.
    pub fn from_path_gzip<P: AsRef<Path>>(path: P) -> io::Result<Self> {
        let encoder = crate::compression::gzip_writer(path.as_ref())?;
        Ok(Self::new(encoder))
    }

    /// Creates a gzip-compressed writer with custom precision.
    pub fn from_path_gzip_with_precision<P: AsRef<Path>>(
        path: P,
        precision: usize,
    ) -> io::Result<Self> {
        let encoder = crate::compression::gzip_writer(path.as_ref())?;
        Ok(Self::with_precision(encoder, precision))
    }
}

// Zstd-compressed writer constructors. Available only with the `zstd`
// Cargo feature.
#[cfg(feature = "zstd")]
impl ConFrameWriter<zstd::stream::write::AutoFinishEncoder<'static, File>> {
    /// Creates a zstd-compressed writer for the given path.
    pub fn from_path_zstd<P: AsRef<Path>>(path: P) -> io::Result<Self> {
        let encoder = crate::compression::zstd_writer(path.as_ref())?;
        Ok(Self::new(encoder))
    }

    /// Creates a zstd-compressed writer with custom precision.
    pub fn from_path_zstd_with_precision<P: AsRef<Path>>(
        path: P,
        precision: usize,
    ) -> io::Result<Self> {
        let encoder = crate::compression::zstd_writer(path.as_ref())?;
        Ok(Self::with_precision(encoder, precision))
    }
}

#[cfg(test)]
mod float_format_tests {
    use super::push_f64_prec;

    fn formatted(v: f64, prec: usize) -> String {
        let mut buf = Vec::new();
        push_f64_prec(&mut buf, v, prec);
        String::from_utf8(buf).expect("utf8")
    }

    #[test]
    fn matches_std_fixed_precision() {
        let vals = [
            0.0,
            -0.0,
            1.0,
            -1.0,
            0.9045,
            6.975_299_999_999_995,
            63.546,
            1.008,
            15.3456,
            90.0,
            218.0,
            1e-6,
            -1.23456789,
            10.0,
        ];
        for prec in [0usize, 6] {
            for v in vals {
                let got = formatted(v, prec);
                let exp = format!("{v:.prec$}");
                assert_eq!(got, exp, "v={v:?} prec={prec}");
            }
        }
    }
}