rudb_csv/

reader.rs

//! A CSV file as chunks.
//!
//! The same shape as `rudb-parquet`'s reader on purpose, because the operator above them is the same
//! operator with a different constructor: open, ask what the columns are, say which of them you
//! want, then pull chunks until there are none. A caller that can read one can read the other.
//!
//! The file is read in blocks and a record that straddles a block boundary is carried into the next
//! one, so a file larger than memory reads the same as a small one. The sample the sniffer looks at
//! is the first block, which is also the first block the reader then goes on to use, so opening a
//! file reads its front once.
//!
//! A reader can also cover a stretch of a file rather than all of it, which is what [`crate::split`]
//! hands each thread. It starts where it is told a record starts and takes the records that start
//! before its end, reading past the end only to finish the last of them.

use std::sync::Arc;

use rudb_common::{Error, Field, LogicalType, Result};
use rudb_io::File;
use rudb_vector::{Chunk, VECTOR_SIZE};

use crate::convert::{self, Cells};
use crate::dialect::{self, Dialect, Given};
use crate::infer;
use crate::scan::Records;

/// How much is read at a time, and how much the sniffer gets to look at.
///
/// A megabyte holds well over the twenty thousand rows of the sample for any file with ordinary
/// rows in it, and for a file with enormous rows the sniffer sees fewer of them and says so by
/// getting a wider type rather than by failing.
const BLOCK: usize = 1 << 20;

/// How much is read at a time once a reader of a stretch is past its end.
///
/// All it is finishing there is the one record that crosses the end, which is usually a line, so a
/// whole block would be read to be thrown away. Each read after the first is as large as what is
/// held, so a record that turns out to be long still arrives in a handful of reads.
const TAIL: usize = 64 << 10;

/// A CSV file, positioned at a record boundary.
#[derive(Debug)]
pub struct Reader {
    file: Arc<dyn File>,
    path: String,
    given: Given,
    dialect: Dialect,
    fields: Vec<Field>,
    projection: Vec<usize>,
    buffer: Vec<u8>,
    at: usize,
    offset: u64,
    drained: bool,
    line: u64,
    scratch: Vec<String>,
    records: Records,
    block: usize,
    /// Where the reading started, so that what it has read is the offset less this.
    origin: u64,
    /// The first byte a record may not start at, which is the end of the file for a whole one.
    end: u64,
    /// Where a reader gives up rather than reading on to finish a record, which only a guess
    /// sets. See [`crate::split`].
    cap: u64,
}

impl Reader {
    /// Opens a file, works out how it is written, and positions it at the first row.
    ///
    /// The path is kept because the error a bad value produces names it, the way DuckDB's does.
    ///
    /// # Errors
    ///
    /// When the file cannot be read, and when the first block of it does not hold one whole record,
    /// which is a single line longer than a megabyte and is not a CSV file anybody meant to write.
    pub fn open(file: Box<dyn File>, path: &str) -> Result<Self> {
        Self::open_with(file, path, Given::default())
    }

    /// The same, with whatever the caller already knows about how the file is written.
    ///
    /// This is where `read_csv('f.csv', delim=';', header=true)` arrives. A given value replaces the
    /// sniffer's answer rather than seeding it, and it replaces it before the sample is split, so the
    /// types and the column names come out of the file read the way the caller said it is written.
    ///
    /// # Errors
    ///
    /// Everything [`Reader::open`] reports.
    pub fn open_with(file: Box<dyn File>, path: &str, given: Given) -> Result<Self> {
        Self::open_sized(file, path, given, BLOCK)
    }

    /// The same, reading `block` bytes at a time, which the tests make small so that a record
    /// crosses a refill every few lines rather than once a megabyte.
    pub(crate) fn open_sized(
        file: Box<dyn File>,
        path: &str,
        given: Given,
        block: usize,
    ) -> Result<Self> {
        let mut reader = Self {
            file: Arc::from(file),
            path: path.to_string(),
            given,
            dialect: Dialect::comma_separated(),
            fields: Vec::new(),
            projection: Vec::new(),
            buffer: Vec::new(),
            at: 0,
            offset: 0,
            drained: false,
            line: 1,
            scratch: Vec::new(),
            records: Records::default(),
            block,
            origin: 0,
            end: u64::MAX,
            cap: u64::MAX,
        };
        reader.fill(0)?;
        let sample = reader.buffer.clone();
        let quote = given.quote.or_else(|| dialect::quote(&sample));
        let delimiter = match given.delimiter {
            Some(byte) => byte,
            None => dialect::delimiter(&sample, quote)?,
        };
        let escape = given.escape.or(quote);
        reader.dialect = Dialect { delimiter, quote, escape, header: false };
        let rows = reader.sample_rows(&sample)?;
        let (header, fields) = describe(&rows, given.header);
        reader.dialect.header = header;
        reader.fields = fields;
        reader.projection = (0..reader.fields.len()).collect();
        if header {
            reader.skip_record()?;
        }
        Ok(reader)
    }

    /// The columns this reader will produce, in order.
    #[must_use]
    pub fn fields(&self) -> Vec<Field> {
        self.projection.iter().map(|&at| self.fields[at].clone()).collect()
    }

    /// Reads only these columns, by position in the file, in this order.
    ///
    /// # Errors
    ///
    /// When a position is past the end of the file's columns.
    pub fn project(&mut self, columns: &[usize]) -> Result<()> {
        for &column in columns {
            if column >= self.fields.len() {
                return Err(Error::io(format!(
                    "column {column} is past the {} the file has",
                    self.fields.len()
                )));
            }
        }
        self.projection = columns.to_vec();
        Ok(())
    }

    /// Reads the projected columns as these types rather than as the ones the sample chose.
    ///
    /// A read that covers several files produces one stream and a stream has one schema, and no
    /// single file's sample is that schema. Every file is sniffed on its own and the answers are
    /// combined by [`crate::across`], so each file is then told what the whole read settled on,
    /// including the first one. Without it a file whose column happens to hold nothing but whole
    /// numbers hands up a BIGINT column into a stream that is DOUBLE because some other file in the
    /// set held a decimal.
    ///
    /// This is not a cast of what was read. The type is what the text is converted with, so saying
    /// it before any row is read converts once rather than converting to the wrong type and again to
    /// the right one. A value that then does not fit is the conversion error, named and lined the
    /// way any other one is.
    ///
    /// # Errors
    ///
    /// When the list is not as long as the projection.
    pub fn retype(&mut self, types: &[LogicalType]) -> Result<()> {
        if types.len() != self.projection.len() {
            return Err(Error::io(format!(
                "{} types for a projection of {} columns",
                types.len(),
                self.projection.len()
            )));
        }
        for (&at, ty) in self.projection.iter().zip(types) {
            self.fields[at].ty = ty.clone();
        }
        Ok(())
    }

    /// How this file is punctuated, which is what the sniffer decided.
    #[must_use]
    pub const fn dialect(&self) -> Dialect {
        self.dialect
    }

    /// The next chunk, or `None` at the end of the file.
    ///
    /// The records are split first, all of them, and converted afterwards a column at a time, which
    /// is also the order the errors come out in: a malformed record anywhere in the chunk is
    /// reported ahead of a value that does not convert, and among values the first projected
    /// column's first bad row is the one named.
    ///
    /// # Errors
    ///
    /// A read error, a malformed record, or a value that does not fit the type the sample chose
    /// for its column.
    pub fn next_chunk(&mut self) -> Result<Option<Chunk>> {
        let rows = self.next_records()?;
        if rows == 0 {
            return Ok(None);
        }
        let first = self.line;
        self.line += rows as u64;
        let cells = Cells { bytes: &self.buffer, records: &self.records, dialect: self.dialect };
        let mut columns = Vec::with_capacity(self.projection.len());
        for &at in &self.projection {
            let field = &self.fields[at];
            let refuse = |text: &str, row: usize| {
                Error::conversion(self.conversion_error(text, field, first + row as u64))
            };
            columns.push(convert::column(&cells, at, &field.ty, &refuse)?);
        }
        Ok(Some(Chunk::with_rows(columns, rows)?))
    }

    /// Splits the next chunk's worth of records into [`Self::records`] and answers how many there
    /// are, which is none at the end.
    ///
    /// The end of the file is the end for a whole file. A reader of a stretch stops at the first
    /// record that starts at or after its end, and it cannot see where a record starts until it has
    /// split it, so a chunk that went past the end is split again a record at a time. That happens
    /// once per stretch, on its last chunk.
    fn next_records(&mut self) -> Result<usize> {
        self.records.clear();
        let mut start = self.at;
        let mut careful = false;
        loop {
            if self.here() >= self.end {
                break;
            }
            let limit = if careful { self.records.len() + 1 } else { VECTOR_SIZE };
            self.at = crate::scan::records(
                &self.buffer,
                self.at,
                self.dialect,
                self.drained,
                limit,
                &mut self.records,
            )?;
            if !careful && self.here() > self.end {
                self.records.clear();
                self.at = start;
                careful = true;
                continue;
            }
            if self.records.len() == VECTOR_SIZE {
                break;
            }
            if careful && self.records.len() == limit {
                continue;
            }
            if self.drained {
                break;
            }
            // The records already read point into the buffer, so the refill keeps everything from
            // the start of the chunk and moves their ranges down by whatever it dropped in front.
            self.fill(start)?;
            self.records.shift(start);
            start = 0;
        }
        Ok(self.records.len())
    }

    /// Where in the file the next record starts.
    pub(crate) fn here(&self) -> u64 {
        self.offset - self.buffer.len() as u64 + self.at as u64
    }

    /// How many bytes of the file this reader has read, sniffing included.
    #[must_use]
    pub fn bytes_read(&self) -> u64 {
        self.offset - self.origin
    }

    /// A reader over the same file with the same answers, that starts at `from`, which has to be
    /// where a record starts, and takes the records that start before `end`.
    ///
    /// `line` is what the line of the record at `from` is called in an error, which is right only
    /// for a caller that knows it. [`crate::split`] does not, and so never shows the error such a
    /// reader makes.
    pub(crate) fn stretch(&self, from: u64, end: u64, line: u64) -> Self {
        Self {
            file: Arc::clone(&self.file),
            path: self.path.clone(),
            given: self.given,
            dialect: self.dialect,
            fields: self.fields.clone(),
            projection: self.projection.clone(),
            buffer: Vec::new(),
            at: 0,
            offset: from,
            drained: false,
            line,
            scratch: Vec::new(),
            records: Records::default(),
            block: self.block,
            origin: from,
            end,
            cap: u64::MAX,
        }
    }

    /// Makes the reader give up once it has read up to `cap`, with an error nobody is shown.
    pub(crate) fn give_up_at(&mut self, cap: u64) {
        self.cap = cap;
    }

    /// Splits records to the end without converting any of them, and answers where the first
    /// record not taken starts.
    pub(crate) fn skim(&mut self) -> Result<u64> {
        while self.next_records()? > 0 {}
        Ok(self.here())
    }

    /// Steps over the chunks that end at or before `target` without converting them, counting their
    /// lines, so that the chunks after are the ones and the lines a whole read would give.
    ///
    /// The chunk that runs past `target` is left to be read, since it is the same chunk a whole read
    /// would convert and a value in it may be the one that fails.
    pub(crate) fn skip_to(&mut self, target: u64) -> Result<()> {
        loop {
            let from = self.here();
            let rows = self.next_records()?;
            if rows == 0 {
                return Ok(());
            }
            if self.here() > target {
                let front = self.offset - self.buffer.len() as u64;
                self.at = usize::try_from(from - front)
                    .map_err(|_| Error::internal("a chunk start outside the buffer"))?;
                return Ok(());
            }
            self.line += rows as u64;
        }
    }

    /// The line the next record is on, as a conversion error counts them.
    pub(crate) const fn line(&self) -> u64 {
        self.line
    }

    /// The file this reads, for a caller that wants to look at bytes the reader has not.
    pub(crate) fn file(&self) -> &dyn File {
        self.file.as_ref()
    }

    /// DuckDB's message for a value that does not fit the type its column was sniffed as.
    ///
    /// Reproduced whole, including the block of settings at the bottom, because that block is the
    /// answer to the question the message raises. Somebody reading it wants to know what was
    /// guessed and how to override the guess, and a shorter message would send them to the
    /// documentation to find out.
    ///
    /// A line of that block says where its value came from, and a value the call gave is `(Set By
    /// User)` rather than `(Auto-Detected)`, measured on `v2.0.0-dev84237` by reading a file with
    /// `delim=';'` past the sample. Telling somebody that what they wrote down was auto-detected is
    /// the one thing the block could say that would send them looking in the wrong place.
    fn conversion_error(&self, text: &str, field: &Field, line: u64) -> String {
        format!(
            "CSV Error on Line: {line}\nOriginal Line: {text}\nError when converting column \
             \"{}\". Could not convert string \"{text}\" to '{}'\n\nColumn {} is being converted \
             as type {}\nThis type was auto-detected from the CSV file.\nPossible solutions:\n* \
             Override the type for this column manually by setting the type explicitly, e.g., \
             types={{'{}': 'VARCHAR'}}\n* Set the sample size to a larger value to enable the \
             auto-detection to scan more values, e.g., sample_size=-1\n* Use a COPY statement to \
             automatically derive types from an existing table.\n* Check whether the null string \
             value is set correctly (e.g., nullstr = 'N/A')\n\n  file = {}\n  delimiter = {}\n  \
             quote = {}\n  escape = {}\n  header = {} {}\n  sample_size = {}\n",
            field.name,
            field.ty,
            field.name,
            field.ty,
            field.name,
            self.path,
            Given::shown(self.given.delimiter, Some(self.dialect.delimiter)),
            Given::shown(self.given.quote, self.dialect.quote),
            Given::shown(self.given.escape, self.dialect.escape),
            self.dialect.header,
            Given::source(self.given.header.is_some()),
            infer::SAMPLE,
        )
    }

    /// The next chunk the way it was read before the records were split a chunk at a time, one
    /// record into owned strings and one cell into a `Value` at a time.
    #[cfg(test)]
    fn next_chunk_by_record(&mut self) -> Result<Option<Chunk>> {
        let mut rows: Vec<Vec<Option<String>>> = Vec::new();
        while rows.len() < VECTOR_SIZE {
            match self.next_record()? {
                Some(fields) => rows.push(fields),
                None => break,
            }
        }
        if rows.is_empty() {
            return Ok(None);
        }
        let mut columns = Vec::with_capacity(self.projection.len());
        for &at in &self.projection {
            let field = &self.fields[at];
            let mut values = Vec::with_capacity(rows.len());
            for (row, held) in rows.iter().enumerate() {
                let text = held.get(at).and_then(Option::as_deref);
                values.push(self.convert(
                    text,
                    field,
                    self.line - rows.len() as u64 + row as u64,
                )?);
            }
            columns.push(rudb_vector::Vector::from_values(field.ty.clone(), &values)?);
        }
        Ok(Some(Chunk::with_rows(columns, rows.len())?))
    }

    /// One value, cast from its text to the column's type.
    #[cfg(test)]
    fn convert(&self, text: Option<&str>, field: &Field, line: u64) -> Result<rudb_common::Value> {
        let Some(text) = text else { return Ok(rudb_common::Value::Null) };
        if field.ty == LogicalType::Varchar {
            return Ok(rudb_common::Value::Varchar(text.to_string()));
        }
        let value = rudb_common::Value::Varchar(text.to_string());
        match rudb_kernels::cast_value(&value, &field.ty, false) {
            Ok(converted) => Ok(converted),
            Err(_) => Err(Error::conversion(self.conversion_error(text, field, line))),
        }
    }

    /// The next record, as one entry per field, with an empty field as a null.
    ///
    /// This and [`Reader::next_chunk_by_record`] are how chunks were read before
    /// [`crate::scan::records`], kept for the tests to hold the new path to the old answers.
    #[cfg(test)]
    fn next_record(&mut self) -> Result<Option<Vec<Option<String>>>> {
        let Some(()) = self.advance()? else { return Ok(None) };
        Ok(Some(
            self.scratch
                .iter()
                .map(|text| if text.is_empty() { None } else { Some(text.clone()) })
                .collect(),
        ))
    }

    /// Reads one record into the scratch, filling the buffer when it has to.
    fn advance(&mut self) -> Result<Option<()>> {
        loop {
            let mut scratch = std::mem::take(&mut self.scratch);
            let outcome = crate::scan::record(
                &self.buffer,
                self.at,
                self.dialect,
                self.drained,
                &mut scratch,
            );
            self.scratch = scratch;
            match outcome? {
                Some(next) => {
                    self.at = next;
                    self.line += 1;
                    return Ok(Some(()));
                }
                None if self.drained => return Ok(None),
                None => self.fill(self.at)?,
            }
        }
    }

    /// Reads one record and throws it away, which is what a header is.
    fn skip_record(&mut self) -> Result<()> {
        self.advance()?;
        Ok(())
    }

    /// Drops the bytes before `keep` and reads another block onto the end.
    ///
    /// `keep` is where the first record anything still points at starts, which is the record being
    /// read for one record at a time and the start of the chunk for a chunk.
    fn fill(&mut self, keep: usize) -> Result<()> {
        if self.offset >= self.cap {
            return Err(Error::io("a record runs further than a guessed start is followed"));
        }
        self.buffer.drain(..keep);
        self.at -= keep;
        let held = self.buffer.len();
        let want = match self.end.checked_sub(self.offset) {
            Some(left) if left > 0 => self.block.min(usize::try_from(left).unwrap_or(usize::MAX)),
            _ => self.block.min(TAIL.max(held)),
        };
        self.buffer.resize(held + want, 0);
        let read = self.file.read_at(self.offset, &mut self.buffer[held..])?;
        self.buffer.truncate(held + read);
        self.offset += read as u64;
        if read == 0 {
            self.drained = true;
        }
        Ok(())
    }

    /// The records the sniffer gets to look at, which is the sample or the file, whichever is
    /// shorter.
    fn sample_rows(&self, sample: &[u8]) -> Result<Vec<Vec<Option<String>>>> {
        let mut rows = Vec::new();
        let mut fields = Vec::new();
        let mut at = 0;
        while rows.len() <= infer::SAMPLE {
            // The end of the block is not the end of the file, so a record the block cut in half is
            // simply not part of the sample.
            let Some(next) = crate::scan::record(sample, at, self.dialect, false, &mut fields)?
            else {
                break;
            };
            at = next;
            rows.push(
                fields
                    .iter()
                    .map(|text| if text.is_empty() { None } else { Some(text.clone()) })
                    .collect(),
            );
        }
        Ok(rows)
    }
}

/// Whether the first row is a header, and what the columns are called and typed.
///
/// The rule is DuckDB's and both halves of it were measured. A file whose columns are all `VARCHAR`
/// once the first row is set aside has a header, because two rows of words is a header and a row.
/// Otherwise the first row is a header exactly when it does not fit the types the rest of the file
/// has, which is what makes `1,2` over `3,4` a file of two rows and `a,b` over `1,2` a file of one.
///
/// `told` is the caller answering the question instead, which is `header=true` or `header=false` on
/// the call. It decides the names and the types as well as the row count, since a first row that is
/// data is a row the types have to fit and a first row that is a header is not.
fn describe(rows: &[Vec<Option<String>>], told: Option<bool>) -> (bool, Vec<Field>) {
    let width = rows.iter().map(Vec::len).max().unwrap_or(0);
    let body = types(&rows[1.min(rows.len())..], width);
    let all_text = body.iter().all(|ty| *ty == LogicalType::Varchar);
    let first_fits = rows.first().is_some_and(|first| {
        first.iter().zip(&body).all(|(text, ty)| match text {
            None => true,
            Some(text) => infer::fits(text, ty),
        })
    });
    // An empty file has no row to take names from, so it has no header whatever it was told.
    let header = !rows.is_empty() && told.unwrap_or(rows.len() > 1 && (all_text || !first_fits));
    if !header {
        let types = types(rows, width);
        let fields = types
            .into_iter()
            .enumerate()
            .map(|(at, ty)| Field::new(format!("column{at}"), ty))
            .collect();
        return (false, fields);
    }
    let names = unique(&rows[0], width);
    let fields = body.into_iter().zip(names).map(|(ty, name)| Field::new(name, ty)).collect();
    (true, fields)
}

/// The column names a header row gives, with the collisions resolved the way DuckDB resolves them.
///
/// A header is text somebody typed and nothing stops it naming two columns the same thing, so the
/// second one gets `_1`, and the count goes up until the name is free. It has to count rather than
/// stop at one, because the suffix can collide too: a file whose header is `a,a,a_1` comes back as
/// `a`, `a_1`, `a_1_1` from the binary, and it is the second column that took the name the third one
/// was written with.
///
/// The comparison ignores case and the written case is kept, which was measured: `a,a,A` comes back
/// as `a`, `a_1`, `A_2`, so `A` collided with `a` and then `A_1` collided with `a_1`. An empty
/// header cell is a column with no name, and it falls back to the generated one rather than to an
/// empty string that no query could write.
fn unique(header: &[Option<String>], width: usize) -> Vec<String> {
    let mut taken: Vec<String> = Vec::with_capacity(width);
    for at in 0..width {
        let base = match header.get(at).and_then(Option::as_deref) {
            Some(written) => written.to_string(),
            None => format!("column{at}"),
        };
        let mut name = base.clone();
        let mut next = 1;
        while taken.iter().any(|held| held.eq_ignore_ascii_case(&name)) {
            name = format!("{base}_{next}");
            next += 1;
        }
        taken.push(name);
    }
    taken
}

/// The type of each of `width` columns, over these rows.
fn types(rows: &[Vec<Option<String>>], width: usize) -> Vec<LogicalType> {
    (0..width)
        .map(|at| {
            let values: Vec<Option<&str>> =
                rows.iter().map(|row| row.get(at).and_then(Option::as_deref)).collect();
            infer::column(&values)
        })
        .collect()
}

#[cfg(test)]
mod tests {
    use super::*;
    use rudb_common::Value;
    use rudb_io::{Filesystem, OpenMode, SimFilesystem};
    use std::path::Path;

    fn read(text: &str) -> Reader {
        let filesystem = SimFilesystem::new();
        let path = Path::new("/t.csv");
        let file = filesystem.open(path, OpenMode::Create).expect("creates");
        file.write_at(0, text.as_bytes()).expect("writes");
        drop(file);
        let file = filesystem.open(path, OpenMode::Read).expect("opens");
        Reader::open(file, "/t.csv").expect("sniffs")
    }

    /// The same file opened with something already known about how it is written.
    fn read_with(text: &str, given: Given) -> Reader {
        let filesystem = SimFilesystem::new();
        let path = Path::new("/t.csv");
        let file = filesystem.open(path, OpenMode::Create).expect("creates");
        file.write_at(0, text.as_bytes()).expect("writes");
        drop(file);
        let file = filesystem.open(path, OpenMode::Read).expect("opens");
        Reader::open_with(file, "/t.csv", given).expect("reads")
    }

    fn names_and_types(reader: &Reader) -> Vec<(String, String)> {
        reader.fields().iter().map(|f| (f.name.clone(), f.ty.to_string())).collect()
    }

    fn all(reader: &mut Reader) -> Vec<Vec<Value>> {
        let mut rows = Vec::new();
        while let Some(chunk) = reader.next_chunk().expect("reads") {
            for row in 0..chunk.len() {
                rows.push((0..chunk.width()).map(|at| chunk.value_at(row, at)).collect());
            }
        }
        rows
    }

    #[test]
    fn a_header_that_names_two_columns_the_same_thing_counts_the_second_one_up() {
        let names: Vec<String> =
            read("a,a,A,a_1\n1,2,3,4\nx,y,z,w\n").fields().into_iter().map(|f| f.name).collect();
        // Measured against the binary, all four of them. The last one is the interesting one: the
        // second column took `a_1`, which is the name the fourth column was written with, so the
        // fourth has to keep counting from its own name rather than from `a`.
        assert_eq!(names, ["a", "a_1", "A_2", "a_1_1"]);
    }

    #[test]
    fn a_header_and_three_types_are_what_duckdb_sniffs_for_the_same_bytes() {
        let reader = read("a,b,c\n1,x,2.5\n2,y,3.5\n");
        assert_eq!(
            names_and_types(&reader),
            [
                ("a".to_string(), "BIGINT".to_string()),
                ("b".to_string(), "VARCHAR".to_string()),
                ("c".to_string(), "DOUBLE".to_string()),
            ]
        );
    }

    #[test]
    fn a_file_with_no_header_gets_the_names_duckdb_gives_it() {
        let reader = read("1,x\n2,y\n");
        assert_eq!(
            names_and_types(&reader),
            [
                ("column0".to_string(), "BIGINT".to_string()),
                ("column1".to_string(), "VARCHAR".to_string()),
            ]
        );
    }

    #[test]
    fn two_rows_of_words_are_a_header_and_a_row() {
        let reader = read("a,b\nc,d\n");
        assert_eq!(reader.fields().iter().map(|f| f.name.clone()).collect::<Vec<_>>(), ["a", "b"]);
    }

    #[test]
    fn one_column_of_words_under_a_row_of_numbers_is_still_a_header() {
        // `a,2` over `3,4`. One column disagreeing is enough, and the second column is then named
        // `2`, which is the text that was in it.
        let reader = read("a,2\n3,4\n");
        assert_eq!(reader.fields().iter().map(|f| f.name.clone()).collect::<Vec<_>>(), ["a", "2"]);
    }

    #[test]
    fn the_rows_are_the_rows_of_the_file() {
        let mut reader = read("a,b\n1,x\n2,y\n");
        assert_eq!(
            all(&mut reader),
            [
                vec![Value::BigInt(1), Value::Varchar("x".into())],
                vec![Value::BigInt(2), Value::Varchar("y".into())],
            ]
        );
    }

    #[test]
    fn an_empty_field_is_a_null_whether_it_was_quoted_or_not() {
        // Measured. `allow_quoted_nulls` is on by default, so `""` is a null and not the empty
        // string, which is the one place a quoted field and a bare one agree about being nothing.
        let mut reader = read("a,b\n1,\n\"\",y\n");
        assert_eq!(
            all(&mut reader),
            [vec![Value::BigInt(1), Value::Null], vec![Value::Null, Value::Varchar("y".into())],]
        );
    }

    #[test]
    fn a_projection_picks_columns_out_by_position_and_can_reorder_them() {
        let mut reader = read("a,b,c\n1,x,2.5\n");
        reader.project(&[2, 0]).expect("projects");
        assert_eq!(reader.fields().iter().map(|f| f.name.clone()).collect::<Vec<_>>(), ["c", "a"]);
        assert_eq!(all(&mut reader), [vec![Value::Double(2.5), Value::BigInt(1)]]);
    }

    #[test]
    fn a_projection_of_nothing_still_counts_the_rows() {
        let mut reader = read("a,b\n1,x\n2,y\n3,z\n");
        reader.project(&[]).expect("projects");
        let chunk = reader.next_chunk().expect("reads").expect("a chunk");
        assert_eq!(chunk.len(), 3);
        assert_eq!(chunk.width(), 0);
    }

    #[test]
    fn a_pipe_separated_file_reads_as_one() {
        let mut reader = read("a|b\n1|x\n");
        assert_eq!(reader.dialect().delimiter, b'|');
        assert_eq!(all(&mut reader), [vec![Value::BigInt(1), Value::Varchar("x".into())]]);
    }

    #[test]
    fn a_quoted_field_with_a_delimiter_in_it_is_one_value() {
        let mut reader = read("a,b\n1,\"x,y\"\n");
        assert_eq!(all(&mut reader), [vec![Value::BigInt(1), Value::Varchar("x,y".into())]]);
    }

    #[test]
    fn more_rows_than_fit_one_chunk_arrive_as_more_than_one_chunk() {
        let mut text = String::from("a\n");
        for row in 0..VECTOR_SIZE + 5 {
            text.push_str(&format!("{row}\n"));
        }
        let mut reader = read(&text);
        let first = reader.next_chunk().expect("reads").expect("a chunk");
        assert_eq!(first.len(), VECTOR_SIZE);
        let second = reader.next_chunk().expect("reads").expect("a second chunk");
        assert_eq!(second.len(), 5);
        assert!(reader.next_chunk().expect("reads").is_none());
    }

    #[test]
    fn a_value_the_sniffer_never_saw_is_an_error_rather_than_a_wider_column() {
        // The value has to be past the sample, because a value inside it would have widened the
        // column to VARCHAR and there would be nothing to fail. Widening after the fact is not an
        // option: the chunks before this one have already gone out with the narrow type on them.
        let mut text = String::from("c\n");
        for row in 0..infer::SAMPLE {
            text.push_str(&format!("{row}\n"));
        }
        text.push_str("oops\n");
        let mut reader = read(&text);
        assert_eq!(reader.fields()[0].ty, LogicalType::BigInt);
        let error = all_or_error(&mut reader).unwrap_err();
        let line = infer::SAMPLE + 2;
        assert!(error.message().starts_with(&format!("CSV Error on Line: {line}")), "{error}");
        assert!(
            error.message().contains("Could not convert string \"oops\" to 'BIGINT'"),
            "{error}"
        );
        assert!(error.message().contains("sample_size = 20480"), "{error}");
    }

    /// Measured. The header row becomes a row, so the names are the generated ones and the first
    /// column holds `a`, `1` and `2`, which is text rather than the BIGINT the sniffer would say.
    #[test]
    fn a_file_told_it_has_no_header_reads_its_first_line_as_a_row() {
        let mut reader =
            read_with("a,b\n1,x\n2,y\n", Given { header: Some(false), ..Given::default() });
        assert_eq!(
            names_and_types(&reader),
            [
                ("column0".to_string(), "VARCHAR".to_string()),
                ("column1".to_string(), "VARCHAR".to_string()),
            ]
        );
        assert_eq!(all(&mut reader).len(), 3);
    }

    /// The other way round, on a file the sniffer would call two rows of data.
    #[test]
    fn a_file_told_it_has_a_header_takes_its_first_line_as_the_names() {
        let reader = read_with("1,2\n3,4\n", Given { header: Some(true), ..Given::default() });
        assert_eq!(reader.fields().iter().map(|f| f.name.clone()).collect::<Vec<_>>(), ["1", "2"]);
    }

    /// A given delimiter is used rather than tried, so a file that is really commas is one column.
    #[test]
    fn a_given_delimiter_is_the_delimiter_whatever_the_file_looks_like() {
        let reader = read_with("a,b\n1,x\n", Given { delimiter: Some(b';'), ..Given::default() });
        assert_eq!(reader.dialect().delimiter, b';');
        assert_eq!(reader.fields().len(), 1);
    }

    /// A quote the sniffer would never find, since it only ever looks for the double quote.
    #[test]
    fn a_given_quote_makes_a_field_that_holds_the_delimiter_one_value() {
        let mut reader =
            read_with("a,b\n1,'x,y'\n", Given { quote: Some(b'\''), ..Given::default() });
        assert_eq!(all(&mut reader), [vec![Value::BigInt(1), Value::Varchar("x,y".into())]]);
    }

    /// The block under a conversion error says which of its lines the caller wrote down.
    #[test]
    fn the_block_says_set_by_user_for_what_the_call_gave_it() {
        let mut text = String::from("c;d\n");
        for row in 0..infer::SAMPLE {
            text.push_str(&format!("{row};x\n"));
        }
        text.push_str("oops;x\n");
        let given = Given { delimiter: Some(b';'), ..Given::default() };
        let mut reader = read_with(&text, given);
        let error = all_or_error(&mut reader).unwrap_err();
        assert!(error.message().contains("delimiter = ; (Set By User)"), "{error}");
        assert!(error.message().contains("header = true (Auto-Detected)"), "{error}");
    }

    #[test]
    fn a_file_told_a_wider_type_than_it_sniffed_reads_its_whole_numbers_as_that_type() {
        // What a glob does to every file it names. This file on its own is BIGINT and the set it
        // belongs to is DOUBLE because some other file in it holds a decimal, so the column comes
        // out DOUBLE and the rows come with it rather than the reader being overruled afterwards.
        let mut reader = read("a\n1\n2\n");
        assert_eq!(reader.fields()[0].ty, LogicalType::BigInt);
        reader.retype(&[LogicalType::Double]).expect("one type for one column");
        assert_eq!(reader.fields()[0].ty, LogicalType::Double);
        assert_eq!(all(&mut reader), [[Value::Double(1.0)], [Value::Double(2.0)]]);
    }

    #[test]
    fn a_type_list_that_is_not_as_long_as_the_projection_is_refused() {
        let mut reader = read("a,b\n1,two\n");
        let error = reader.retype(&[LogicalType::Double]).unwrap_err();
        assert!(error.message().contains("1 types for a projection of 2 columns"), "{error}");
    }

    /// Every chunk a reader hands back, as its length and its values written out, or the error
    /// it stopped on. The values are compared as their debug text so that a NaN equals itself.
    fn drained(reader: &mut Reader, old: bool) -> (Vec<(usize, Vec<String>)>, Option<String>) {
        let mut chunks = Vec::new();
        loop {
            let next = if old { reader.next_chunk_by_record() } else { reader.next_chunk() };
            match next {
                Ok(Some(chunk)) => {
                    let mut values = Vec::new();
                    for row in 0..chunk.len() {
                        for at in 0..chunk.width() {
                            values.push(format!("{:?}", chunk.value_at(row, at)));
                        }
                    }
                    chunks.push((chunk.len(), values));
                }
                Ok(None) => return (chunks, None),
                Err(error) => return (chunks, Some(error.to_string())),
            }
        }
    }

    /// A small generator, since the crate has no dependencies to take one from.
    struct Rng(u64);

    impl Rng {
        fn next(&mut self) -> u64 {
            self.0 ^= self.0 << 13;
            self.0 ^= self.0 >> 7;
            self.0 ^= self.0 << 17;
            self.0
        }

        fn below(&mut self, n: usize) -> usize {
            (self.next() % n as u64) as usize
        }
    }

    /// A typed CSV file with a header, whose values are mostly what their column says and now and
    /// then something only the cast knows what to do with, or something nothing can convert.
    fn typed_file(rng: &mut Rng, rows: usize) -> String {
        const ODD: [&str; 27] = [
            "",
            " 1",
            "1 ",
            "1e3",
            "0x10",
            "inf",
            "-nan",
            "abc",
            "\"12\"",
            "\"a\"\"b\"",
            "\"x,y\"",
            "\"x\ny\"",
            "h\u{e9}llo",
            "99999999999999999999",
            "9999999999999999999",
            "-",
            "+5",
            "007",
            "2020-02-30",
            "2020-02-29",
            "0000-01-01",
            "TRUE",
            "no",
            "1_000",
            "1.5e-3",
            "-0",
            "\"\"",
        ];
        let width = 1 + rng.below(6);
        let kinds: Vec<usize> = (0..width).map(|_| rng.below(5)).collect();
        let mut text: String = (0..width).map(|at| format!("c{at}")).collect::<Vec<_>>().join(",");
        text.push('\n');
        for _ in 0..rows {
            let mut fields = Vec::with_capacity(width);
            for &kind in &kinds {
                let odd = rng.below(60) == 0;
                fields.push(if odd {
                    ODD[rng.below(ODD.len())].to_string()
                } else {
                    let n = rng.next();
                    match kind {
                        0 => format!("{}", (n % 2_000_001) as i64 - 1_000_000),
                        1 => format!("{}.{:02}", n % 100_000, n % 100),
                        2 => format!("{}-{:02}-{:02}", 1990 + n % 20, 1 + n % 12, 1 + n % 28),
                        3 => ["true", "false", "t", "F"][(n % 4) as usize].to_string(),
                        _ => ["x", "hello world", "a longer piece of text", "\"q,\"\"q\""]
                            [(n % 4) as usize]
                            .to_string(),
                    }
                });
            }
            if rng.below(200) == 0 {
                fields.pop();
            }
            if rng.below(200) == 0 {
                fields.push("extra".to_string());
            }
            text.push_str(&fields.join(","));
            text.push_str(["\n", "\n", "\n", "\r\n", "\r"][rng.below(5)]);
        }
        if rng.below(4) == 0 {
            text.pop();
        }
        text
    }

    fn open_sized(text: &[u8], block: usize) -> Result<Reader> {
        let filesystem = SimFilesystem::new();
        let path = Path::new("/t.csv");
        let file = filesystem.open(path, OpenMode::Create).expect("creates");
        file.write_at(0, text).expect("writes");
        drop(file);
        let file = filesystem.open(path, OpenMode::Read).expect("opens");
        Reader::open_sized(file, "/t.csv", Given::default(), block)
    }

    /// The chunk at a time reader and the record at a time one, over generated files, read with
    /// blocks small enough that records straddle the refills, projected and retyped at random so
    /// that every type with a parser of its own gets values it takes and values it has to pass on.
    #[test]
    fn generated_files_read_the_same_a_chunk_at_a_time_as_a_record_at_a_time() {
        let types = [
            LogicalType::BigInt,
            LogicalType::Integer,
            LogicalType::SmallInt,
            LogicalType::TinyInt,
            LogicalType::UBigInt,
            LogicalType::UInteger,
            LogicalType::USmallInt,
            LogicalType::UTinyInt,
            LogicalType::Double,
            LogicalType::Float,
            LogicalType::Date,
            LogicalType::Boolean,
            LogicalType::Varchar,
            LogicalType::Timestamp,
            LogicalType::Decimal { width: 18, scale: 3 },
        ];
        let mut rng = Rng(0x2545_f491_4f6c_dd1d);
        for case in 0..200 {
            let rows = if case % 100 == 0 { 8192 + rng.below(1000) } else { rng.below(200) };
            let mut text = typed_file(&mut rng, rows).into_bytes();
            if rng.below(20) == 0 {
                // A quoted field with rubbish after it somewhere, which is a malformed record.
                let at = rng.below(text.len() + 1);
                text.splice(at..at, *b",\"x\"y,");
            }
            for block in [1 << 20, 32 + rng.below(400)] {
                let (Ok(mut new), Ok(mut old)) =
                    (open_sized(&text, block), open_sized(&text, block))
                else {
                    continue;
                };
                assert_eq!(new.fields(), old.fields());
                let width = new.fields().len();
                if width > 0 && rng.below(2) == 0 {
                    let columns: Vec<usize> =
                        (0..rng.below(width + 2)).map(|_| rng.below(width)).collect();
                    new.project(&columns).expect("projects");
                    old.project(&columns).expect("projects");
                    let wanted: Vec<LogicalType> =
                        columns.iter().map(|_| types[rng.below(types.len())].clone()).collect();
                    new.retype(&wanted).expect("retypes");
                    old.retype(&wanted).expect("retypes");
                }
                let expected = drained(&mut old, true);
                let found = drained(&mut new, false);
                assert_eq!(found.1, expected.1, "case {case}, block {block}");
                assert_eq!(found.0, expected.0, "case {case}, block {block}");
            }
        }
    }

    /// How much faster the chunk at a time reader is, on a file shaped like TPC-H's `lineitem`.
    ///
    /// Not run by default, because it is a measurement rather than a check. Run it with
    /// `cargo test --release -p rudb-csv -- --ignored --nocapture reads_lineitem`.
    #[test]
    #[ignore = "a measurement, run by hand"]
    fn reads_lineitem_faster_a_chunk_at_a_time() {
        use rudb_io::RealFilesystem;
        use std::time::Instant;

        const ROWS: usize = 200_000;
        let mut rng = Rng(0x1234_5678_9abc_def1);
        let mut text = String::from(
            "l_orderkey,l_partkey,l_suppkey,l_linenumber,l_quantity,l_extendedprice,l_discount,\
             l_tax,l_returnflag,l_linestatus,l_shipdate,l_commitdate,l_receiptdate,\
             l_shipinstruct,l_shipmode,l_comment\n",
        );
        let words = ["carefully", "final", "deposits", "furiously", "regular", "ideas", "sleep"];
        for row in 0..ROWS {
            let n = rng.next();
            let date = |shift: u64| {
                format!(
                    "{}-{:02}-{:02}",
                    1992 + (n >> shift) % 7,
                    1 + (n >> shift) % 12,
                    1 + (n >> shift) % 28
                )
            };
            let comment: Vec<&str> =
                (0..3 + n % 4).map(|k| words[((n >> (k * 3)) % 7) as usize]).collect();
            text.push_str(&format!(
                "{},{},{},{},{}.00,{}.{:02},0.0{},0.0{},{},{},{},{},{},{},{},{}\n",
                row / 4 + 1,
                n % 200_000,
                n % 10_000,
                row % 4 + 1,
                1 + n % 50,
                900 + n % 100_000,
                n % 100,
                n % 10,
                (n >> 7) % 9,
                ["A", "N", "R"][(n % 3) as usize],
                ["O", "F"][(n % 2) as usize],
                date(3),
                date(11),
                date(19),
                ["DELIVER IN PERSON", "NONE", "TAKE BACK RETURN"][(n % 3) as usize],
                ["TRUCK", "MAIL", "AIR", "SHIP"][(n % 4) as usize],
                comment.join(" "),
            ));
        }
        let path = std::env::temp_dir().join(format!("rudb-lineitem-{}.csv", std::process::id()));
        std::fs::write(&path, &text).expect("writes");
        let megabytes = text.len() as f64 / 1e6;
        let filesystem = RealFilesystem::new();
        let mut best = [f64::MAX; 2];
        for _ in 0..5 {
            for (slot, old) in [(0, true), (1, false)] {
                let file = filesystem.open(&path, OpenMode::Read).expect("opens");
                let mut reader = Reader::open(file, "lineitem.csv").expect("sniffs");
                let started = Instant::now();
                let mut rows = 0;
                loop {
                    let next =
                        if old { reader.next_chunk_by_record() } else { reader.next_chunk() };
                    let Some(chunk) = next.expect("reads") else { break };
                    rows += chunk.len();
                }
                assert_eq!(rows, ROWS);
                best[slot] = best[slot].min(started.elapsed().as_secs_f64());
            }
        }
        std::fs::remove_file(&path).expect("removes");
        let (old, new) = (megabytes / best[0], megabytes / best[1]);
        println!(
            "{ROWS} rows, {megabytes:.1} MB: a record at a time {old:.1} MB/s, a chunk at a time"
        );
        println!("{new:.1} MB/s, {:.2}x", best[0] / best[1]);
    }

    fn all_or_error(reader: &mut Reader) -> Result<Vec<Vec<Value>>> {
        let mut rows = Vec::new();
        while let Some(chunk) = reader.next_chunk()? {
            for row in 0..chunk.len() {
                rows.push((0..chunk.width()).map(|at| chunk.value_at(row, at)).collect());
            }
        }
        Ok(rows)
    }
}