//! Cuts markdown into retrievable passages that carry their page, chapter
//! and role.

use std::io::{BufWriter, Read, Write};
use std::path::PathBuf;
use std::process::ExitCode;

use chunkmd::Budgets;
use clap::Parser;
use corpus_core::{exit, ndjson};

/// Cut markdown into chunks, one JSON record per line.
///
/// Reads markdown on stdin, or from a path. Page markers (`<!-- Page 41 -->`)
/// and headings become the page and chapter on every chunk, so a passage can
/// always be cited back to where it came from.
#[derive(Parser)]
#[command(
    version,
    about,
    after_help = "\
Example:

    mdscrub book.md | chunkmd --target 1024 --overlap 128 > chunks.ndjson

The defaults suit a model with a 512-token window; a larger model wants
--target 1024 --overlap 128 --min-tokens 80 --hard-max 1280."
)]
struct Args {
    /// Markdown file to read. Reads stdin when absent.
    path: Option<PathBuf>,

    /// Flush a chunk at this many tokens.
    #[arg(long, default_value_t = Budgets::default().target, value_name = "TOKENS")]
    target: usize,

    /// Carry this much of each chunk into the next one.
    #[arg(long, default_value_t = Budgets::default().overlap, value_name = "TOKENS")]
    overlap: usize,

    /// Drop chunks smaller than this.
    #[arg(long, default_value_t = Budgets::default().min_tokens, value_name = "TOKENS")]
    min_tokens: usize,

    /// Never emit a chunk larger than this.
    #[arg(long, default_value_t = Budgets::default().hard_max, value_name = "TOKENS")]
    hard_max: usize,
}

fn main() -> ExitCode {
    let args = Args::parse();
    let budgets = Budgets {
        target: args.target,
        overlap: args.overlap,
        min_tokens: args.min_tokens,
        hard_max: args.hard_max,
    };
    if let Err(why) = sane(&budgets) {
        eprintln!("chunkmd: {why}");
        return ExitCode::from(exit::USAGE);
    }

    let markdown = match read(args.path.as_deref()) {
        Ok(markdown) => markdown,
        Err((code, why)) => {
            eprintln!("chunkmd: {why}");
            return ExitCode::from(code);
        }
    };

    let mut out = BufWriter::new(std::io::stdout().lock());
    for chunk in chunkmd::chunk(&markdown, &budgets) {
        if let Err(e) = ndjson::write_line(&mut out, &chunk) {
            // A consumer that stopped reading is not a failure: `| head` is a
            // normal way to look at a stream.
            if e.is_broken_pipe() {
                return ExitCode::from(exit::OK);
            }
            eprintln!("chunkmd: {e}");
            return ExitCode::from(exit::IO);
        }
    }
    match out.flush() {
        Ok(()) => ExitCode::from(exit::OK),
        Err(e) if e.kind() == std::io::ErrorKind::BrokenPipe => ExitCode::from(exit::OK),
        Err(e) => {
            eprintln!("chunkmd: {e}");
            ExitCode::from(exit::IO)
        }
    }
}

/// Budgets that cannot produce anything sensible, rejected before reading
/// input rather than after.
fn sane(budgets: &Budgets) -> Result<(), String> {
    if budgets.target == 0 || budgets.hard_max == 0 {
        return Err("--target and --hard-max must be above zero".into());
    }
    if budgets.overlap >= budgets.target {
        return Err(format!(
            "--overlap {} must be below --target {}, or a chunk is all overlap",
            budgets.overlap, budgets.target
        ));
    }
    if budgets.hard_max < budgets.target {
        return Err(format!(
            "--hard-max {} must be at least --target {}",
            budgets.hard_max, budgets.target
        ));
    }
    if budgets.min_tokens > budgets.hard_max {
        return Err(format!(
            "--min-tokens {} is above --hard-max {}, so every chunk would be dropped",
            budgets.min_tokens, budgets.hard_max
        ));
    }
    Ok(())
}

/// Read the input, distinguishing a file that cannot be read from one that
/// is not text: the first is an I/O failure and the second is bad input, and
/// a caller routing on the exit code needs to tell them apart.
fn read(path: Option<&std::path::Path>) -> Result<String, (u8, String)> {
    let bytes = match path {
        Some(path) => {
            std::fs::read(path).map_err(|e| (exit::IO, format!("{}: {e}", path.display())))
        }
        None => {
            let mut bytes = Vec::new();
            std::io::stdin()
                .read_to_end(&mut bytes)
                .map(|_| bytes)
                .map_err(|e| (exit::IO, e.to_string()))
        }
    }?;
    String::from_utf8(bytes).map_err(|e| (exit::DATA, format!("input is not UTF-8: {e}")))
}