use anyhow::{bail, Context, Result};
use clap::{Parser, ValueEnum};
use rand::rngs::StdRng;
use rand::seq::SliceRandom;
use rand::Rng;
use rand::SeedableRng;
use rayon::prelude::*;
use serde::{Deserialize, Serialize};
use serde_json::json;
use std::collections::{HashMap, HashSet};
use std::fs::{self, File};
use std::io::{BufRead, BufReader, BufWriter, Write};
use std::path::{Path, PathBuf};
use std::time::Instant;

const ENTITIES: [Entity; 7] = [
    Entity::Group,
    Entity::Title,
    Entity::Season,
    Entity::Episode,
    Entity::Special,
    Entity::Resolution,
    Entity::Source,
];

#[derive(Parser, Debug)]
#[command(
    about = "Generate pre-encoded AniFileBERT virtual BIO permutation shards",
    version
)]
struct Args {
    #[arg(long)]
    input: PathBuf,

    #[arg(long)]
    vocab_file: PathBuf,

    #[arg(long)]
    output_dir: PathBuf,

    #[arg(long, default_value_t = 128)]
    max_length: usize,

    #[arg(long, default_value_t = 25_000)]
    shard_size: usize,

    #[arg(long, default_value_t = 0)]
    limit_rows: usize,

    #[arg(long, default_value_t = 0)]
    samples_per_source: usize,

    #[arg(long, default_value_t = 42)]
    seed: u64,

    #[arg(long, default_value_t = 0)]
    threads: usize,

    #[arg(long, default_value = "global")]
    separator_mode: SeparatorMode,

    #[arg(long, default_value = "global")]
    bracket_mode: BracketMode,

    #[arg(long, value_delimiter = ',', default_value = " , - ,.,_,-,～,~")]
    separators: Vec<String>,

    #[arg(
        long,
        value_delimiter = ',',
        default_value = "none,square,round,corner,angle"
    )]
    bracket_styles: Vec<String>,

    #[arg(long, default_value_t = true)]
    include_original: bool,

    #[arg(long, default_value_t = true)]
    include_special_fixtures: bool,

    #[arg(long, help = "Only count rows; do not write shard files")]
    dry_run: bool,
}

#[derive(Clone, Copy, Debug, Serialize, ValueEnum)]
enum SeparatorMode {
    Global,
    PerGap,
}

#[derive(Clone, Copy, Debug, Serialize, ValueEnum)]
enum BracketMode {
    Global,
    PerPart,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash, Ord, PartialOrd, Serialize)]
enum Entity {
    Group,
    Title,
    Season,
    Episode,
    Special,
    Resolution,
    Source,
}

impl Entity {
    fn index(self) -> usize {
        match self {
            Entity::Group => 0,
            Entity::Title => 1,
            Entity::Season => 2,
            Entity::Episode => 3,
            Entity::Special => 4,
            Entity::Resolution => 5,
            Entity::Source => 6,
        }
    }

    fn from_name(name: &str) -> Option<Self> {
        match name {
            "GROUP" => Some(Entity::Group),
            "TITLE" => Some(Entity::Title),
            "SEASON" => Some(Entity::Season),
            "EPISODE" => Some(Entity::Episode),
            "SPECIAL" => Some(Entity::Special),
            "RESOLUTION" => Some(Entity::Resolution),
            "SOURCE" => Some(Entity::Source),
            _ => None,
        }
    }

    fn b_label(self) -> &'static str {
        match self {
            Entity::Group => "B-GROUP",
            Entity::Title => "B-TITLE",
            Entity::Season => "B-SEASON",
            Entity::Episode => "B-EPISODE",
            Entity::Special => "B-SPECIAL",
            Entity::Resolution => "B-RESOLUTION",
            Entity::Source => "B-SOURCE",
        }
    }

    fn i_label(self) -> &'static str {
        match self {
            Entity::Group => "I-GROUP",
            Entity::Title => "I-TITLE",
            Entity::Season => "I-SEASON",
            Entity::Episode => "I-EPISODE",
            Entity::Special => "I-SPECIAL",
            Entity::Resolution => "I-RESOLUTION",
            Entity::Source => "I-SOURCE",
        }
    }
}

#[derive(Clone, Debug)]
struct Bracket {
    name: String,
    open: String,
    close: String,
}

impl Bracket {
    fn from_name(name: &str) -> Result<Self> {
        let trimmed = name.trim();
        let pair = match trimmed {
            "none" => ("", ""),
            "square" => ("[", "]"),
            "round" => ("(", ")"),
            "corner" => ("【", "】"),
            "angle" => ("《", "》"),
            custom if custom.contains('|') => {
                let mut parts = custom.splitn(2, '|');
                let open = parts.next().unwrap_or_default();
                let close = parts.next().unwrap_or_default();
                return Ok(Self {
                    name: custom.to_string(),
                    open: open.to_string(),
                    close: close.to_string(),
                });
            }
            other => bail!("unknown bracket style '{other}'"),
        };
        Ok(Self {
            name: trimmed.to_string(),
            open: pair.0.to_string(),
            close: pair.1.to_string(),
        })
    }
}

#[derive(Deserialize)]
struct InputRow {
    filename: Option<String>,
    tokens: Vec<String>,
    labels: Vec<String>,
    tokenizer_variant: Option<String>,
}

#[derive(Clone)]
struct SourceSample {
    row_index: usize,
    filename: String,
    tokens: Vec<String>,
    labels: Vec<String>,
    fields: Vec<Vec<String>>,
}

#[derive(Clone)]
struct GenConfig {
    max_length: usize,
    shard_size: usize,
    separator_mode: SeparatorMode,
    bracket_mode: BracketMode,
    separators: Vec<String>,
    brackets: Vec<Bracket>,
    include_original: bool,
    samples_per_source: usize,
    seed: u64,
}

#[derive(Clone)]
struct Vocab {
    ids: HashMap<String, u16>,
    pad_id: u16,
    unk_id: u16,
    cls_id: u16,
    sep_id: u16,
}

#[derive(Serialize)]
struct ShardManifest {
    rows: usize,
    input_ids: String,
    attention_mask: String,
    labels: String,
}

struct ShardWriter {
    output_dir: PathBuf,
    worker_id: usize,
    shard_seq: usize,
    shard_size: usize,
    max_length: usize,
    input_ids: Vec<u16>,
    attention_mask: Vec<u8>,
    labels: Vec<i16>,
    rows: usize,
    total_rows: u64,
    shards: Vec<ShardManifest>,
}

impl ShardWriter {
    fn new(output_dir: &Path, worker_id: usize, shard_size: usize, max_length: usize) -> Self {
        let capacity = shard_size.saturating_mul(max_length);
        Self {
            output_dir: output_dir.to_path_buf(),
            worker_id,
            shard_seq: 0,
            shard_size,
            max_length,
            input_ids: Vec::with_capacity(capacity),
            attention_mask: Vec::with_capacity(capacity),
            labels: Vec::with_capacity(capacity),
            rows: 0,
            total_rows: 0,
            shards: Vec::new(),
        }
    }

    fn add(&mut self, input_ids: &[u16], attention_mask: &[u8], labels: &[i16]) -> Result<()> {
        if input_ids.len() != self.max_length
            || attention_mask.len() != self.max_length
            || labels.len() != self.max_length
        {
            bail!("encoded sample has wrong shape");
        }
        self.input_ids.extend_from_slice(input_ids);
        self.attention_mask.extend_from_slice(attention_mask);
        self.labels.extend_from_slice(labels);
        self.rows += 1;
        self.total_rows += 1;
        if self.rows >= self.shard_size {
            self.flush()?;
        }
        Ok(())
    }

    fn flush(&mut self) -> Result<()> {
        if self.rows == 0 {
            return Ok(());
        }

        let base = format!("part-w{:03}-s{:06}", self.worker_id, self.shard_seq);
        let input_name = format!("{base}.input_ids.npy");
        let mask_name = format!("{base}.attention_mask.npy");
        let label_name = format!("{base}.labels.npy");
        write_npy_u16(
            &self.output_dir.join(&input_name),
            &self.input_ids,
            self.rows,
            self.max_length,
        )?;
        write_npy_u8(
            &self.output_dir.join(&mask_name),
            &self.attention_mask,
            self.rows,
            self.max_length,
        )?;
        write_npy_i16(
            &self.output_dir.join(&label_name),
            &self.labels,
            self.rows,
            self.max_length,
        )?;
        self.shards.push(ShardManifest {
            rows: self.rows,
            input_ids: input_name,
            attention_mask: mask_name,
            labels: label_name,
        });
        self.input_ids.clear();
        self.attention_mask.clear();
        self.labels.clear();
        self.rows = 0;
        self.shard_seq += 1;
        Ok(())
    }
}

fn main() -> Result<()> {
    let args = Args::parse();
    if args.max_length < 4 {
        bail!("--max-length must be at least 4");
    }
    if args.shard_size == 0 {
        bail!("--shard-size must be positive");
    }
    if args.threads > 0 {
        rayon::ThreadPoolBuilder::new()
            .num_threads(args.threads)
            .build_global()
            .context("failed to configure rayon thread pool")?;
    }

    let started = Instant::now();
    let vocab = load_vocab(&args.vocab_file)?;
    let brackets = args
        .bracket_styles
        .iter()
        .map(|style| Bracket::from_name(style))
        .collect::<Result<Vec<_>>>()?;
    let separators = args
        .separators
        .iter()
        .map(|sep| normalize_separator_arg(sep))
        .collect::<Vec<_>>();
    let cfg = GenConfig {
        max_length: args.max_length,
        shard_size: args.shard_size,
        separator_mode: args.separator_mode,
        bracket_mode: args.bracket_mode,
        separators,
        brackets,
        include_original: args.include_original,
        samples_per_source: args.samples_per_source,
        seed: args.seed,
    };

    let mut samples = load_samples(&args.input, args.limit_rows)?;
    let source_rows = samples.len();
    let mut rng = StdRng::seed_from_u64(args.seed);
    samples.shuffle(&mut rng);

    if args.dry_run {
        let generated: u128 = samples
            .par_iter()
            .map(|sample| count_variants(sample, &cfg))
            .sum();
        let special_fixtures = if args.include_special_fixtures {
            count_special_fixtures(&cfg) as u128
        } else {
            0
        };
        let manifest = json!({
            "format": "anifilebert.virtual_dataset.preview.v1",
            "input": args.input,
            "vocab_file": args.vocab_file,
            "source_rows": source_rows,
            "estimated_rows": generated + special_fixtures,
            "source_variant_rows": generated,
            "special_fixture_rows": special_fixtures,
            "max_length": cfg.max_length,
            "separator_mode": cfg.separator_mode,
            "bracket_mode": cfg.bracket_mode,
            "separators": cfg.separators,
            "brackets": cfg.brackets.iter().map(|b| &b.name).collect::<Vec<_>>(),
            "include_original": cfg.include_original,
            "samples_per_source": cfg.samples_per_source,
            "include_special_fixtures": args.include_special_fixtures,
            "seed": args.seed,
            "elapsed_seconds": started.elapsed().as_secs_f64(),
        });
        println!("{}", serde_json::to_string_pretty(&manifest)?);
        return Ok(());
    }

    fs::create_dir_all(&args.output_dir).with_context(|| {
        format!(
            "failed to create output directory {}",
            args.output_dir.display()
        )
    })?;

    let chunk_count = rayon::current_num_threads().max(1) * 4;
    let chunk_size = samples.len().div_ceil(chunk_count).max(1);
    let chunks = samples
        .chunks(chunk_size)
        .enumerate()
        .collect::<Vec<(usize, &[SourceSample])>>();

    let mut worker_results = chunks
        .par_iter()
        .map(|(chunk_idx, chunk)| {
            let mut writer =
                ShardWriter::new(&args.output_dir, *chunk_idx, cfg.shard_size, cfg.max_length);
            for sample in *chunk {
                generate_for_sample(sample, &cfg, &vocab, &mut writer)?;
            }
            writer.flush()?;
            Ok::<_, anyhow::Error>((writer.total_rows, writer.shards))
        })
        .collect::<Result<Vec<_>>>()?;

    let mut total_rows: u64 = 0;
    let mut shards: Vec<ShardManifest> = Vec::new();
    for (rows, mut worker_shards) in worker_results.drain(..) {
        total_rows += rows;
        shards.append(&mut worker_shards);
    }

    let special_rows = if args.include_special_fixtures {
        let mut writer = ShardWriter::new(
            &args.output_dir,
            chunk_count + 1,
            cfg.shard_size,
            cfg.max_length,
        );
        for special in built_in_specials() {
            let parts = vec![PartChoice {
                entity: Entity::Special,
                value: special,
            }];
            emit_syntax_variants(&parts, &cfg, &vocab, &mut writer)?;
        }
        writer.flush()?;
        total_rows += writer.total_rows;
        shards.append(&mut writer.shards);
        writer.total_rows
    } else {
        0
    };

    shards.sort_by(|a, b| a.input_ids.cmp(&b.input_ids));
    let manifest = json!({
        "format": "anifilebert.virtual_dataset.shards.v1",
        "input": args.input,
        "vocab_file": args.vocab_file,
        "source_rows": source_rows,
        "total_rows": total_rows,
        "special_fixture_rows": special_rows,
        "max_length": cfg.max_length,
        "shard_size": cfg.shard_size,
        "tokenizer_variant": "char",
        "encoding": {
            "input_ids_dtype": "uint16",
            "attention_mask_dtype": "uint8",
            "labels_dtype": "int16",
            "layout": "row_major_npy"
        },
        "special_tokens": {
            "pad_id": vocab.pad_id,
            "unk_id": vocab.unk_id,
            "cls_id": vocab.cls_id,
            "sep_id": vocab.sep_id
        },
        "generation": {
            "separator_mode": cfg.separator_mode,
            "bracket_mode": cfg.bracket_mode,
            "separators": cfg.separators,
            "brackets": cfg.brackets.iter().map(|b| &b.name).collect::<Vec<_>>(),
            "include_original": cfg.include_original,
            "samples_per_source": cfg.samples_per_source,
            "include_special_fixtures": args.include_special_fixtures,
            "seed": args.seed,
            "threads": rayon::current_num_threads()
        },
        "shards": shards,
        "elapsed_seconds": started.elapsed().as_secs_f64(),
    });
    let manifest_path = args.output_dir.join("manifest.json");
    fs::write(&manifest_path, serde_json::to_string_pretty(&manifest)?)
        .with_context(|| format!("failed to write {}", manifest_path.display()))?;
    println!("{}", serde_json::to_string_pretty(&manifest)?);
    Ok(())
}

fn normalize_separator_arg(value: &str) -> String {
    match value {
        "\\t" => "\t".to_string(),
        "\\s" => " ".to_string(),
        other => other.to_string(),
    }
}

fn load_vocab(path: &Path) -> Result<Vocab> {
    let text = fs::read_to_string(path)
        .with_context(|| format!("failed to read vocab file {}", path.display()))?;
    let raw: HashMap<String, u64> =
        serde_json::from_str(&text).context("failed to parse vocab JSON")?;
    let mut ids = HashMap::with_capacity(raw.len());
    for (token, id) in raw {
        if id > u16::MAX as u64 {
            bail!("vocab id {id} for token '{token}' exceeds uint16 storage");
        }
        ids.insert(token, id as u16);
    }
    let pad_id = *ids.get("[PAD]").context("vocab is missing [PAD]")?;
    let unk_id = *ids.get("[UNK]").context("vocab is missing [UNK]")?;
    let cls_id = *ids.get("[CLS]").context("vocab is missing [CLS]")?;
    let sep_id = *ids.get("[SEP]").context("vocab is missing [SEP]")?;
    Ok(Vocab {
        ids,
        pad_id,
        unk_id,
        cls_id,
        sep_id,
    })
}

fn load_samples(path: &Path, limit_rows: usize) -> Result<Vec<SourceSample>> {
    let file = File::open(path).with_context(|| format!("failed to open {}", path.display()))?;
    let reader = BufReader::new(file);
    let mut samples = Vec::new();
    for (idx, line) in reader.lines().enumerate() {
        if limit_rows > 0 && samples.len() >= limit_rows {
            break;
        }
        let line = line.with_context(|| format!("failed reading line {}", idx + 1))?;
        if line.trim().is_empty() {
            continue;
        }
        let row: InputRow = serde_json::from_str(&line)
            .with_context(|| format!("failed to parse JSONL line {}", idx + 1))?;
        if let Some(variant) = row.tokenizer_variant.as_deref() {
            if variant != "char" {
                bail!(
                    "line {} has tokenizer_variant={variant}; virtual shard generation currently requires char data",
                    idx + 1
                );
            }
        }
        if row.tokens.len() != row.labels.len() {
            bail!(
                "line {} has mismatched token/label lengths: {} vs {}",
                idx + 1,
                row.tokens.len(),
                row.labels.len()
            );
        }
        let filename = row.filename.clone().unwrap_or_else(|| row.tokens.join(""));
        let fields = extract_fields(&row.tokens, &row.labels);
        samples.push(SourceSample {
            row_index: idx,
            filename,
            tokens: row.tokens,
            labels: row.labels,
            fields,
        });
    }
    Ok(samples)
}

fn extract_fields(tokens: &[String], labels: &[String]) -> Vec<Vec<String>> {
    let mut fields: Vec<Vec<String>> = (0..ENTITIES.len()).map(|_| Vec::new()).collect();
    let mut seen: Vec<HashSet<String>> = (0..ENTITIES.len()).map(|_| HashSet::new()).collect();
    let mut active_entity: Option<Entity> = None;
    let mut active_text = String::new();

    let flush = |entity: Option<Entity>,
                 text: &mut String,
                 fields: &mut Vec<Vec<String>>,
                 seen: &mut Vec<HashSet<String>>| {
        if let Some(entity) = entity {
            let value = text.trim().to_string();
            if !value.is_empty() && seen[entity.index()].insert(value.clone()) {
                fields[entity.index()].push(value);
            }
        }
        text.clear();
    };

    for (token, label) in tokens.iter().zip(labels.iter()) {
        if let Some(entity) = label.strip_prefix("B-").and_then(Entity::from_name) {
            flush(active_entity, &mut active_text, &mut fields, &mut seen);
            active_entity = Some(entity);
            active_text.push_str(token);
        } else if let Some(entity) = label.strip_prefix("I-").and_then(Entity::from_name) {
            if active_entity == Some(entity) {
                active_text.push_str(token);
            } else {
                flush(active_entity, &mut active_text, &mut fields, &mut seen);
                active_entity = Some(entity);
                active_text.push_str(token);
            }
        } else {
            flush(active_entity, &mut active_text, &mut fields, &mut seen);
            active_entity = None;
        }
    }
    flush(active_entity, &mut active_text, &mut fields, &mut seen);
    fields
}

fn count_variants(sample: &SourceSample, cfg: &GenConfig) -> u128 {
    let mut count = if cfg.include_original { 1 } else { 0 };
    let available = ENTITIES
        .iter()
        .copied()
        .filter(|entity| !sample.fields[entity.index()].is_empty())
        .collect::<Vec<_>>();
    let n = available.len();
    if n == 0 {
        return count;
    }
    if cfg.samples_per_source > 0 {
        return count + cfg.samples_per_source as u128;
    }
    for mask in 1usize..(1usize << n) {
        let selected = available
            .iter()
            .enumerate()
            .filter_map(|(idx, entity)| ((mask & (1usize << idx)) != 0).then_some(*entity))
            .collect::<Vec<_>>();
        let m = selected.len();
        let value_product: u128 = selected
            .iter()
            .map(|entity| sample.fields[entity.index()].len() as u128)
            .product();
        let perm_count = factorial(m as u32);
        let sep_factor = if m <= 1 {
            1
        } else {
            match cfg.separator_mode {
                SeparatorMode::Global => cfg.separators.len() as u128,
                SeparatorMode::PerGap => (cfg.separators.len() as u128).pow((m - 1) as u32),
            }
        };
        let bracket_factor = match cfg.bracket_mode {
            BracketMode::Global => cfg.brackets.len() as u128,
            BracketMode::PerPart => (cfg.brackets.len() as u128).pow(m as u32),
        };
        count += value_product * perm_count * sep_factor * bracket_factor;
    }
    count
}

fn count_special_fixtures(cfg: &GenConfig) -> usize {
    let bracket_factor = match cfg.bracket_mode {
        BracketMode::Global => cfg.brackets.len(),
        BracketMode::PerPart => cfg.brackets.len(),
    };
    built_in_specials().len() * bracket_factor
}

fn factorial(n: u32) -> u128 {
    (1..=n as u128).product::<u128>().max(1)
}

fn generate_for_sample(
    sample: &SourceSample,
    cfg: &GenConfig,
    vocab: &Vocab,
    writer: &mut ShardWriter,
) -> Result<()> {
    if cfg.include_original {
        let (input_ids, attention_mask, labels) =
            encode_original_sample(sample, vocab, cfg.max_length)?;
        writer.add(&input_ids, &attention_mask, &labels)?;
    }

    if cfg.samples_per_source > 0 {
        generate_sampled_variants(sample, cfg, vocab, writer)?;
        return Ok(());
    }

    let available = ENTITIES
        .iter()
        .copied()
        .filter(|entity| !sample.fields[entity.index()].is_empty())
        .collect::<Vec<_>>();
    let n = available.len();
    for mask in 1usize..(1usize << n) {
        let mut selected = available
            .iter()
            .enumerate()
            .filter_map(|(idx, entity)| ((mask & (1usize << idx)) != 0).then_some(*entity))
            .collect::<Vec<_>>();
        permute_entities(&mut selected, 0, &mut |order| {
            let mut parts: Vec<PartChoice> = Vec::with_capacity(order.len());
            for_each_value_combo(order, &sample.fields, 0, &mut parts, &mut |combo| {
                emit_syntax_variants(combo, cfg, vocab, writer)
            })
        })?;
    }
    Ok(())
}

fn generate_sampled_variants(
    sample: &SourceSample,
    cfg: &GenConfig,
    vocab: &Vocab,
    writer: &mut ShardWriter,
) -> Result<()> {
    let mut rng = StdRng::seed_from_u64(
        cfg.seed ^ ((sample.row_index as u64).wrapping_mul(0x9E37_79B9_7F4A_7C15)),
    );
    let available = ENTITIES
        .iter()
        .copied()
        .filter(|entity| !sample.fields[entity.index()].is_empty())
        .collect::<Vec<_>>();
    if available.is_empty() {
        return Ok(());
    }

    let mut seen = HashSet::new();
    let mut emitted = 0usize;
    let budget = cfg.samples_per_source;
    let max_unique_attempts = budget.saturating_mul(32).max(64);
    let mut attempts = 0usize;

    let mut templates: Vec<Vec<PartChoice>> = Vec::new();
    if let Some(title) = sample.fields[Entity::Title.index()].first() {
        templates.push(vec![PartChoice {
            entity: Entity::Title,
            value: title.clone(),
        }]);
        if let Some(season) = sample.fields[Entity::Season.index()].first() {
            templates.push(vec![
                PartChoice {
                    entity: Entity::Title,
                    value: title.clone(),
                },
                PartChoice {
                    entity: Entity::Season,
                    value: season.clone(),
                },
            ]);
        }
    }
    if let Some(episode) = sample.fields[Entity::Episode.index()].first() {
        templates.push(vec![PartChoice {
            entity: Entity::Episode,
            value: episode.clone(),
        }]);
    }
    if let Some(special) = sample.fields[Entity::Special.index()].first() {
        templates.push(vec![PartChoice {
            entity: Entity::Special,
            value: special.clone(),
        }]);
    }
    if let (Some(title), Some(special)) = (
        sample.fields[Entity::Title.index()].first(),
        sample.fields[Entity::Special.index()].first(),
    ) {
        templates.push(vec![
            PartChoice {
                entity: Entity::Title,
                value: title.clone(),
            },
            PartChoice {
                entity: Entity::Special,
                value: special.clone(),
            },
        ]);
    }

    for parts in templates {
        if emitted >= budget {
            break;
        }
        emit_sample_variant(
            parts,
            cfg,
            vocab,
            writer,
            &mut seen,
            &mut emitted,
            &mut rng,
            false,
        )?;
    }

    while emitted < budget && attempts < max_unique_attempts {
        attempts += 1;
        let subset_size = match rng.gen_range(0..100) {
            0..=29 => 1,
            30..=54 => 2,
            55..=74 => 3,
            75..=89 => 4.min(available.len()),
            _ => available.len().min(5),
        }
        .max(1)
        .min(available.len());

        let mut chosen = available
            .choose_multiple(&mut rng, subset_size)
            .copied()
            .collect::<Vec<_>>();
        chosen.shuffle(&mut rng);
        if !chosen
            .iter()
            .any(|entity| matches!(entity, Entity::Title | Entity::Episode | Entity::Special))
        {
            if let Some(fallback) = available
                .iter()
                .copied()
                .find(|entity| matches!(entity, Entity::Title | Entity::Episode | Entity::Special))
            {
                if !chosen.contains(&fallback) {
                    chosen.push(fallback);
                }
            }
        }

        let mut parts = Vec::with_capacity(chosen.len());
        for entity in chosen {
            let values = &sample.fields[entity.index()];
            let value = values.choose(&mut rng).cloned().unwrap_or_default();
            parts.push(PartChoice { entity, value });
        }
        parts.shuffle(&mut rng);
        emit_sample_variant(
            parts,
            cfg,
            vocab,
            writer,
            &mut seen,
            &mut emitted,
            &mut rng,
            false,
        )?;
    }

    while emitted < budget {
        let subset_size = match rng.gen_range(0..100) {
            0..=29 => 1,
            30..=54 => 2,
            55..=74 => 3,
            75..=89 => 4.min(available.len()),
            _ => available.len().min(5),
        }
        .max(1)
        .min(available.len());

        let mut chosen = available
            .choose_multiple(&mut rng, subset_size)
            .copied()
            .collect::<Vec<_>>();
        chosen.shuffle(&mut rng);
        if !chosen
            .iter()
            .any(|entity| matches!(entity, Entity::Title | Entity::Episode | Entity::Special))
        {
            if let Some(fallback) = available
                .iter()
                .copied()
                .find(|entity| matches!(entity, Entity::Title | Entity::Episode | Entity::Special))
            {
                if !chosen.contains(&fallback) {
                    chosen.push(fallback);
                }
            }
        }

        let mut parts = Vec::with_capacity(chosen.len());
        for entity in chosen {
            let values = &sample.fields[entity.index()];
            let value = values.choose(&mut rng).cloned().unwrap_or_default();
            parts.push(PartChoice { entity, value });
        }
        parts.shuffle(&mut rng);
        emit_sample_variant(
            parts,
            cfg,
            vocab,
            writer,
            &mut seen,
            &mut emitted,
            &mut rng,
            true,
        )?;
    }
    Ok(())
}

fn emit_sample_variant(
    parts: Vec<PartChoice>,
    cfg: &GenConfig,
    vocab: &Vocab,
    writer: &mut ShardWriter,
    seen: &mut HashSet<String>,
    emitted: &mut usize,
    rng: &mut StdRng,
    allow_duplicate: bool,
) -> Result<()> {
    if *emitted >= cfg.samples_per_source {
        return Ok(());
    }
    if parts.is_empty() {
        return Ok(());
    }
    let separators = match cfg.separator_mode {
        SeparatorMode::Global => {
            let sep = cfg
                .separators
                .choose(rng)
                .cloned()
                .unwrap_or_else(|| " ".to_string());
            if parts.len() > 1 {
                vec![sep; parts.len() - 1]
            } else {
                Vec::new()
            }
        }
        SeparatorMode::PerGap => {
            let mut values = Vec::with_capacity(parts.len().saturating_sub(1));
            for _ in 0..parts.len().saturating_sub(1) {
                values.push(
                    cfg.separators
                        .choose(rng)
                        .cloned()
                        .unwrap_or_else(|| " ".to_string()),
                );
            }
            values
        }
    };
    let brackets = match cfg.bracket_mode {
        BracketMode::Global => {
            let bracket = cfg
                .brackets
                .choose(rng)
                .cloned()
                .unwrap_or_else(|| Bracket {
                    name: "none".to_string(),
                    open: String::new(),
                    close: String::new(),
                });
            vec![bracket; parts.len()]
        }
        BracketMode::PerPart => {
            let mut values = Vec::with_capacity(parts.len());
            for _ in 0..parts.len() {
                values.push(
                    cfg.brackets
                        .choose(rng)
                        .cloned()
                        .unwrap_or_else(|| Bracket {
                            name: "none".to_string(),
                            open: String::new(),
                            close: String::new(),
                        }),
                );
            }
            values
        }
    };
    let text = render_variant_text(&parts, &separators, &brackets);
    if !allow_duplicate && !seen.insert(text) {
        return Ok(());
    }
    let (input_ids, attention_mask, labels) =
        encode_generated_sample(&parts, &separators, &brackets, vocab, cfg.max_length)?;
    writer.add(&input_ids, &attention_mask, &labels)?;
    *emitted += 1;
    Ok(())
}

fn permute_entities<F>(values: &mut [Entity], start: usize, callback: &mut F) -> Result<()>
where
    F: FnMut(&[Entity]) -> Result<()>,
{
    if start >= values.len() {
        return callback(values);
    }
    for idx in start..values.len() {
        values.swap(start, idx);
        permute_entities(values, start + 1, callback)?;
        values.swap(start, idx);
    }
    Ok(())
}

#[derive(Clone)]
struct PartChoice {
    entity: Entity,
    value: String,
}

fn for_each_value_combo<F>(
    order: &[Entity],
    fields: &[Vec<String>],
    idx: usize,
    current: &mut Vec<PartChoice>,
    callback: &mut F,
) -> Result<()>
where
    F: FnMut(&[PartChoice]) -> Result<()>,
{
    if idx >= order.len() {
        return callback(current);
    }
    let entity = order[idx];
    for value in &fields[entity.index()] {
        current.push(PartChoice {
            entity,
            value: value.clone(),
        });
        for_each_value_combo(order, fields, idx + 1, current, callback)?;
        current.pop();
    }
    Ok(())
}

fn emit_syntax_variants(
    parts: &[PartChoice],
    cfg: &GenConfig,
    vocab: &Vocab,
    writer: &mut ShardWriter,
) -> Result<()> {
    let gaps = parts.len().saturating_sub(1);
    let mut separators = Vec::with_capacity(gaps);
    for_each_separator_combo(gaps, cfg, 0, &mut separators, &mut |sep_combo| {
        let mut brackets = Vec::with_capacity(parts.len());
        for_each_bracket_combo(parts.len(), cfg, 0, &mut brackets, &mut |bracket_combo| {
            let (input_ids, attention_mask, labels) =
                encode_generated_sample(parts, sep_combo, bracket_combo, vocab, cfg.max_length)?;
            writer.add(&input_ids, &attention_mask, &labels)
        })
    })
}

fn for_each_separator_combo<F>(
    gaps: usize,
    cfg: &GenConfig,
    idx: usize,
    current: &mut Vec<String>,
    callback: &mut F,
) -> Result<()>
where
    F: FnMut(&[String]) -> Result<()>,
{
    if gaps == 0 {
        return callback(current);
    }
    match cfg.separator_mode {
        SeparatorMode::Global => {
            if idx == 0 {
                for sep in &cfg.separators {
                    current.clear();
                    current.resize(gaps, sep.clone());
                    callback(current)?;
                }
            }
            Ok(())
        }
        SeparatorMode::PerGap => {
            if idx >= gaps {
                return callback(current);
            }
            for sep in &cfg.separators {
                current.push(sep.clone());
                for_each_separator_combo(gaps, cfg, idx + 1, current, callback)?;
                current.pop();
            }
            Ok(())
        }
    }
}

fn for_each_bracket_combo<F>(
    parts: usize,
    cfg: &GenConfig,
    idx: usize,
    current: &mut Vec<Bracket>,
    callback: &mut F,
) -> Result<()>
where
    F: FnMut(&[Bracket]) -> Result<()>,
{
    match cfg.bracket_mode {
        BracketMode::Global => {
            if idx == 0 {
                for bracket in &cfg.brackets {
                    current.clear();
                    current.resize(parts, bracket.clone());
                    callback(current)?;
                }
            }
            Ok(())
        }
        BracketMode::PerPart => {
            if idx >= parts {
                return callback(current);
            }
            for bracket in &cfg.brackets {
                current.push(bracket.clone());
                for_each_bracket_combo(parts, cfg, idx + 1, current, callback)?;
                current.pop();
            }
            Ok(())
        }
    }
}

fn render_variant_text(
    parts: &[PartChoice],
    separators: &[String],
    brackets: &[Bracket],
) -> String {
    let mut text = String::new();
    for (idx, part) in parts.iter().enumerate() {
        text.push_str(&brackets[idx].open);
        text.push_str(&part.value);
        text.push_str(&brackets[idx].close);
        if idx < separators.len() {
            text.push_str(&separators[idx]);
        }
    }
    text
}

fn encode_original_sample(
    sample: &SourceSample,
    vocab: &Vocab,
    max_length: usize,
) -> Result<(Vec<u16>, Vec<u8>, Vec<i16>)> {
    let mut input_ids = vec![vocab.pad_id; max_length];
    let mut attention_mask = vec![0u8; max_length];
    let mut labels = vec![-100i16; max_length];

    input_ids[0] = vocab.cls_id;
    attention_mask[0] = 1;
    let available = max_length.saturating_sub(2);
    let token_count = sample.tokens.len().min(available);
    for idx in 0..token_count {
        input_ids[idx + 1] = token_id(vocab, &sample.tokens[idx]);
        attention_mask[idx + 1] = 1;
        labels[idx + 1] = label_id(&sample.labels[idx]).with_context(|| {
            format!(
                "unknown label '{}' on source row {} ({})",
                sample.labels[idx],
                sample.row_index + 1,
                sample.filename
            )
        })?;
    }
    let sep_pos = token_count + 1;
    input_ids[sep_pos] = vocab.sep_id;
    attention_mask[sep_pos] = 1;
    Ok((input_ids, attention_mask, labels))
}

fn encode_generated_sample(
    parts: &[PartChoice],
    separators: &[String],
    brackets: &[Bracket],
    vocab: &Vocab,
    max_length: usize,
) -> Result<(Vec<u16>, Vec<u8>, Vec<i16>)> {
    let mut input_ids = vec![vocab.pad_id; max_length];
    let mut attention_mask = vec![0u8; max_length];
    let mut labels = vec![-100i16; max_length];
    input_ids[0] = vocab.cls_id;
    attention_mask[0] = 1;

    let available = max_length.saturating_sub(2);
    let mut pos = 1usize;
    for (idx, part) in parts.iter().enumerate() {
        let bracket = &brackets[idx];
        append_o_text(
            &bracket.open,
            vocab,
            available,
            &mut pos,
            &mut input_ids,
            &mut attention_mask,
            &mut labels,
        );
        append_entity_text(
            &part.value,
            part.entity,
            vocab,
            available,
            &mut pos,
            &mut input_ids,
            &mut attention_mask,
            &mut labels,
        )?;
        append_o_text(
            &bracket.close,
            vocab,
            available,
            &mut pos,
            &mut input_ids,
            &mut attention_mask,
            &mut labels,
        );
        if idx < separators.len() {
            append_o_text(
                &separators[idx],
                vocab,
                available,
                &mut pos,
                &mut input_ids,
                &mut attention_mask,
                &mut labels,
            );
        }
    }

    let sep_pos = pos.min(max_length - 1);
    input_ids[sep_pos] = vocab.sep_id;
    attention_mask[sep_pos] = 1;
    labels[sep_pos] = -100;
    Ok((input_ids, attention_mask, labels))
}

fn append_o_text(
    text: &str,
    vocab: &Vocab,
    available: usize,
    pos: &mut usize,
    input_ids: &mut [u16],
    attention_mask: &mut [u8],
    labels: &mut [i16],
) {
    for ch in text.chars() {
        if *pos > available {
            return;
        }
        let token = ch.to_string();
        input_ids[*pos] = token_id(vocab, &token);
        attention_mask[*pos] = 1;
        labels[*pos] = 0;
        *pos += 1;
    }
}

fn append_entity_text(
    text: &str,
    entity: Entity,
    vocab: &Vocab,
    available: usize,
    pos: &mut usize,
    input_ids: &mut [u16],
    attention_mask: &mut [u8],
    labels: &mut [i16],
) -> Result<()> {
    let b = label_id(entity.b_label()).context("missing B label")?;
    let i = label_id(entity.i_label()).context("missing I label")?;
    let mut first = true;
    for ch in text.chars() {
        if *pos > available {
            return Ok(());
        }
        let token = ch.to_string();
        input_ids[*pos] = token_id(vocab, &token);
        attention_mask[*pos] = 1;
        labels[*pos] = if first { b } else { i };
        first = false;
        *pos += 1;
    }
    Ok(())
}

fn token_id(vocab: &Vocab, token: &str) -> u16 {
    *vocab.ids.get(token).unwrap_or(&vocab.unk_id)
}

fn label_id(label: &str) -> Option<i16> {
    Some(match label {
        "O" => 0,
        "B-TITLE" => 1,
        "I-TITLE" => 2,
        "B-SEASON" => 3,
        "I-SEASON" => 4,
        "B-EPISODE" => 5,
        "I-EPISODE" => 6,
        "B-SPECIAL" => 7,
        "I-SPECIAL" => 8,
        "B-GROUP" => 9,
        "I-GROUP" => 10,
        "B-RESOLUTION" => 11,
        "I-RESOLUTION" => 12,
        "B-SOURCE" => 13,
        "I-SOURCE" => 14,
        _ => return None,
    })
}

fn built_in_specials() -> Vec<String> {
    let mut values = Vec::new();
    values.push("Menu".to_string());
    for idx in 1..=24 {
        values.push(format!("Menu{idx:02}"));
        values.push(format!("Menu {idx:02}"));
        values.push(format!("BDMenu{idx:02}"));
        values.push(format!("BD Menu{idx:02}"));
        values.push(format!("Menu{idx:02}-01"));
        values.push(format!("ED E{idx:02}"));
    }
    for idx in 1..=6 {
        values.push(format!("OP{idx:02}"));
        values.push(format!("NCOP{idx:02}"));
        values.push(format!("NCED{idx:02}"));
    }
    for idx in 1..=12 {
        values.push(format!("CM{idx:02}"));
        values.push(format!("PV{idx:02}"));
    }
    values
}

fn write_npy_u16(path: &Path, data: &[u16], rows: usize, cols: usize) -> Result<()> {
    let mut writer = BufWriter::new(
        File::create(path).with_context(|| format!("failed to create {}", path.display()))?,
    );
    write_npy_header(&mut writer, "<u2", rows, cols)?;
    for value in data {
        writer.write_all(&value.to_le_bytes())?;
    }
    Ok(())
}

fn write_npy_u8(path: &Path, data: &[u8], rows: usize, cols: usize) -> Result<()> {
    let mut writer = BufWriter::new(
        File::create(path).with_context(|| format!("failed to create {}", path.display()))?,
    );
    write_npy_header(&mut writer, "|u1", rows, cols)?;
    writer.write_all(data)?;
    Ok(())
}

fn write_npy_i16(path: &Path, data: &[i16], rows: usize, cols: usize) -> Result<()> {
    let mut writer = BufWriter::new(
        File::create(path).with_context(|| format!("failed to create {}", path.display()))?,
    );
    write_npy_header(&mut writer, "<i2", rows, cols)?;
    for value in data {
        writer.write_all(&value.to_le_bytes())?;
    }
    Ok(())
}

fn write_npy_header<W: Write>(writer: &mut W, descr: &str, rows: usize, cols: usize) -> Result<()> {
    let mut header = format!(
        "{{'descr': '{}', 'fortran_order': False, 'shape': ({}, {}), }}",
        descr, rows, cols
    )
    .into_bytes();
    let preamble_len = 10usize;
    let pad_len = (16 - ((preamble_len + header.len() + 1) % 16)) % 16;
    header.extend(std::iter::repeat(b' ').take(pad_len));
    header.push(b'\n');
    if header.len() > u16::MAX as usize {
        bail!("npy header too large");
    }
    writer.write_all(b"\x93NUMPY")?;
    writer.write_all(&[1, 0])?;
    writer.write_all(&(header.len() as u16).to_le_bytes())?;
    writer.write_all(&header)?;
    Ok(())
}