//! Text tokenization for BM25.
//!
//! Converts text into tokens (terms) for indexing and search.
//!
//! # Tokenization Pipeline
//!
//! ```text
//! Raw Text
//!   ↓
//! 1. Lowercase        "Rust Programming" → "rust programming"
//!   ↓
//! 2. Split on whitespace/punctuation  → ["rust", "programming"]
//!   ↓
//! 3. Remove stopwords (optional)      → ["rust", "programming"] (no change)
//!   ↓
//! 4. Stem (optional)                  → ["rust", "program"]
//!   ↓
//! Tokens
//! ```
//!
//! # Configuration Options
//!
//! | Option | Effect | Use Case |
//! |--------|--------|----------|
//! | Lowercase | Normalize case | Recommended (default: true) |
//! | Stopwords | Remove common words (the, a, is) | Optional (can hurt recall) |
//! | Stemming | Reduce to root form (running → run) | Optional (lossy) |
//! | Min length | Filter short tokens | Remove noise (default: 2) |
//!
//! # Example
//!
//! ```rust,ignore
//! let config = TokenizerConfig::default();
//! let tokenizer = Tokenizer::new(config);
//!
//! let tokens = tokenizer.tokenize("Rust is a systems programming language!");
//! // Result: ["rust", "systems", "programming", "language"]
//! ```

use crate::types::error::Result;
use std::collections::{HashMap, HashSet};

/// Tokenizer configuration.
#[derive(Debug, Clone)]
pub struct TokenizerConfig {
    /// Convert to lowercase
    pub lowercase: bool,

    /// Remove stopwords (common words like "the", "is", "a")
    pub remove_stopwords: bool,

    /// Apply stemming (reduce to root form)
    pub stemming: bool,

    /// Minimum token length
    pub min_token_length: usize,

    /// Custom stopword list (if None, use built-in English stopwords)
    pub stopwords: Option<HashSet<String>>,
}

impl Default for TokenizerConfig {
    fn default() -> Self {
        Self {
            lowercase: true,
            remove_stopwords: false, // Conservative default (preserve all terms)
            stemming: false,         // Conservative default (exact matching)
            min_token_length: 2,
            stopwords: None,
        }
    }
}

/// Text tokenizer for BM25.
#[derive(Debug, Clone)]
pub struct Tokenizer {
    config: TokenizerConfig,
    stopwords: HashSet<String>,
}

impl Tokenizer {
    /// Create a new tokenizer.
    ///
    /// # Arguments
    ///
    /// * `config` - Tokenizer configuration
    ///
    /// # Example
    ///
    /// ```rust,ignore
    /// let tokenizer = Tokenizer::new(TokenizerConfig::default());
    /// ```
    pub fn new(config: TokenizerConfig) -> Self {
        let stopwords = config
            .stopwords
            .clone()
            .unwrap_or_else(|| default_stopwords());

        Self { config, stopwords }
    }

    /// Tokenize text into terms.
    ///
    /// # Arguments
    ///
    /// * `text` - Input text
    ///
    /// # Returns
    ///
    /// Vector of tokens (lowercased, filtered, stemmed as per config)
    ///
    /// # Example
    ///
    /// ```rust,ignore
    /// let tokens = tokenizer.tokenize("The quick brown fox jumps!");
    /// // Result: ["quick", "brown", "fox", "jumps"] (stopword "the" removed)
    /// ```
    pub fn tokenize(&self, text: &str) -> Vec<String> {
        todo!("Split text, apply filters, return tokens")
    }

    /// Tokenize and compute term frequencies.
    ///
    /// # Arguments
    ///
    /// * `text` - Input text
    ///
    /// # Returns
    ///
    /// HashMap of (term -> frequency)
    ///
    /// # Example
    ///
    /// ```rust,ignore
    /// let term_freqs = tokenizer.term_frequencies("rust rust python");
    /// assert_eq!(term_freqs["rust"], 2);
    /// assert_eq!(term_freqs["python"], 1);
    /// ```
    pub fn term_frequencies(&self, text: &str) -> HashMap<String, u32> {
        let tokens = self.tokenize(text);
        let mut freqs = HashMap::new();
        for token in tokens {
            *freqs.entry(token).or_insert(0) += 1;
        }
        freqs
    }

    /// Split text into raw tokens (before filtering).
    ///
    /// Splits on whitespace and punctuation.
    ///
    /// # Arguments
    ///
    /// * `text` - Input text
    ///
    /// # Returns
    ///
    /// Vector of raw tokens
    fn split(&self, text: &str) -> Vec<String> {
        todo!("Split on whitespace and punctuation")
    }

    /// Apply stemming to a token.
    ///
    /// Uses a simple rule-based stemmer (Porter-like).
    ///
    /// # Arguments
    ///
    /// * `token` - Input token
    ///
    /// # Returns
    ///
    /// Stemmed token
    fn stem(&self, token: &str) -> String {
        todo!("Apply simple stemming rules (e.g., remove -ing, -ed, -s)")
    }

    /// Check if token is a stopword.
    ///
    /// # Arguments
    ///
    /// * `token` - Token to check
    ///
    /// # Returns
    ///
    /// `true` if token is a stopword
    fn is_stopword(&self, token: &str) -> bool {
        self.stopwords.contains(token)
    }
}

/// Default English stopwords.
///
/// Common words that typically don't contribute to document relevance.
///
/// # Returns
///
/// HashSet of stopwords
fn default_stopwords() -> HashSet<String> {
    [
        "a", "an", "and", "are", "as", "at", "be", "by", "for", "from", "has", "he", "in", "is",
        "it", "its", "of", "on", "that", "the", "to", "was", "will", "with",
    ]
    .iter()
    .map(|&s| s.to_string())
    .collect()
}

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

    #[test]
    fn test_tokenize_basic() {
        todo!("Test basic tokenization (split, lowercase)")
    }

    #[test]
    fn test_tokenize_with_stopwords() {
        todo!("Test stopword removal")
    }

    #[test]
    fn test_tokenize_with_stemming() {
        todo!("Test stemming (running → run)")
    }

    #[test]
    fn test_tokenize_min_length() {
        todo!("Test minimum token length filter")
    }

    #[test]
    fn test_term_frequencies() {
        todo!("Test term frequency counting")
    }

    #[test]
    fn test_split_punctuation() {
        todo!("Test that punctuation is handled correctly")
    }

    #[test]
    fn test_stem() {
        todo!("Test stemming rules")
    }

    #[test]
    fn test_is_stopword() {
        todo!("Test stopword detection")
    }
}
