//! Asymmetric Numeral Systems (ANS) entropy coding.
//!
//! This module provides ANS-based compression for achieving near-optimal
//! compression ratios. ANS is used as the backbone for "bits-back" coding
//! in ROC (Random Order Coding).
//!
//! # Theory
//!
//! ANS was introduced by Jarek Duda (2009) as an entropy coder that:
//! - Approaches the theoretical entropy bound H(X)
//! - Encodes in ~1 bit per symbol overhead
//! - Supports arithmetic coding-like compression with table-based speed
//!
//! # Implementation Status
//!
//! Currently a placeholder. Full implementation will use the `constriction`
//! crate for ANS primitives.

use crate::error::CompressionError;

/// ANS encoder state.
pub struct AnsEncoder {
    state: u64,
    precision: u32,
}

impl AnsEncoder {
    /// Create a new ANS encoder with given precision.
    pub fn new(precision: u32) -> Self {
        Self {
            state: precision as u64, // Initial state = L
            precision,
        }
    }

    /// Encode a symbol with given cumulative frequency.
    ///
    /// # Arguments
    ///
    /// * `cum_freq` - Cumulative frequency of symbol (0..total)
    /// * `freq` - Frequency of symbol
    /// * `total` - Total frequency (power of 2 for fast division)
    pub fn encode(
        &mut self,
        cum_freq: u32,
        freq: u32,
        _total: u32,
    ) -> Result<(), CompressionError> {
        // Placeholder: actual ANS encoding would be:
        // state = (state / freq) * total + (state % freq) + cum_freq
        self.state = self.state.wrapping_add(cum_freq as u64 + freq as u64);
        Ok(())
    }

    /// Finalize encoding and return compressed bytes.
    pub fn finish(self) -> Vec<u8> {
        self.state.to_le_bytes().to_vec()
    }
}

/// ANS decoder state.
pub struct AnsDecoder {
    state: u64,
    #[allow(dead_code)]
    precision: u32,
}

impl AnsDecoder {
    /// Create a new ANS decoder from compressed data.
    pub fn new(data: &[u8], precision: u32) -> Result<Self, CompressionError> {
        if data.len() < 8 {
            return Err(CompressionError::DecompressionFailed(
                "ANS data too short".to_string(),
            ));
        }

        let state = u64::from_le_bytes(data[..8].try_into().unwrap());
        Ok(Self { state, precision })
    }

    /// Decode a symbol given the frequency table.
    ///
    /// Returns (symbol, cum_freq, freq).
    pub fn decode(&mut self, _total: u32) -> Result<(u32, u32, u32), CompressionError> {
        // Placeholder: actual ANS decoding would be:
        // slot = state % total
        // symbol = lookup(slot)
        // cum_freq, freq = freq_table[symbol]
        // state = freq * (state / total) + slot - cum_freq
        let symbol = (self.state & 0xFFFF) as u32;
        self.state >>= 16;
        Ok((symbol, 0, 1))
    }
}

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

    #[test]
    fn test_encoder_decoder_stub() {
        // Just verify the stubs compile and run
        let mut encoder = AnsEncoder::new(4096);
        encoder.encode(0, 1, 256).unwrap();
        let data = encoder.finish();

        let decoder = AnsDecoder::new(&data, 4096);
        assert!(decoder.is_ok());
    }
}
