//! Protein sequence encoder with amino acid embeddings and positional encoding.

use alphafold_shared::AminoAcid;
use serde::{Deserialize, Serialize};

/// Embedding dimension for amino acids.
pub const EMBEDDING_DIM: usize = 256;

/// Maximum sequence length supported.
pub const MAX_SEQ_LEN: usize = 2500;

/// Number of amino acid types (20 standard + 1 unknown).
pub const NUM_AMINO_ACIDS: usize = 21;

/// Protein sequence encoder.
#[derive(Debug, Clone)]
pub struct ProteinEncoder {
    /// Amino acid embedding weights [21 x `EMBEDDING_DIM`]
    pub aa_embeddings: Vec<Vec<f32>>,
    /// Positional encoding frequencies
    pub pos_frequencies: Vec<f32>,
}

impl Default for ProteinEncoder {
    fn default() -> Self {
        Self::new()
    }
}

impl ProteinEncoder {
    /// Create a new protein encoder with initialized embeddings.
    #[must_use]
    pub fn new() -> Self {
        use rand::SeedableRng;
        use rand_distr::{Distribution, Normal};

        let mut rng = rand::rngs::StdRng::seed_from_u64(42);
        let normal = Normal::new(0.0_f32, 0.02).unwrap();

        // Initialize amino acid embeddings
        let aa_embeddings: Vec<Vec<f32>> = (0..NUM_AMINO_ACIDS)
            .map(|_| {
                (0..EMBEDDING_DIM)
                    .map(|_| normal.sample(&mut rng))
                    .collect()
            })
            .collect();

        // Precompute positional encoding frequencies
        let pos_frequencies: Vec<f32> = (0..EMBEDDING_DIM / 2)
            .map(|i| 1.0 / 10000_f32.powf(2.0 * i as f32 / EMBEDDING_DIM as f32))
            .collect();

        Self {
            aa_embeddings,
            pos_frequencies,
        }
    }

    /// Encode a sequence of amino acids to embeddings.
    pub fn encode(&self, sequence: &str) -> SequenceEmbedding {
        let amino_acids: Vec<AminoAcid> =
            sequence.chars().filter_map(AminoAcid::from_code).collect();

        let seq_len = amino_acids.len();
        let mut embeddings = Vec::with_capacity(seq_len);

        for (pos, aa) in amino_acids.iter().enumerate() {
            let aa_idx = aa.embedding_idx();
            let aa_emb = &self.aa_embeddings[aa_idx];

            // Add positional encoding
            let mut emb = vec![0.0_f32; EMBEDDING_DIM];
            for (i, &freq) in self.pos_frequencies.iter().enumerate() {
                let pos_f = pos as f32;
                emb[2 * i] = aa_emb[2 * i] + (pos_f * freq).sin();
                emb[2 * i + 1] = aa_emb[2 * i + 1] + (pos_f * freq).cos();
            }

            embeddings.push(emb);
        }

        SequenceEmbedding {
            embeddings,
            sequence_length: seq_len,
            embedding_dim: EMBEDDING_DIM,
        }
    }

    /// Encode pairwise residue features.
    #[must_use]
    pub fn encode_pairs(&self, seq_len: usize) -> PairEmbedding {
        let pair_dim = EMBEDDING_DIM / 2;
        let mut pair_embeddings = vec![vec![vec![0.0_f32; pair_dim]; seq_len]; seq_len];

        for i in 0..seq_len {
            for j in 0..seq_len {
                // Relative position encoding
                let rel_pos = (i as i32 - j as i32).clamp(-32, 32);
                let rel_pos_idx = (rel_pos + 32) as usize;

                // Simple relative position feature
                for k in 0..pair_dim {
                    let freq = self.pos_frequencies[k.min(self.pos_frequencies.len() - 1)];
                    pair_embeddings[i][j][k] = ((rel_pos_idx as f32) * freq).sin();
                }
            }
        }

        PairEmbedding {
            embeddings: pair_embeddings,
            sequence_length: seq_len,
            pair_dim,
        }
    }
}

/// Encoded sequence representation.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SequenceEmbedding {
    /// Embeddings for each residue [`seq_len` x `embedding_dim`]
    pub embeddings: Vec<Vec<f32>>,
    /// Sequence length
    pub sequence_length: usize,
    /// Embedding dimension
    pub embedding_dim: usize,
}

impl SequenceEmbedding {
    /// Get embedding for a specific residue.
    #[must_use]
    pub fn get(&self, idx: usize) -> Option<&[f32]> {
        self.embeddings.get(idx).map(std::vec::Vec::as_slice)
    }

    /// Apply layer normalization to embeddings.
    pub fn layer_norm(&mut self, eps: f32) {
        for emb in &mut self.embeddings {
            let mean: f32 = emb.iter().sum::<f32>() / emb.len() as f32;
            let var: f32 = emb.iter().map(|x| (x - mean).powi(2)).sum::<f32>() / emb.len() as f32;
            let std = (var + eps).sqrt();

            for x in emb.iter_mut() {
                *x = (*x - mean) / std;
            }
        }
    }
}

/// Pairwise residue embedding.
#[derive(Debug, Clone)]
pub struct PairEmbedding {
    /// Pairwise embeddings [`seq_len` x `seq_len` x `pair_dim`]
    pub embeddings: Vec<Vec<Vec<f32>>>,
    /// Sequence length
    pub sequence_length: usize,
    /// Pair embedding dimension
    pub pair_dim: usize,
}

impl PairEmbedding {
    /// Get pairwise embedding for residues i and j.
    #[must_use]
    pub fn get(&self, i: usize, j: usize) -> Option<&[f32]> {
        self.embeddings
            .get(i)
            .and_then(|row| row.get(j))
            .map(std::vec::Vec::as_slice)
    }

    /// Update pair embedding with outer product of sequence embeddings.
    pub fn outer_product_update(&mut self, seq_emb: &SequenceEmbedding) {
        let seq_len = self.sequence_length;

        for i in 0..seq_len {
            for j in 0..seq_len {
                if let (Some(emb_i), Some(emb_j)) = (seq_emb.get(i), seq_emb.get(j)) {
                    // Simple outer product projection
                    for k in 0..self.pair_dim {
                        let idx_i = k * 2 % seq_emb.embedding_dim;
                        let idx_j = (k * 2 + 1) % seq_emb.embedding_dim;
                        self.embeddings[i][j][k] += emb_i[idx_i] * emb_j[idx_j] * 0.1;
                    }
                }
            }
        }
    }
}

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

    #[test]
    fn test_encoder_creation() {
        let encoder = ProteinEncoder::new();
        assert_eq!(encoder.aa_embeddings.len(), NUM_AMINO_ACIDS);
        assert_eq!(encoder.aa_embeddings[0].len(), EMBEDDING_DIM);
    }

    #[test]
    fn test_sequence_encoding() {
        let encoder = ProteinEncoder::new();
        let embedding = encoder.encode("ACDEF");

        assert_eq!(embedding.sequence_length, 5);
        assert_eq!(embedding.embedding_dim, EMBEDDING_DIM);
        assert_eq!(embedding.embeddings.len(), 5);
    }

    #[test]
    fn test_pair_encoding() {
        let encoder = ProteinEncoder::new();
        let pairs = encoder.encode_pairs(10);

        assert_eq!(pairs.sequence_length, 10);
        assert_eq!(pairs.embeddings.len(), 10);
        assert_eq!(pairs.embeddings[0].len(), 10);
    }

    #[test]
    fn test_layer_norm() {
        let encoder = ProteinEncoder::new();
        let mut embedding = encoder.encode("AAA");
        embedding.layer_norm(1e-5);

        // After layer norm, mean should be ~0 and std ~1
        for emb in &embedding.embeddings {
            let mean: f32 = emb.iter().sum::<f32>() / emb.len() as f32;
            assert!(mean.abs() < 0.01);
        }
    }
}