src/encoder.rs

// SPF Smart Gateway - Transformer Encoder
// Copyright 2026 Joseph Stone - All Rights Reserved
//
// N stacked encoder layers: self-attention → layer_norm → FFN → layer_norm
// Sinusoidal positional encoding. Bidirectional attention (no causal mask).
// Input: token embeddings → Output: contextualized representations.
//
// Depends on: tensor.rs, attention.rs, ffn.rs (Layers 0-1)

use crate::tensor::Tensor;
use crate::attention::{AttentionCache, AttentionConfig, MultiHeadAttention};
use crate::ffn::{FfnCache, FfnConfig, FeedForward};

// ============================================================================
// ACTIVATION CACHE (for backward pass — P2-C)
// ============================================================================

/// Cached activations from one encoder layer forward pass
pub struct EncoderLayerCache {
    /// Input before LN1 (for layer_norm_backward)
    pub ln1_input: Tensor,
    /// Attention cache (Q, K, V, attn_weights, input, scale)
    pub attn_cache: AttentionCache,
    /// Input before LN2 (after attention residual)
    pub ln2_input: Tensor,
    /// FFN cache (input, hidden_pre_gelu)
    pub ffn_cache: FfnCache,
    /// Original layer input (for residual backward)
    pub residual_input: Tensor,
}

// ============================================================================
// ENCODER CONFIGURATION
// ============================================================================

/// Configuration for the encoder stack
#[derive(Debug, Clone)]
pub struct EncoderConfig {
    /// Number of encoder layers
    pub n_layers: usize,
    /// Model dimension
    pub d_model: usize,
    /// Number of attention heads
    pub n_heads: usize,
    /// Feed-forward hidden dimension (default: 4× d_model)
    pub d_ff: usize,
    /// Maximum sequence length (for positional encoding)
    pub max_seq_len: usize,
    /// Layer norm epsilon
    pub ln_eps: f32,
}

impl EncoderConfig {
    /// SPF Writer default: 6 layers, 256 dim, 8 heads
    pub fn spf_writer() -> Self {
        Self {
            n_layers: 6,
            d_model: 256,
            n_heads: 8,
            d_ff: 1024,
            max_seq_len: 2048,
            ln_eps: 1e-5,
        }
    }

    /// Smaller config for testing
    pub fn small() -> Self {
        Self {
            n_layers: 2,
            d_model: 64,
            n_heads: 4,
            d_ff: 256,
            max_seq_len: 512,
            ln_eps: 1e-5,
        }
    }
}

// ============================================================================
// SINGLE ENCODER LAYER
// ============================================================================

/// One encoder layer: self-attention + FFN, each with residual + layer norm.
///
/// Pre-norm architecture (norm before sublayer, used by modern transformers):
///   x → LayerNorm → SelfAttention → + residual → LayerNorm → FFN → + residual
pub struct EncoderLayer {
    /// Multi-head self-attention (bidirectional)
    pub self_attn: MultiHeadAttention,
    /// Feed-forward network
    pub ffn: FeedForward,
    /// Layer norm before attention
    pub ln1_gamma: Tensor,
    pub ln1_beta: Tensor,
    /// Layer norm before FFN
    pub ln2_gamma: Tensor,
    pub ln2_beta: Tensor,
    /// Epsilon for layer norm
    ln_eps: f32,
}

impl EncoderLayer {
    /// Initialize a single encoder layer
    pub fn new(d_model: usize, n_heads: usize, d_ff: usize, ln_eps: f32, seed: u64) -> Self {
        let attn_config = AttentionConfig {
            d_model,
            n_heads,
            causal: false, // Encoder uses bidirectional attention
        };
        let ffn_config = FfnConfig { d_model, d_ff };

        Self {
            self_attn: MultiHeadAttention::new(attn_config, seed),
            ffn: FeedForward::new(ffn_config, seed + 100),
            ln1_gamma: Tensor::ones(&[d_model]),
            ln1_beta: Tensor::zeros(&[d_model]),
            ln2_gamma: Tensor::ones(&[d_model]),
            ln2_beta: Tensor::zeros(&[d_model]),
            ln_eps,
        }
    }

    /// Forward pass: [batch, seq, d_model] → [batch, seq, d_model]
    pub fn forward(&self, x: &Tensor) -> Result<Tensor, String> {
        // Pre-norm self-attention with residual
        let normed = x.layer_norm(&self.ln1_gamma, &self.ln1_beta, self.ln_eps)?;
        let attn_out = self.self_attn.forward(&normed)?;
        let x = x.add(&attn_out)?;

        // Pre-norm FFN with residual
        let normed = x.layer_norm(&self.ln2_gamma, &self.ln2_beta, self.ln_eps)?;
        let ffn_out = self.ffn.forward(&normed)?;
        x.add(&ffn_out)
    }

    /// Forward pass with cached activations for backward.
    /// Output is IDENTICAL to forward(). Cache is additional data only.
    pub fn forward_with_cache(&self, x: &Tensor) -> Result<(Tensor, EncoderLayerCache), String> {
        let residual_input = x.clone();

        // Pre-norm self-attention with residual
        let ln1_input = x.clone();
        let normed = x.layer_norm(&self.ln1_gamma, &self.ln1_beta, self.ln_eps)?;
        let (attn_out, attn_cache) = self.self_attn.forward_with_cache(&normed)?;
        let x = x.add(&attn_out)?;

        // Pre-norm FFN with residual
        let ln2_input = x.clone();
        let normed = x.layer_norm(&self.ln2_gamma, &self.ln2_beta, self.ln_eps)?;
        let (ffn_out, ffn_cache) = self.ffn.forward_with_cache(&normed)?;
        let output = x.add(&ffn_out)?;

        let cache = EncoderLayerCache {
            ln1_input,
            attn_cache,
            ln2_input,
            ffn_cache,
            residual_input,
        };

        Ok((output, cache))
    }

    /// Total parameters in this layer
    pub fn num_params(&self) -> usize {
        let d = self.ln1_gamma.numel();
        self.self_attn.num_params() + self.ffn.num_params() + 4 * d // 2 LN × (gamma + beta)
    }

    /// Collect all weight tensors for serialization
    pub fn weights(&self) -> Vec<&Tensor> {
        let mut w = self.self_attn.weights();
        w.extend(self.ffn.weights());
        w.extend([&self.ln1_gamma, &self.ln1_beta, &self.ln2_gamma, &self.ln2_beta]);
        w
    }

    /// Collect all weight tensors mutably for optimizer updates
    pub fn weights_mut(&mut self) -> Vec<&mut Tensor> {
        let mut w = self.self_attn.weights_mut();
        w.extend(self.ffn.weights_mut());
        w.extend([&mut self.ln1_gamma, &mut self.ln1_beta, &mut self.ln2_gamma, &mut self.ln2_beta]);
        w
    }
}

// ============================================================================
// POSITIONAL ENCODING
// ============================================================================

/// Generate sinusoidal positional encoding table.
/// PE(pos, 2i) = sin(pos / 10000^(2i/d_model))
/// PE(pos, 2i+1) = cos(pos / 10000^(2i/d_model))
///
/// Returns: [max_seq_len, d_model] tensor
pub fn sinusoidal_positional_encoding(max_seq_len: usize, d_model: usize) -> Tensor {
    let mut data = vec![0.0f32; max_seq_len * d_model];
    for pos in 0..max_seq_len {
        for i in 0..d_model / 2 {
            let angle = pos as f32 / (10000.0_f32).powf(2.0 * i as f32 / d_model as f32);
            data[pos * d_model + 2 * i] = angle.sin();
            data[pos * d_model + 2 * i + 1] = angle.cos();
        }
        // Handle odd d_model
        if d_model % 2 == 1 {
            let angle = pos as f32 / (10000.0_f32).powf((d_model - 1) as f32 / d_model as f32);
            data[pos * d_model + d_model - 1] = angle.sin();
        }
    }
    Tensor { data, shape: vec![max_seq_len, d_model] }
}

// ============================================================================
// ENCODER STACK
// ============================================================================

/// Full encoder: positional encoding + N encoder layers + final layer norm.
pub struct Encoder {
    pub config: EncoderConfig,
    /// Positional encoding table [max_seq_len, d_model]
    pub pos_encoding: Tensor,
    /// Stack of encoder layers
    pub layers: Vec<EncoderLayer>,
    /// Final layer norm (applied after all layers)
    pub final_ln_gamma: Tensor,
    pub final_ln_beta: Tensor,
}

impl Encoder {
    /// Initialize encoder with given config
    pub fn new(config: EncoderConfig, seed: u64) -> Self {
        let pos_encoding = sinusoidal_positional_encoding(config.max_seq_len, config.d_model);

        let layers: Vec<EncoderLayer> = (0..config.n_layers)
            .map(|i| {
                EncoderLayer::new(
                    config.d_model,
                    config.n_heads,
                    config.d_ff,
                    config.ln_eps,
                    seed + (i as u64) * 1000,
                )
            })
            .collect();

        Self {
            final_ln_gamma: Tensor::ones(&[config.d_model]),
            final_ln_beta: Tensor::zeros(&[config.d_model]),
            pos_encoding,
            layers,
            config,
        }
    }

    /// Forward pass: embeddings [batch, seq_len, d_model] → encoded [batch, seq_len, d_model]
    ///
    /// 1. Add positional encoding to input embeddings
    /// 2. Pass through N encoder layers
    /// 3. Apply final layer norm
    pub fn forward(&self, embeddings: &Tensor) -> Result<Tensor, String> {
        let seq_len = embeddings.shape[1];
        if seq_len > self.config.max_seq_len {
            return Err(format!(
                "Sequence length {} exceeds max {}",
                seq_len, self.config.max_seq_len
            ));
        }

        // Slice positional encoding to actual sequence length
        let pos_enc = self.pos_encoding.slice(0, seq_len)?;

        // Add positional encoding (broadcasts across batch dimension)
        let batch = embeddings.shape[0];
        let d_model = embeddings.shape[2];
        let mut x_data = embeddings.data.clone();
        for b in 0..batch {
            for s in 0..seq_len {
                for d in 0..d_model {
                    x_data[(b * seq_len + s) * d_model + d] += pos_enc.data[s * d_model + d];
                }
            }
        }
        let mut x = Tensor::from_data(x_data, embeddings.shape.clone())?;

        // Pass through encoder layers
        for layer in &self.layers {
            x = layer.forward(&x)?;
        }

        // Final layer norm
        x.layer_norm(&self.final_ln_gamma, &self.final_ln_beta, self.config.ln_eps)
    }

    /// Total parameters in the encoder
    pub fn num_params(&self) -> usize {
        let layer_params: usize = self.layers.iter().map(|l| l.num_params()).sum();
        layer_params + 2 * self.config.d_model // final LN gamma + beta
    }

    /// Collect all weight tensors
    pub fn weights(&self) -> Vec<&Tensor> {
        let mut w: Vec<&Tensor> = Vec::new();
        for layer in &self.layers {
            w.extend(layer.weights());
        }
        w.push(&self.final_ln_gamma);
        w.push(&self.final_ln_beta);
        w
    }

    /// Collect all weight tensors mutably
    pub fn weights_mut(&mut self) -> Vec<&mut Tensor> {
        let mut w: Vec<&mut Tensor> = Vec::new();
        for layer in &mut self.layers {
            w.extend(layer.weights_mut());
        }
        w.push(&mut self.final_ln_gamma);
        w.push(&mut self.final_ln_beta);
        w
    }
}

// ============================================================================
// TESTS
// ============================================================================

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

    #[test]
    fn test_positional_encoding_shape() {
        let pe = sinusoidal_positional_encoding(100, 64);
        assert_eq!(pe.shape, vec![100, 64]);
    }

    #[test]
    fn test_positional_encoding_values() {
        let pe = sinusoidal_positional_encoding(10, 8);
        // Position 0 should have sin(0)=0, cos(0)=1 for first pair
        assert!((pe.data[0] - 0.0).abs() < 1e-5); // sin(0)
        assert!((pe.data[1] - 1.0).abs() < 1e-5); // cos(0)
        // All values should be in [-1, 1]
        assert!(pe.data.iter().all(|&v| v >= -1.0 && v <= 1.0));
    }

    #[test]
    fn test_encoder_layer_shape() {
        let layer = EncoderLayer::new(64, 4, 256, 1e-5, 42);
        let x = Tensor::randn(&[2, 8, 64], 99);
        let out = layer.forward(&x).unwrap();
        assert_eq!(out.shape, vec![2, 8, 64]);
    }

    #[test]
    fn test_encoder_layer_residual() {
        let layer = EncoderLayer::new(64, 4, 256, 1e-5, 42);
        let x = Tensor::randn(&[1, 4, 64], 99);
        let out = layer.forward(&x).unwrap();
        // With residual connections, output should not be identical to input
        // but should be close in magnitude (not exploding)
        let diff: f32 = x.data.iter().zip(&out.data)
            .map(|(a, b)| (a - b).abs())
            .sum::<f32>() / x.numel() as f32;
        assert!(diff < 10.0, "Output diverged too far from input: {}", diff);
    }

    #[test]
    fn test_encoder_full_forward() {
        let config = EncoderConfig::small();
        let encoder = Encoder::new(config, 42);
        let x = Tensor::randn(&[1, 8, 64], 99);
        let out = encoder.forward(&x).unwrap();
        assert_eq!(out.shape, vec![1, 8, 64]);
        assert!(out.data.iter().all(|v| v.is_finite()));
    }

    #[test]
    fn test_encoder_seq_exceeds_max() {
        let config = EncoderConfig { max_seq_len: 10, ..EncoderConfig::small() };
        let encoder = Encoder::new(config, 42);
        let x = Tensor::randn(&[1, 20, 64], 99); // seq=20 > max=10
        assert!(encoder.forward(&x).is_err());
    }

    #[test]
    fn test_encoder_num_params() {
        let config = EncoderConfig::small(); // 2 layers, d=64, ff=256
        let encoder = Encoder::new(config, 42);
        let params = encoder.num_params();
        // Each layer: attn(16640) + ffn(33088) + 4×64(LN) = 49984
        // 2 layers + final LN = 2×49984 + 128 = 100096
        assert_eq!(params, 100096);
    }

    #[test]
    fn test_encoder_weights_collection() {
        let config = EncoderConfig::small();
        let encoder = Encoder::new(config, 42);
        let weights = encoder.weights();
        // Each layer: 8(attn) + 4(ffn) + 4(LN) = 16. ×2 layers + 2 final LN = 34
        assert_eq!(weights.len(), 34);
    }
}