src/decoder.rs

// SPF Smart Gateway - Transformer Decoder
// Copyright 2026 Joseph Stone - All Rights Reserved
//
// N stacked decoder layers: masked self-attention → cross-attention → FFN
// Each sublayer has residual connection + layer norm (pre-norm architecture).
// Causal masking prevents attending to future tokens.
// Cross-attention allows decoder to attend to encoder output.
//
// 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 decoder layer forward pass (causal/self-only mode)
pub struct DecoderLayerCache {
    /// Input before LN1 (for layer_norm_backward)
    pub ln1_input: Tensor,
    /// Self-attention cache (Q, K, V, attn_weights, input, scale)
    pub self_attn_cache: AttentionCache,
    /// Input before LN3 (before FFN — ln2 unused in causal path)
    pub ln3_input: Tensor,
    /// FFN cache (input, hidden_pre_gelu)
    pub ffn_cache: FfnCache,
    /// Original layer input (for residual backward)
    pub residual_input: Tensor,
}

// ============================================================================
// DECODER CONFIGURATION
// ============================================================================

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

impl DecoderConfig {
    /// SPF Writer default: matches encoder config
    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 DECODER LAYER
// ============================================================================

/// One decoder layer with three sublayers:
///
/// 1. Masked self-attention (causal — can't see future tokens)
/// 2. Cross-attention (attends to encoder output)
/// 3. Feed-forward network
///
/// Pre-norm architecture:
///   x → LN → MaskedSelfAttn → +residual
///     → LN → CrossAttn → +residual
///     → LN → FFN → +residual
pub struct DecoderLayer {
    /// Masked multi-head self-attention (causal)
    pub self_attn: MultiHeadAttention,
    /// Cross-attention to encoder output
    pub cross_attn: MultiHeadAttention,
    /// Feed-forward network
    pub ffn: FeedForward,
    /// Layer norm before self-attention
    pub ln1_gamma: Tensor,
    pub ln1_beta: Tensor,
    /// Layer norm before cross-attention
    pub ln2_gamma: Tensor,
    pub ln2_beta: Tensor,
    /// Layer norm before FFN
    pub ln3_gamma: Tensor,
    pub ln3_beta: Tensor,
    /// Epsilon for layer norm
    pub ln_eps: f32,
}

impl DecoderLayer {
    /// Initialize a single decoder layer
    pub fn new(d_model: usize, n_heads: usize, d_ff: usize, ln_eps: f32, seed: u64) -> Self {
        let self_attn_config = AttentionConfig {
            d_model,
            n_heads,
            causal: true, // Decoder self-attention is causal
        };
        let cross_attn_config = AttentionConfig {
            d_model,
            n_heads,
            causal: false, // Cross-attention is bidirectional over encoder output
        };
        let ffn_config = FfnConfig { d_model, d_ff };

        Self {
            self_attn: MultiHeadAttention::new(self_attn_config, seed),
            cross_attn: MultiHeadAttention::new(cross_attn_config, seed + 50),
            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]),
            ln3_gamma: Tensor::ones(&[d_model]),
            ln3_beta: Tensor::zeros(&[d_model]),
            ln_eps,
        }
    }

    /// Forward pass with encoder output for cross-attention
    /// x: decoder input [batch, dec_seq, d_model]
    /// encoder_output: [batch, enc_seq, d_model]
    pub fn forward(&self, x: &Tensor, encoder_output: &Tensor) -> Result<Tensor, String> {
        // 1. Masked self-attention with residual
        let normed = x.layer_norm(&self.ln1_gamma, &self.ln1_beta, self.ln_eps)?;
        let self_attn_out = self.self_attn.forward(&normed)?;
        let x = x.add(&self_attn_out)?;

        // 2. Cross-attention with residual
        let normed = x.layer_norm(&self.ln2_gamma, &self.ln2_beta, self.ln_eps)?;
        let cross_attn_out = self.cross_attn.forward_cross(&normed, encoder_output)?;
        let x = x.add(&cross_attn_out)?;

        // 3. FFN with residual
        let normed = x.layer_norm(&self.ln3_gamma, &self.ln3_beta, self.ln_eps)?;
        let ffn_out = self.ffn.forward(&normed)?;
        x.add(&ffn_out)
    }

    /// Forward pass without cross-attention (decoder-only / causal LM mode)
    /// Used when running as a causal language model without encoder.
    /// The Researcher transformer uses this mode.
    pub fn forward_self_only(&self, x: &Tensor) -> Result<Tensor, String> {
        // 1. Masked self-attention with residual
        let normed = x.layer_norm(&self.ln1_gamma, &self.ln1_beta, self.ln_eps)?;
        let self_attn_out = self.self_attn.forward(&normed)?;
        let x = x.add(&self_attn_out)?;

        // Skip cross-attention (no encoder output)

        // 2. FFN with residual (use ln3 — ln2 unused in this path)
        let normed = x.layer_norm(&self.ln3_gamma, &self.ln3_beta, self.ln_eps)?;
        let ffn_out = self.ffn.forward(&normed)?;
        x.add(&ffn_out)
    }

    /// Forward pass without cross-attention, with cached activations for backward.
    /// Output is IDENTICAL to forward_self_only(). Cache is additional data only.
    pub fn forward_self_only_with_cache(&self, x: &Tensor) -> Result<(Tensor, DecoderLayerCache), String> {
        let residual_input = x.clone();

        // 1. Masked self-attention with residual
        let ln1_input = x.clone();
        let normed = x.layer_norm(&self.ln1_gamma, &self.ln1_beta, self.ln_eps)?;
        let (self_attn_out, self_attn_cache) = self.self_attn.forward_with_cache(&normed)?;
        let x = x.add(&self_attn_out)?;

        // Skip cross-attention (no encoder output)

        // 2. FFN with residual (use ln3 — ln2 unused in this path)
        let ln3_input = x.clone();
        let normed = x.layer_norm(&self.ln3_gamma, &self.ln3_beta, self.ln_eps)?;
        let (ffn_out, ffn_cache) = self.ffn.forward_with_cache(&normed)?;
        let output = x.add(&ffn_out)?;

        let cache = DecoderLayerCache {
            ln1_input,
            self_attn_cache,
            ln3_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_attn + cross_attn + ffn + 3 layer norms × (gamma + beta)
        self.self_attn.num_params() + self.cross_attn.num_params() + self.ffn.num_params() + 6 * d
    }

    /// Collect all weight tensors
    pub fn weights(&self) -> Vec<&Tensor> {
        let mut w = self.self_attn.weights();
        w.extend(self.cross_attn.weights());
        w.extend(self.ffn.weights());
        w.extend([
            &self.ln1_gamma, &self.ln1_beta,
            &self.ln2_gamma, &self.ln2_beta,
            &self.ln3_gamma, &self.ln3_beta,
        ]);
        w
    }

    /// Collect all weight tensors mutably
    pub fn weights_mut(&mut self) -> Vec<&mut Tensor> {
        let mut w = self.self_attn.weights_mut();
        w.extend(self.cross_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,
            &mut self.ln3_gamma, &mut self.ln3_beta,
        ]);
        w
    }
}

// ============================================================================
// DECODER STACK
// ============================================================================

/// Full decoder: positional encoding + N decoder layers + final layer norm.
pub struct Decoder {
    pub config: DecoderConfig,
    /// Positional encoding table [max_seq_len, d_model]
    pub pos_encoding: Tensor,
    /// Stack of decoder layers
    pub layers: Vec<DecoderLayer>,
    /// Final layer norm
    pub final_ln_gamma: Tensor,
    pub final_ln_beta: Tensor,
}

impl Decoder {
    /// Initialize decoder with given config
    pub fn new(config: DecoderConfig, seed: u64) -> Self {
        // Reuse encoder's sinusoidal positional encoding
        let pos_encoding = crate::encoder::sinusoidal_positional_encoding(
            config.max_seq_len,
            config.d_model,
        );

        let layers: Vec<DecoderLayer> = (0..config.n_layers)
            .map(|i| {
                DecoderLayer::new(
                    config.d_model,
                    config.n_heads,
                    config.d_ff,
                    config.ln_eps,
                    seed + (i as u64) * 2000,
                )
            })
            .collect();

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

    /// Forward pass with encoder output (encoder-decoder mode)
    /// embeddings: [batch, dec_seq, d_model]
    /// encoder_output: [batch, enc_seq, d_model]
    pub fn forward(
        &self,
        embeddings: &Tensor,
        encoder_output: &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
            ));
        }

        let mut x = self.add_positional_encoding(embeddings)?;

        for layer in &self.layers {
            x = layer.forward(&x, encoder_output)?;
        }

        x.layer_norm(&self.final_ln_gamma, &self.final_ln_beta, self.config.ln_eps)
    }

    /// Forward pass without encoder (decoder-only / causal LM mode)
    /// Used by the Researcher transformer and for autoregressive generation
    pub fn forward_causal(&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
            ));
        }

        let mut x = self.add_positional_encoding(embeddings)?;

        for layer in &self.layers {
            x = layer.forward_self_only(&x)?;
        }

        x.layer_norm(&self.final_ln_gamma, &self.final_ln_beta, self.config.ln_eps)
    }

    /// Forward pass without encoder, with cached activations for backward.
    /// Output is IDENTICAL to forward_causal(). Caches are additional data only.
    pub fn forward_causal_with_cache(&self, embeddings: &Tensor) -> Result<(Tensor, Vec<DecoderLayerCache>), 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
            ));
        }

        let mut x = self.add_positional_encoding(embeddings)?;
        let mut layer_caches = Vec::with_capacity(self.layers.len());

        for layer in &self.layers {
            let (out, cache) = layer.forward_self_only_with_cache(&x)?;
            x = out;
            layer_caches.push(cache);
        }

        let output = x.layer_norm(&self.final_ln_gamma, &self.final_ln_beta, self.config.ln_eps)?;
        Ok((output, layer_caches))
    }

    /// Add sinusoidal positional encoding to embeddings
    fn add_positional_encoding(&self, embeddings: &Tensor) -> Result<Tensor, String> {
        let batch = embeddings.shape[0];
        let seq_len = embeddings.shape[1];
        let d_model = embeddings.shape[2];

        let pos_enc = self.pos_encoding.slice(0, seq_len)?;

        let mut data = embeddings.data.clone();
        for b in 0..batch {
            for s in 0..seq_len {
                for d in 0..d_model {
                    data[(b * seq_len + s) * d_model + d] += pos_enc.data[s * d_model + d];
                }
            }
        }
        Tensor::from_data(data, embeddings.shape.clone())
    }

    /// Total parameters in the decoder
    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
    }

    /// 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_decoder_layer_with_encoder() {
        let layer = DecoderLayer::new(64, 4, 256, 1e-5, 42);
        let dec_input = Tensor::randn(&[1, 4, 64], 99);   // decoder: 4 tokens
        let enc_output = Tensor::randn(&[1, 8, 64], 100);  // encoder: 8 tokens
        let out = layer.forward(&dec_input, &enc_output).unwrap();
        assert_eq!(out.shape, vec![1, 4, 64]); // follows decoder seq_len
        assert!(out.data.iter().all(|v| v.is_finite()));
    }

    #[test]
    fn test_decoder_layer_self_only() {
        let layer = DecoderLayer::new(64, 4, 256, 1e-5, 42);
        let x = Tensor::randn(&[1, 6, 64], 99);
        let out = layer.forward_self_only(&x).unwrap();
        assert_eq!(out.shape, vec![1, 6, 64]);
        assert!(out.data.iter().all(|v| v.is_finite()));
    }

    #[test]
    fn test_decoder_full_forward() {
        let config = DecoderConfig::small();
        let decoder = Decoder::new(config, 42);
        let dec_emb = Tensor::randn(&[1, 4, 64], 99);
        let enc_out = Tensor::randn(&[1, 8, 64], 100);
        let out = decoder.forward(&dec_emb, &enc_out).unwrap();
        assert_eq!(out.shape, vec![1, 4, 64]);
        assert!(out.data.iter().all(|v| v.is_finite()));
    }

    #[test]
    fn test_decoder_causal_forward() {
        let config = DecoderConfig::small();
        let decoder = Decoder::new(config, 42);
        let x = Tensor::randn(&[1, 6, 64], 99);
        let out = decoder.forward_causal(&x).unwrap();
        assert_eq!(out.shape, vec![1, 6, 64]);
        assert!(out.data.iter().all(|v| v.is_finite()));
    }

    #[test]
    fn test_decoder_seq_exceeds_max() {
        let config = DecoderConfig { max_seq_len: 10, ..DecoderConfig::small() };
        let decoder = Decoder::new(config, 42);
        let x = Tensor::randn(&[1, 20, 64], 99);
        let enc = Tensor::randn(&[1, 5, 64], 100);
        assert!(decoder.forward(&x, &enc).is_err());
    }

    #[test]
    fn test_decoder_causal_seq_exceeds_max() {
        let config = DecoderConfig { max_seq_len: 10, ..DecoderConfig::small() };
        let decoder = Decoder::new(config, 42);
        let x = Tensor::randn(&[1, 20, 64], 99);
        assert!(decoder.forward_causal(&x).is_err());
    }

    #[test]
    fn test_decoder_num_params() {
        let config = DecoderConfig::small(); // 2 layers, d=64, ff=256
        let decoder = Decoder::new(config, 42);
        let params = decoder.num_params();
        // Each layer: self_attn(16640) + cross_attn(16640) + ffn(33088) + 6×64(LN) = 66752
        // 2 layers + final LN: 2×66752 + 128 = 133632
        assert_eq!(params, 133632);
    }

    #[test]
    fn test_decoder_batch() {
        let config = DecoderConfig::small();
        let decoder = Decoder::new(config, 42);
        let x = Tensor::randn(&[3, 4, 64], 99);   // batch=3
        let enc = Tensor::randn(&[3, 6, 64], 100);
        let out = decoder.forward(&x, &enc).unwrap();
        assert_eq!(out.shape, vec![3, 4, 64]);
    }

    #[test]
    fn test_decoder_weights_count() {
        let config = DecoderConfig::small();
        let decoder = Decoder::new(config, 42);
        let weights = decoder.weights();
        // Each layer: 8(self_attn) + 8(cross_attn) + 4(ffn) + 6(LN) = 26
        // 2 layers + 2 final LN = 54
        assert_eq!(weights.len(), 54);
    }

    #[test]
    fn test_decoder_layer_params() {
        let layer = DecoderLayer::new(64, 4, 256, 1e-5, 42);
        // self_attn: 16640, cross_attn: 16640, ffn: 33088, 6×64 LN: 384
        assert_eq!(layer.num_params(), 66752);
    }
}