src/attention.rs

// SPF Smart Gateway - Multi-Head Self-Attention
// Copyright 2026 Joseph Stone - All Rights Reserved
//
// Scaled dot-product attention with multi-head projection.
// Supports both causal (decoder) and bidirectional (encoder) masking.
// Pure Rust — builds on tensor.rs only.
//
// Depends on: tensor.rs (Layer 0)

use crate::tensor::Tensor;

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

/// Cached activations from attention forward pass (for backward)
pub struct AttentionCache {
    /// Q projections [batch, n_heads, seq, d_head]
    pub q: Tensor,
    /// K projections [batch, n_heads, seq, d_head]
    pub k: Tensor,
    /// V projections [batch, n_heads, seq, d_head]
    pub v: Tensor,
    /// Attention weights [batch, n_heads, seq, seq]
    pub attn_weights: Tensor,
    /// Input to projections [batch*seq, d_model]
    pub input: Tensor,
    /// Scale factor (1/sqrt(d_head))
    pub scale: f32,
}

// ============================================================================
// ATTENTION CONFIGURATION
// ============================================================================

/// Configuration for multi-head attention
#[derive(Debug, Clone)]
pub struct AttentionConfig {
    /// Model dimension (total across all heads)
    pub d_model: usize,
    /// Number of attention heads
    pub n_heads: usize,
    /// Whether to apply causal masking (decoder-style)
    pub causal: bool,
}

impl AttentionConfig {
    /// Dimension per head: d_model / n_heads
    pub fn d_head(&self) -> usize {
        self.d_model / self.n_heads
    }
}

// ============================================================================
// MULTI-HEAD ATTENTION
// ============================================================================

/// Multi-head self-attention layer.
///
/// Contains Q, K, V projection weights and output projection.
/// All weights stored as 2D tensors [d_model, d_model].
pub struct MultiHeadAttention {
    pub config: AttentionConfig,
    /// Query projection: [d_model, d_model]
    pub w_q: Tensor,
    /// Key projection: [d_model, d_model]
    pub w_k: Tensor,
    /// Value projection: [d_model, d_model]
    pub w_v: Tensor,
    /// Output projection: [d_model, d_model]
    pub w_o: Tensor,
    /// Query bias: [d_model]
    pub b_q: Tensor,
    /// Key bias: [d_model]
    pub b_k: Tensor,
    /// Value bias: [d_model]
    pub b_v: Tensor,
    /// Output bias: [d_model]
    pub b_o: Tensor,
}

impl MultiHeadAttention {
    /// Initialize with Xavier/Glorot uniform scaling
    /// Scale = sqrt(6 / (fan_in + fan_out)) for uniform distribution
    /// For d_model=256: scale ≈ 0.108
    pub fn new(config: AttentionConfig, seed: u64) -> Self {
        let d = config.d_model;
        let scale = (6.0 / (d + d) as f32).sqrt();

        Self {
            w_q: Tensor::randn(&[d, d], seed).scale(scale),
            w_k: Tensor::randn(&[d, d], seed + 1).scale(scale),
            w_v: Tensor::randn(&[d, d], seed + 2).scale(scale),
            w_o: Tensor::randn(&[d, d], seed + 3).scale(scale),
            b_q: Tensor::zeros(&[d]),
            b_k: Tensor::zeros(&[d]),
            b_v: Tensor::zeros(&[d]),
            b_o: Tensor::zeros(&[d]),
            config,
        }
    }

    /// Forward pass: input [batch, seq_len, d_model] → output [batch, seq_len, d_model]
    ///
    /// Steps:
    /// 1. Project input to Q, K, V using learned weights
    /// 2. Split into multiple heads
    /// 3. Compute scaled dot-product attention per head
    /// 4. Concatenate heads and project output
    pub fn forward(&self, x: &Tensor) -> Result<Tensor, String> {
        if x.ndim() != 3 {
            return Err(format!("Attention expects 3D input [batch, seq, d_model], got {}D", x.ndim()));
        }
        let batch = x.shape[0];
        let seq_len = x.shape[1];
        let d_model = x.shape[2];

        if d_model != self.config.d_model {
            return Err(format!(
                "Input d_model {} doesn't match config {}",
                d_model, self.config.d_model
            ));
        }

        let n_heads = self.config.n_heads;
        let d_head = self.config.d_head();

        // Reshape input to [batch * seq_len, d_model] for matmul
        let x_2d = x.reshape(&[batch * seq_len, d_model])?;

        // Project Q, K, V: [batch*seq, d_model] × [d_model, d_model] = [batch*seq, d_model]
        let q = x_2d.matmul(&self.w_q.transpose_2d()?)?.add(&self.expand_bias(&self.b_q, batch * seq_len))?;
        let k = x_2d.matmul(&self.w_k.transpose_2d()?)?.add(&self.expand_bias(&self.b_k, batch * seq_len))?;
        let v = x_2d.matmul(&self.w_v.transpose_2d()?)?.add(&self.expand_bias(&self.b_v, batch * seq_len))?;

        // Reshape to [batch, seq_len, n_heads, d_head]
        let q = q.reshape(&[batch, seq_len, n_heads, d_head])?;
        let k = k.reshape(&[batch, seq_len, n_heads, d_head])?;
        let v = v.reshape(&[batch, seq_len, n_heads, d_head])?;

        // Transpose to [batch, n_heads, seq_len, d_head] for attention computation
        let q = self.transpose_heads(&q, batch, seq_len, n_heads, d_head)?;
        let k = self.transpose_heads(&k, batch, seq_len, n_heads, d_head)?;
        let v = self.transpose_heads(&v, batch, seq_len, n_heads, d_head)?;

        // Compute attention for each batch × head combination
        let scale = 1.0 / (d_head as f32).sqrt();
        let mut attn_output = Vec::with_capacity(batch * n_heads * seq_len * d_head);

        for b in 0..batch {
            for h in 0..n_heads {
                let bh_offset = (b * n_heads + h) * seq_len * d_head;

                // Extract Q, K, V slices for this batch/head: [seq_len, d_head]
                let q_slice = Tensor::from_data(
                    q.data[bh_offset..bh_offset + seq_len * d_head].to_vec(),
                    vec![seq_len, d_head],
                )?;
                let k_slice = Tensor::from_data(
                    k.data[bh_offset..bh_offset + seq_len * d_head].to_vec(),
                    vec![seq_len, d_head],
                )?;
                let v_slice = Tensor::from_data(
                    v.data[bh_offset..bh_offset + seq_len * d_head].to_vec(),
                    vec![seq_len, d_head],
                )?;

                // Attention scores: Q × K^T / sqrt(d_head) → [seq_len, seq_len]
                let k_t = k_slice.transpose_2d()?;
                let scores = q_slice.matmul(&k_t)?.scale(scale);

                // Apply causal mask if needed (set future positions to -inf)
                let scores = if self.config.causal {
                    self.apply_causal_mask(&scores, seq_len)?
                } else {
                    scores
                };

                // Softmax over last dimension → attention weights
                let weights = scores.softmax()?;

                // Weighted sum of values: weights × V → [seq_len, d_head]
                let head_out = weights.matmul(&v_slice)?;
                attn_output.extend_from_slice(&head_out.data);
            }
        }

        // Output is [batch, n_heads, seq_len, d_head]
        let attn_out = Tensor::from_data(attn_output, vec![batch, n_heads, seq_len, d_head])?;

        // Transpose back to [batch, seq_len, n_heads, d_head]
        let attn_out = self.transpose_heads_back(&attn_out, batch, seq_len, n_heads, d_head)?;

        // Reshape to [batch * seq_len, d_model] for output projection
        let concat = attn_out.reshape(&[batch * seq_len, d_model])?;

        // Output projection
        let output = concat.matmul(&self.w_o.transpose_2d()?)?.add(&self.expand_bias(&self.b_o, batch * seq_len))?;

        // Reshape back to [batch, seq_len, d_model]
        output.reshape(&[batch, seq_len, d_model])
    }

    /// Forward pass that also returns cached activations for backward.
    /// Output is IDENTICAL to forward(). Cache is additional data only.
    pub fn forward_with_cache(&self, x: &Tensor) -> Result<(Tensor, AttentionCache), String> {
        if x.ndim() != 3 {
            return Err(format!("Attention expects 3D input [batch, seq, d_model], got {}D", x.ndim()));
        }
        let batch = x.shape[0];
        let seq_len = x.shape[1];
        let d_model = x.shape[2];

        if d_model != self.config.d_model {
            return Err(format!("Input d_model {} doesn't match config {}", d_model, self.config.d_model));
        }

        let n_heads = self.config.n_heads;
        let d_head = self.config.d_head();

        let x_2d = x.reshape(&[batch * seq_len, d_model])?;
        let input_cache = x_2d.clone();

        let q = x_2d.matmul(&self.w_q.transpose_2d()?)?.add(&self.expand_bias(&self.b_q, batch * seq_len))?;
        let k = x_2d.matmul(&self.w_k.transpose_2d()?)?.add(&self.expand_bias(&self.b_k, batch * seq_len))?;
        let v = x_2d.matmul(&self.w_v.transpose_2d()?)?.add(&self.expand_bias(&self.b_v, batch * seq_len))?;

        let q = q.reshape(&[batch, seq_len, n_heads, d_head])?;
        let k = k.reshape(&[batch, seq_len, n_heads, d_head])?;
        let v = v.reshape(&[batch, seq_len, n_heads, d_head])?;

        let q = self.transpose_heads(&q, batch, seq_len, n_heads, d_head)?;
        let k = self.transpose_heads(&k, batch, seq_len, n_heads, d_head)?;
        let v = self.transpose_heads(&v, batch, seq_len, n_heads, d_head)?;

        let scale = 1.0 / (d_head as f32).sqrt();
        let mut attn_output = Vec::with_capacity(batch * n_heads * seq_len * d_head);
        let mut all_weights = Vec::with_capacity(batch * n_heads * seq_len * seq_len);

        for b in 0..batch {
            for h in 0..n_heads {
                let bh_offset = (b * n_heads + h) * seq_len * d_head;
                let q_slice = Tensor::from_data(
                    q.data[bh_offset..bh_offset + seq_len * d_head].to_vec(),
                    vec![seq_len, d_head],
                )?;
                let k_slice = Tensor::from_data(
                    k.data[bh_offset..bh_offset + seq_len * d_head].to_vec(),
                    vec![seq_len, d_head],
                )?;
                let v_slice = Tensor::from_data(
                    v.data[bh_offset..bh_offset + seq_len * d_head].to_vec(),
                    vec![seq_len, d_head],
                )?;

                let scores = q_slice.matmul(&k_slice.transpose_2d()?)?.scale(scale);
                let scores = if self.config.causal {
                    self.apply_causal_mask(&scores, seq_len)?
                } else {
                    scores
                };
                let weights = scores.softmax()?;
                let head_out = weights.matmul(&v_slice)?;
                all_weights.extend_from_slice(&weights.data);
                attn_output.extend_from_slice(&head_out.data);
            }
        }

        let attn_out = Tensor::from_data(attn_output, vec![batch, n_heads, seq_len, d_head])?;
        let attn_weights = Tensor::from_data(all_weights, vec![batch, n_heads, seq_len, seq_len])?;
        let attn_out = self.transpose_heads_back(&attn_out, batch, seq_len, n_heads, d_head)?;
        let concat = attn_out.reshape(&[batch * seq_len, d_model])?;
        let output = concat.matmul(&self.w_o.transpose_2d()?)?.add(&self.expand_bias(&self.b_o, batch * seq_len))?;
        let output = output.reshape(&[batch, seq_len, d_model])?;

        let cache = AttentionCache {
            q: q,
            k: k,
            v: v,
            attn_weights,
            input: input_cache,
            scale,
        };

        Ok((output, cache))
    }

    /// Forward pass with cross-attention (for decoder attending to encoder output)
    /// q_input: decoder state [batch, dec_seq, d_model]
    /// kv_input: encoder output [batch, enc_seq, d_model]
    pub fn forward_cross(
        &self,
        q_input: &Tensor,
        kv_input: &Tensor,
    ) -> Result<Tensor, String> {
        let batch = q_input.shape[0];
        let dec_seq = q_input.shape[1];
        let enc_seq = kv_input.shape[1];
        let d_model = q_input.shape[2];
        let n_heads = self.config.n_heads;
        let d_head = self.config.d_head();

        // Project Q from decoder, K/V from encoder
        let q_2d = q_input.reshape(&[batch * dec_seq, d_model])?;
        let kv_2d = kv_input.reshape(&[batch * enc_seq, d_model])?;

        let q = q_2d.matmul(&self.w_q.transpose_2d()?)?.add(&self.expand_bias(&self.b_q, batch * dec_seq))?;
        let k = kv_2d.matmul(&self.w_k.transpose_2d()?)?.add(&self.expand_bias(&self.b_k, batch * enc_seq))?;
        let v = kv_2d.matmul(&self.w_v.transpose_2d()?)?.add(&self.expand_bias(&self.b_v, batch * enc_seq))?;

        let q = q.reshape(&[batch, dec_seq, n_heads, d_head])?;
        let k = k.reshape(&[batch, enc_seq, n_heads, d_head])?;
        let v = v.reshape(&[batch, enc_seq, n_heads, d_head])?;

        let q = self.transpose_heads(&q, batch, dec_seq, n_heads, d_head)?;
        let k = self.transpose_heads(&k, batch, enc_seq, n_heads, d_head)?;
        let v = self.transpose_heads(&v, batch, enc_seq, n_heads, d_head)?;

        let scale = 1.0 / (d_head as f32).sqrt();
        let mut attn_output = Vec::with_capacity(batch * n_heads * dec_seq * d_head);

        for b in 0..batch {
            for h in 0..n_heads {
                let q_off = (b * n_heads + h) * dec_seq * d_head;
                let k_off = (b * n_heads + h) * enc_seq * d_head;

                let q_slice = Tensor::from_data(
                    q.data[q_off..q_off + dec_seq * d_head].to_vec(),
                    vec![dec_seq, d_head],
                )?;
                let k_slice = Tensor::from_data(
                    k.data[k_off..k_off + enc_seq * d_head].to_vec(),
                    vec![enc_seq, d_head],
                )?;
                let v_slice = Tensor::from_data(
                    v.data[k_off..k_off + enc_seq * d_head].to_vec(),
                    vec![enc_seq, d_head],
                )?;

                // Q[dec_seq, d_head] × K^T[d_head, enc_seq] → [dec_seq, enc_seq]
                let scores = q_slice.matmul(&k_slice.transpose_2d()?)?.scale(scale);
                // No causal mask for cross-attention — decoder can attend to all encoder positions
                let weights = scores.softmax()?;
                let head_out = weights.matmul(&v_slice)?;
                attn_output.extend_from_slice(&head_out.data);
            }
        }

        let attn_out = Tensor::from_data(attn_output, vec![batch, n_heads, dec_seq, d_head])?;
        let attn_out = self.transpose_heads_back(&attn_out, batch, dec_seq, n_heads, d_head)?;
        let concat = attn_out.reshape(&[batch * dec_seq, d_model])?;
        let output = concat.matmul(&self.w_o.transpose_2d()?)?.add(&self.expand_bias(&self.b_o, batch * dec_seq))?;
        output.reshape(&[batch, dec_seq, d_model])
    }

    // ========================================================================
    // INTERNAL HELPERS
    // ========================================================================

    /// Transpose [batch, seq, n_heads, d_head] → [batch, n_heads, seq, d_head]
    fn transpose_heads(
        &self, t: &Tensor, batch: usize, seq: usize, n_heads: usize, d_head: usize,
    ) -> Result<Tensor, String> {
        let mut out = vec![0.0f32; batch * n_heads * seq * d_head];
        for b in 0..batch {
            for s in 0..seq {
                for h in 0..n_heads {
                    for d in 0..d_head {
                        let src_idx = ((b * seq + s) * n_heads + h) * d_head + d;
                        let dst_idx = ((b * n_heads + h) * seq + s) * d_head + d;
                        out[dst_idx] = t.data[src_idx];
                    }
                }
            }
        }
        Tensor::from_data(out, vec![batch, n_heads, seq, d_head])
    }

    /// Transpose [batch, n_heads, seq, d_head] → [batch, seq, n_heads, d_head]
    fn transpose_heads_back(
        &self, t: &Tensor, batch: usize, seq: usize, n_heads: usize, d_head: usize,
    ) -> Result<Tensor, String> {
        let mut out = vec![0.0f32; batch * seq * n_heads * d_head];
        for b in 0..batch {
            for h in 0..n_heads {
                for s in 0..seq {
                    for d in 0..d_head {
                        let src_idx = ((b * n_heads + h) * seq + s) * d_head + d;
                        let dst_idx = ((b * seq + s) * n_heads + h) * d_head + d;
                        out[dst_idx] = t.data[src_idx];
                    }
                }
            }
        }
        Tensor::from_data(out, vec![batch, seq, n_heads * d_head])
    }

    /// Apply causal mask: set attention scores for future positions to -inf
    fn apply_causal_mask(&self, scores: &Tensor, seq_len: usize) -> Result<Tensor, String> {
        let mut data = scores.data.clone();
        for i in 0..seq_len {
            for j in (i + 1)..seq_len {
                data[i * seq_len + j] = f32::NEG_INFINITY;
            }
        }
        Tensor::from_data(data, scores.shape.clone())
    }

    /// Expand bias [d_model] to [n_rows, d_model] for addition after matmul
    fn expand_bias(&self, bias: &Tensor, n_rows: usize) -> Tensor {
        let d = bias.numel();
        let mut data = Vec::with_capacity(n_rows * d);
        for _ in 0..n_rows {
            data.extend_from_slice(&bias.data);
        }
        Tensor { data, shape: vec![n_rows, d] }
    }

    /// Total number of parameters in this attention layer
    pub fn num_params(&self) -> usize {
        let d = self.config.d_model;
        // 4 weight matrices [d,d] + 4 bias vectors [d]
        4 * d * d + 4 * d
    }

    /// Collect all weight tensors (for serialization / gradient updates)
    pub fn weights(&self) -> Vec<&Tensor> {
        vec![&self.w_q, &self.w_k, &self.w_v, &self.w_o,
             &self.b_q, &self.b_k, &self.b_v, &self.b_o]
    }

    /// Collect all weight tensors mutably (for optimizer updates)
    pub fn weights_mut(&mut self) -> Vec<&mut Tensor> {
        vec![&mut self.w_q, &mut self.w_k, &mut self.w_v, &mut self.w_o,
             &mut self.b_q, &mut self.b_k, &mut self.b_v, &mut self.b_o]
    }
}

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

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

    fn test_config() -> AttentionConfig {
        AttentionConfig {
            d_model: 64,
            n_heads: 4,
            causal: false,
        }
    }

    #[test]
    fn test_attention_output_shape() {
        let attn = MultiHeadAttention::new(test_config(), 42);
        let x = Tensor::randn(&[2, 8, 64], 99); // batch=2, seq=8, d=64
        let out = attn.forward(&x).unwrap();
        assert_eq!(out.shape, vec![2, 8, 64]);
    }

    #[test]
    fn test_attention_causal_mask() {
        let config = AttentionConfig {
            d_model: 64,
            n_heads: 4,
            causal: true,
        };
        let attn = MultiHeadAttention::new(config, 42);
        let x = Tensor::randn(&[1, 4, 64], 99);
        let out = attn.forward(&x).unwrap();
        assert_eq!(out.shape, vec![1, 4, 64]);
        // Values should be finite (causal mask doesn't break softmax)
        assert!(out.data.iter().all(|v| v.is_finite()));
    }

    #[test]
    fn test_cross_attention_shape() {
        let config = AttentionConfig {
            d_model: 64,
            n_heads: 4,
            causal: false,
        };
        let attn = MultiHeadAttention::new(config, 42);
        let dec = Tensor::randn(&[1, 4, 64], 99);  // decoder: seq=4
        let enc = Tensor::randn(&[1, 8, 64], 100);  // encoder: seq=8
        let out = attn.forward_cross(&dec, &enc).unwrap();
        assert_eq!(out.shape, vec![1, 4, 64]); // output follows decoder seq_len
    }

    #[test]
    fn test_num_params() {
        let attn = MultiHeadAttention::new(test_config(), 42);
        // d=64: 4×64×64 + 4×64 = 16384 + 256 = 16640
        assert_eq!(attn.num_params(), 16640);
    }

    #[test]
    fn test_d_head() {
        let config = AttentionConfig { d_model: 256, n_heads: 8, causal: false };
        assert_eq!(config.d_head(), 32);
    }

    #[test]
    fn test_dimension_mismatch() {
        let attn = MultiHeadAttention::new(test_config(), 42);
        let x = Tensor::randn(&[1, 4, 32], 99); // wrong d_model
        assert!(attn.forward(&x).is_err());
    }

    #[test]
    fn test_weights_count() {
        let attn = MultiHeadAttention::new(test_config(), 42);
        assert_eq!(attn.weights().len(), 8); // 4 weight matrices + 4 biases
    }
}