# Autograd Engine

ggmlR includes a PyTorch-style dynamic autograd engine built on top of ggml tensors. Gradients are computed via reverse-mode AD on a tape recorded inside `with_grad_tape({})`.

Tensor layout: all autograd tensors are column-major — shape is `[features, batch]`, matching ggml’s native C layout.

    library(ggmlR)

## 1. Core primitives

### `ag_tensor` and `ag_param`

    # ag_tensor — non-trainable input (e.g. data)
    x <- ag_tensor(matrix(1:6 / 6, nrow = 2L))   # [2, 3]

    # ag_param — trainable parameter (gradient accumulated)
    W <- ag_param(matrix(rnorm(4), nrow = 2L))    # [2, 2]
    b <- ag_param(matrix(0.0, 2L, 1L))

### Forward pass and gradient tape

    with_grad_tape({
      h    <- ag_relu(ag_add(ag_matmul(W, x), b))   # [2, 3]
      loss <- ag_mean(ag_mul(h, h))                  # scalar MSE-like
    })

    grads <- backward(loss)   # returns named list of gradients

    cat("dL/dW:\n"); print(grads[["W"]])

    ## dL/dW:

    ## NULL

    cat("dL/db:\n"); print(grads[["b"]])

    ## dL/db:

    ## NULL

`backward()` returns gradients keyed by the parameter’s variable name.

## 2. Built-in operations

Function | Description
--- | ---
`ag_matmul(A, B)` | Matrix multiply
`ag_add(A, B)` | Element-wise add (broadcast supported)
`ag_mul(A, B)` | Element-wise multiply
`ag_relu(x)` | ReLU activation
`ag_sigmoid(x)` | Sigmoid activation
`ag_tanh(x)` | Tanh activation
`ag_gelu(x)` | GELU activation
`ag_softmax(x)` | Softmax (column-wise)
`ag_sum(x)` | Sum all elements → scalar
`ag_mean(x)` | Mean all elements → scalar
`ag_transpose(x)` | Transpose [m,n] → [n,m]
`ag_reshape(x, dims)` | Reshape tensor
`ag_softmax_cross_entropy_loss(logits, y)` | Fused softmax + cross-entropy

## 3. `ag_sequential` — module API

`ag_sequential` stacks layers into a callable module with `.forward()` and `.parameters()`.

    data(iris)
    set.seed(42)

    x_all <- t(scale(as.matrix(iris[, 1:4])))        # [4, 150]
    y_oh  <- model.matrix(~ Species - 1, iris)
    y_all <- t(y_oh)                                  # [3, 150]

    idx  <- sample(150L)
    x_tr <- x_all[, idx[1:120]];  x_vl <- x_all[, idx[121:150]]
    y_tr <- y_all[, idx[1:120]];  y_vl <- y_all[, idx[121:150]]

    model <- ag_sequential(
      ag_linear(4L,  64L, activation = "relu"),
      ag_batch_norm(64L),
      ag_dropout(0.3),
      ag_linear(64L, 32L, activation = "relu"),
      ag_linear(32L,  3L)
    )

    params <- model$parameters()
    cat("Parameter tensors:", length(params), "\n")

    ## Parameter tensors: 8

## 4. Optimizers

    # Adam (default lr = 1e-3)
    opt <- optimizer_adam(params, lr = 1e-3)

    # SGD with momentum
    opt_sgd <- optimizer_sgd(params, lr = 0.05, momentum = 0.9)

### Training loop

    BS <- 32L
    n  <- ncol(x_tr)

    ag_train(model)   # set training mode (enables dropout, batch norm train)
    set.seed(1)

    for (ep in seq_len(150L)) {
      perm <- sample(n)
      for (b in seq_len(ceiling(n / BS))) {
        idx <- perm[seq((b-1L)*BS + 1L, min(b*BS, n))]
        xb  <- ag_tensor(x_tr[, idx, drop = FALSE])
        yb  <- y_tr[, idx, drop = FALSE]

        with_grad_tape({
          loss <- ag_softmax_cross_entropy_loss(model$forward(xb), yb)
        })
        grads <- backward(loss)
        opt$step(grads)
        opt$zero_grad()
      }

      if (ep %% 50L == 0L)
        cat(sprintf("epoch %3d  loss %.4f\n", ep, loss$data[1]))
    }

    ## epoch  50  loss 0.1060
    ## epoch 100  loss 0.1083
    ## epoch 150  loss 0.1306

## 5. LR schedulers

    opt2 <- optimizer_adam(params, lr = 1e-3)

    # Cosine annealing: lr goes from lr_max to lr_min over T_max epochs
    sch_cos <- lr_scheduler_cosine(opt2, T_max = 150L, lr_min = 1e-5)

    # Step decay: multiply lr by gamma every step_size epochs
    sch_step <- lr_scheduler_step(opt2, step_size = 30L, gamma = 0.5)

    # Call after each epoch:
    # sch_cos$step()

## 6. Gradient clipping

    with_grad_tape({
      loss <- ag_softmax_cross_entropy_loss(model$forward(ag_tensor(x_tr)), y_tr)
    })
    grads <- backward(loss)

    # Clip global gradient norm to max_norm
    clip_grad_norm(params, grads, max_norm = 5.0)

    opt$step(grads)
    opt$zero_grad()

## 7. Dataloader

`ag_dataloader` shuffles and batches column-major data matrices:

    dl <- ag_dataloader(x_tr, y_tr, batch_size = BS, shuffle = TRUE)

    ag_train(model)
    for (ep in seq_len(100L)) {
      for (batch in dl$epoch()) {
        with_grad_tape({
          loss <- ag_softmax_cross_entropy_loss(model$forward(batch$x), batch$y$data)
        })
        grads <- backward(loss)
        opt$step(grads);  opt$zero_grad()
      }
    }

## 8. Eval mode and inference

    ag_eval(model)   # disables dropout, switches batch norm to inference stats

    # Forward in chunks to avoid memory pressure
    predict_cm <- function(mod, x_cm, chunk = 64L) {
      n   <- ncol(x_cm)
      out <- NULL
      for (s in seq(1L, n, by = chunk)) {
        e  <- min(s + chunk - 1L, n)
        lg <- mod$forward(ag_tensor(x_cm[, s:e, drop = FALSE]))$data
        ev <- exp(lg - apply(lg, 2, max))
        sm <- ev / colSums(ev)
        out <- if (is.null(out)) sm else cbind(out, sm)
      }
      out
    }

    probs <- predict_cm(model, x_vl)          # [3, 30]
    preds <- apply(probs, 2, which.max)
    truth <- apply(y_vl, 1, which.max)
    cat(sprintf("Val accuracy: %.4f\n", mean(preds == truth)))

    ## Val accuracy: 0.1000

## 9. Raw `ag_param` — full manual control

For complete flexibility, build the network from scratch:

    set.seed(7)
    W1 <- ag_param(matrix(rnorm(64*4) * sqrt(2/4),  64, 4))
    b1 <- ag_param(matrix(0.0, 64, 1))
    W2 <- ag_param(matrix(rnorm(3*64) * sqrt(2/64),  3, 64))
    b2 <- ag_param(matrix(0.0,  3, 1))

    forward <- function(x)
      ag_add(ag_matmul(W2, ag_relu(ag_add(ag_matmul(W1, x), b1))), b2)

    opt_raw <- optimizer_adam(list(W1=W1, b1=b1, W2=W2, b2=b2), lr = 1e-3)

    for (ep in seq_len(200L)) {
      perm <- sample(n)
      for (b in seq_len(ceiling(n / BS))) {
        idx <- perm[seq((b-1L)*BS+1L, min(b*BS, n))]
        xb  <- ag_tensor(x_tr[, idx, drop = FALSE])
        yb  <- y_tr[, idx, drop = FALSE]
        with_grad_tape({ loss_r <- ag_softmax_cross_entropy_loss(forward(xb), yb) })
        gr <- backward(loss_r)
        opt_raw$step(gr);  opt_raw$zero_grad()
      }
    }

## 10. Mixed precision

    # f16 on GPU, f32 on CPU
    device <- tryCatch({ ag_device("gpu"); "gpu" }, error = function(e) "cpu")
    ag_dtype(if (device == "gpu") "f16" else "f32")

    # All subsequent ag_param / ag_tensor use the selected dtype

See also `vignette("gpu-vulkan", package = "ggmlR")` for device management.

## 11. Data-parallel training

For multi-GPU or faster single-GPU training see `dp_train()`:

    # Full example: inst/examples/dp_train_demo.R

See also `vignette("data-parallel-training", package = "ggmlR")`.