tenferro-internal-ops 0.7.1

//! AD rules for `StdTensorOp::Gather` and `StdTensorOp::Scatter`.
//!
//! The forward rules (`linearize_gather` / `linearize_scatter`) push the
//! tangent through the same indexing op, reusing the integer index operand
//! from the primal because indices are non-differentiable. The backward
//! rules (`transpose_gather` / `transpose_scatter`) flow the cotangent
//! through the inverse op by swapping `Gather` ↔ `Scatter`, with a
//! mechanical config inversion that mirrors:
//!
//! - `start_index_map` ↔ `scatter_dims_to_operand_dims`
//! - `collapsed_slice_dims` ↔ `inserted_window_dims`
//! - `offset_dims` ↔ `update_window_dims`
//! - `index_vector_dim` carried over unchanged
//!
//! Scatter uses the StableHLO add-scatter semantics: the output is
//! `operand + scatter_add(zeros, indices, updates, config)`, so the
//! output depends on both the operand (identity passthrough plus the
//! base value at scattered slots) and the updates (accumulated at the
//! scattered slots). The forward rule treats the scatter as linear in
//! both operand and updates; the backward rule sends the output
//! cotangent to the operand as an identity passthrough and to the
//! updates through the inverse `Gather`.
//!
//! Indexing AD follows the repository AD contract's JAX/StableHLO-style
//! `promise_in_bounds` rule: gradients are guaranteed only for in-bounds
//! starts and indices. Primal out-of-bounds clamping or dropped scatter
//! windows must not be reviewed as an AD finite-difference promise.
//!
//! Closing the core op vocabulary under AD is a Stage 7 prerequisite (the
//! tropical fused backward emits `Gather` / `Scatter` on this vocabulary).

use crate::ad::ADRuleResult;
use computegraph::types::{LocalValueId, OperationRole, ValueKey, ValueRef};
use tenferro_tensor::{GatherConfig, ScatterConfig};

use crate::ad::context::ShapeGuardContext;
use crate::ad::zeros::{build_zero_like, SymbolicZero};
use crate::ad::PrimitiveRuleBuilder;
use crate::dim_expr::DimExpr;
use crate::shape_extent::ShapeExtent;
use crate::std_tensor_op::StdTensorOp;
use crate::sym_dim::SymDim;

/// Forward-mode AD rule for `Gather(operand, start_indices, config)`.
///
/// `start_indices` is integer-valued and non-differentiable, so its tangent
/// is ignored. The output tangent is the same gather applied to the
/// operand's tangent, reusing the primal indices.
pub fn linearize_gather(
    builder: &mut dyn PrimitiveRuleBuilder,
    primal_in: &[ValueKey<StdTensorOp>],
    tangent_in: &[Option<LocalValueId>],
    config: &GatherConfig,
) -> Vec<Option<LocalValueId>> {
    match tangent_in[0] {
        Some(d_operand) => {
            let out = builder.add_operation(
                StdTensorOp::Gather(config.clone()),
                vec![
                    ValueRef::Local(d_operand),
                    ValueRef::External(primal_in[1].clone()),
                ],
                OperationRole::Linearized {
                    active_mask: vec![true, false],
                },
            );
            vec![Some(out[0])]
        }
        None => vec![None],
    }
}

/// Forward-mode AD rule for `GatherDynamicSliceSizes`.
///
/// The operand tangent flows through the same gather. Start indices and all
/// shape-source inputs are non-differentiable and are reused from the primal.
#[allow(clippy::too_many_arguments)]
pub fn linearize_gather_dynamic_slice_sizes(
    builder: &mut dyn PrimitiveRuleBuilder,
    primal_in: &[ValueKey<StdTensorOp>],
    tangent_in: &[Option<LocalValueId>],
    offset_dims: &[usize],
    collapsed_slice_dims: &[usize],
    start_index_map: &[usize],
    index_vector_dim: usize,
    slice_sizes: &[DimExpr],
) -> Vec<Option<LocalValueId>> {
    match tangent_in[0] {
        Some(d_operand) => {
            let mut inputs = Vec::with_capacity(primal_in.len());
            inputs.push(ValueRef::Local(d_operand));
            inputs.extend(primal_in.iter().skip(1).cloned().map(ValueRef::External));

            let mut active_mask = vec![false; primal_in.len()];
            active_mask[0] = true;

            let out = builder.add_operation(
                StdTensorOp::GatherDynamicSliceSizes {
                    offset_dims: offset_dims.to_vec(),
                    collapsed_slice_dims: collapsed_slice_dims.to_vec(),
                    start_index_map: start_index_map.to_vec(),
                    index_vector_dim,
                    slice_sizes: slice_sizes.to_vec(),
                },
                inputs,
                OperationRole::Linearized { active_mask },
            );
            vec![Some(out[0])]
        }
        None => vec![None],
    }
}

/// Forward-mode AD rule for `DynamicSlice`.
///
/// The source tensor tangent flows through the same dynamic slice. Runtime
/// start indices are integer-valued and non-differentiable.
pub fn linearize_dynamic_slice(
    builder: &mut dyn PrimitiveRuleBuilder,
    primal_in: &[ValueKey<StdTensorOp>],
    tangent_in: &[Option<LocalValueId>],
    slice_sizes: &[usize],
) -> Vec<Option<LocalValueId>> {
    match tangent_in[0] {
        Some(d_operand) => {
            let out = builder.add_operation(
                StdTensorOp::DynamicSlice {
                    slice_sizes: slice_sizes.to_vec(),
                },
                vec![
                    ValueRef::Local(d_operand),
                    ValueRef::External(primal_in[1].clone()),
                ],
                OperationRole::Linearized {
                    active_mask: vec![true, false],
                },
            );
            vec![Some(out[0])]
        }
        None => vec![None],
    }
}

/// Forward-mode AD rule for `DynamicUpdateSlice`.
///
/// The operand and update inputs are differentiable. Runtime start indices are
/// integer-valued and non-differentiable.
pub fn linearize_dynamic_update_slice(
    builder: &mut dyn PrimitiveRuleBuilder,
    primal_in: &[ValueKey<StdTensorOp>],
    tangent_in: &[Option<LocalValueId>],
    ctx: &mut ShapeGuardContext,
) -> ADRuleResult<Vec<Option<LocalValueId>>> {
    if tangent_in[0].is_none() && tangent_in[1].is_none() {
        return Ok(vec![None]);
    }

    let operand = ValueRef::External(primal_in[0].clone());
    let update = ValueRef::External(primal_in[1].clone());
    let d_operand = match tangent_in[0] {
        Some(tangent) => tangent,
        None => {
            let meta = ctx.metadata_of(&operand)?;
            SymbolicZero::from_meta(operand, meta).instantiate(builder)
        }
    };
    let d_update = match tangent_in[1] {
        Some(tangent) => tangent,
        None => {
            let meta = ctx.metadata_of(&update)?;
            SymbolicZero::from_meta(update, meta).instantiate(builder)
        }
    };

    let out = builder.add_operation(
        StdTensorOp::DynamicUpdateSlice,
        vec![
            ValueRef::Local(d_operand),
            ValueRef::Local(d_update),
            ValueRef::External(primal_in[2].clone()),
        ],
        OperationRole::Linearized {
            active_mask: vec![tangent_in[0].is_some(), tangent_in[1].is_some(), false],
        },
    );
    Ok(vec![Some(out[0])])
}

/// Reverse-mode AD rule for `Gather(operand, start_indices, config)`.
///
/// Emits a `Scatter` whose config inverts the `GatherConfig` so that the
/// cotangent values are scattered into the positions the forward gather
/// read from. The scatter uses add-mode semantics, so multiple gathers
/// reading the same slot accumulate additively in the operand cotangent.
/// Because the underlying scatter now follows StableHLO add-scatter
/// semantics (output starts from `operand`), the inverse scatter must
/// use a zero operand so the operand cotangent is only the sum over
/// gather reads, not `original operand + sum over gather reads`.
pub fn transpose_gather(
    builder: &mut dyn PrimitiveRuleBuilder,
    cotangent_out: &[Option<LocalValueId>],
    inputs: &[ValueRef<StdTensorOp>],
    mode: &OperationRole,
    config: &GatherConfig,
    ctx: &mut ShapeGuardContext,
) -> ADRuleResult<Vec<Option<LocalValueId>>> {
    let ct = match cotangent_out[0] {
        Some(ct) => ct,
        None => return Ok(vec![None, None]),
    };

    let active_mask = match mode {
        OperationRole::Linearized { active_mask } => active_mask.clone(),
        OperationRole::Primary => return Ok(vec![None, None]),
    };

    if !active_mask.first().copied().unwrap_or(false) {
        return Ok(vec![None, None]);
    }

    let inverse_config = ScatterConfig {
        update_window_dims: config.offset_dims.clone(),
        inserted_window_dims: config.collapsed_slice_dims.clone(),
        scatter_dims_to_operand_dims: config.start_index_map.clone(),
        index_vector_dim: config.index_vector_dim,
    };

    let operand_meta = ctx.metadata_of(&inputs[0])?;
    let operand_rank = operand_meta.rank();
    let operand_dtype = operand_meta.dtype;
    let zero_operand = build_zero_like(builder, operand_dtype, inputs[0].clone(), operand_rank);

    let out = builder.add_operation(
        StdTensorOp::Scatter(inverse_config),
        vec![
            ValueRef::Local(zero_operand),
            inputs[1].clone(),
            ValueRef::Local(ct),
        ],
        OperationRole::Linearized {
            active_mask: vec![false, false, true],
        },
    );

    Ok(vec![Some(out[0]), None])
}

/// Reverse-mode AD rule for `GatherDynamicSliceSizes`.
///
/// The operand cotangent is the same inverse scatter as for concrete
/// `Gather`. Shape-source inputs only parameterize the primal slice sizes, so
/// their cotangents are always inactive.
#[allow(clippy::too_many_arguments)]
pub fn transpose_gather_dynamic_slice_sizes(
    builder: &mut dyn PrimitiveRuleBuilder,
    cotangent_out: &[Option<LocalValueId>],
    inputs: &[ValueRef<StdTensorOp>],
    mode: &OperationRole,
    offset_dims: &[usize],
    collapsed_slice_dims: &[usize],
    start_index_map: &[usize],
    index_vector_dim: usize,
    ctx: &mut ShapeGuardContext,
) -> ADRuleResult<Vec<Option<LocalValueId>>> {
    let mut result = vec![None; inputs.len()];
    let ct = match cotangent_out[0] {
        Some(ct) => ct,
        None => return Ok(result),
    };

    let active_mask = match mode {
        OperationRole::Linearized { active_mask } => active_mask,
        OperationRole::Primary => return Ok(result),
    };

    if !active_mask.first().copied().unwrap_or(false) {
        return Ok(result);
    }

    let inverse_config = ScatterConfig {
        update_window_dims: offset_dims.to_vec(),
        inserted_window_dims: collapsed_slice_dims.to_vec(),
        scatter_dims_to_operand_dims: start_index_map.to_vec(),
        index_vector_dim,
    };

    let operand_meta = ctx.metadata_of(&inputs[0])?;
    let operand_rank = operand_meta.rank();
    let operand_dtype = operand_meta.dtype;
    let zero_operand = build_zero_like(builder, operand_dtype, inputs[0].clone(), operand_rank);

    let out = builder.add_operation(
        StdTensorOp::Scatter(inverse_config),
        vec![
            ValueRef::Local(zero_operand),
            inputs[1].clone(),
            ValueRef::Local(ct),
        ],
        OperationRole::Linearized {
            active_mask: vec![false, false, true],
        },
    );

    result[0] = Some(out[0]);
    Ok(result)
}

/// Reverse-mode AD rule for `DynamicSlice`.
///
/// The transpose of `DynamicSlice(x, starts, sizes)` writes the cotangent back
/// into a zero tensor shaped like `x` using the same start-adjustment semantics.
pub fn transpose_dynamic_slice(
    builder: &mut dyn PrimitiveRuleBuilder,
    cotangent_out: &[Option<LocalValueId>],
    inputs: &[ValueRef<StdTensorOp>],
    mode: &OperationRole,
    ctx: &mut ShapeGuardContext,
) -> ADRuleResult<Vec<Option<LocalValueId>>> {
    let ct = match cotangent_out[0] {
        Some(ct) => ct,
        None => return Ok(vec![None, None]),
    };

    let active_mask = match mode {
        OperationRole::Linearized { active_mask } => active_mask,
        OperationRole::Primary => return Ok(vec![None, None]),
    };
    if !active_mask.first().copied().unwrap_or(false) {
        return Ok(vec![None, None]);
    }

    let operand_meta = ctx.metadata_of(&inputs[0])?;
    let zero_operand = build_zero_like(
        builder,
        operand_meta.dtype,
        inputs[0].clone(),
        operand_meta.rank(),
    );
    let out = builder.add_operation(
        StdTensorOp::DynamicUpdateSlice,
        vec![
            ValueRef::Local(zero_operand),
            ValueRef::Local(ct),
            inputs[1].clone(),
        ],
        OperationRole::Linearized {
            active_mask: vec![false, true, false],
        },
    );
    Ok(vec![Some(out[0]), None])
}

/// Reverse-mode AD rule for `DynamicUpdateSlice`.
///
/// Operand cotangent keeps the output cotangent outside the update window and
/// zeros the updated window. Update cotangent is the matching dynamic slice of
/// the output cotangent.
pub fn transpose_dynamic_update_slice(
    builder: &mut dyn PrimitiveRuleBuilder,
    cotangent_out: &[Option<LocalValueId>],
    inputs: &[ValueRef<StdTensorOp>],
    mode: &OperationRole,
    ctx: &mut ShapeGuardContext,
) -> ADRuleResult<Vec<Option<LocalValueId>>> {
    let ct = match cotangent_out[0] {
        Some(ct) => ct,
        None => return Ok(vec![None, None, None]),
    };

    let active_mask = match mode {
        OperationRole::Linearized { active_mask } => active_mask,
        OperationRole::Primary => return Ok(vec![None, None, None]),
    };

    let mut result = vec![None, None, None];
    if active_mask.first().copied().unwrap_or(false) {
        let update_meta = ctx.metadata_of(&inputs[1])?;
        let zero_update = build_zero_like(
            builder,
            update_meta.dtype,
            inputs[1].clone(),
            update_meta.rank(),
        );
        let operand_ct = builder.add_operation(
            StdTensorOp::DynamicUpdateSlice,
            vec![
                ValueRef::Local(ct),
                ValueRef::Local(zero_update),
                inputs[2].clone(),
            ],
            OperationRole::Linearized {
                active_mask: vec![true, false, false],
            },
        )[0];
        result[0] = Some(operand_ct);
    }

    if active_mask.get(1).copied().unwrap_or(false) {
        // DynamicSlice carries concrete slice sizes. If the update shape is not
        // exact, this transpose path cannot safely encode the update cotangent.
        let Some(update_shape) = exact_usize_shape(ctx, &inputs[1])? else {
            return Ok(result);
        };
        let update_ct = builder.add_operation(
            StdTensorOp::DynamicSlice {
                slice_sizes: update_shape,
            },
            vec![ValueRef::Local(ct), inputs[2].clone()],
            OperationRole::Linearized {
                active_mask: vec![true, false],
            },
        )[0];
        result[1] = Some(update_ct);
    }

    Ok(result)
}

/// Forward-mode AD rule for `Scatter(operand, scatter_indices, updates, config)`.
///
/// `scatter_indices` is integer-valued and non-differentiable.
///
/// Under StableHLO add-scatter semantics the scatter output is
/// `operand + scatter_add(zeros, indices, updates, config)`, so the
/// forward tangent factors into an identity contribution from
/// `operand_dot` and a scattered contribution from `updates_dot`.
/// Four cases:
///
/// - Both `operand_dot = None`, `updates_dot = None` → `[None]`.
/// - Only `operand_dot = Some` → identity passthrough `[Some(operand_dot)]`
///   (scattering a zero updates tangent adds nothing).
/// - Only `updates_dot = Some` → emit
///   `Scatter(zeros_like(operand), indices, updates_dot, config)` so the
///   output tangent is only the scattered contribution.
/// - Both `Some` → emit
///   `Scatter(operand_dot, indices, updates_dot, config)`; by linearity
///   this captures both contributions in a single scatter.
pub fn linearize_scatter(
    builder: &mut dyn PrimitiveRuleBuilder,
    primal_in: &[ValueKey<StdTensorOp>],
    tangent_in: &[Option<LocalValueId>],
    config: &ScatterConfig,
    ctx: &mut ShapeGuardContext,
) -> ADRuleResult<Vec<Option<LocalValueId>>> {
    let d_operand = tangent_in[0];
    let d_updates = tangent_in[2];

    match (d_operand, d_updates) {
        (None, None) => Ok(vec![None]),
        (Some(d_op), None) => Ok(vec![Some(d_op)]),
        (None, Some(d_up)) => {
            let operand_key = ValueRef::External(primal_in[0].clone());
            let operand_meta = ctx.metadata_of(&operand_key)?;
            let zero_operand =
                SymbolicZero::from_meta(operand_key.clone(), operand_meta).instantiate(builder);
            let out = builder.add_operation(
                StdTensorOp::Scatter(config.clone()),
                vec![
                    ValueRef::Local(zero_operand),
                    ValueRef::External(primal_in[1].clone()),
                    ValueRef::Local(d_up),
                ],
                OperationRole::Linearized {
                    active_mask: vec![false, false, true],
                },
            );
            Ok(vec![Some(out[0])])
        }
        (Some(d_op), Some(d_up)) => {
            let out = builder.add_operation(
                StdTensorOp::Scatter(config.clone()),
                vec![
                    ValueRef::Local(d_op),
                    ValueRef::External(primal_in[1].clone()),
                    ValueRef::Local(d_up),
                ],
                OperationRole::Linearized {
                    active_mask: vec![true, false, true],
                },
            );
            Ok(vec![Some(out[0])])
        }
    }
}

/// Reverse-mode AD rule for `Scatter(operand, scatter_indices, updates, config)`.
///
/// Under StableHLO add-scatter semantics the output depends on both the
/// operand (identity passthrough for non-scattered slots, plus the base
/// value at scattered slots) and the updates (accumulated at the
/// scattered slots). The cotangent therefore flows back to both:
///
/// - operand cotangent: identity passthrough of `cot_out`. The first
///   entry of the active mask gates whether it is returned.
/// - scatter_indices cotangent: always `None` (integer-valued).
/// - updates cotangent: `Gather(cot_out, scatter_indices, inverse_config)`
///   with `slice_sizes` derived from the primal `updates`' shape. The
///   third entry of the active mask gates whether it is returned.
pub fn transpose_scatter(
    builder: &mut dyn PrimitiveRuleBuilder,
    cotangent_out: &[Option<LocalValueId>],
    inputs: &[ValueRef<StdTensorOp>],
    mode: &OperationRole,
    config: &ScatterConfig,
    ctx: &mut ShapeGuardContext,
) -> ADRuleResult<Vec<Option<LocalValueId>>> {
    let ct = match cotangent_out[0] {
        Some(ct) => ct,
        None => return Ok(vec![None, None, None]),
    };

    let active_mask = match mode {
        OperationRole::Linearized { active_mask } => active_mask.clone(),
        OperationRole::Primary => return Ok(vec![None, None, None]),
    };

    let operand_active = active_mask.first().copied().unwrap_or(false);
    let updates_active = active_mask.get(2).copied().unwrap_or(false);

    let mut result = vec![None, None, None];

    if operand_active {
        result[0] = Some(ct);
    }

    if updates_active {
        let operand_rank = ctx.rank_of(&inputs[0])?;
        let updates_extents = ctx.extents_of(&inputs[2])?.to_vec();
        let Some(inverse) =
            compute_inverse_gather(operand_rank, &updates_extents, config, &inputs[2])
        else {
            return Ok(result);
        };

        let out = match inverse {
            InverseGather::Concrete(config) => builder.add_operation(
                StdTensorOp::Gather(config),
                vec![ValueRef::Local(ct), inputs[1].clone()],
                OperationRole::Linearized {
                    active_mask: vec![true, false],
                },
            ),
            InverseGather::Dynamic {
                offset_dims,
                collapsed_slice_dims,
                start_index_map,
                index_vector_dim,
                slice_sizes,
                shape_sources,
            } => {
                let mut gather_inputs = vec![ValueRef::Local(ct), inputs[1].clone()];
                let mut active_mask = vec![true, false];
                for shape_source in shape_sources {
                    gather_inputs.push(shape_source);
                    active_mask.push(false);
                }
                builder.add_operation(
                    StdTensorOp::GatherDynamicSliceSizes {
                        offset_dims,
                        collapsed_slice_dims,
                        start_index_map,
                        index_vector_dim,
                        slice_sizes,
                    },
                    gather_inputs,
                    OperationRole::Linearized { active_mask },
                )
            }
        };
        result[2] = Some(out[0]);
    }

    Ok(result)
}

enum InverseGather {
    Concrete(GatherConfig),
    Dynamic {
        offset_dims: Vec<usize>,
        collapsed_slice_dims: Vec<usize>,
        start_index_map: Vec<usize>,
        index_vector_dim: usize,
        slice_sizes: Vec<DimExpr>,
        shape_sources: Vec<ValueRef<StdTensorOp>>,
    },
}

/// Build the inverse gather used in `transpose_scatter`.
///
/// For each operand dim `d`:
/// - if `d ∈ inserted_window_dims`: `slice_sizes[d] = 1` (the dim is
///   collapsed / indexed, not windowed)
/// - otherwise: `d` is the `k`-th entry of operand-window dims
///   `(0..rank) \ inserted_window_dims`; the size comes from the primal
///   updates tensor at the corresponding `update_window_dims[k]` axis
fn compute_inverse_gather(
    operand_rank: usize,
    updates_extents: &[ShapeExtent<SymDim>],
    config: &ScatterConfig,
    updates_ref: &ValueRef<StdTensorOp>,
) -> Option<InverseGather> {
    let rank = operand_rank;
    let operand_window_dims: Vec<usize> = (0..rank)
        .filter(|dim| !config.inserted_window_dims.contains(dim))
        .collect();
    if operand_window_dims.len() != config.update_window_dims.len() {
        return None;
    }

    let mut concrete_slice_sizes = vec![1usize; rank];
    let mut dynamic_slice_sizes = vec![DimExpr::Const(1); rank];
    let mut has_dynamic_slice_size = false;
    for (k, &operand_dim) in operand_window_dims.iter().enumerate() {
        let update_axis = config.update_window_dims[k];
        let extent = updates_extents.get(update_axis)?;
        let exact_value = match extent {
            ShapeExtent::Exact(dim) => dim.constant_value(),
            ShapeExtent::UpperBound(_) | ShapeExtent::Unknown => None,
        };
        if let Some(value) = exact_value {
            concrete_slice_sizes[operand_dim] = value;
            dynamic_slice_sizes[operand_dim] = DimExpr::Const(value);
        } else {
            // Non-exact or symbolic update extents are read from the runtime
            // updates tensor shape instead of being treated as concrete sizes.
            has_dynamic_slice_size = true;
            dynamic_slice_sizes[operand_dim] = DimExpr::InputDim {
                input_idx: 2,
                axis: update_axis,
            };
        }
    }

    if has_dynamic_slice_size {
        Some(InverseGather::Dynamic {
            offset_dims: config.update_window_dims.clone(),
            collapsed_slice_dims: config.inserted_window_dims.clone(),
            start_index_map: config.scatter_dims_to_operand_dims.clone(),
            index_vector_dim: config.index_vector_dim,
            slice_sizes: dynamic_slice_sizes,
            shape_sources: vec![updates_ref.clone()],
        })
    } else {
        Some(InverseGather::Concrete(GatherConfig {
            offset_dims: config.update_window_dims.clone(),
            collapsed_slice_dims: config.inserted_window_dims.clone(),
            start_index_map: config.scatter_dims_to_operand_dims.clone(),
            index_vector_dim: config.index_vector_dim,
            slice_sizes: concrete_slice_sizes,
        }))
    }
}

fn exact_usize_shape(
    ctx: &mut ShapeGuardContext,
    value: &ValueRef<StdTensorOp>,
) -> ADRuleResult<Option<Vec<usize>>> {
    let Some(shape) = ctx.exact_shape_of(value)? else {
        return Ok(None);
    };
    Ok(shape.into_iter().map(|dim| dim.constant_value()).collect())
}