# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
def validate_kv_sharing_target(
    current_layer_name, target_layer_name, static_forward_context
):
    error_msg = (
        f"Specified KV sharing target layer for {current_layer_name} "
        f"is not valid: target layer {target_layer_name} "
    )

    if current_layer_name == target_layer_name:
        raise ValueError(error_msg + "cannot be the same as the current layer.")

    if target_layer_name not in static_forward_context:
        from vllm.model_executor.models.utils import extract_layer_index

        # If target layer name is not in the static fwd context, it means either
        # a) the target layer does not come BEFORE the current layer, or
        # b) the target layer is not an Attention layer that exists in the model
        current_layer_idx = extract_layer_index(current_layer_name)
        target_layer_idx = extract_layer_index(target_layer_name)
        if current_layer_idx <= target_layer_idx:
            raise ValueError(error_msg + "must come before the current layer.")
        else:
            raise ValueError(error_msg + "is not a valid Attention layer in the model.")

    # Currently KV sharing is only supported between layers of the same type
    target_layer_attn_type = static_forward_context[target_layer_name].attn_type
    expected = static_forward_context[current_layer_name].attn_type
    if target_layer_attn_type != expected:
        raise ValueError(
            error_msg + f"must be the same type as the current layer ({expected})."
        )
