// Package attestationutil contains useful helpers for converting
// attestations into indexed form.
package attestationutil

import (
	"context"
	"sort"

	ethpb "github.com/prysmaticlabs/ethereumapis/eth/v1alpha1"
	"github.com/prysmaticlabs/go-bitfield"
	"go.opencensus.io/trace"
)

// ConvertToIndexed converts attestation to (almost) indexed-verifiable form.
//
// Note about spec pseudocode definition. The state was used by get_attesting_indices to determine
// the attestation committee. Now that we provide this as an argument, we no longer need to provide
// a state.
//
// Spec pseudocode definition:
//   def get_indexed_attestation(state: BeaconState, attestation: Attestation) -> IndexedAttestation:
//    """
//    Return the indexed attestation corresponding to ``attestation``.
//    """
//    attesting_indices = get_attesting_indices(state, attestation.data, attestation.aggregation_bits)
//
//    return IndexedAttestation(
//        attesting_indices=sorted(attesting_indices),
//        data=attestation.data,
//        signature=attestation.signature,
//    )
func ConvertToIndexed(ctx context.Context, attestation *ethpb.Attestation, committee []uint64) *ethpb.IndexedAttestation {
	ctx, span := trace.StartSpan(ctx, "core.ConvertToIndexed")
	defer span.End()

	attIndices := AttestingIndices(attestation.AggregationBits, committee)

	sort.Slice(attIndices, func(i, j int) bool {
		return attIndices[i] < attIndices[j]
	})
	inAtt := &ethpb.IndexedAttestation{
		Data:             attestation.Data,
		Signature:        attestation.Signature,
		AttestingIndices: attIndices,
	}
	return inAtt
}

// AttestingIndices returns the attesting participants indices from the attestation data. The
// committee is provided as an argument rather than a direct implementation from the spec definition.
// Having the committee as an argument allows for re-use of beacon committees when possible.
//
// Spec pseudocode definition:
//   def get_attesting_indices(state: BeaconState,
//                          data: AttestationData,
//                          bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]) -> Set[ValidatorIndex]:
//    """
//    Return the set of attesting indices corresponding to ``data`` and ``bits``.
//    """
//    committee = get_beacon_committee(state, data.slot, data.index)
//    return set(index for i, index in enumerate(committee) if bits[i])
func AttestingIndices(bf bitfield.Bitfield, committee []uint64) []uint64 {
	indices := make([]uint64, 0, len(committee))
	for _, idx := range bf.BitIndices() {
		if idx < len(committee) {
			indices = append(indices, committee[idx])
		}
	}
	return indices
}