Add ComputeProposerIndex (#6297)

This is described in
https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/beacon-chain.md#compute_proposer_index.

Part of https://github.com/ledgerwatch/erigon/issues/5965

I compared with the Prysm tests to confirm the implementation is
correct:
2e49fdb3d2/beacon-chain/core/helpers/validators_test.go (L506-L614)

cmd/erigon-cl/core/transition/beacon_state_utils.go

@@ -4,6 +4,9 @@ import (
 	"crypto/sha256"
 	"encoding/binary"
 	"fmt"
+
+	"github.com/ledgerwatch/erigon/cl/clparams"
+	"github.com/ledgerwatch/erigon/cl/cltypes"
 )
 
 const SHUFFLE_ROUND_COUNT = uint8(90)
@@ -50,3 +53,34 @@ func ComputeShuffledIndex(ind, ind_count uint64, seed [32]byte) (uint64, error)
 	}
 	return ind, nil
 }
+
+func ComputePropserIndex(state *cltypes.BeaconStateBellatrix, indices []uint64, seed [32]byte) (uint64, error) {
+	if len(indices) == 0 {
+		return 0, fmt.Errorf("must have >0 indices")
+	}
+	maxRandomByte := uint64(1<<8 - 1)
+	i := uint64(0)
+	total := uint64(len(indices))
+	hash := sha256.New()
+	buf := make([]byte, 8)
+	for {
+		shuffled, err := ComputeShuffledIndex(i%total, total, seed)
+		if err != nil {
+			return 0, err
+		}
+		candidateIndex := indices[shuffled]
+		if candidateIndex >= uint64(len(state.Validators)) {
+			return 0, fmt.Errorf("candidate index out of range: %d for validator set of length: %d", candidateIndex, len(state.Validators))
+		}
+		binary.LittleEndian.PutUint64(buf, i/32)
+		input := append(seed[:], buf...)
+		hash.Reset()
+		hash.Write(input)
+		randomByte := uint64(hash.Sum(nil)[i%32])
+		effectiveBalance := state.Validators[candidateIndex].EffectiveBalance
+		if effectiveBalance*maxRandomByte >= clparams.MainnetBeaconConfig.MaxEffectiveBalance*randomByte {
+			return candidateIndex, nil
+		}
+		i += 1
+	}
+}

cmd/erigon-cl/core/transition/beacon_state_utils_test.go

@@ -2,6 +2,8 @@ package transition
 
 import (
 	"testing"
+
+	"github.com/ledgerwatch/erigon/cl/cltypes"
 )
 
 func TestComputeShuffledIndex(t *testing.T) {
@@ -34,3 +36,102 @@ func TestComputeShuffledIndex(t *testing.T) {
 		})
 	}
 }
+
+func TestComputeProposerIndex(t *testing.T) {
+	seed := [32]byte{}
+	copy(seed[:], []byte("seed"))
+	testCases := []struct {
+		description string
+		state       *cltypes.BeaconStateBellatrix
+		indices     []uint64
+		seed        [32]byte
+		expected    uint64
+		wantErr     bool
+	}{
+		{
+			description: "success",
+			state: &cltypes.BeaconStateBellatrix{
+				Validators: []*cltypes.Validator{
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+				},
+			},
+			indices:  []uint64{0, 1, 2, 3, 4},
+			seed:     seed,
+			expected: 2,
+		},
+		{
+			description: "single_active_index",
+			state: &cltypes.BeaconStateBellatrix{
+				Validators: []*cltypes.Validator{
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+				},
+			},
+			indices:  []uint64{3},
+			seed:     seed,
+			expected: 3,
+		},
+		{
+			description: "second_half_active",
+			state: &cltypes.BeaconStateBellatrix{
+				Validators: []*cltypes.Validator{
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+				},
+			},
+			indices:  []uint64{5, 6, 7, 8, 9},
+			seed:     seed,
+			expected: 7,
+		},
+		{
+			description: "zero_active_indices",
+			indices:     []uint64{},
+			seed:        seed,
+			wantErr:     true,
+		},
+		{
+			description: "active_index_out_of_range",
+			indices:     []uint64{100},
+			state: &cltypes.BeaconStateBellatrix{
+				Validators: []*cltypes.Validator{
+					{EffectiveBalance: testBeaconConfig.MaxEffectiveBalance},
+				},
+			},
+			seed:    seed,
+			wantErr: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.description, func(t *testing.T) {
+			got, err := ComputePropserIndex(tc.state, tc.indices, tc.seed)
+			if tc.wantErr {
+				if err == nil {
+					t.Errorf("unexpected success, wanted error")
+				}
+				return
+			}
+			if err != nil {
+				t.Errorf("unexpected error: %v", err)
+			}
+			if got != tc.expected {
+				t.Errorf("unexpected result: got %d, want %d", got, tc.expected)
+			}
+		})
+	}
+}