package bloom

import (
	"encoding/binary"

	"codeberg.org/lindenii/furgit/internal/intconv"
)

// Settings describe the changed-paths Bloom filter parameters stored in
// commit-graph BDAT chunks.
//
// Obviously, they must match the repository's commit-graph settings to
// interpret filters correctly.
type Settings struct {
	HashVersion    uint32
	NumHashes      uint32
	BitsPerEntry   uint32
	MaxChangePaths uint32
}

// ParseSettings reads Bloom filter settings from a BDAT chunk header.
func ParseSettings(bdat []byte) (*Settings, error) {
	if len(bdat) < DataHeaderSize {
		return nil, ErrInvalid
	}

	settings := &Settings{
		HashVersion:    binary.BigEndian.Uint32(bdat[0:4]),
		NumHashes:      binary.BigEndian.Uint32(bdat[4:8]),
		BitsPerEntry:   binary.BigEndian.Uint32(bdat[8:12]),
		MaxChangePaths: DefaultMaxChange,
	}

	switch settings.HashVersion {
	case 1, 2:
	default:
		return nil, ErrInvalid
	}

	if settings.NumHashes == 0 {
		return nil, ErrInvalid
	}

	_, err := intconv.Uint32ToInt(settings.NumHashes)
	if err != nil {
		return nil, ErrInvalid
	}

	return settings, nil
}