1 files changed, 151 insertions, 0 deletions
diff --git a/internal/format/packidx/bloom/write.go b/internal/format/packidx/bloom/write.go
new file mode 100644
index 00000000..a9efbaae
--- /dev/null
+++ b/internal/format/packidx/bloom/write.go
@@ -0,0 +1,151 @@
+package bloom
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"math/bits"
+
+	"lindenii.org/go/furgit/object/id"
+	"lindenii.org/go/lgo/intconv"
+)
+
+// ErrInvalidParameters reports that
+// the parameters supplied for a filter build
+// are not representable in the format.
+var ErrInvalidParameters = errors.New("internal/format/packidx/bloom: invalid parameters")
+
+// defaultK is the probe count used by [RecommendParams].
+//
+// With 512-bit buckets it keeps the false positive rate near one percent
+// at the target bucket load.
+const defaultK = 8
+
+// targetLoad is the object count per bucket that [RecommendParams] aims for.
+const targetLoad = 48
+
+// Builder accumulates object IDs into an in-memory Bloom filter
+// and serializes it.
+//
+// Labels: MT-Unsafe.
+type Builder struct {
+	// data is the full filter file, header included.
+	data []byte
+
+	// buckets aliases the bucket region of data, after the header.
+	buckets []byte
+
+	log2B    uint
+	k        int
+	hashSize int
+}
+
+// NewBuilder creates a filter builder
+// for bucketCount buckets and k probes per object ID.
+//
+// bucketCount must be a nonzero power of two,
+// k must be nonzero,
+// and log2(bucketCount) + 9*k must not exceed the hash length in bits.
+func NewBuilder(objectFormat id.ObjectFormat, bucketCount uint32, k uint16) (*Builder, error) {
+	hashID, err := hashFunctionID(objectFormat)
+	if err != nil {
+		return nil, err
+	}
+
+	hashSize := objectFormat.Size()
+
+	log2B, err := checkParams(bucketCount, k, hashSize)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrInvalidParameters, err)
+	}
+
+	total, err := intconv.Uint64ToInt(uint64(HeaderLen) + uint64(BucketLen)*uint64(bucketCount))
+	if err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrInvalidParameters, err)
+	}
+
+	data := make([]byte, total)
+	binary.BigEndian.PutUint32(data[0:], signature)
+	binary.BigEndian.PutUint32(data[4:], version)
+	binary.BigEndian.PutUint32(data[8:], hashID)
+	binary.BigEndian.PutUint32(data[12:], bucketCount)
+	binary.BigEndian.PutUint16(data[16:], k)
+
+	return &Builder{
+		data:     data,
+		buckets:  data[HeaderLen:],
+		log2B:    log2B,
+		k:        int(k),
+		hashSize: hashSize,
+	}, nil
+}
+
+// Add records oid in the filter.
+//
+// oid must be exactly the filter's hash size;
+// Add panics otherwise.
+func (b *Builder) Add(oid []byte) {
+	if len(oid) != b.hashSize {
+		panic("internal/format/packidx/bloom: invalid object ID length")
+	}
+
+	base := int(binary.BigEndian.Uint32(oid[:4])>>(32-b.log2B)) * BucketLen
+
+	for i := range b.k {
+		word, mask := probe(oid, b.log2B, i)
+
+		off := base + word*8
+		set := binary.BigEndian.Uint64(b.buckets[off:]) | mask
+		binary.BigEndian.PutUint64(b.buckets[off:], set)
+	}
+}
+
+// Bytes returns the serialized filter.
+//
+// Labels: Life-Parent, Mut-No.
+func (b *Builder) Bytes() []byte {
+	return b.data
+}
+
+// WriteTo writes the serialized filter to w.
+func (b *Builder) WriteTo(w io.Writer) (int64, error) {
+	n, err := w.Write(b.data)
+	if err != nil {
+		return int64(n), fmt.Errorf("internal/format/packidx/bloom: %w", err)
+	}
+
+	return int64(n), nil
+}
+
+// RecommendParams returns filter parameters for an index of n objects,
+// targeting a false positive rate near one percent.
+func RecommendParams(objectFormat id.ObjectFormat, n int) (bucketCount uint32, k uint16, err error) {
+	hashSize := objectFormat.Size()
+	if hashSize == 0 {
+		return 0, 0, id.ErrInvalidObjectFormat
+	}
+
+	const maxPow2 = uint32(1) << 31
+
+	wanted := uint64(0)
+	if n > 0 {
+		wanted = (uint64(n) + targetLoad - 1) / targetLoad
+	}
+
+	switch {
+	case wanted <= 1:
+		bucketCount = 1
+	case wanted > uint64(maxPow2):
+		bucketCount = maxPow2
+	default:
+		bucketCount = uint32(1) << bits.Len64(wanted-1)
+	}
+
+	_, err = checkParams(bucketCount, defaultK, hashSize)
+	if err != nil {
+		return 0, 0, fmt.Errorf("%w: %w", ErrInvalidParameters, err)
+	}
+
+	return bucketCount, defaultK, nil
+}