diff --git a/pkg/obikmer/kmer_set_builder.go b/pkg/obikmer/kmer_set_builder.go
index 10b0bf0..565eaf5 100644
--- a/pkg/obikmer/kmer_set_builder.go
+++ b/pkg/obikmer/kmer_set_builder.go
@@ -16,7 +16,9 @@ import (
 type BuilderOption func(*builderConfig)
 
 type builderConfig struct {
-	minFreq int // 0 means no frequency filtering (simple dedup)
+	minFreq      int // 0 means no frequency filtering (simple dedup)
+	maxFreq      int // 0 means no upper bound
+	saveFreqTopN int // >0 means save the N most frequent k-mers per set to CSV
 }
 
 // WithMinFrequency activates frequency filtering mode.
@@ -27,6 +29,22 @@ func WithMinFrequency(minFreq int) BuilderOption {
 	}
 }
 
+// WithMaxFrequency sets the upper frequency bound.
+// Only k-mers seen <= maxFreq times are kept in the final index.
+func WithMaxFrequency(maxFreq int) BuilderOption {
+	return func(c *builderConfig) {
+		c.maxFreq = maxFreq
+	}
+}
+
+// WithSaveFreqKmers saves the N most frequent k-mers per set to a CSV file
+// (top_kmers.csv in each set directory).
+func WithSaveFreqKmers(n int) BuilderOption {
+	return func(c *builderConfig) {
+		c.saveFreqTopN = n
+	}
+}
+
 // KmerSetGroupBuilder constructs a KmerSetGroup on disk.
 // During construction, super-kmers are written to temporary .skm files
 // partitioned by minimizer. On Close(), each partition is finalized
@@ -283,8 +301,17 @@ func (b *KmerSetGroupBuilder) Close() (*KmerSetGroup, error) {
 
 	// Process partitions in parallel
 	counts := make([][]uint64, b.n)
+	spectra := make([][]map[int]uint64, b.n)
+	var topKmers [][]*TopNKmers
 	for s := 0; s < b.n; s++ {
 		counts[s] = make([]uint64, b.P)
+		spectra[s] = make([]map[int]uint64, b.P)
+	}
+	if b.config.saveFreqTopN > 0 {
+		topKmers = make([][]*TopNKmers, b.n)
+		for s := 0; s < b.n; s++ {
+			topKmers[s] = make([]*TopNKmers, b.P)
+		}
 	}
 
 	nWorkers := runtime.NumCPU()
@@ -307,13 +334,18 @@ func (b *KmerSetGroupBuilder) Close() (*KmerSetGroup, error) {
 		go func() {
 			defer wg.Done()
 			for j := range jobs {
-				if err := b.finalizePartition(j.setIdx, j.partIdx, &counts[j.setIdx][j.partIdx]); err != nil {
+				partSpec, partTop, err := b.finalizePartition(j.setIdx, j.partIdx, &counts[j.setIdx][j.partIdx])
+				if err != nil {
 					errMu.Lock()
 					if firstErr == nil {
 						firstErr = err
 					}
 					errMu.Unlock()
 				}
+				spectra[j.setIdx][j.partIdx] = partSpec
+				if topKmers != nil {
+					topKmers[j.setIdx][j.partIdx] = partTop
+				}
 			}
 		}()
 	}
@@ -330,6 +362,41 @@ func (b *KmerSetGroupBuilder) Close() (*KmerSetGroup, error) {
 		return nil, firstErr
 	}
 
+	// Aggregate per-partition spectra into per-set spectra and write spectrum.bin
+	for s := 0; s < b.n; s++ {
+		globalIdx := b.startIndex + s
+		setSpectrum := make(map[int]uint64)
+		for p := 0; p < b.P; p++ {
+			if spectra[s][p] != nil {
+				MergeSpectraMaps(setSpectrum, spectra[s][p])
+			}
+		}
+		if len(setSpectrum) > 0 {
+			specPath := filepath.Join(b.dir, fmt.Sprintf("set_%d", globalIdx), "spectrum.bin")
+			if err := WriteSpectrum(specPath, MapToSpectrum(setSpectrum)); err != nil {
+				return nil, fmt.Errorf("obikmer: write spectrum set=%d: %w", globalIdx, err)
+			}
+		}
+	}
+
+	// Aggregate per-partition top-N k-mers and write CSV
+	if topKmers != nil {
+		for s := 0; s < b.n; s++ {
+			globalIdx := b.startIndex + s
+			merged := NewTopNKmers(b.config.saveFreqTopN)
+			for p := 0; p < b.P; p++ {
+				merged.MergeTopN(topKmers[s][p])
+			}
+			results := merged.Results()
+			if len(results) > 0 {
+				csvPath := filepath.Join(b.dir, fmt.Sprintf("set_%d", globalIdx), "top_kmers.csv")
+				if err := WriteTopKmersCSV(csvPath, results, b.k); err != nil {
+					return nil, fmt.Errorf("obikmer: write top kmers set=%d: %w", globalIdx, err)
+				}
+			}
+		}
+	}
+
 	// 3. Build KmerSetGroup and write metadata
 	newCounts := make([]uint64, b.n)
 	for s := 0; s < b.n; s++ {
@@ -383,8 +450,10 @@ func (b *KmerSetGroupBuilder) Close() (*KmerSetGroup, error) {
 }
 
 // finalizePartition processes a single partition: load SKM, extract k-mers,
-// sort, dedup/count, write KDI.
-func (b *KmerSetGroupBuilder) finalizePartition(setIdx, partIdx int, count *uint64) error {
+// sort, dedup/count, write KDI. Returns a partial frequency spectrum
+// (frequency → count of distinct k-mers) computed before filtering,
+// and optionally the top-N most frequent k-mers.
+func (b *KmerSetGroupBuilder) finalizePartition(setIdx, partIdx int, count *uint64) (map[int]uint64, *TopNKmers, error) {
 	// setIdx is local (0..n-1); build dirs use local index, output dirs use global
 	skmPath := filepath.Join(b.dir, ".build",
 		fmt.Sprintf("set_%d", setIdx),
@@ -399,7 +468,7 @@ func (b *KmerSetGroupBuilder) finalizePartition(setIdx, partIdx int, count *uint
 	reader, err := NewSkmReader(skmPath)
 	if err != nil {
 		// If file doesn't exist or is empty, write empty KDI
-		return b.writeEmptyKdi(kdiPath, count)
+		return nil, nil, b.writeEmptyKdi(kdiPath, count)
 	}
 
 	var kmers []uint64
@@ -415,7 +484,7 @@ func (b *KmerSetGroupBuilder) finalizePartition(setIdx, partIdx int, count *uint
 	reader.Close()
 
 	if len(kmers) == 0 {
-		return b.writeEmptyKdi(kdiPath, count)
+		return nil, nil, b.writeEmptyKdi(kdiPath, count)
 	}
 
 	// Sort
@@ -424,15 +493,23 @@ func (b *KmerSetGroupBuilder) finalizePartition(setIdx, partIdx int, count *uint
 	// Write KDI based on mode
 	w, err := NewKdiWriter(kdiPath)
 	if err != nil {
-		return err
+		return nil, nil, err
 	}
 
 	minFreq := b.config.minFreq
 	if minFreq <= 0 {
 		minFreq = 1 // simple dedup
 	}
+	maxFreq := b.config.maxFreq // 0 means no upper bound
 
-	// Linear scan: count consecutive identical values
+	// Prepare top-N collector if requested
+	var topN *TopNKmers
+	if b.config.saveFreqTopN > 0 {
+		topN = NewTopNKmers(b.config.saveFreqTopN)
+	}
+
+	// Linear scan: count consecutive identical values and accumulate spectrum
+	partSpectrum := make(map[int]uint64)
 	i := 0
 	for i < len(kmers) {
 		val := kmers[i]
@@ -440,17 +517,21 @@ func (b *KmerSetGroupBuilder) finalizePartition(setIdx, partIdx int, count *uint
 		for i+c < len(kmers) && kmers[i+c] == val {
 			c++
 		}
-		if c >= minFreq {
+		partSpectrum[c]++
+		if topN != nil {
+			topN.Add(val, c)
+		}
+		if c >= minFreq && (maxFreq <= 0 || c <= maxFreq) {
 			if err := w.Write(val); err != nil {
 				w.Close()
-				return err
+				return nil, nil, err
 			}
 		}
 		i += c
 	}
 
 	*count = w.Count()
-	return w.Close()
+	return partSpectrum, topN, w.Close()
 }
 
 func (b *KmerSetGroupBuilder) writeEmptyKdi(path string, count *uint64) error {
diff --git a/pkg/obikmer/kmer_set_disk.go b/pkg/obikmer/kmer_set_disk.go
index 6d8f7e2..9613ebe 100644
--- a/pkg/obikmer/kmer_set_disk.go
+++ b/pkg/obikmer/kmer_set_disk.go
@@ -706,6 +706,21 @@ func (ksg *KmerSetGroup) PartitionPath(setIndex, partIndex int) string {
 	return ksg.partitionPath(setIndex, partIndex)
 }
 
+// SpectrumPath returns the path to the spectrum.bin file for the given set.
+func (ksg *KmerSetGroup) SpectrumPath(setIndex int) string {
+	return filepath.Join(ksg.path, fmt.Sprintf("set_%d", setIndex), "spectrum.bin")
+}
+
+// Spectrum reads the k-mer frequency spectrum for the given set.
+// Returns nil, nil if no spectrum file exists.
+func (ksg *KmerSetGroup) Spectrum(setIndex int) (*KmerSpectrum, error) {
+	path := ksg.SpectrumPath(setIndex)
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		return nil, nil
+	}
+	return ReadSpectrum(path)
+}
+
 // IsCompatibleWith returns true if the other group has the same k, m, and partitions.
 func (ksg *KmerSetGroup) IsCompatibleWith(other *KmerSetGroup) bool {
 	return ksg.k == other.k && ksg.m == other.m && ksg.partitions == other.partitions
@@ -847,6 +862,15 @@ func (ksg *KmerSetGroup) CopySetsByIDTo(ids []string, destDir string, force bool
 			}
 		}
 
+		// Copy spectrum.bin if it exists
+		srcSpecPath := ksg.SpectrumPath(srcIdx)
+		if _, err := os.Stat(srcSpecPath); err == nil {
+			destSpecPath := filepath.Join(destSetDir, "spectrum.bin")
+			if err := copyFile(srcSpecPath, destSpecPath); err != nil {
+				return nil, fmt.Errorf("obikmer: copy spectrum of set %q: %w", srcID, err)
+			}
+		}
+
 		// Update destination metadata
 		dest.setsIDs = append(dest.setsIDs, srcID)
 		dest.counts = append(dest.counts, ksg.counts[srcIdx])
diff --git a/pkg/obikmer/spectrum.go b/pkg/obikmer/spectrum.go
new file mode 100644
index 0000000..c5b5733
--- /dev/null
+++ b/pkg/obikmer/spectrum.go
@@ -0,0 +1,253 @@
+package obikmer
+
+import (
+	"bufio"
+	"container/heap"
+	"encoding/csv"
+	"fmt"
+	"os"
+	"sort"
+	"strconv"
+)
+
+// KSP file magic bytes: "KSP\x01" (K-mer SPectrum v1)
+var kspMagic = [4]byte{'K', 'S', 'P', 0x01}
+
+// SpectrumEntry represents one entry in a k-mer frequency spectrum.
+type SpectrumEntry struct {
+	Frequency int    // how many times a k-mer was observed
+	Count     uint64 // how many distinct k-mers have this frequency
+}
+
+// KmerSpectrum represents the frequency distribution of k-mers.
+// Entries are sorted by Frequency in ascending order and only include
+// non-zero counts.
+type KmerSpectrum struct {
+	Entries []SpectrumEntry
+}
+
+// MaxFrequency returns the highest frequency in the spectrum, or 0 if empty.
+func (s *KmerSpectrum) MaxFrequency() int {
+	if len(s.Entries) == 0 {
+		return 0
+	}
+	return s.Entries[len(s.Entries)-1].Frequency
+}
+
+// ToMap converts a KmerSpectrum back to a map for easy lookup.
+func (s *KmerSpectrum) ToMap() map[int]uint64 {
+	m := make(map[int]uint64, len(s.Entries))
+	for _, e := range s.Entries {
+		m[e.Frequency] = e.Count
+	}
+	return m
+}
+
+// MapToSpectrum converts a map[int]uint64 to a sorted KmerSpectrum.
+func MapToSpectrum(m map[int]uint64) *KmerSpectrum {
+	entries := make([]SpectrumEntry, 0, len(m))
+	for freq, count := range m {
+		if count > 0 {
+			entries = append(entries, SpectrumEntry{Frequency: freq, Count: count})
+		}
+	}
+	sort.Slice(entries, func(i, j int) bool {
+		return entries[i].Frequency < entries[j].Frequency
+	})
+	return &KmerSpectrum{Entries: entries}
+}
+
+// MergeSpectraMaps adds all entries from b into a.
+func MergeSpectraMaps(a, b map[int]uint64) {
+	for freq, count := range b {
+		a[freq] += count
+	}
+}
+
+// WriteSpectrum writes a KmerSpectrum to a binary file.
+//
+// Format:
+//
+//	[magic: 4 bytes "KSP\x01"]
+//	[n_entries: varint]
+//	For each entry (sorted by frequency ascending):
+//	  [frequency: varint]
+//	  [count: varint]
+func WriteSpectrum(path string, spectrum *KmerSpectrum) error {
+	f, err := os.Create(path)
+	if err != nil {
+		return fmt.Errorf("create spectrum file: %w", err)
+	}
+	w := bufio.NewWriterSize(f, 65536)
+
+	// Magic
+	if _, err := w.Write(kspMagic[:]); err != nil {
+		f.Close()
+		return err
+	}
+
+	// Number of entries
+	if _, err := EncodeVarint(w, uint64(len(spectrum.Entries))); err != nil {
+		f.Close()
+		return err
+	}
+
+	// Entries
+	for _, e := range spectrum.Entries {
+		if _, err := EncodeVarint(w, uint64(e.Frequency)); err != nil {
+			f.Close()
+			return err
+		}
+		if _, err := EncodeVarint(w, e.Count); err != nil {
+			f.Close()
+			return err
+		}
+	}
+
+	if err := w.Flush(); err != nil {
+		f.Close()
+		return err
+	}
+	return f.Close()
+}
+
+// ReadSpectrum reads a KmerSpectrum from a binary file.
+func ReadSpectrum(path string) (*KmerSpectrum, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	r := bufio.NewReaderSize(f, 65536)
+
+	// Check magic
+	var magic [4]byte
+	if _, err := r.Read(magic[:]); err != nil {
+		return nil, fmt.Errorf("read spectrum magic: %w", err)
+	}
+	if magic != kspMagic {
+		return nil, fmt.Errorf("invalid spectrum file magic: %v", magic)
+	}
+
+	// Number of entries
+	nEntries, err := DecodeVarint(r)
+	if err != nil {
+		return nil, fmt.Errorf("read spectrum entry count: %w", err)
+	}
+
+	entries := make([]SpectrumEntry, nEntries)
+	for i := uint64(0); i < nEntries; i++ {
+		freq, err := DecodeVarint(r)
+		if err != nil {
+			return nil, fmt.Errorf("read spectrum freq at entry %d: %w", i, err)
+		}
+		count, err := DecodeVarint(r)
+		if err != nil {
+			return nil, fmt.Errorf("read spectrum count at entry %d: %w", i, err)
+		}
+		entries[i] = SpectrumEntry{
+			Frequency: int(freq),
+			Count:     count,
+		}
+	}
+
+	return &KmerSpectrum{Entries: entries}, nil
+}
+
+// KmerFreq associates a k-mer (encoded as uint64) with its observed frequency.
+type KmerFreq struct {
+	Kmer uint64
+	Freq int
+}
+
+// kmerFreqHeap is a min-heap of KmerFreq ordered by Freq (lowest first).
+// Used to maintain a top-N most frequent k-mers set.
+type kmerFreqHeap []KmerFreq
+
+func (h kmerFreqHeap) Len() int            { return len(h) }
+func (h kmerFreqHeap) Less(i, j int) bool  { return h[i].Freq < h[j].Freq }
+func (h kmerFreqHeap) Swap(i, j int)       { h[i], h[j] = h[j], h[i] }
+func (h *kmerFreqHeap) Push(x interface{}) { *h = append(*h, x.(KmerFreq)) }
+func (h *kmerFreqHeap) Pop() interface{} {
+	old := *h
+	n := len(old)
+	x := old[n-1]
+	*h = old[:n-1]
+	return x
+}
+
+// TopNKmers maintains a collection of the N most frequent k-mers
+// using a min-heap. Thread-safe usage requires external synchronization.
+type TopNKmers struct {
+	n int
+	h kmerFreqHeap
+}
+
+// NewTopNKmers creates a new top-N collector.
+func NewTopNKmers(n int) *TopNKmers {
+	return &TopNKmers{
+		n: n,
+		h: make(kmerFreqHeap, 0, n+1),
+	}
+}
+
+// Add considers a k-mer with the given frequency for inclusion in the top-N.
+func (t *TopNKmers) Add(kmer uint64, freq int) {
+	if t.n <= 0 {
+		return
+	}
+	if len(t.h) < t.n {
+		heap.Push(&t.h, KmerFreq{Kmer: kmer, Freq: freq})
+	} else if freq > t.h[0].Freq {
+		t.h[0] = KmerFreq{Kmer: kmer, Freq: freq}
+		heap.Fix(&t.h, 0)
+	}
+}
+
+// Results returns the collected k-mers sorted by frequency descending.
+func (t *TopNKmers) Results() []KmerFreq {
+	result := make([]KmerFreq, len(t.h))
+	copy(result, t.h)
+	sort.Slice(result, func(i, j int) bool {
+		return result[i].Freq > result[j].Freq
+	})
+	return result
+}
+
+// MergeTopN merges another TopNKmers into this one.
+func (t *TopNKmers) MergeTopN(other *TopNKmers) {
+	if other == nil {
+		return
+	}
+	for _, kf := range other.h {
+		t.Add(kf.Kmer, kf.Freq)
+	}
+}
+
+// WriteTopKmersCSV writes the top k-mers to a CSV file.
+// Columns: sequence, frequency
+func WriteTopKmersCSV(path string, topKmers []KmerFreq, k int) error {
+	f, err := os.Create(path)
+	if err != nil {
+		return fmt.Errorf("create top-kmers file: %w", err)
+	}
+	defer f.Close()
+
+	w := csv.NewWriter(f)
+	defer w.Flush()
+
+	if err := w.Write([]string{"sequence", "frequency"}); err != nil {
+		return err
+	}
+
+	buf := make([]byte, k)
+	for _, kf := range topKmers {
+		seq := DecodeKmer(kf.Kmer, k, buf)
+		if err := w.Write([]string{string(seq), strconv.Itoa(kf.Freq)}); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
diff --git a/pkg/obitools/obik/index.go b/pkg/obitools/obik/index.go
index a1a3e03..cdce3e7 100644
--- a/pkg/obitools/obik/index.go
+++ b/pkg/obitools/obik/index.go
@@ -31,11 +31,19 @@ func runIndex(ctx context.Context, opt *getoptions.GetOpt, args []string) error
 		return fmt.Errorf("invalid min-occurrence: %d (must be >= 1)", minOcc)
 	}
 
+	maxOcc := CLIMaxOccurrence()
+
 	// Build options
 	var opts []obikmer.BuilderOption
 	if minOcc > 1 {
 		opts = append(opts, obikmer.WithMinFrequency(minOcc))
 	}
+	if maxOcc > 0 {
+		opts = append(opts, obikmer.WithMaxFrequency(maxOcc))
+	}
+	if topN := CLISaveFreqKmer(); topN > 0 {
+		opts = append(opts, obikmer.WithSaveFreqKmers(topN))
+	}
 
 	// Determine whether to append to existing group or create new
 	var builder *obikmer.KmerSetGroupBuilder
@@ -50,7 +58,11 @@ func runIndex(ctx context.Context, opt *getoptions.GetOpt, args []string) error
 		}
 	} else {
 		// New group
-		log.Infof("Creating new kmer index: k=%d, m=%d, min-occurrence=%d", k, m, minOcc)
+		if maxOcc > 0 {
+			log.Infof("Creating new kmer index: k=%d, m=%d, occurrence=[%d,%d]", k, m, minOcc, maxOcc)
+		} else {
+			log.Infof("Creating new kmer index: k=%d, m=%d, min-occurrence=%d", k, m, minOcc)
+		}
 		builder, err = obikmer.NewKmerSetGroupBuilder(outDir, k, m, 1, -1, opts...)
 		if err != nil {
 			return fmt.Errorf("failed to create kmer index builder: %w", err)
@@ -99,6 +111,9 @@ func runIndex(ctx context.Context, opt *getoptions.GetOpt, args []string) error
 	if minOcc > 1 {
 		ksg.SetAttribute("min_occurrence", minOcc)
 	}
+	if maxOcc > 0 {
+		ksg.SetAttribute("max_occurrence", maxOcc)
+	}
 
 	if err := ksg.SaveMetadata(); err != nil {
 		return fmt.Errorf("failed to save metadata: %w", err)
diff --git a/pkg/obitools/obik/obik.go b/pkg/obitools/obik/obik.go
index cdc792e..3b5c7bc 100644
--- a/pkg/obitools/obik/obik.go
+++ b/pkg/obitools/obik/obik.go
@@ -48,6 +48,12 @@ func OptionSet(opt *getoptions.GetOpt) {
 	SetSelectionOptionSet(rmCmd)
 	rmCmd.SetCommandFn(runRm)
 
+	// spectrum: output k-mer frequency spectrum as CSV
+	spectrumCmd := opt.NewCommand("spectrum", "Output k-mer frequency spectrum as CSV")
+	SetSelectionOptionSet(spectrumCmd)
+	obiconvert.OutputModeOptionSet(spectrumCmd, false)
+	spectrumCmd.SetCommandFn(runSpectrum)
+
 	// super: extract super k-mers from sequences
 	superCmd := opt.NewCommand("super", "Extract super k-mers from sequence files")
 	obiconvert.InputOptionSet(superCmd)
diff --git a/pkg/obitools/obik/options.go b/pkg/obitools/obik/options.go
index a7766dd..7a15863 100644
--- a/pkg/obitools/obik/options.go
+++ b/pkg/obitools/obik/options.go
@@ -38,7 +38,9 @@ var _indexId = ""
 var _metadataFormat = "toml"
 var _setTag = make(map[string]string, 0)
 var _minOccurrence = 1
+var _maxOccurrence = 0
 var _saveFullFilter = false
+var _saveFreqKmer = 0
 
 // KmerIndexOptionSet defines every option related to kmer index building.
 func KmerIndexOptionSet(options *getoptions.GetOpt) {
@@ -64,8 +66,14 @@ func KmerIndexOptionSet(options *getoptions.GetOpt) {
 	options.IntVar(&_minOccurrence, "min-occurrence", _minOccurrence,
 		options.Description("Minimum number of occurrences for a k-mer to be kept (default 1 = keep all)."))
 
+	options.IntVar(&_maxOccurrence, "max-occurrence", _maxOccurrence,
+		options.Description("Maximum number of occurrences for a k-mer to be kept (default 0 = no upper bound)."))
+
 	options.BoolVar(&_saveFullFilter, "save-full-filter", _saveFullFilter,
 		options.Description("When using --min-occurrence > 1, save the full frequency filter instead of just the filtered index."))
+
+	options.IntVar(&_saveFreqKmer, "save-freq-kmer", _saveFreqKmer,
+		options.Description("Save the N most frequent k-mers per set to a CSV file (top_kmers.csv)."))
 }
 
 // CLIKmerSize returns the k-mer size.
@@ -114,11 +122,21 @@ func CLIMinOccurrence() int {
 	return _minOccurrence
 }
 
+// CLIMaxOccurrence returns the maximum occurrence threshold (0 = no upper bound).
+func CLIMaxOccurrence() int {
+	return _maxOccurrence
+}
+
 // CLISaveFullFilter returns whether to save the full frequency filter.
 func CLISaveFullFilter() bool {
 	return _saveFullFilter
 }
 
+// CLISaveFreqKmer returns the number of top frequent k-mers to save (0 = disabled).
+func CLISaveFreqKmer() int {
+	return _saveFreqKmer
+}
+
 // CLIOutputDirectory returns the output directory path.
 func CLIOutputDirectory() string {
 	return obiconvert.CLIOutPutFileName()
diff --git a/pkg/obitools/obik/spectrum.go b/pkg/obitools/obik/spectrum.go
new file mode 100644
index 0000000..a226b4f
--- /dev/null
+++ b/pkg/obitools/obik/spectrum.go
@@ -0,0 +1,121 @@
+package obik
+
+import (
+	"context"
+	"encoding/csv"
+	"fmt"
+	"os"
+	"strconv"
+
+	log "github.com/sirupsen/logrus"
+
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obikmer"
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obitools/obiconvert"
+	"github.com/DavidGamba/go-getoptions"
+)
+
+// runSpectrum implements the "obik spectrum" subcommand.
+// It outputs k-mer frequency spectra as CSV with one column per set.
+func runSpectrum(ctx context.Context, opt *getoptions.GetOpt, args []string) error {
+	if len(args) < 1 {
+		return fmt.Errorf("usage: obik spectrum [options] <index_directory>")
+	}
+
+	ksg, err := obikmer.OpenKmerSetGroup(args[0])
+	if err != nil {
+		return fmt.Errorf("failed to open kmer index: %w", err)
+	}
+
+	// Determine which sets to include
+	patterns := CLISetPatterns()
+	var indices []int
+	if len(patterns) > 0 {
+		indices, err = ksg.MatchSetIDs(patterns)
+		if err != nil {
+			return fmt.Errorf("failed to match set patterns: %w", err)
+		}
+		if len(indices) == 0 {
+			return fmt.Errorf("no sets match the given patterns")
+		}
+	} else {
+		// All sets
+		indices = make([]int, ksg.Size())
+		for i := range indices {
+			indices[i] = i
+		}
+	}
+
+	// Read spectra for selected sets
+	spectraMaps := make([]map[int]uint64, len(indices))
+	maxFreq := 0
+	for i, idx := range indices {
+		spectrum, err := ksg.Spectrum(idx)
+		if err != nil {
+			return fmt.Errorf("failed to read spectrum for set %d: %w", idx, err)
+		}
+		if spectrum == nil {
+			log.Warnf("No spectrum data for set %d (%s)", idx, ksg.SetIDOf(idx))
+			spectraMaps[i] = make(map[int]uint64)
+			continue
+		}
+		spectraMaps[i] = spectrum.ToMap()
+		if mf := spectrum.MaxFrequency(); mf > maxFreq {
+			maxFreq = mf
+		}
+	}
+
+	if maxFreq == 0 {
+		return fmt.Errorf("no spectrum data found in any selected set")
+	}
+
+	// Determine output destination
+	outFile := obiconvert.CLIOutPutFileName()
+	var w *csv.Writer
+	if outFile == "" || outFile == "-" {
+		w = csv.NewWriter(os.Stdout)
+	} else {
+		f, err := os.Create(outFile)
+		if err != nil {
+			return fmt.Errorf("failed to create output file: %w", err)
+		}
+		defer f.Close()
+		w = csv.NewWriter(f)
+	}
+	defer w.Flush()
+
+	// Build header: frequency, set_id_1, set_id_2, ...
+	header := make([]string, 1+len(indices))
+	header[0] = "frequency"
+	for i, idx := range indices {
+		id := ksg.SetIDOf(idx)
+		if id == "" {
+			id = fmt.Sprintf("set_%d", idx)
+		}
+		header[i+1] = id
+	}
+	if err := w.Write(header); err != nil {
+		return err
+	}
+
+	// Write rows for each frequency from 1 to maxFreq
+	record := make([]string, 1+len(indices))
+	for freq := 1; freq <= maxFreq; freq++ {
+		record[0] = strconv.Itoa(freq)
+		hasData := false
+		for i := range indices {
+			count := spectraMaps[i][freq]
+			record[i+1] = strconv.FormatUint(count, 10)
+			if count > 0 {
+				hasData = true
+			}
+		}
+		// Only write rows where at least one set has a non-zero count
+		if hasData {
+			if err := w.Write(record); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}