Introduce new sequence matching logic and corpus handling

This commit introduces the core matching functionality, including parallel corpus loading, the `Corpus` data structure, and implementations for `matchNaive` and `matchApat`. It also defines new matching rules, variant handling, and comprehensive unit and integration tests to validate the new logic and parameter handling.
2026-11-09 09:24:41 +00:00 · 2026-11-03 22:35:51 +11:00
1 parent f75e774bd5
commit ad065f6853
@@ -250,3 +250,54 @@ suffisait. **Corrigé** (accord de l'utilisateur) : `globalLockerCounter` est un

 Suite : étape 3 (recherche approchée des mots stricts, <= e substitutions, deux brins, dans exemples
 et contre-exemples), puis paires et indices.
+
+## 11. Mode A, étape 3 : recherche approchée (2026-11-03)
+
+Code : `pkg/obiprimer/corpus.go` (`Corpus`, `LoadCorpus` : exemples + contre-exemples en mémoire, ordre
+d'entrée conservé, ignorés comptés seulement, comme ecoPrimers), `match.go` (types `Match`,
+`WordMatches`, `MatchParams`, harnais parallèle `matchCorpus` indépendant du nombre de workers),
+`match_naive.go` (oracle en Go pur), `match_apat.go` (variante `obiapat`). `FindStrictWords` travaille
+maintenant sur un `Corpus`.
+
+**Règle d'appariement** (celle d'ecoPrimers, figée dans `MatchParams`) : substitutions seules ; toute base
+hors A, C, G, T (U lu comme T) dans la séquence compte comme une erreur ; toutes les fenêtres à <= e
+erreurs sont rapportées, chevauchantes comprises ; fenêtres à cheval sur la jonction si circulaire.
+**`obiapat` ne suit pas cette règle de lui-même** : il lit les codes IUPAC dans la séquence (un N
+correspond à toutes les bases). La variante `obiapat` remplace donc les symboles ambigus par `z` (rien
+ne lui correspond), sur une copie, seulement pour les séquences qui en contiennent.
+
+Vérification de justesse :
+- oracle validé à la main sur 12 cas (brins, N, IUPAC, U, jonction, chevauchement, séquence trop courte) ;
+- `obiapat` == oracle position par position et nombre d'erreurs compris, 6 configurations (k = 12, 18,
+  25 ; e = 0 à 4 ; linéaire/circulaire ; séquences avec n, r, y, k, m ; mots plantés sur les deux brins
+  avec 0 à 3 substitutions et à la jonction) ;
+- **bout en bout** : pour chaque mot strict, nombre de séquences exemples touchées == celui que
+  ecoPrimers a journalisé (`e = 3`) : **1 207 mots Cyprinidae et 255 mots Teleostei, 0 différence**.
+  Les nombres d'ecoPrimers sont dans `testdata/ecoprimers_*.words.tsv` ; test opt-in
+  `TestMatchApatAgainstEcoPrimers` (`OBIPRIMERS_TUTORIAL`, `OBIPRIMERS_WORKERS=n` pour fixer les cœurs).
+  (Cyprinidae : 1 207 des 1 208 mots d'ecoPrimers, le mot en trop de la section 10 est exclu.)
+
+Mesures, `e = 3`, génomes circulaires, M3 Max :
+
+| Jeu | Mots x séquences | Matches | 16 workers | 1 worker | Mémoire ajoutée |
+|---|---|---|---|---|---|
+| Cyprinidae | 1 207 x 325 (5,4 Mb) | 416 386 | 3,8 s | non mesuré | 53 Mo |
+| Teleostei | 255 x 3 909 (65 Mb) | 999 407 | 10,7 s | **113 s** (293 Mb/s par passe mot-brin) | 151 Mo |
+
+ecoPrimers complet (non décomposé par étape) : 134 s et 350 s, 1,9 et 2,1 Go.
+
+Profil monothread (Teleostei, `go tool pprof`) : 74 % dans `runtime.cgocall` (le balayage C), 11 % en
+surcoût de transition cgo (`pthread_cond_signal`), tri/GC marginaux. Il y a 2 millions d'appels cgo
+(255 mots x 2 brins x 3 909 séquences).
+
+Variantes du protocole :
+1. `obiapat` mot par mot, parallélisé par séquence : **retenue comme référence de production** pour l'instant.
+2. Arrêt anticipé sous le quorum de 90 % : **écartée, mesuré inutile** (Teleostei : 255/255 mots
+   conservés ; Cyprinidae : 1 201/1 208). Le parallélisme par séquence est déjà dans la variante 1.
+3. Bit-parallèle multi-mots en Go (SIMD) : **non écrite**. Cible : les ~74 % de balayage C.
+4. Index de voisinage / Aho-Corasick : écartée pour e >= 2 par le calcul (graines de 4 à 5 bases,
+   ~250 000 candidats par graine et par mot sur 65 Mb) ; `obicorazick` reste pertinent pour e = 0.
+   Non mesurée.
+
+Décision en suspens : écrire la variante 3 maintenant, ou d'abord finir le pipeline (paires et indices)
+et profiler l'ensemble pour savoir si l'appariement domine encore.
@@ -0,0 +1,108 @@
+package obiprimer
+
+import (
+	"cmp"
+	"fmt"
+	"slices"
+	"sync"
+
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obidefault"
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiiter"
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiseq"
+)
+
+// Counts gives the number of sequences by role.
+type Counts struct {
+	Examples        int
+	CounterExamples int
+	Ignored         int
+}
+
+// Corpus is the set of sequences used by the primer search: the examples and
+// the counterexamples, in input order. Sequences with the Ignored role are not
+// kept (as in ecoPrimers), only counted.
+//
+// The primer search needs several passes over the sequences (words are
+// searched approximately in every one of them), so the corpus lives in memory.
+type Corpus struct {
+	Seqs    []*obiseq.BioSequence
+	Roles   []Role // Roles[i] is Example or CounterExample
+	Ignored int    // number of sequences dropped by the selection
+}
+
+// Counts returns the number of sequences by role.
+func (c *Corpus) Counts() Counts {
+	n := Counts{Ignored: c.Ignored}
+	for _, r := range c.Roles {
+		if r == Example {
+			n.Examples++
+		} else {
+			n.CounterExamples++
+		}
+	}
+	return n
+}
+
+type corpusEntry struct {
+	batch, index int
+	seq          *obiseq.BioSequence
+	role         Role
+}
+
+// LoadCorpus reads every sequence of seqs, once, and keeps those selected as
+// examples or counterexamples. The order of the input is preserved whatever
+// the number of reader goroutines.
+func LoadCorpus(seqs obiiter.IBioSequence, sel Selection) (*Corpus, error) {
+	var mu sync.Mutex
+	var entries []corpusEntry
+	ignored := 0
+
+	consumer := func(it obiiter.IBioSequence, wg *sync.WaitGroup) {
+		defer wg.Done()
+		var local []corpusEntry
+		localIgnored := 0
+		for it.Next() {
+			batch := it.Get()
+			for i, seq := range batch.Slice() {
+				if role := sel.Role(seq); role == Ignored {
+					localIgnored++
+				} else {
+					local = append(local, corpusEntry{batch.Order(), i, seq, role})
+				}
+			}
+		}
+		mu.Lock()
+		entries = append(entries, local...)
+		ignored += localIgnored
+		mu.Unlock()
+	}
+
+	var wg sync.WaitGroup
+	for i := 1; i < obidefault.ParallelWorkers(); i++ {
+		wg.Add(1)
+		go consumer(seqs.Split(), &wg)
+	}
+	wg.Add(1)
+	go consumer(seqs, &wg)
+	wg.Wait()
+
+	slices.SortFunc(entries, func(a, b corpusEntry) int {
+		if c := cmp.Compare(a.batch, b.batch); c != 0 {
+			return c
+		}
+		return cmp.Compare(a.index, b.index)
+	})
+
+	c := &Corpus{
+		Seqs:    make([]*obiseq.BioSequence, len(entries)),
+		Roles:   make([]Role, len(entries)),
+		Ignored: ignored,
+	}
+	for i, e := range entries {
+		c.Seqs[i], c.Roles[i] = e.seq, e.role
+	}
+	if c.Counts().Examples == 0 {
+		return nil, fmt.Errorf("obiprimer: no example sequence selected")
+	}
+	return c, nil
+}
@@ -0,0 +1,156 @@
+package obiprimer
+
+import (
+	"cmp"
+	"fmt"
+	"slices"
+	"sync"
+
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obidefault"
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obikmer"
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiseq"
+)
+
+// Match is one approximate occurrence of a word in a corpus sequence.
+//
+// Pos is the 0-based start of the window on the sequence as given (forward
+// coordinates). When Reverse is false the window reads like the word; when it
+// is true the window reads like the reverse complement of the word, i.e. the
+// word occurs on the other strand. Errors is the number of substitutions.
+type Match struct {
+	Seq     uint32 // index in the Corpus
+	Pos     uint32
+	Errors  uint8
+	Reverse bool
+}
+
+// WordMatches gathers every occurrence of one canonical word, sorted by
+// (Seq, Pos, Reverse).
+type WordMatches struct {
+	Word    uint64
+	Matches []Match
+}
+
+// MatchParams parameterizes the approximate search.
+//
+// Rules, those of ecoPrimers: only substitutions are allowed; a base other
+// than A, C, G, T (or U, read as T) in the sequence never matches, it counts as
+// one error; every window with at most MaxErrors errors is reported, windows
+// overlapping each other included; on circular sequences windows may span the
+// junction.
+type MatchParams struct {
+	K         int
+	MaxErrors int
+	Circular  bool
+}
+
+func (p MatchParams) validate() error {
+	if p.K < 2 || p.K > 31 {
+		return fmt.Errorf("obiprimer: word size must be between 2 and 31, got %d", p.K)
+	}
+	if p.MaxErrors < 0 || p.MaxErrors >= p.K {
+		return fmt.Errorf("obiprimer: error count must be in [0,%d[, got %d", p.K, p.MaxErrors)
+	}
+	return nil
+}
+
+// hit is a Match before its sequence index is known to be final.
+type hit struct {
+	word int
+	m    Match
+}
+
+// scanFunc finds every occurrence of every word, on both strands, in one
+// sequence and reports them through emit.
+type scanFunc func(seq *obiseq.BioSequence, emit func(word, pos, errors int, reverse bool))
+
+// scannerMaker returns an independent scanFunc; each worker gets its own so
+// that scanners can keep private buffers.
+type scannerMaker func() scanFunc
+
+// wordStrings returns, for each canonical word, the word and its reverse
+// complement as lowercase strings.
+func wordStrings(words []uint64, k int) (fwd, rev []string) {
+	fwd = make([]string, len(words))
+	rev = make([]string, len(words))
+	for i, w := range words {
+		fwd[i] = string(obikmer.DecodeKmer(w, k, nil))
+		rev[i] = string(obikmer.DecodeKmer(obikmer.ReverseComplement(w, k), k, nil))
+	}
+	return
+}
+
+// matchCorpus runs the scanners over every sequence of the corpus in parallel
+// and assembles the per-word results. The output does not depend on the number
+// of workers.
+func matchCorpus(c *Corpus, words []uint64, mk scannerMaker) []WordMatches {
+	perSeq := make([][]hit, len(c.Seqs))
+
+	jobs := make(chan int)
+	var wg sync.WaitGroup
+	for range obidefault.ParallelWorkers() {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			scan := mk()
+			for i := range jobs {
+				var hits []hit
+				scan(c.Seqs[i], func(word, pos, errors int, reverse bool) {
+					hits = append(hits, hit{word, Match{uint32(i), uint32(pos), uint8(errors), reverse}})
+				})
+				perSeq[i] = hits
+			}
+		}()
+	}
+	for i := range c.Seqs {
+		jobs <- i
+	}
+	close(jobs)
+	wg.Wait()
+
+	res := make([]WordMatches, len(words))
+	for i, w := range words {
+		res[i].Word = w
+	}
+	for _, hits := range perSeq {
+		for _, h := range hits {
+			res[h.word].Matches = append(res[h.word].Matches, h.m)
+		}
+	}
+	for i := range res {
+		slices.SortFunc(res[i].Matches, func(a, b Match) int {
+			if c := cmp.Compare(a.Seq, b.Seq); c != 0 {
+				return c
+			}
+			if c := cmp.Compare(a.Pos, b.Pos); c != 0 {
+				return c
+			}
+			if a.Reverse != b.Reverse {
+				if !a.Reverse {
+					return -1
+				}
+				return 1
+			}
+			return 0
+		})
+	}
+	return res
+}
+
+// SequencesMatched returns, for the matches of one word, the number of
+// distinct example and counterexample sequences it occurs in.
+func (w WordMatches) SequencesMatched(c *Corpus) (examples, counterExamples int) {
+	last := ^uint32(0)
+	for _, m := range w.Matches {
+		if m.Seq == last {
+			continue
+		}
+		last = m.Seq
+		if c.Roles[m.Seq] == Example {
+			examples++
+		} else {
+			counterExamples++
+		}
+	}
+	return
+}
@@ -0,0 +1,87 @@
+package obiprimer
+
+import (
+	"fmt"
+
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiapat"
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiseq"
+)
+
+// apatSafe returns seq when it only holds a, c, g, t; otherwise a copy where u
+// becomes t and every other symbol becomes z. obiapat reads IUPAC codes in the
+// sequence (an n matches every base); z is the symbol it matches with nothing,
+// which gives back the ecoPrimers rule "an ambiguous base is an error".
+func apatSafe(seq *obiseq.BioSequence) *obiseq.BioSequence {
+	raw := seq.Sequence()
+	clean := true
+	for _, b := range raw {
+		if b != 'a' && b != 'c' && b != 'g' && b != 't' {
+			clean = false
+			break
+		}
+	}
+	if clean {
+		return seq
+	}
+	out := make([]byte, len(raw))
+	for i, b := range raw {
+		switch b {
+		case 'a', 'c', 'g', 't':
+			out[i] = b
+		case 'A', 'C', 'G', 'T':
+			out[i] = b + 'a' - 'A'
+		case 'u', 'U':
+			out[i] = 't'
+		default:
+			out[i] = 'z'
+		}
+	}
+	return obiseq.NewBioSequence(seq.Id(), out, "")
+}
+
+// matchApat searches the words with obiapat, the bit-parallel Baeza-Yates /
+// Manber matcher of OBITools (the engine of ecoPrimers and obipcr).
+func matchApat(c *Corpus, words []uint64, p MatchParams) ([]WordMatches, error) {
+	if err := p.validate(); err != nil {
+		return nil, err
+	}
+	fwd, rev := wordStrings(words, p.K)
+	pats := make([][2]obiapat.ApatPattern, len(words))
+	for i := range words {
+		var err error
+		if pats[i][0], err = obiapat.MakeApatPattern(fwd[i], p.MaxErrors, false); err != nil {
+			return nil, fmt.Errorf("obiprimer: pattern %s: %w", fwd[i], err)
+		}
+		if pats[i][1], err = obiapat.MakeApatPattern(rev[i], p.MaxErrors, false); err != nil {
+			return nil, fmt.Errorf("obiprimer: pattern %s: %w", rev[i], err)
+		}
+	}
+
+	mk := func() scanFunc {
+		var recycle []obiapat.ApatSequence
+		return func(seq *obiseq.BioSequence, emit func(word, pos, errors int, reverse bool)) {
+			n := seq.Len()
+			if n < p.K {
+				return
+			}
+			aseq, err := obiapat.MakeApatSequence(apatSafe(seq), p.Circular, recycle...)
+			if err != nil {
+				panic(fmt.Sprintf("obiprimer: cannot encode sequence %s: %v", seq.Id(), err))
+			}
+			recycle = append(recycle[:0], aseq)
+
+			for w := range pats {
+				for strand := range 2 {
+					for _, m := range pats[w][strand].FindAllIndex(aseq, 0, -1) {
+						// On circular sequences obiapat also scans a copy of the
+						// start: those hits repeat the ones at start-n.
+						if m[0] >= 0 && m[0] < n {
+							emit(w, m[0], m[2], strand == 1)
+						}
+					}
+				}
+			}
+		}
+	}
+	return matchCorpus(c, words, mk), nil
+}
@@ -0,0 +1,78 @@
+package obiprimer
+
+import "git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiseq"
+
+// baseCode maps a nucleotide to 0..3 (A,C,G,T; U is T) and everything else to 4,
+// which never equals a word base.
+var baseCode = func() (t [256]byte) {
+	for i := range t {
+		t[i] = 4
+	}
+	for i, c := range "ACGT" {
+		t[c], t[c+'a'-'A'] = byte(i), byte(i)
+	}
+	t['U'], t['u'] = 3, 3
+	return
+}()
+
+func codesOf(s string) []byte {
+	out := make([]byte, len(s))
+	for i := range out {
+		out[i] = baseCode[s[i]]
+	}
+	return out
+}
+
+// matchNaive is the reference implementation: every window of every sequence
+// is compared with every word, base by base. It is slow and exists to define
+// the exact semantics (see MatchParams) that faster implementations must
+// reproduce.
+func matchNaive(c *Corpus, words []uint64, p MatchParams) ([]WordMatches, error) {
+	if err := p.validate(); err != nil {
+		return nil, err
+	}
+	fwd, rev := wordStrings(words, p.K)
+	fc, rc := make([][]byte, len(words)), make([][]byte, len(words))
+	for i := range words {
+		fc[i], rc[i] = codesOf(fwd[i]), codesOf(rev[i])
+	}
+
+	mk := func() scanFunc {
+		var codes []byte
+		return func(seq *obiseq.BioSequence, emit func(word, pos, errors int, reverse bool)) {
+			raw := seq.Sequence()
+			n := len(raw)
+			if n < p.K {
+				return
+			}
+			codes = codes[:0]
+			for _, b := range raw {
+				codes = append(codes, baseCode[b])
+			}
+			windows := n - p.K + 1
+			if p.Circular {
+				codes = append(codes, codes[:p.K-1]...)
+				windows = n
+			}
+			for pos := range windows {
+				win := codes[pos : pos+p.K]
+				for w := range words {
+					for strand, pat := range [2][]byte{fc[w], rc[w]} {
+						errs := 0
+						for j, b := range pat {
+							if win[j] != b {
+								if errs++; errs > p.MaxErrors {
+									break
+								}
+							}
+						}
+						if errs <= p.MaxErrors {
+							emit(w, pos, errs, strand == 1)
+						}
+					}
+				}
+			}
+		}
+	}
+	return matchCorpus(c, words, mk), nil
+}
@@ -0,0 +1,285 @@
+package obiprimer
+
+import (
+	"bufio"
+	"fmt"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obidefault"
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiformats"
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obikmer"
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiseq"
+)
+
+func corpusOf(roles []Role, seqs ...string) *Corpus {
+	c := &Corpus{Roles: roles}
+	for i, s := range seqs {
+		c.Seqs = append(c.Seqs, obiseq.NewBioSequence(fmt.Sprintf("s%d", i), []byte(s), ""))
+	}
+	return c
+}
+
+func canon(word string) uint64 { return obikmer.EncodeCanonicalKmer([]byte(word), len(word)) }
+
+// The reference implementation is checked by hand first: everything else is
+// compared with it.
+func TestNaiveMatchSemantics(t *testing.T) {
+	const word = "AACCGGTA" // not a palindrome; reverse complement TACCGGTT
+	cases := []struct {
+		name     string
+		seq      string
+		circular bool
+		e        int
+		want     []Match
+	}{
+		{"exact forward", "ggaaccggtacc", false, 0, []Match{{0, 2, 0, false}}},
+		{"exact reverse strand", "ggtaccggttcc", false, 0, []Match{{0, 2, 0, true}}},
+		{"one error is not found at e=0", "ggaaccgctacc", false, 0, nil},
+		{"one error found at e=1 with its count", "ggaaccgctacc", false, 1, []Match{{0, 2, 1, false}}},
+		{"an N is an error", "ggaaccgntacc", false, 1, []Match{{0, 2, 1, false}}},
+		{"an N is never a match at e=0", "ggaaccgntacc", false, 0, nil},
+		{"an IUPAC code is an error, not a wildcard", "ggaaccgrtacc", false, 0, nil},
+		{"U reads as T", "ggaaccgguacc", false, 0, []Match{{0, 2, 0, false}}},
+		{"junction ignored on a linear sequence", "ccggtagggaa", false, 0, nil},
+		{"junction found on a circular sequence", "ccggtagggaa", true, 0, []Match{{0, 9, 0, false}}},
+		{"overlapping windows are all reported", "aaccggtaaccggta", false, 0, []Match{{0, 0, 0, false}, {0, 7, 0, false}}},
+		{"too short", "aaccgg", true, 3, nil},
+	}
+	for _, tc := range cases {
+		c := corpusOf([]Role{Example}, tc.seq)
+		got, err := matchNaive(c, []uint64{canon(word)}, MatchParams{K: len(word), MaxErrors: tc.e, Circular: tc.circular})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !reflect.DeepEqual(got[0].Matches, tc.want) {
+			t.Errorf("%s: got %v, want %v", tc.name, got[0].Matches, tc.want)
+		}
+	}
+}
+
+func TestMatchParamsValidation(t *testing.T) {
+	c := corpusOf([]Role{Example}, "acgtacgtacgtacgt")
+	for _, p := range []MatchParams{{K: 1}, {K: 32}, {K: 8, MaxErrors: -1}, {K: 8, MaxErrors: 8}} {
+		if _, err := matchNaive(c, nil, p); err == nil {
+			t.Errorf("%+v must be rejected", p)
+		}
+		if _, err := matchApat(c, nil, p); err == nil {
+			t.Errorf("%+v must be rejected by obiapat variant", p)
+		}
+	}
+}
+
+func randSeq(rng *rand.Rand, n int, alphabet string) []byte {
+	b := make([]byte, n)
+	for i := range b {
+		b[i] = alphabet[rng.Intn(len(alphabet))]
+	}
+	return b
+}
+
+func mutate(rng *rand.Rand, s []byte, n int) []byte {
+	out := append([]byte(nil), s...)
+	for _, i := range rng.Perm(len(out))[:n] {
+		out[i] = "acgt"[rng.Intn(4)]
+	}
+	return out
+}
+
+// randomCorpus builds sequences with words planted on both strands, with 0..3
+// substitutions, across the circular junction, and with ambiguity codes.
+func randomCorpus(rng *rand.Rand, words []string, nseq int) *Corpus {
+	c := &Corpus{}
+	for i := 0; i < nseq; i++ {
+		alphabet := "acgt"
+		if i%3 == 0 {
+			alphabet = "acgtacgtacgtacgtacgtnnrykm"
+		}
+		s := randSeq(rng, 30+rng.Intn(500), alphabet)
+		for range 1 + rng.Intn(4) {
+			w := []byte(words[rng.Intn(len(words))])
+			if rng.Intn(2) == 0 {
+				w = []byte(revcomp(string(w)))
+			}
+			w = mutate(rng, []byte(string(w)), rng.Intn(4))
+			for j := range w {
+				w[j] += 'a' - 'A'
+			}
+			at := rng.Intn(len(s) - len(w) + 1)
+			if rng.Intn(5) == 0 { // across the junction
+				s = append(s[:0:0], append(append([]byte(nil), w[len(w)/2:]...), append(s[:len(s)-len(w)/2], w[:len(w)/2]...)...)...)
+				continue
+			}
+			copy(s[at:], w)
+		}
+		c.Seqs = append(c.Seqs, obiseq.NewBioSequence(fmt.Sprintf("r%d", i), s, ""))
+		c.Roles = append(c.Roles, Example)
+	}
+	return c
+}
+
+// Every implementation must return exactly what the reference returns.
+func TestMatchersAgreeWithReference(t *testing.T) {
+	for _, tc := range []struct {
+		k, e     int
+		circular bool
+	}{{18, 0, false}, {18, 1, true}, {18, 3, false}, {18, 3, true}, {12, 2, true}, {25, 4, false}} {
+		t.Run(fmt.Sprintf("k%d-e%d-circ%v", tc.k, tc.e, tc.circular), func(t *testing.T) {
+			rng := rand.New(rand.NewSource(int64(tc.k*100 + tc.e)))
+			var wordStr []string
+			var words []uint64
+			for range 7 {
+				w := string(randSeq(rng, tc.k, "ACGT"))
+				wordStr = append(wordStr, w)
+				words = append(words, canon(w))
+			}
+			c := randomCorpus(rng, wordStr, 40)
+			p := MatchParams{K: tc.k, MaxErrors: tc.e, Circular: tc.circular}
+
+			want, err := matchNaive(c, words, p)
+			if err != nil {
+				t.Fatal(err)
+			}
+			total := 0
+			for _, w := range want {
+				total += len(w.Matches)
+			}
+			if total < 10 {
+				t.Fatalf("test data too poor: only %d reference matches", total)
+			}
+
+			got, err := matchApat(c, words, p)
+			if err != nil {
+				t.Fatal(err)
+			}
+			for i := range want {
+				if !reflect.DeepEqual(got[i].Matches, want[i].Matches) {
+					t.Fatalf("word %d: obiapat gives %v\nreference gives %v", i, got[i].Matches, want[i].Matches)
+				}
+			}
+		})
+	}
+}
+
+func TestSequencesMatched(t *testing.T) {
+	c := corpusOf([]Role{Example, Example, CounterExample, CounterExample}, "a", "a", "a", "a")
+	w := WordMatches{Matches: []Match{{0, 1, 0, false}, {0, 9, 0, true}, {1, 3, 0, false}, {3, 0, 0, false}}}
+	if e, x := w.SequencesMatched(c); e != 2 || x != 1 {
+		t.Errorf("got %d examples, %d counterexamples", e, x)
+	}
+}
+
+func readEcoCounts(t *testing.T, path string) map[string]int {
+	t.Helper()
+	f, err := os.Open(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	counts := map[string]int{}
+	sc := bufio.NewScanner(f)
+	for sc.Scan() {
+		var w string
+		var in, out int
+		if _, err := fmt.Sscanf(sc.Text(), "%s\t%d\t%d", &w, &in, &out); err != nil {
+			t.Fatal(err)
+		}
+		counts[w] = in
+	}
+	return counts
+}
+
+// Opt-in comparison with ecoPrimers on the tutorial data (same variable as the
+// strict word test). For every strict word, ecoPrimers logs the number of
+// example sequences in which the fuzzy search (-e 3, both strands, circular)
+// finds it; testdata/ecoprimers_*.words.tsv holds those numbers.
+func TestMatchApatAgainstEcoPrimers(t *testing.T) {
+	dir := os.Getenv("OBIPRIMERS_TUTORIAL")
+	if dir == "" {
+		t.Skip("OBIPRIMERS_TUTORIAL not set")
+	}
+	if v := os.Getenv("OBIPRIMERS_WORKERS"); v != "" { // e.g. 1, to time a single core
+		n, err := strconv.Atoi(v)
+		if err != nil {
+			t.Fatal(err)
+		}
+		obidefault.SetMaxCPU(n)
+	}
+	taxonomy, err := obiformats.LoadTaxonomy(filepath.Join(dir, "ncbitaxo_20250211.tgz"), false, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, tc := range []struct{ name, taxid string }{{"Cyprinidae", "7953"}, {"Teleostei", "32443"}} {
+		t.Run(tc.name, func(t *testing.T) {
+			restrict, err := BelongingTo(taxonomy, []string{tc.taxid})
+			if err != nil {
+				t.Fatal(err)
+			}
+			seqs, err := obiformats.ReadSequencesFromFile(filepath.Join(dir, "mito.vert.fasta.gz"))
+			if err != nil {
+				t.Fatal(err)
+			}
+			corpus, err := LoadCorpus(seqs, Selection{Restrict: restrict})
+			if err != nil {
+				t.Fatal(err)
+			}
+			sw, err := FindStrictWords(corpus, WordSearch{PrimerLength: 18, Quorum: 0.7, Circular: true, TmpDir: t.TempDir()})
+			if err != nil {
+				t.Fatal(err)
+			}
+			words := make([]uint64, len(sw.Words))
+			for i, w := range sw.Words {
+				words[i] = w.Kmer
+			}
+
+			var before, after runtime.MemStats
+			runtime.GC()
+			runtime.ReadMemStats(&before)
+			start := time.Now()
+			res, err := matchApat(corpus, words, MatchParams{K: 18, MaxErrors: 3, Circular: true})
+			elapsed := time.Since(start)
+			runtime.ReadMemStats(&after)
+			if err != nil {
+				t.Fatal(err)
+			}
+			nm := 0
+			for _, r := range res {
+				nm += len(r.Matches)
+			}
+			bases := 0
+			for _, s := range corpus.Seqs {
+				bases += s.Len()
+			}
+			t.Logf("obiapat: %d words x %d sequences (%.1f Mb) e=3 in %s (%.0f Mb/s per word-strand pass); %d matches; heap +%.0f MB, allocated %.0f MB",
+				len(words), len(corpus.Seqs), float64(bases)/1e6, elapsed.Round(time.Millisecond),
+				float64(bases)*float64(len(words))*2/1e6/elapsed.Seconds(), nm,
+				float64(after.HeapAlloc-before.HeapAlloc)/1e6, float64(after.TotalAlloc-before.TotalAlloc)/1e6)
+
+			eco := readEcoCounts(t, filepath.Join("testdata", "ecoprimers_"+tc.name+".words.tsv"))
+			compared, differ := 0, 0
+			for i, r := range res {
+				w := strings.ToUpper(string(obikmer.DecodeKmer(words[i], 18, nil)))
+				want, ok := eco[w]
+				if !ok {
+					t.Errorf("word %s is not in the ecoPrimers log", w)
+					continue
+				}
+				got, _ := r.SequencesMatched(corpus)
+				compared++
+				if got != want {
+					differ++
+					t.Errorf("word %s: %d example sequences, ecoPrimers %d", w, got, want)
+				}
+			}
+			t.Logf("%d words compared with ecoPrimers, %d differ", compared, differ)
+		})
+	}
+}
@@ -0,0 +1,255 @@
+AAAACTCGTGCCAGCCAC	3891	0
+AAACCCAAAGGACTTGGC	3890	0
+AAACGAACCCTTAATAGC	3875	0
+AAACTCGTGCCAGCCACC	3894	0
+AAACTGGGATTAGATACC	3896	0
+AAAGTCCTACGTGATCTG	3901	0
+AAATCATGATGCAAAAGG	3894	0
+AACAAACGAACCCTTAAT	3888	0
+AACATCGAGGTCGTAAAC	3893	0
+AACATGGTAAGTGTACCG	3894	0
+AACCAAAAACTCTTGGTG	3854	0
+AACCCAAAGGACTTGGCG	3888	0
+AACCCTTAATAGCGGCTG	3858	0
+AACCGCGGTGGCTGGCAC	3896	0
+AACGAACCCTTAATAGCG	3860	0
+AACGATTAAAGTCCTACG	3700	0
+AACGTCAGGTCGAGGTGT	3894	0
+AACTCAGATCACGTAGGA	3899	0
+AACTCGTGCCAGCCACCG	3893	0
+AACTGGGATTAGATACCC	3896	0
+AAGAGTTTTTGGTTCCTA	3848	0
+AAGCCTCGCCTGTTTACC	3876	0
+AAGGGTTCGTTTGTTCAA	3889	0
+AAGGTAGCGCAATCACTT	3889	0
+AAGTCCTACGTGATCTGA	3900	0
+AAGTCGTAACATGGTAAG	3893	0
+AAGTGCACCTTCCGGTAC	3888	0
+AAGTGTACCGGAAGGTGC	3889	0
+AATAGCGGCTGCACCATT	3839	0
+AATCACTTGTCTTTTAAA	3856	0
+AATCATGATGCAAAAGGT	3893	0
+AATCCAGGTCAGTTTCTA	3903	0
+AATCGTTGAACAAACGAA	3876	0
+AATGTAGCCCATTTCTTC	3871	0
+ACAAACGAACCCTTAATA	3885	0
+ACAAACTGGGATTAGATA	3893	0
+ACAAGTGATTGCGCTACC	3889	0
+ACACACCGCCCGTCACTC	3891	0
+ACACCGCCCGTCACTCTC	3890	0
+ACACTTACCATGTTACGA	3893	0
+ACATCCTAATGGTGCAGC	3858	0
+ACATCGAGGTCGTAAACC	3894	0
+ACATGGTAAGTGTACCGG	3894	0
+ACCAAAAACTCTTGGTGC	3875	0
+ACCAAGAGTTTTTGGTTC	3857	0
+ACCATGTTACGACTTGCC	3891	0
+ACCATTAGGATGTCCTGA	3891	0
+ACCCAAAGGACTTGGCGG	3886	0
+ACCCGTCTCTGTGGCAAA	3892	0
+ACCCTAGGGATAACAGCG	3895	0
+ACCCTTAATAGCGGCTGC	3861	0
+ACCGCCCGTCACTCTCCC	3893	0
+ACCGCGGTGGCTGGCACG	3895	0
+ACCGGAAGGTGCACTTGG	3887	0
+ACCTCGATGTTGGATCAG	3891	0
+ACCTTCCGGTACACTTAC	3893	0
+ACGAACCCTTAATAGCGG	3859	0
+ACGACCTCGATGTTGGAT	3889	0
+ACGATTAAAGTCCTACGT	3701	0
+ACGTCAGGTCGAGGTGTA	3894	0
+ACGTGATCTGAGTTCAGA	3901	0
+ACTCAGATCACGTAGGAC	3899	0
+ACTCGTGCCAGCCACCGC	3894	0
+ACTCTTTTGCCACAGAGA	3867	0
+ACTGGGATTAGATACCCC	3896	0
+ACTTACCATGTTACGACT	3894	0
+ACTTTAATCGTTGAACAA	3876	0
+AGAACAGGCTCCTCTAGG	3893	0
+AGACAAGTGATTGCGCTA	3883	0
+AGAGGAGCCTGTTCTAGA	3876	0
+AGAGTGACGGGCGGTGTG	3890	0
+AGAGTTTTTGGTTCCTAA	3851	0
+AGATAGAAACTGACCTGG	3900	0
+AGATCACGTAGGACTTTA	3901	0
+AGCCGCTATTAAGGGTTC	3856	0
+AGCCTCGCCTGTTTACCA	3889	0
+AGCCTGTTCTAGAACCGA	3871	0
+AGCGGCTGCACCATTAGG	3846	0
+AGGAACCAAAAACTCTTG	3842	0
+AGGACATCCTAATGGTGC	3866	0
+AGGACTTTAATCGTTGAA	3688	0
+AGGAGCCTGTTCTAGAAC	3874	0
+AGGATGTCCTGATCCAAC	3897	0
+AGGCAAGTCGTAACATGG	3868	0
+AGGCGATGTTTTTGGTAA	3894	0
+AGGGATAACAGCGCAATC	3896	0
+AGGGGAGGCAAGTCGTAA	3890	0
+AGGGGGTTTACGACCTCG	3894	0
+AGGGTTCGTTTGTTCAAC	3888	0
+AGGTAGCGCAATCACTTG	3888	0
+AGTCCTACGTGATCTGAG	3900	0
+AGTGGGGTATCTAATCCC	3896	0
+AGTTACCCTAGGGATAAC	3895	0
+AGTTTTTGGTTCCTAAGA	3848	0
+ATAGAAACTGACCTGGAT	3901	0
+ATAGCGGCTGCACCATTA	3837	0
+ATAGTGGGGTATCTAATC	3895	0
+ATCACGTAGGACTTTAAT	3880	0
+ATCACTTGTCTTTTAAAT	3856	0
+ATCAGGACATCCTAATGG	3892	0
+ATCATGATGCAAAAGGTA	3894	0
+ATCCTAATGGTGCAGCCG	3852	0
+ATCGAGGTCGTAAACCCC	3894	0
+ATCGGTTCTAGAACAGGC	3862	0
+ATCGTTGAACAAACGAAC	3877	0
+ATCTGAGTTCAGACCGGA	3902	0
+ATGATGCAAAAGGTACGA	3891	0
+ATGGTAAGTGTACCGGAA	3895	0
+ATGGTGCAGCCGCTATTA	3840	0
+ATGTAGCCCATTTCTTCC	3871	0
+ATGTCCTGATCCAACATC	3892	0
+ATGTTACGACTTGCCTCC	3864	0
+ATGTTGGATCAGGACATC	3893	0
+ATGTTTTTGGTAAACAGG	3887	0
+ATTAGATACCCCACTATG	3896	0
+ATTAGGATGTCCTGATCC	3893	0
+ATTGCGCTGTTATCCCTA	3896	0
+CAAAAACTCTTGGTGCAA	3886	0
+CAAACGAACCCTTAATAG	3882	0
+CAAACTGGGATTAGATAC	3895	0
+CAACATCGAGGTCGTAAA	3894	0
+CAACGATTAAAGTCCTAC	3691	0
+CAAGTCGTAACATGGTAA	3893	0
+CAAGTGCACCTTCCGGTA	3888	0
+CACCATTAGGATGTCCTG	3890	0
+CACCGCCCGTCACTCTCC	3888	0
+CACCTTCCGGTACACTTA	3891	0
+CACGTAGGACTTTAATCG	3702	0
+CACTCTTTTGCCACAGAG	3856	0
+CACTTACCATGTTACGAC	3894	0
+CATCCTAATGGTGCAGCC	3852	0
+CATCGAGGTCGTAAACCC	3894	0
+CATGATGCAAAAGGTACG	3890	0
+CATGGTAAGTGTACCGGA	3894	0
+CATGTTACGACTTGCCTC	3866	0
+CATTAGGATGTCCTGATC	3893	0
+CCAAAAACTCTTGGTGCA	3883	0
+CCAACATCGAGGTCGTAA	3894	0
+CCAAGAGTTTTTGGTTCC	3844	0
+CCACTCTTTTGCCACAGA	3879	0
+CCCACTCTTTTGCCACAG	3879	0
+CCCGTCTCTGTGGCAAAA	3894	0
+CCCTAGGGATAACAGCGC	3898	0
+CCCTTAATAGCGGCTGCA	3853	0
+CCGCCCGTCACTCTCCCC	3890	0
+CCGCGGTGGCTGGCACGA	3895	0
+CCGGTAAAACTCGTGCCA	3847	0
+CCGGTCTGAACTCAGATC	3904	0
+CCGTCTCTGTGGCAAAAG	3894	0
+CCTACGTGATCTGAGTTC	3899	0
+CCTAGGGATAACAGCGCA	3896	0
+CCTCGATGTTGGATCAGG	3892	0
+CCTCGCCTGTTTACCAAA	3892	0
+CCTGTTCTAGAACCGATA	3862	0
+CCTTAATAGCGGCTGCAC	3850	0
+CCTTCCGGTACACTTACC	3893	0
+CGAACCCTTAATAGCGGC	3855	0
+CGACCTCGATGTTGGATC	3890	0
+CGACGGCGGTATATAGGC	3890	0
+CGATGTTGGATCAGGACA	3892	0
+CGATGTTTTTGGTAAACA	3886	0
+CGCCTGTTTACCAAAAAC	3889	0
+CGCGGTGGCTGGCACGAG	3894	0
+CGCTTAGCTGTTAACTAA	3864	0
+CGGTAAAACTCGTGCCAG	3856	0
+CGGTCTGAACTCAGATCA	3904	0
+CGGTTCTAGAACAGGCTC	3873	0
+CGTAACATGGTAAGTGTA	3892	0
+CGTCAGGTCGAGGTGTAG	3893	0
+CGTCTCTGTGGCAAAAGA	3891	0
+CGTGATCTGAGTTCAGAC	3902	0
+CGTTGAACAAACGAACCC	3887	0
+CGTTTGTTCAACGATTAA	3877	0
+CTAAATCATGATGCAAAA	3896	0
+CTAATGGTGCAGCCGCTA	3840	0
+CTACGTGATCTGAGTTCA	3899	0
+CTAGAACAGGCTCCTCTA	3877	0
+CTAGAGGAGCCTGTTCTA	3891	0
+CTAGGGATAACAGCGCAA	3897	0
+CTCCGGTCTGAACTCAGA	3903	0
+CTCGATGTTGGATCAGGA	3892	0
+CTCGCCTGTTTACCAAAA	3892	0
+CTCGTACCTTTTGCATCA	3889	0
+CTCTTTTGCCACAGAGAC	3869	0
+CTGAACTCAGATCACGTA	3900	0
+CTGCACCATTAGGATGTC	3861	0
+CTGGGATTAGATACCCCA	3895	0
+CTGTTATCCCTAGGGTAA	3897	0
+CTGTTTACCAAAAACATC	3886	0
+CTTAATAGCGGCTGCACC	3848	0
+CTTAGCTGTTAACTAAGA	3878	0
+CTTAGGAACCAAAAACTC	3847	0
+CTTAGTTAACAGCTAAGC	3875	0
+CTTCCGGTACACTTACCA	3891	0
+CTTGGATTTGCACCAAGA	3899	0
+CTTTAATCGTTGAACAAA	3876	0
+CTTTTGCATCATGATTTA	3896	0
+GAACAAACGAACCCTTAA	3890	0
+GAAGGAACTCGGCAAACA	3889	0
+GACAAGTGATTGCGCTAC	3889	0
+GACCTCGATGTTGGATCA	3890	0
+GACTTTAATCGTTGAACA	3686	0
+GAGGAGCCTGTTCTAGAA	3876	0
+GAGGCGATGTTTTTGGTA	3893	0
+GATAGAAACTGACCTGGA	3900	0
+GATCACGTAGGACTTTAA	3900	0
+GATTAAAGTCCTACGTGA	3704	0
+GCAAGTCGTAACATGGTA	3891	0
+GCATAGTGGGGTATCTAA	3896	0
+GCCGGTAAAACTCGTGCC	3827	0
+GCCTCGCCTGTTTACCAA	3890	0
+GCCTGTTTACCAAAAACA	3889	0
+GCGATGTTTTTGGTAAAC	3888	0
+GCGCTTAGCTGTTAACTA	3899	0
+GCGGCTGCACCATTAGGA	3850	0
+GCTAAATCATGATGCAAA	3893	0
+GCTACACCTCGACCTGAC	3893	0
+GCTGGCACGAGTTTTACC	3881	0
+GCTGTTATCCCTAGGGTA	3896	0
+GGACATCCTAATGGTGCA	3865	0
+GGACTTTAATCGTTGAAC	3687	0
+GGAGCCTGTTCTAGAACC	3874	0
+GGATGTCCTGATCCAACA	3895	0
+GGATTAGATACCCCACTA	3896	0
+GGATTGCGCTGTTATCCC	3895	0
+GGCGATGTTTTTGGTAAA	3890	0
+GGCTGGCACGAGTTTTAC	3880	0
+GGGAAGAAATGGGCTACA	3872	0
+GGGAGGCAAGTCGTAACA	3891	0
+GGGGAGGCAAGTCGTAAC	3890	0
+GGGGGTTTACGACCTCGA	3894	0
+GGTACACTTACCATGTTA	3893	0
+GGTCTGAACTCAGATCAC	3902	0
+GGTCTTAGGAACCAAAAA	3862	0
+GGTTCGTTTGTTCAACGA	3877	0
+GTAACATGGTAAGTGTAC	3893	0
+GTACCTTTTGCATCATGA	3892	0
+GTCCTGATCCAACATCGA	3892	0
+GTCTTAGGAACCAAAAAC	3853	0
+GTGGGGTATCTAATCCCA	3895	0
+GTTACCCTAGGGATAACA	3896	0
+GTTTGTTCAACGATTAAA	3877	0
+TAAAACTCGTGCCAGCCA	3887	0
+TAACCGCGGTGGCTGGCA	3895	0
+TAAGGGTTCGTTTGTTCA	3889	0
+TAATCGTTGAACAAACGA	3876	0
+TACGACCTCGATGTTGGA	3892	0
+TAGGACTTTAATCGTTGA	3689	0
+TAGGATGTCCTGATCCAA	3897	0
+TCCCACTCTTTTGCCACA	3866	0
+TCGCCTGTTTACCAAAAA	3891	0
+TGGATCAGGACATCCTAA	3894	0
+TGGTCTTAGGAACCAAAA	3862	0
+TGGTGCAGCCGCTATTAA	3847	0
+TTGGTCTTAGGAACCAAA	3862	0
@@ -4,10 +4,8 @@ import (
 	"fmt"
 	"math"
 	"sync"
-	"sync/atomic"

 	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obidefault"
-	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiiter"
 	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obikmer"
 )

@@ -20,13 +18,6 @@ type WordSearch struct {
 	TmpDir       string  // parent of the temporary directory ("" = system default)
 }

-// Counts gives the number of sequences by role.
-type Counts struct {
-	Examples        int
-	CounterExamples int
-	Ignored         int
-}
-
 // StrictWords is the result of the strict word search: the canonical words of
 // PrimerLength bases present, without any error, in at least MinSequences
 // example sequences.
@@ -44,12 +35,12 @@ func MinSequences(examples int, quorum float64) int {
 	return int(math.Floor(float64(examples)*quorum + 1e-9))
 }

-// FindStrictWords reads every sequence of seqs once, indexes the example
-// sequences, and returns the words reaching the quorum. Sequences no longer
-// than the word size are counted as examples but cannot contain any word, as
-// in ecoPrimers. Words are canonical (the smaller of the word and its reverse
-// complement), so both strands are searched.
-func FindStrictWords(seqs obiiter.IBioSequence, sel Selection, cfg WordSearch) (*StrictWords, error) {
+// FindStrictWords indexes the example sequences of the corpus and returns the
+// words reaching the quorum. Sequences no longer than the word size are
+// counted as examples but cannot contain any word, as in ecoPrimers. Words are
+// canonical (the smaller of the word and its reverse complement), so both
+// strands are searched.
+func FindStrictWords(c *Corpus, cfg WordSearch) (*StrictWords, error) {
 	if cfg.PrimerLength < 2 || cfg.PrimerLength > 31 {
 		return nil, fmt.Errorf("obiprimer: primer length must be between 2 and 31, got %d", cfg.PrimerLength)
 	}
@@ -66,50 +57,27 @@ func FindStrictWords(seqs obiiter.IBioSequence, sel Selection, cfg WordSearch) (
 	}
 	defer counter.Close()

-	var nextID atomic.Uint32
-	var examples, counters, ignored atomic.Int64
-
-	consumer := func(it obiiter.IBioSequence, wg *sync.WaitGroup) {
-		defer wg.Done()
-		for it.Next() {
-			for _, seq := range it.Get().Slice() {
-				switch sel.Role(seq) {
-				case Example:
-					examples.Add(1)
-					if seq.Len() > cfg.PrimerLength {
-						counter.AddSequence(nextID.Add(1)-1, seq.Sequence(), cfg.Circular)
-					}
-				case CounterExample:
-					counters.Add(1)
-				default:
-					ignored.Add(1)
+	jobs := make(chan int)
+	var wg sync.WaitGroup
+	for range obidefault.ParallelWorkers() {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for i := range jobs {
+				if seq := c.Seqs[i]; c.Roles[i] == Example && seq.Len() > cfg.PrimerLength {
+					counter.AddSequence(uint32(i), seq.Sequence(), cfg.Circular)
 				}
 			}
-		}
+		}()
 	}
-
-	var wg sync.WaitGroup
-	for i := 1; i < obidefault.ParallelWorkers(); i++ {
-		wg.Add(1)
-		go consumer(seqs.Split(), &wg)
+	for i := range c.Seqs {
+		jobs <- i
 	}
-	wg.Add(1)
-	go consumer(seqs, &wg)
+	close(jobs)
 	wg.Wait()

-	res := &StrictWords{
-		PrimerLength: cfg.PrimerLength,
-		Counts: Counts{
-			Examples:        int(examples.Load()),
-			CounterExamples: int(counters.Load()),
-			Ignored:         int(ignored.Load()),
-		},
-	}
+	res := &StrictWords{PrimerLength: cfg.PrimerLength, Counts: c.Counts()}
 	res.MinSequences = MinSequences(res.Counts.Examples, cfg.Quorum)
-	if res.Counts.Examples == 0 {
-		return nil, fmt.Errorf("obiprimer: no example sequence selected")
-	}
-
 	res.Words, err = counter.CountAtLeast(max(res.MinSequences, 1))
 	if err != nil {
 		return nil, err
@@ -107,8 +107,11 @@ func TestFindStrictWords(t *testing.T) {
 	add("zz1", randDNA(rng, 50)+motif)

 	sel := Selection{Restrict: idPrefix("ex").Or(idPrefix("cx")), Ignore: idPrefix("cx"), Except: idPrefix("cx")}
-	res, err := FindStrictWords(obiiter.IBatchOver("t", data, 3), sel,
-		WordSearch{PrimerLength: k, Quorum: 0.7, Partitions: 8, TmpDir: t.TempDir()})
+	corpus, err := LoadCorpus(obiiter.IBatchOver("t", data, 3), sel)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res, err := FindStrictWords(corpus, WordSearch{PrimerLength: k, Quorum: 0.7, Partitions: 8, TmpDir: t.TempDir()})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -151,14 +154,17 @@ func TestFindStrictWordsParameters(t *testing.T) {
 	it := func() obiiter.IBioSequence {
 		return obiiter.IBatchOver("t", obiseq.BioSequenceSlice{obiseq.NewBioSequence("a", []byte("acgtacgtacgtacgtacgtacgt"), "")}, 1)
 	}
-	if _, err := FindStrictWords(it(), Selection{}, WordSearch{PrimerLength: 1, Quorum: 0.7}); err == nil {
+	corpus, err := LoadCorpus(it(), Selection{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := FindStrictWords(corpus, WordSearch{PrimerLength: 1, Quorum: 0.7}); err == nil {
 		t.Error("primer length 1 must fail")
 	}
-	if _, err := FindStrictWords(it(), Selection{}, WordSearch{PrimerLength: 18, Quorum: 0}); err == nil {
+	if _, err := FindStrictWords(corpus, WordSearch{PrimerLength: 18, Quorum: 0}); err == nil {
 		t.Error("quorum 0 must fail")
 	}
-	if _, err := FindStrictWords(it(), Selection{Restrict: idPrefix("zz")},
-		WordSearch{PrimerLength: 18, Quorum: 0.7}); err == nil {
+	if _, err := LoadCorpus(it(), Selection{Restrict: idPrefix("zz")}); err == nil {
 		t.Error("no example must fail")
 	}
 }
@@ -206,7 +212,11 @@ func TestStrictWordsAgainstEcoPrimers(t *testing.T) {
 			if err != nil {
 				t.Fatal(err)
 			}
-			res, err := FindStrictWords(seqs, Selection{Restrict: restrict},
+			corpus, err := LoadCorpus(seqs, Selection{Restrict: restrict})
+			if err != nil {
+				t.Fatal(err)
+			}
+			res, err := FindStrictWords(corpus,
 				WordSearch{PrimerLength: 18, Quorum: 0.7, Circular: true, TmpDir: t.TempDir()})
 			if err != nil {
 				t.Fatal(err)