diff --git a/index.c b/index.c
index 7e55c4e..47a62f8 100644
--- a/index.c
+++ b/index.c
@@ -325,7 +325,7 @@ mm_idx_t *mm_idx_load(FILE *fp)
 
 	if (fread(magic, 1, 4, fp) != 4) return 0;
 	if (strncmp(magic, MM_IDX_MAGIC, 4) != 0) return 0;
-	if (fread(x, 4, 5, fp) != 6) return 0;
+	if (fread(x, 4, 5, fp) != 5) return 0;
 	mi = mm_idx_init(x[0], x[1], x[2], x[4]);
 	mi->n_seq = x[3];
 	mi->seq = (mm_idx_seq_t*)calloc(mi->n_seq, sizeof(mm_idx_seq_t));
diff --git a/main.c b/main.c
index ec1f94c..66dc1e8 100644
--- a/main.c
+++ b/main.c
@@ -7,7 +7,7 @@
 #include "bseq.h"
 #include "minimap.h"
 
-#define MM_VERSION "0.2-r124-dirty"
+#define MM_VERSION "2.0-r14-pre"
 
 void liftrlimit()
 {
@@ -34,7 +34,7 @@ int main(int argc, char *argv[])
 	mm_realtime0 = realtime();
 	mm_mapopt_init(&opt);
 
-	while ((c = getopt(argc, argv, "w:k:B:b:t:r:c:f:Vv:NOg:I:d:lRPST:m:L:Dx:H")) >= 0) {
+	while ((c = getopt(argc, argv, "w:k:B:b:t:r:c:f:Vv:NOg:I:d:lRPST:L:Dx:H")) >= 0) {
 		if (c == 'w') w = atoi(optarg);
 		else if (c == 'k') k = atoi(optarg);
 		else if (c == 'b') b = atoi(optarg);
@@ -43,7 +43,6 @@ int main(int argc, char *argv[])
 		else if (c == 'd') fnw = optarg; // the above are indexing related options, except -I
 		else if (c == 'r') opt.radius = atoi(optarg);
 		else if (c == 'c') opt.min_cnt = atoi(optarg);
-		else if (c == 'm') opt.merge_frac = atof(optarg);
 		else if (c == 'f') f = atof(optarg);
 		else if (c == 't') n_threads = atoi(optarg);
 		else if (c == 'v') mm_verbose = atoi(optarg);
@@ -72,7 +71,6 @@ int main(int argc, char *argv[])
 			if (strcmp(optarg, "ava10k") == 0) {
 				opt.flag |= MM_F_AVA | MM_F_NO_SELF;
 				opt.min_match = 100;
-				opt.merge_frac = 0.0;
 				w = 5;
 			}
 		}
@@ -93,7 +91,6 @@ int main(int argc, char *argv[])
 		fprintf(stderr, "  Mapping:\n");
 		fprintf(stderr, "    -f FLOAT   filter out top FLOAT fraction of repetitive minimizers [%.3f]\n", f);
 		fprintf(stderr, "    -r INT     bandwidth [%d]\n", opt.radius);
-		fprintf(stderr, "    -m FLOAT   merge two chains if FLOAT fraction of minimizers are shared [%.2f]\n", opt.merge_frac);
 		fprintf(stderr, "    -c INT     retain a mapping if it consists of >=INT minimizers [%d]\n", opt.min_cnt);
 		fprintf(stderr, "    -L INT     min matching length [%d]\n", opt.min_match);
 		fprintf(stderr, "    -g INT     split a mapping if there is a gap longer than INT [%d]\n", opt.max_gap);
@@ -129,13 +126,12 @@ int main(int argc, char *argv[])
 					__func__, realtime() - mm_realtime0, cputime() / (realtime() - mm_realtime0), mi->n_seq);
 		if (fpw) {
 			mm_idx_dump(fpw, mi);
-			if (mm_verbose >= 2)
+			if (mm_verbose >= 3)
 				fprintf(stderr, "[M::%s::%.3f*%.2f] dumpped the (partial) index to disk\n", __func__, realtime() - mm_realtime0, cputime() / (realtime() - mm_realtime0));
 		}
-		/*
+		if (argc != optind + 1) mm_mapopt_update(&opt, mi);
 		for (i = optind + 1; i < argc; ++i)
 			mm_map_file(mi, argv[i], &opt, n_threads, mini_batch_size);
-		*/
 		mm_idx_destroy(mi);
 	}
 	if (fpw) fclose(fpw);
diff --git a/map.c b/map.c
index 216db2d..2ab91a6 100644
--- a/map.c
+++ b/map.c
@@ -1,25 +1,49 @@
 #include <stdlib.h>
+#include <string.h>
+#include "kthread.h"
 #include "kvec.h"
 #include "kalloc.h"
 #include "sdust.h"
 #include "minimap.h"
+#include "bseq.h"
 
 void mm_mapopt_init(mm_mapopt_t *opt)
 {
+	opt->n_frag_mini = 100;
+	opt->max_occ_frac = 1e-5f;
+	opt->mid_occ_frac = 1e-3f;
+	opt->sdust_thres = 0;
+
 	opt->radius = 500;
 	opt->max_gap = 10000;
 	opt->min_cnt = 4;
 	opt->min_match = 40;
-	opt->sdust_thres = 0;
 	opt->flag = MM_F_WITH_REP;
-	opt->merge_frac = .5;
 }
 
+void mm_mapopt_update(mm_mapopt_t *opt, const mm_idx_t *mi)
+{
+	opt->max_occ = mm_idx_cal_max_occ(mi, opt->max_occ_frac);
+	opt->mid_occ = mm_idx_cal_max_occ(mi, opt->mid_occ_frac);
+	if (mm_verbose >= 3)
+		fprintf(stderr, "[M::%s::%.3f*%.2f] mid_occ = %d; max_occ = %d\n", __func__, realtime() - mm_realtime0, cputime() / (realtime() - mm_realtime0),
+				opt->mid_occ, opt->max_occ);
+}
+
+typedef struct {
+	uint32_t n, is_alloc;
+	uint32_t qpos, span;
+	union {
+		const uint64_t *cr;
+		uint64_t *r;
+	} x;
+} mm_match_t;
+
 struct mm_tbuf_s {
 	sdust_buf_t *sdb;
 	mm128_v mini;
 	kvec_t(mm_reg1_t) reg;
-	void *km;
+	void *km, *km_fixed;
 };
 
 mm_tbuf_t *mm_tbuf_init(void)
@@ -27,6 +51,7 @@ mm_tbuf_t *mm_tbuf_init(void)
 	mm_tbuf_t *b;
 	b = (mm_tbuf_t*)calloc(1, sizeof(mm_tbuf_t));
 	b->km = km_init();
+	b->km_fixed = km_init();
 	b->sdb = sdust_buf_init(b->km);
 	return b;
 }
@@ -34,36 +59,178 @@ mm_tbuf_t *mm_tbuf_init(void)
 void mm_tbuf_destroy(mm_tbuf_t *b)
 {
 	if (b == 0) return;
+	km_destroy(b->km_fixed);
 	km_destroy(b->km);
 	free(b);
 }
 
-const mm_reg1_t *mm_map(const mm_idx_t *mi, int l_seq, const char *seq, int *n_regs, mm_tbuf_t *b, const mm_mapopt_t *opt, const char *name)
+static void mm_dust_minier(mm128_v *mini, int l_seq, const char *seq, int sdust_thres, sdust_buf_t *sdb)
 {
-	int j;
+	int n_dreg, j, k, u = 0;
+	const uint64_t *dreg;
+	if (sdust_thres <= 0 || sdb == 0) return;
+	dreg = sdust_core((const uint8_t*)seq, l_seq, sdust_thres, 64, &n_dreg, sdb);
+	for (j = k = 0; j < mini->n; ++j) { // squeeze out minimizers that significantly overlap with LCRs
+		int32_t qpos = (uint32_t)mini->a[j].y>>1, span = mini->a[j].x&0xff;
+		int32_t s = qpos - (span - 1), e = s + span;
+		while (u < n_dreg && (uint32_t)dreg[u] <= s) ++u;
+		if (u < n_dreg && dreg[u]>>32 < e) {
+			int v, l = 0;
+			for (v = u; v < n_dreg && dreg[v]>>32 < e; ++v) { // iterate over LCRs overlapping this minimizer
+				int ss = s > dreg[v]>>32? s : dreg[v]>>32;
+				int ee = e < (uint32_t)dreg[v]? e : (uint32_t)dreg[v];
+				l += ee - ss;
+			}
+			if (l <= span>>1) mini->a[k++] = mini->a[j]; // keep the minimizer if less than half of it falls in masked region
+		}
+	}
+	mini->n = k;
+}
 
-	b->mini.n = 0;
-	mm_sketch(b->km, seq, l_seq, mi->w, mi->k, 0, mi->is_hpc, &b->mini);
-	if (opt->sdust_thres > 0) { // squeeze out minimizers that significantly overlap with LCRs
-		int n_dreg, k, u = 0;
-		const uint64_t *dreg;
-		dreg = sdust_core((const uint8_t*)seq, l_seq, opt->sdust_thres, 64, &n_dreg, b->sdb);
-		for (j = k = 0; j < b->mini.n; ++j) {
-			int32_t qpos = (uint32_t)b->mini.a[j].y>>1, span = b->mini.a[j].x&0xff;
-			int32_t s = qpos - (span - 1), e = s + span;
-			while (u < n_dreg && (uint32_t)dreg[u] <= s) ++u;
-			if (u < n_dreg && dreg[u]>>32 < e) {
-				int v, l = 0;
-				for (v = u; v < n_dreg && dreg[v]>>32 < e; ++v) { // iterate over LCRs overlapping this minimizer
-					int ss = s > dreg[v]>>32? s : dreg[v]>>32;
-					int ee = e < (uint32_t)dreg[v]? e : (uint32_t)dreg[v];
-					l += ee - ss;
-				}
-				if (l <= mi->k>>1) b->mini.a[k++] = b->mini.a[j];
+void mm_pair_thin(mm_tbuf_t *b, mm_match_t *_m1, mm_match_t *_m2)
+{
+}
+
+void mm_map_frag(const mm_mapopt_t *opt, const mm_idx_t *mi, mm_tbuf_t *b, uint32_t m_st, uint32_t m_en)
+{
+	int i, n = m_en - m_st, last = -1, last2 = -1;
+	mm_match_t *m;
+
+	// convert to local representation
+	m = (mm_match_t*)kmalloc(b->km_fixed, (m_en - m_st) * sizeof(mm_match_t));
+	for (i = 0; i < n; ++i) {
+		int t;
+		mm128_t *p = &b->mini.a[i + m_st];
+		m[i].is_alloc = 0;
+		m[i].qpos = (uint32_t)p->y;
+		m[i].span = p->x & 0xff;
+		m[i].x.cr = mm_idx_get(mi, p->x, &t);
+		m[i].n = t;
+	}
+
+	// pair k-mer thinning
+	for (i = 0; i < n; ++i) {
+		if (m[i].n >= opt->mid_occ && m[i].n < opt->max_occ) {
+			if (last2 < 0) last2 = i;
+			if (last < 0 || m[last].n < m[i].n) last = i;
+			if (last >= 0 && m[last].span + (m[last].qpos>>1) <= m[i].qpos>>1) {
+				mm_pair_thin(b, &m[last], &m[i]);
+				last2 = last = -1;
+			} else if (last2 >= 0 && m[last].span + (m[last].qpos>>1) <= m[i].qpos>>1) {
+				mm_pair_thin(b, &m[last2], &m[i]);
+				last2 = last = -1;
 			}
 		}
-		b->mini.n = k;
+	}
+	kfree(b->km_fixed, m);
+}
+
+const mm_reg1_t *mm_map(const mm_idx_t *mi, int l_seq, const char *seq, int *n_regs, mm_tbuf_t *b, const mm_mapopt_t *opt, const char *name)
+{
+	uint32_t proc_mini = 0;
+	b->mini.n = 0;
+	mm_sketch(b->km, seq, l_seq, mi->w, mi->k, 0, mi->is_hpc, &b->mini);
+	if (opt->sdust_thres > 0)
+		mm_dust_minier(&b->mini, l_seq, seq, opt->sdust_thres, b->sdb);
+	while (proc_mini < b->mini.n) {
+		uint32_t n = b->mini.n - proc_mini < opt->n_frag_mini * 1.5f? b->mini.n - proc_mini : opt->n_frag_mini;
+		mm_map_frag(opt, mi, b, proc_mini, proc_mini + n);
+		proc_mini += n;
 	}
 	*n_regs = b->reg.n;
 	return b->reg.a;
 }
+
+/**************************
+ * Multi-threaded mapping *
+ **************************/
+
+typedef struct {
+	int mini_batch_size, n_processed, n_threads;
+	const mm_mapopt_t *opt;
+	bseq_file_t *fp;
+	const mm_idx_t *mi;
+} pipeline_t;
+
+typedef struct {
+	const pipeline_t *p;
+    int n_seq;
+	bseq1_t *seq;
+	int *n_reg;
+	mm_reg1_t **reg;
+	mm_tbuf_t **buf;
+} step_t;
+
+static void worker_for(void *_data, long i, int tid) // kt_for() callback
+{
+    step_t *step = (step_t*)_data;
+	const mm_reg1_t *regs;
+	int n_regs;
+
+	regs = mm_map(step->p->mi, step->seq[i].l_seq, step->seq[i].seq, &n_regs, step->buf[tid], step->p->opt, step->seq[i].name);
+	step->n_reg[i] = n_regs;
+	if (n_regs > 0) {
+		step->reg[i] = (mm_reg1_t*)malloc(n_regs * sizeof(mm_reg1_t));
+		memcpy(step->reg[i], regs, n_regs * sizeof(mm_reg1_t));
+	}
+}
+
+static void *worker_pipeline(void *shared, int step, void *in)
+{
+	int i, j;
+    pipeline_t *p = (pipeline_t*)shared;
+    if (step == 0) { // step 0: read sequences
+        step_t *s;
+        s = (step_t*)calloc(1, sizeof(step_t));
+		s->seq = bseq_read(p->fp, p->mini_batch_size, &s->n_seq);
+		if (s->seq) {
+			s->p = p;
+			for (i = 0; i < s->n_seq; ++i)
+				s->seq[i].rid = p->n_processed++;
+			s->buf = (mm_tbuf_t**)calloc(p->n_threads, sizeof(mm_tbuf_t*));
+			for (i = 0; i < p->n_threads; ++i)
+				s->buf[i] = mm_tbuf_init();
+			s->n_reg = (int*)calloc(s->n_seq, sizeof(int));
+			s->reg = (mm_reg1_t**)calloc(s->n_seq, sizeof(mm_reg1_t*));
+			return s;
+		} else free(s);
+    } else if (step == 1) { // step 1: map
+		kt_for(p->n_threads, worker_for, in, ((step_t*)in)->n_seq);
+		return in;
+    } else if (step == 2) { // step 2: output
+        step_t *s = (step_t*)in;
+		const mm_idx_t *mi = p->mi;
+		for (i = 0; i < p->n_threads; ++i) mm_tbuf_destroy(s->buf[i]);
+		free(s->buf);
+		for (i = 0; i < s->n_seq; ++i) {
+			bseq1_t *t = &s->seq[i];
+			for (j = 0; j < s->n_reg[i]; ++j) {
+				mm_reg1_t *r = &s->reg[i][j];
+				if (r->len < p->opt->min_match) continue;
+				printf("%s\t%d\t%d\t%d\t%c\t", t->name, t->l_seq, r->qs, r->qe, "+-"[r->rev]);
+				if (mi->seq[r->rid].name) fputs(mi->seq[r->rid].name, stdout);
+				else printf("%d", r->rid + 1);
+				printf("\t%d\t%d\t%d\t%d\t%d\t255\tcm:i:%d\n", mi->seq[r->rid].len, r->rs, r->re, r->len,
+						r->re - r->rs > r->qe - r->qs? r->re - r->rs : r->qe - r->qs, r->cnt);
+			}
+			free(s->reg[i]);
+			free(s->seq[i].seq); free(s->seq[i].name);
+		}
+		free(s->reg); free(s->n_reg); free(s->seq);
+		free(s);
+	}
+    return 0;
+}
+
+int mm_map_file(const mm_idx_t *idx, const char *fn, const mm_mapopt_t *opt, int n_threads, int mini_batch_size)
+{
+	pipeline_t pl;
+	memset(&pl, 0, sizeof(pipeline_t));
+	pl.fp = bseq_open(fn);
+	if (pl.fp == 0) return -1;
+	pl.opt = opt, pl.mi = idx;
+	pl.n_threads = n_threads, pl.mini_batch_size = mini_batch_size;
+	kt_pipeline(n_threads == 1? 1 : 2, worker_pipeline, &pl, 3);
+	bseq_close(pl.fp);
+	return 0;
+}
diff --git a/minimap.h b/minimap.h
index cd6a57c..37139d7 100644
--- a/minimap.h
+++ b/minimap.h
@@ -50,13 +50,18 @@ typedef struct {
 } mm_reg1_t;
 
 typedef struct {
+	int n_frag_mini;
+	float max_occ_frac;
+	float mid_occ_frac;
+	int sdust_thres;  // score threshold for SDUST; 0 to disable
+	int flag;    // see MM_F_* macros
 	int radius;  // bandwidth to cluster hits
 	int max_gap; // break a chain if there are no minimizers in a max_gap window
 	int min_cnt; // minimum number of minimizers to start a chain
 	int min_match;
-	int sdust_thres;  // score threshold for SDUST; 0 to disable
-	int flag;    // see MM_F_* macros
-	float merge_frac; // merge two chains if merge_frac fraction of minimzers are shared between the chains
+
+	int max_occ;
+	int mid_occ;
 } mm_mapopt_t;
 
 extern int mm_verbose;
@@ -81,7 +86,7 @@ void mm_sketch(void *km, const char *str, int len, int w, int k, uint32_t rid, i
 mm_idx_t *mm_idx_init(int w, int k, int b, int is_hpc);
 void mm_idx_destroy(mm_idx_t *mi);
 mm_idx_t *mm_idx_gen(struct bseq_file_s *fp, int w, int k, int b, int is_hpc, int mini_batch_size, int n_threads, uint64_t batch_size, int keep_name);
-void mm_idx_set_max_occ(mm_idx_t *mi, float f);
+uint32_t mm_idx_cal_max_occ(const mm_idx_t *mi, float f);
 const uint64_t *mm_idx_get(const mm_idx_t *mi, uint64_t minier, int *n);
 
 mm_idx_t *mm_idx_build(const char *fn, int w, int k, int is_hpc, int n_threads);
@@ -92,6 +97,7 @@ mm_idx_t *mm_idx_load(FILE *fp);
 
 // mapping
 void mm_mapopt_init(mm_mapopt_t *opt);
+void mm_mapopt_update(mm_mapopt_t *opt, const mm_idx_t *mi);
 mm_tbuf_t *mm_tbuf_init(void);
 void mm_tbuf_destroy(mm_tbuf_t *b);
 const mm_reg1_t *mm_map(const mm_idx_t *mi, int l_seq, const char *seq, int *n_regs, mm_tbuf_t *b, const mm_mapopt_t *opt, const char *name);