diff --git a/bwamem.c b/bwamem.c
index 91931bb..52c4a18 100644
--- a/bwamem.c
+++ b/bwamem.c
@@ -1,4 +1,6 @@
 #include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
 #include "bwamem.h"
 
 memopt_t *mem_opt_init()
@@ -8,6 +10,7 @@ memopt_t *mem_opt_init()
 	o->a = 1; o->b = 9; o->q = 16; o->r = 1; o->w = 100;
 	o->min_seed_len = 17;
 	o->max_occ = 10;
+	o->max_chain_gap = 10000;
 	return o;
 }
 
@@ -52,3 +55,75 @@ int smem_next(smem_i *itr)
 	return itr->start;
 }
 
+#include "kbtree.h"
+
+#define chain_lt(a, b) ((a).pos < (b).pos)
+KBTREE_INIT(chn, memchain1_t, chain_lt)
+
+static int test_and_merge(const memopt_t *opt, memchain1_t *c, const memseed_t *p)
+{
+	int64_t qend, rend, x, y;
+	const memseed_t *last = &c->seeds[c->n-1];
+	qend = last->qbeg + last->len;
+	rend = last->rbeg + last->len;
+	if (p->qbeg > c->seeds[0].qbeg && p->qbeg + p->len < qend && p->rbeg > c->seeds[0].rbeg && p->rbeg + p->len < rend)
+		return 1; // contained seed; do nothing
+	x = p->qbeg - last->qbeg; // always positive
+	y = p->rbeg - last->rbeg;
+	if (y > 0 && x - y <= opt->w && y - x <= opt->w && x - last->len < opt->max_chain_gap && y - last->len < opt->max_chain_gap) {
+		if (c->n == c->m) {
+			c->m <<= 1;
+			c->seeds = realloc(c->seeds, c->m * sizeof(memseed_t));
+		}
+		c->seeds[c->n++] = *p;
+		return 1;
+	}
+	return 0;
+}
+
+void mem_insert_seed(const memopt_t *opt, kbtree_t(chn) *tree, smem_i *itr)
+{
+	while (smem_next(itr) > 0) {
+		int i;
+		for (i = 0; i < itr->matches->n; ++i) {
+			bwtintv_t *p = &itr->matches->a[i];
+			int slen = (uint32_t)p->info - (p->info>>32); // seed length
+			int64_t k;
+			if (slen >= opt->min_seed_len || p->x[2] > opt->max_occ) continue;
+			for (k = 0; k < p->x[2]; ++k) {
+				memchain1_t tmp, *lower, *upper;
+				memseed_t c1;
+				int to_add = 0;
+				c1.rbeg = tmp.pos = bwt_sa(itr->bwt, p->x[0] + k);
+				c1.qbeg = p->info>>32;
+				c1.len  = slen;
+				if (kb_size(tree)) {
+					kb_intervalp(chn, tree, &tmp, &lower, &upper);
+					if (!test_and_merge(opt, lower, &c1)) to_add = 1;
+				} to_add = 1;
+				if (to_add) {
+					tmp.n = 1; tmp.m = 4;
+					tmp.seeds = calloc(tmp.m, sizeof(memseed_t));
+					kb_putp(chn, tree, &tmp);
+				}
+			}
+		}
+	}
+}
+
+memchain_t mem_collect_seed(const memopt_t *opt, const bwt_t *bwt, int len, const uint8_t *seq)
+{
+	memchain_t chain;
+	smem_i *itr;
+	kbtree_t(chn) *tree;
+
+	memset(&chain, 0, sizeof(memchain_t));
+	if (len < opt->min_seed_len) return chain; // if the query is shorter than the seed length, no match
+	tree = kb_init(chn, KB_DEFAULT_SIZE);
+	itr = smem_itr_init(bwt);
+	smem_set_query(itr, 1, len, seq);
+
+	smem_itr_destroy(itr);
+	kb_destroy(chn, tree);
+	return chain;
+}
diff --git a/bwamem.h b/bwamem.h
index da636a0..cc86ef2 100644
--- a/bwamem.h
+++ b/bwamem.h
@@ -10,11 +10,26 @@ typedef struct {
 	bwtintv_v *tmpvec[2], *matches;
 } smem_i;
 
+typedef struct {
+	int64_t qbeg, rbeg, len;
+} memseed_t;
+
 typedef struct {
 	int a, b, q, r, w;
-	int min_seed_len, max_occ;
+	int min_seed_len, max_occ, max_chain_gap;
 } memopt_t;
 
+typedef struct {
+	int n, m;
+	int64_t pos;
+	memseed_t *seeds;
+} memchain1_t;
+
+typedef struct {
+	int n, m;
+	memchain1_t *chains;
+} memchain_t;
+
 #ifdef __cplusplus
 extern "C" {
 #endif