diff --git a/python/cminimap2.h b/python/cminimap2.h
index b7e90b5..2eb9faa 100644
--- a/python/cminimap2.h
+++ b/python/cminimap2.h
@@ -8,28 +8,27 @@ typedef struct {
 	const char *ctg;
 	int32_t ctg_start, ctg_end;
 	int32_t qry_start, qry_end;
+	int32_t blen, NM, ctg_len;
+	uint8_t mapq, is_primary;
+	int8_t strand, trans_strand;
 	int32_t n_cigar32;
-	uint8_t strand, mapq, is_primary;
 	uint32_t *cigar32;
 } mm_hitpy_t;
 
-static inline mm_hitpy_t *mm_reg2hitpy(const mm_idx_t *mi, int n_regs, mm_reg1_t *regs)
+static inline void mm_reg2hitpy(const mm_idx_t *mi, mm_reg1_t *r, mm_hitpy_t *h)
 {
-	mm_hitpy_t *h;
-	int i;
-	h = (mm_hitpy_t*)calloc(n_regs, sizeof(mm_hitpy_t));
-	for (i = 0; i < n_regs; ++i) {
-		mm_reg1_t *r = &regs[i];
-		h[i].ctg = mi->seq[r->rid].name;
-		h[i].ctg_start = r->rs, h[i].ctg_end = r->re;
-		h[i].qry_start = r->qs, h[i].qry_end = r->qe;
-		h[i].strand = r->rev;
-		h[i].mapq = r->mapq;
-		h[i].is_primary = (r->id == r->parent);
-		h[i].n_cigar32 = r->p->n_cigar;
-		h[i].cigar32 = r->p->cigar;
-	}
-	return h;
+	h->ctg = mi->seq[r->rid].name;
+	h->ctg_len = mi->seq[r->rid].len;
+	h->ctg_start = r->rs, h->ctg_end = r->re;
+	h->qry_start = r->qs, h->qry_end = r->qe;
+	h->strand = r->rev? -1 : 1;
+	h->mapq = r->mapq;
+	h->blen = r->p->blen;
+	h->NM = r->p->n_diff;
+	h->trans_strand = r->p->trans_strand == 1? 1 : r->p->trans_strand == 2? -1 : 0;
+	h->is_primary = (r->id == r->parent);
+	h->n_cigar32 = r->p->n_cigar;
+	h->cigar32 = r->p->cigar;
 }
 
 static inline void mm_free_reg1(mm_reg1_t *r)
diff --git a/python/cminimap2.pxd b/python/cminimap2.pxd
index b4511fe..b19c347 100644
--- a/python/cminimap2.pxd
+++ b/python/cminimap2.pxd
@@ -79,12 +79,13 @@ cdef extern from "minimap.h":
 #
 cdef extern from "cminimap2.h":
 	ctypedef struct mm_hitpy_t:
-		const char *ctg
-		int32_t ctg_start, ctg_end
-		int32_t qry_start, qry_end
-		int32_t n_cigar32
-		uint8_t strand, mapq, is_primary
-		uint32_t *cigar32
+		const char *ctg;
+		int32_t ctg_start, ctg_end;
+		int32_t qry_start, qry_end;
+		int32_t blen, NM, ctg_len;
+		uint8_t strand, mapq, is_primary, trans_strand;
+		int32_t n_cigar32;
+		uint32_t *cigar32;
 	
-	mm_hitpy_t *mm_reg2hitpy(const mm_idx_t *mi, int n_regs, mm_reg1_t *regs)
+	void mm_reg2hitpy(const mm_idx_t *mi, mm_reg1_t *r, mm_hitpy_t *h)
 	void mm_free_reg1(mm_reg1_t *r)
diff --git a/python/minimap2.pyx b/python/minimap2.pyx
index ca347be..6aa1a81 100644
--- a/python/minimap2.pyx
+++ b/python/minimap2.pyx
@@ -3,55 +3,70 @@ from libc.stdlib cimport free
 cimport cminimap2
 
 cdef class Alignment:
-	cdef const char *_ctg
-	cdef int _r_st, _r_en
+	cdef int _ctg_len, _r_st, _r_en
 	cdef int _q_st, _q_en
-	cdef int8_t _is_rev, _is_primary
-	cdef uint8_t _mapq
-	cdef _cigar
+	cdef int _NM, _blen
+	cdef int8_t _strand, _trans_strand
+	cdef uint8_t _mapq, _is_primary
+	cdef _ctg, _cigar # these are python objects
 
-	def __cinit__(self, ctg, cs, ce, strand, qs, qe, mapq, cigar, is_primary):
-		self._ctg, self._r_st, self._r_en = ctg, cs, ce
-		self._is_rev, self._q_st, self._q_en = strand, qs, qe
+	def __cinit__(self, ctg, cl, cs, ce, strand, qs, qe, mapq, cigar, is_primary, blen, NM, trans_strand):
+		self._ctg, self._ctg_len, self._r_st, self._r_en = ctg, cl, cs, ce
+		self._strand, self._q_st, self._q_en = strand, qs, qe
+		self._NM, self._blen = NM, blen
 		self._mapq = mapq
 		self._cigar = cigar
 		self._is_primary = is_primary
+		self._trans_strand = trans_strand
 
 	@property
-	def ctg(self):
-		return self._ctg
+	def ctg(self): return self._ctg
 
 	@property
-	def r_st(self):
-		return self._r_st
+	def ctg_len(self): return self._ctg_len
 
 	@property
-	def r_en(self):
-		return self._r_en
+	def r_st(self): return self._r_st
 
 	@property
-	def is_rev(self):
-		return (self._is_rev != 0)
+	def r_en(self): return self._r_en
 
 	@property
-	def is_primary(self):
-		return (self._is_primary != 0)
+	def strand(self): return self.strand
 
 	@property
-	def q_st(self):
-		return self._q_st
+	def trans_strand(self): return self.trans_strand
 
 	@property
-	def q_en(self):
-		return self._q_en
+	def NM(self): return self._NM
 
 	@property
-	def mapq(self):
-		return self._mapq
+	def is_primary(self): return (self._is_primary != 0)
 
 	@property
-	def cigar(self):
-		return self._cigar
+	def q_st(self): return self._q_st
+
+	@property
+	def q_en(self): return self._q_en
+
+	@property
+	def mapq(self): return self._mapq
+
+	@property
+	def cigar(self): return self._cigar
+
+	@property
+	def cigar_str(self):
+		return "".join(map(lambda x: str(x[0]) + 'MIDNSH'[x[1]], self._cigar))
+
+	def __str__(self):
+		if self._strand > 0: strand = '+'
+		elif self._strand < 0: strand = '-'
+		else: strand = '?'
+		if self._is_primary != 0: tp = 'tp:A:P'
+		else: tp = 'tp:A:S'
+		return "\t".join([str(self._q_st), str(self._q_en), strand, self._ctg, str(self._ctg_len), str(self._r_st), str(self._r_en),
+				str(self._blen - self._NM), str(self._blen), str(self._mapq), "NM:i:" + str(self._NM), tp, "cg:Z:" + self.cigar_str])
 
 cdef class ThreadBuffer:
 	cdef cminimap2.mm_tbuf_t *_b
@@ -67,46 +82,55 @@ cdef class Aligner:
 	cdef public cminimap2.mm_idxopt_t idx_opt
 	cdef public cminimap2.mm_mapopt_t map_opt
 
-	def __cinit__(self, fn, preset=None):
-		self._config(preset)
+	def __cinit__(self, fn_idx_in, preset=None, k=None, w=None, min_cnt=None, min_chain_score=None, min_dp_score=None, bw=None, best_n=None, n_threads=3, fn_idx_out=None):
 		cdef cminimap2.mm_idx_reader_t *r;
-		r = cminimap2.mm_idx_reader_open(fn, &self.idx_opt, NULL)
-		self._idx = cminimap2.mm_idx_reader_read(r, 3) # NB: ONLY read the first part
-		cminimap2.mm_idx_reader_close(r)
-		cminimap2.mm_mapopt_update(&self.map_opt, self._idx)
+
+		self._config(preset)
+		self.idx_opt.batch_size = 0x7fffffffffffffffL # always build a uni-part index
+		if k is not None: self.idx_opt.k = k
+		if w is not None: self.idx_opt.w = w
+		if min_cnt is not None: self.map_opt.min_cnt = min_cnt
+		if min_chain_score is not None: self.map_opt.min_chain_score = min_chain_score
+		if min_dp_score is not None: self.map_opt.min_dp_max = min_dp_score
+		if bw is not None: self.map_opt.bw = bw
+		if best_n is not None: self.best_n = best_n
+
+		if fn_idx_out is None:
+			r = cminimap2.mm_idx_reader_open(fn_idx_in, &self.idx_opt, NULL)
+		else:
+			r = cminimap2.mm_idx_reader_open(fn_idx_in, &self.idx_opt, fn_idx_out)
+		if r is not NULL:
+			self._idx = cminimap2.mm_idx_reader_read(r, n_threads) # NB: ONLY read the first part
+			cminimap2.mm_idx_reader_close(r)
+			cminimap2.mm_mapopt_update(&self.map_opt, self._idx)
 	
 	def __dealloc__(self):
 		if self._idx is not NULL:
 			cminimap2.mm_idx_destroy(self._idx)
 
-	def _config(self, preset=None):
-		cminimap2.mm_set_opt(NULL, &self.idx_opt, &self.map_opt)
+	cdef _config(self, preset=None):
+		cminimap2.mm_set_opt(NULL, &self.idx_opt, &self.map_opt) # set the default options
 		if preset is not None:
-			cminimap2.mm_set_opt(preset, &self.idx_opt, &self.map_opt)
+			cminimap2.mm_set_opt(preset, &self.idx_opt, &self.map_opt) # preset
 		self.map_opt.flag |= 4 # always perform alignment
 
 	def map(self, seq, buf=None):
 		cdef cminimap2.mm_reg1_t *regs
+		cdef cminimap2.mm_hitpy_t h
 		cdef ThreadBuffer b
 		cdef int n_regs
 
-		if self._idx is NULL:
-			return None
+		if self._idx is NULL: return None
 		if buf is None: b = ThreadBuffer()
 		else: b = buf
 		regs = cminimap2.mm_map(self._idx, len(seq), seq, &n_regs, b._b, &self.map_opt, NULL)
-		hits = cminimap2.mm_reg2hitpy(self._idx, n_regs, regs)
 
-		arr = []
 		for i in range(n_regs):
-			h = hits[i]
+			cminimap2.mm_reg2hitpy(self._idx, &regs[i], &h)
 			cigar = []
 			for k in range(h.n_cigar32):
 				c = h.cigar32[k]
 				cigar.append([c>>4, c&0xf])
-			arr.append(Alignment(h.ctg, h.ctg_start, h.ctg_end, h.strand, h.qry_start, h.qry_end, h.mapq, cigar, h.is_primary))
+			yield Alignment(h.ctg, h.ctg_len, h.ctg_start, h.ctg_end, h.strand, h.qry_start, h.qry_end, h.mapq, cigar, h.is_primary, h.blen, h.NM, h.trans_strand)
 			cminimap2.mm_free_reg1(&regs[i])
 		free(regs)
-		free(hits)
-
-		return arr