diff --git a/kdq.h b/kdq.h
new file mode 100644
index 00000000..edd55b57
--- /dev/null
+++ b/kdq.h
@@ -0,0 +1,128 @@
+#ifndef __AC_KDQ_H
+#define __AC_KDQ_H
+
+#include <stdlib.h>
+#include <string.h>
+
+#define __KDQ_TYPE(type) \
+	typedef struct { \
+		size_t front:58, bits:6, count, mask; \
+		type *a; \
+	} kdq_##type##_t;
+
+#define kdq_t(type) kdq_##type##_t
+#define kdq_size(q) ((q)->count)
+#define kdq_first(q) ((q)->a[(q)->front])
+#define kdq_last(q) ((q)->a[((q)->front + (q)->count - 1) & (q)->mask])
+#define kdq_at(q, i) ((q)->a[((q)->front + (i)) & (q)->mask])
+
+#define __KDQ_IMPL(type, SCOPE) \
+	SCOPE kdq_##type##_t *kdq_init_##type() \
+	{ \
+		kdq_##type##_t *q; \
+		q = (kdq_##type##_t*)calloc(1, sizeof(kdq_##type##_t)); \
+		q->bits = 2, q->mask = (1ULL<<q->bits) - 1; \
+		q->a = (type*)malloc((1<<q->bits) * sizeof(type)); \
+		return q; \
+	} \
+	SCOPE void kdq_destroy_##type(kdq_##type##_t *q) \
+	{ \
+		if (q == 0) return; \
+		free(q->a); free(q); \
+	} \
+	SCOPE int kdq_resize_##type(kdq_##type##_t *q, int new_bits) \
+	{ \
+		size_t new_size = 1ULL<<new_bits, old_size = 1ULL<<q->bits; \
+		if (new_size < q->count) { /* not big enough */ \
+			int i; \
+			for (i = 0; i < 64; ++i) \
+				if (1ULL<<i > q->count) break; \
+			new_bits = i, new_size = 1ULL<<new_bits; \
+		} \
+		if (new_bits == q->bits) return q->bits; /* unchanged */ \
+		if (new_bits > q->bits) q->a = (type*)realloc(q->a, (1ULL<<new_bits) * sizeof(type)); \
+		if (q->front + q->count <= old_size) { /* unwrapped */ \
+			if (q->front + q->count > new_size) /* only happens for shrinking */ \
+				memmove(q->a, q->a + new_size, (q->front + q->count - new_size) * sizeof(type)); \
+		} else { /* wrapped */ \
+			memmove(q->a + (new_size - (old_size - q->front)), q->a + q->front, (old_size - q->front) * sizeof(type)); \
+			q->front = new_size - (old_size - q->front); \
+		} \
+		q->bits = new_bits, q->mask = (1ULL<<q->bits) - 1; \
+		if (new_bits < q->bits) q->a = (type*)realloc(q->a, (1ULL<<new_bits) * sizeof(type)); \
+		return q->bits; \
+	} \
+	SCOPE type *kdq_pushp_##type(kdq_##type##_t *q) \
+	{ \
+		if (q->count == 1ULL<<q->bits) kdq_resize_##type(q, q->bits + 1); \
+		return &q->a[((q->count++) + q->front) & (q)->mask]; \
+	} \
+	SCOPE void kdq_push_##type(kdq_##type##_t *q, type v) \
+	{ \
+		if (q->count == 1ULL<<q->bits) kdq_resize_##type(q, q->bits + 1); \
+		q->a[((q->count++) + q->front) & (q)->mask] = v; \
+	} \
+	SCOPE type *kdq_unshiftp_##type(kdq_##type##_t *q) \
+	{ \
+		if (q->count == 1ULL<<q->bits) kdq_resize_##type(q, q->bits + 1); \
+		++q->count; \
+		q->front = q->front? q->front - 1 : (1ULL<<q->bits) - 1; \
+		return &q->a[q->front]; \
+	} \
+	SCOPE void kdq_unshift_##type(kdq_##type##_t *q, type v) \
+	{ \
+		type *p; \
+		p = kdq_unshiftp_##type(q); \
+		*p = v; \
+	} \
+	SCOPE type *kdq_pop_##type(kdq_##type##_t *q) \
+	{ \
+		return q->count? &q->a[((--q->count) + q->front) & q->mask] : 0; \
+	} \
+	SCOPE type *kdq_shift_##type(kdq_##type##_t *q) \
+	{ \
+		type *d = 0; \
+		if (q->count == 0) return 0; \
+		d = &q->a[q->front++]; \
+		q->front &= q->mask; \
+		--q->count; \
+		return d; \
+	}
+
+#define KDQ_INIT2(type, SCOPE) \
+	__KDQ_TYPE(type) \
+	__KDQ_IMPL(type, SCOPE)
+
+#ifndef klib_unused
+#if (defined __clang__ && __clang_major__ >= 3) || (defined __GNUC__ && __GNUC__ >= 3)
+#define klib_unused __attribute__ ((__unused__))
+#else
+#define klib_unused
+#endif
+#endif /* klib_unused */
+
+#define KDQ_INIT(type) KDQ_INIT2(type, static inline klib_unused)
+
+#define KDQ_DECLARE(type) \
+	__KDQ_TYPE(type) \
+	kdq_##type##_t *kdq_init_##type(); \
+	void kdq_destroy_##type(kdq_##type##_t *q); \
+	int kdq_resize_##type(kdq_##type##_t *q, int new_bits); \
+	type *kdq_pushp_##type(kdq_##type##_t *q); \
+	void kdq_push_##type(kdq_##type##_t *q, type v); \
+	type *kdq_unshiftp_##type(kdq_##type##_t *q); \
+	void kdq_unshift_##type(kdq_##type##_t *q, type v); \
+	type *kdq_pop_##type(kdq_##type##_t *q); \
+	type *kdq_shift_##type(kdq_##type##_t *q);
+
+#define kdq_init(type) kdq_init_##type()
+#define kdq_destroy(type, q) kdq_destroy_##type(q)
+#define kdq_resize(type, q, new_bits) kdq_resize_##type(q, new_bits)
+#define kdq_pushp(type, q) kdq_pushp_##type(q)
+#define kdq_push(type, q, v) kdq_push_##type(q, v)
+#define kdq_pop(type, q) kdq_pop_##type(q)
+#define kdq_unshiftp(type, q) kdq_unshiftp_##type(q)
+#define kdq_unshift(type, q, v) kdq_unshift_##type(q, v)
+#define kdq_shift(type, q) kdq_shift_##type(q)
+
+#endif