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+/*  Copyright (C) CZ.NIC, z.s.p.o. <knot-resolver@labs.nic.cz>
+ *  SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+/**
+ *
+ * @file array.h
+ * @brief A set of simple macros to make working with dynamic arrays easier.
+ *
+ * @note The C has no generics, so it is implemented mostly using macros.
+ * Be aware of that, as direct usage of the macros in the evaluating macros
+ * may lead to different expectations:
+ *
+ * @code{.c}
+ *     MIN(array_push(arr, val), other)
+ * @endcode
+ *
+ * May evaluate the code twice, leading to unexpected behaviour.
+ * This is a price to pay for the absence of proper generics.
+ *
+ * # Example usage:
+ *
+ * @code{.c}
+ *      array_t(const char*) arr;
+ *      array_init(arr);
+ *
+ *      // Reserve memory in advance
+ *      if (array_reserve(arr, 2) < 0) {
+ *          return ENOMEM;
+ *      }
+ *
+ *      // Already reserved, cannot fail
+ *      array_push(arr, "princess");
+ *      array_push(arr, "leia");
+ *
+ *      // Not reserved, may fail
+ *      if (array_push(arr, "han") < 0) {
+ *          return ENOMEM;
+ *      }
+ *
+ *      // It does not hide what it really is
+ *      for (size_t i = 0; i < arr.len; ++i) {
+ *          printf("%s\n", arr.at[i]);
+ *      }
+ *
+ *      // Random delete
+ *      array_del(arr, 0);
+ * @endcode
+ * \addtogroup generics
+ * @{
+ */
+
+#pragma once
+#include <stdlib.h>
+
+/** Choose array length when it overflows. */
+static inline size_t array_next_count(size_t elm_size, size_t want, size_t have)
+{
+	if (want >= have * 2) // We amortized enough and maybe more won't be needed.
+		return want;
+	const size_t want_b = want * elm_size;
+	if (want_b < 64) // Short arrays are cheap to copy; get just one extra.
+		return want + 1;
+	if (want_b < 1024) // 50% growth amortizes to roughly 3 copies per element.
+		return want + want / 2;
+	return want * 2; // Doubling growth amortizes to roughly 2 copies per element.
+}
+
+/** @internal Incremental memory reservation */
+static inline int array_std_reserve(void *baton, void **mem, size_t elm_size, size_t want, size_t *have)
+{
+	if (*have >= want) {
+		return 0;
+	}
+	/* Simplified Qt containers growth strategy */
+	size_t next_size = array_next_count(elm_size, want, *have);
+	void *mem_new = realloc(*mem, next_size * elm_size);
+	if (mem_new != NULL) {
+		*mem = mem_new;
+		*have = next_size;
+		return 0;
+	}
+	return -1;
+}
+
+/** @internal Wrapper for stdlib free. */
+static inline void array_std_free(void *baton, void *p)
+{
+	free(p);
+}
+
+/** Declare an array structure. */
+#define array_t(type) struct {type * at; size_t len; size_t cap; }
+
+/** Zero-initialize the array. */
+#define array_init(array) ((array).at = NULL, (array).len = (array).cap = 0)
+
+/** Free and zero-initialize the array (plain malloc/free). */
+#define array_clear(array) \
+	array_clear_mm(array, array_std_free, NULL)
+
+/** Make the array empty and free pointed-to memory.
+ * Mempool usage: pass mm_free and a knot_mm_t* . */
+#define array_clear_mm(array, free, baton) \
+	(free)((baton), (array).at), array_init(array)
+
+/** Reserve capacity for at least n elements.
+ * @return 0 if success, <0 on failure */
+#define array_reserve(array, n) \
+	array_reserve_mm(array, n, array_std_reserve, NULL)
+
+/** Reserve capacity for at least n elements.
+ * Mempool usage: pass kr_memreserve and a knot_mm_t* .
+ * @return 0 if success, <0 on failure */
+#define array_reserve_mm(array, n, reserve, baton) \
+	(reserve)((baton), (void **) &(array).at, sizeof((array).at[0]), (n), &(array).cap)
+
+/**
+ * Push value at the end of the array, resize it if necessary.
+ * Mempool usage: pass kr_memreserve and a knot_mm_t* .
+ * @note May fail if the capacity is not reserved.
+ * @return element index on success, <0 on failure
+ */
+#define array_push_mm(array, val, reserve, baton) \
+	(int)((array).len < (array).cap ? ((array).at[(array).len] = val, (array).len++) \
+		: (array_reserve_mm(array, ((array).cap + 1), reserve, baton) < 0 ? -1 \
+			: ((array).at[(array).len] = val, (array).len++)))
+
+/**
+ * Push value at the end of the array, resize it if necessary (plain malloc/free).
+ * @note May fail if the capacity is not reserved.
+ * @return element index on success, <0 on failure
+ */
+#define array_push(array, val) \
+	array_push_mm(array, val, array_std_reserve, NULL)
+
+/**
+ * Pop value from the end of the array.
+ */
+#define array_pop(array) \
+ 	(array).len -= 1
+
+/**
+ * Remove value at given index.
+ * @return 0 on success, <0 on failure
+ */
+#define array_del(array, i) \
+	(int)((i) < (array).len ? ((array).len -= 1,(array).at[i] = (array).at[(array).len], 0) : -1)
+
+/**
+ * Return last element of the array.
+ * @warning Undefined if the array is empty.
+ */
+#define array_tail(array) \
+    (array).at[(array).len - 1]
+
+/** @} */