reftable: add a heap-based priority queue for reftable records

This is needed to create a merged view multiple reftables

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/pq_test.c

@@ -0,0 +1,82 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "system.h"
+
+#include "basics.h"
+#include "constants.h"
+#include "pq.h"
+#include "record.h"
+#include "reftable-tests.h"
+#include "test_framework.h"
+
+void merged_iter_pqueue_check(struct merged_iter_pqueue pq)
+{
+	int i;
+	for (i = 1; i < pq.len; i++) {
+		int parent = (i - 1) / 2;
+
+		EXPECT(pq_less(&pq.heap[parent], &pq.heap[i]));
+	}
+}
+
+static void test_pq(void)
+{
+	char *names[54] = { NULL };
+	int N = ARRAY_SIZE(names) - 1;
+
+	struct merged_iter_pqueue pq = { NULL };
+	const char *last = NULL;
+
+	int i = 0;
+	for (i = 0; i < N; i++) {
+		char name[100];
+		snprintf(name, sizeof(name), "%02d", i);
+		names[i] = xstrdup(name);
+	}
+
+	i = 1;
+	do {
+		struct reftable_record rec =
+			reftable_new_record(BLOCK_TYPE_REF);
+		struct pq_entry e = { 0 };
+
+		reftable_record_as_ref(&rec)->refname = names[i];
+		e.rec = rec;
+		merged_iter_pqueue_add(&pq, e);
+		merged_iter_pqueue_check(pq);
+		i = (i * 7) % N;
+	} while (i != 1);
+
+	while (!merged_iter_pqueue_is_empty(pq)) {
+		struct pq_entry e = merged_iter_pqueue_remove(&pq);
+		struct reftable_ref_record *ref =
+			reftable_record_as_ref(&e.rec);
+
+		merged_iter_pqueue_check(pq);
+
+		if (last) {
+			EXPECT(strcmp(last, ref->refname) < 0);
+		}
+		last = ref->refname;
+		ref->refname = NULL;
+		reftable_free(ref);
+	}
+
+	for (i = 0; i < N; i++) {
+		reftable_free(names[i]);
+	}
+
+	merged_iter_pqueue_release(&pq);
+}
+
+int pq_test_main(int argc, const char *argv[])
+{
+	RUN_TEST(test_pq);
+	return 0;
+}