The choice of array implementation is deliberate, for future iterator support
---
 lib/Makefile.local    |   1 +
 lib/notmuch-private.h |  11 ++++
 lib/string-map.c      | 153 ++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 165 insertions(+)
 create mode 100644 lib/string-map.c

diff --git a/lib/Makefile.local b/lib/Makefile.local
index beb9635..9280880 100644
--- a/lib/Makefile.local
+++ b/lib/Makefile.local
@@ -40,6 +40,7 @@ libnotmuch_c_srcs =		\
 	$(dir)/messages.c	\
 	$(dir)/sha1.c		\
 	$(dir)/built-with.c	\
+	$(dir)/string-map.c    \
 	$(dir)/tags.c
 
 libnotmuch_cxx_srcs =		\
diff --git a/lib/notmuch-private.h b/lib/notmuch-private.h
index df63905..5abde88 100644
--- a/lib/notmuch-private.h
+++ b/lib/notmuch-private.h
@@ -540,6 +540,17 @@ _notmuch_string_list_sort (notmuch_string_list_t *list);
 /* string-map.c */
 typedef struct _notmuch_string_map  notmuch_string_map_t;
 
+notmuch_string_map_t *
+_notmuch_string_map_create (const void *ctx);
+
+void
+_notmuch_string_map_append (notmuch_string_map_t *map,
+			    const char *key,
+			    const char *value);
+
+const char *
+_notmuch_string_map_get (notmuch_string_map_t *map, const char *key);
+
 /* tags.c */
 
 notmuch_tags_t *
diff --git a/lib/string-map.c b/lib/string-map.c
new file mode 100644
index 0000000..0491a10
--- /dev/null
+++ b/lib/string-map.c
@@ -0,0 +1,153 @@
+/* string-map.c - associative arrays of strings
+ *
+ *
+ * Copyright © 2016 David Bremner
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see http://www.gnu.org/licenses/ .
+ *
+ * Author: David Bremner <david@tethera.net>
+ */
+
+#include "notmuch-private.h"
+
+/* Create a new notmuch_string_map_t object, with 'ctx' as its
+ * talloc owner.
+ *
+ * This function can return NULL in case of out-of-memory.
+ */
+
+typedef struct _notmuch_string_pair_t {
+    char *key;
+    char *value;
+} notmuch_string_pair_t;
+
+struct _notmuch_string_map {
+    notmuch_bool_t sorted;
+    size_t length;
+    notmuch_string_pair_t *pairs;
+};
+
+notmuch_string_map_t *
+_notmuch_string_map_create (const void *ctx)
+{
+    notmuch_string_map_t *map;
+
+    map = talloc (ctx, notmuch_string_map_t);
+    if (unlikely (map == NULL))
+	return NULL;
+
+    map->length = 0;
+    map->pairs = NULL;
+    map->sorted = TRUE;
+
+    return map;
+}
+
+void
+_notmuch_string_map_append (notmuch_string_map_t *map,
+			    const char *key,
+			    const char *value)
+{
+
+    map->length++;
+    map->sorted = FALSE;
+
+    if (map->pairs)
+	map->pairs = talloc_realloc (map, map->pairs, notmuch_string_pair_t, map->length + 1);
+    else
+	map->pairs = talloc_array (map, notmuch_string_pair_t, map->length + 1);
+
+    map->pairs[map->length - 1].key = talloc_strdup (map, key);
+    map->pairs[map->length - 1].value = talloc_strdup (map, value);
+
+    /* Add sentinel */
+    map->pairs[map->length].key = NULL;
+    map->pairs[map->length].value = NULL;
+
+}
+
+static int
+cmppair (const void *pa, const void *pb)
+{
+    notmuch_string_pair_t *a = (notmuch_string_pair_t *) pa;
+    notmuch_string_pair_t *b = (notmuch_string_pair_t *) pb;
+
+    return strcmp (a->key, b->key);
+}
+
+static void
+_notmuch_string_map_sort (notmuch_string_map_t *map)
+{
+    if (map->length == 0)
+	return;
+
+    if (map->sorted)
+	return;
+
+    qsort (map->pairs, map->length, sizeof (notmuch_string_pair_t), cmppair);
+
+    map->sorted = TRUE;
+}
+
+static notmuch_bool_t
+string_cmp (const char *a, const char *b, notmuch_bool_t exact)
+{
+    if (exact)
+	return (strcmp (a, b));
+    else
+	return (strncmp (a, b, strlen (a)));
+}
+
+static notmuch_string_pair_t *
+bsearch_first (notmuch_string_pair_t *array, size_t len, const char *key, notmuch_bool_t exact)
+{
+    size_t first = 0;
+    size_t last = len - 1;
+    size_t mid;
+
+    if (len <= 0)
+	return NULL;
+
+    while (last > first) {
+	mid = (first + last) / 2;
+	int sign = string_cmp (key, array[mid].key, exact);
+
+	if (sign <= 0)
+	    last = mid;
+	else
+	    first = mid + 1;
+    }
+
+
+    if (string_cmp (key, array[first].key, exact) == 0)
+	return array + first;
+    else
+	return NULL;
+
+}
+
+const char *
+_notmuch_string_map_get (notmuch_string_map_t *map, const char *key)
+{
+    notmuch_string_pair_t *pair;
+
+    /* this means that calling append invalidates iterators */
+    _notmuch_string_map_sort (map);
+
+    pair = bsearch_first (map->pairs, map->length, key, TRUE);
+    if (! pair)
+	return NULL;
+
+    return pair->value;
+}
-- 
2.8.1