diff -r 000000000000 -r 3f655b1b0d93 bookloupe/counters.c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/bookloupe/counters.c	Sat Sep 21 18:28:40 2013 +0100
@@ -0,0 +1,106 @@
+#include <stdlib.h>
+#include <glib.h>
+#include "bookloupe.h"
+#include "counters.h"
+
+struct matching_counter {
+    int open,close;
+};
+
+static struct matching_counter *matching_counter_new(void)
+{
+    return g_slice_new0(struct matching_counter);
+}
+
+static void matching_counter_free(struct matching_counter *counter)
+{
+    g_slice_free(struct matching_counter,counter);
+}
+
+static gint compar_unichars(gconstpointer a,gconstpointer b,gpointer unused)
+{
+    /*
+     * Unicode code points only go up to 0x10FFFF and thus this cannot overflow.
+     */
+    return GPOINTER_TO_INT(a)-GPOINTER_TO_INT(b);
+}
+
+/*
+ * For matching characters, we maintain a count of the opens and closes.
+ * In the simplest case, we are dealing with a matching pair such as [ and ]
+ * where there is a 1:1 mapping between an instance of [ with an open and
+ * between an instance of ] with a close. matching_ket() is
+ * responsible for selecting an arbitary base character of a matching pair.
+ */
+static gpointer matching_key(gunichar ch)
+{
+    gunichar mirrored;
+    if (g_unichar_get_mirror_char(ch,&mirrored))
+	if (ch<mirrored)
+	    return GINT_TO_POINTER((gint)ch);
+	else
+	    return GINT_TO_POINTER((gint)mirrored);
+    else if (ch==CHAR_SQUOTE || ch==CHAR_OPEN_SQUOTE)
+	return GINT_TO_POINTER((gint)CHAR_SQUOTE);
+    else if (ch==CHAR_LS_QUOTE || ch==CHAR_RS_QUOTE)
+	return GINT_TO_POINTER((gint)CHAR_LS_QUOTE);
+    else
+    {
+	g_warning("Matching pair not found for U+%04"G_GINT32_FORMAT"X",ch);
+	return GINT_TO_POINTER((gint)ch);
+    }
+}
+
+void increment_matching(struct counters *counters,gunichar ch,gboolean open)
+{
+    gpointer key,orig_key;
+    struct matching_counter *value;
+    if (!counters->matching)
+	counters->matching=g_tree_new_full(compar_unichars,NULL,NULL,
+	  (GDestroyNotify)matching_counter_free);
+    key=matching_key(ch);
+    if (!g_tree_lookup_extended(counters->matching,key,&orig_key,
+      (gpointer *)&value))
+    {
+	value=matching_counter_new();
+	g_tree_insert(counters->matching,key,value);
+    }
+    if (open)
+	value->open++;
+    else
+	value->close++;
+}
+
+int matching_count(const struct counters *counters,gunichar ch,gboolean open)
+{
+    struct matching_counter *value;
+    if (!counters->matching)
+	return 0;
+    value=g_tree_lookup(counters->matching,matching_key(ch));
+    if (!value)
+	return 0;
+    return open?value->open:value->close;
+}
+
+/*
+ * Return open count - closed count
+ */
+int matching_difference(const struct counters *counters,gunichar ch)
+{
+    struct matching_counter *value;
+    if (!counters->matching)
+	return 0;
+    value=g_tree_lookup(counters->matching,matching_key(ch));
+    if (!value)
+	return 0;
+    return value->open-value->close;
+}
+
+void counters_destroy(struct counters *counters)
+{
+    if (counters->matching)
+    {
+	g_tree_destroy(counters->matching);
+	counters->matching=NULL;
+    }
+}