From cff6963f31759a056a6e01e2bb2b8953a2a51cf8 Mon Sep 17 00:00:00 2001
From: Ben Pfaff <blp@cs.stanford.edu>
Date: Sat, 29 Jan 2022 16:56:30 -0800
Subject: [PATCH] mode: New order statistic.

---
 src/math/automake.mk |  1 +
 src/math/mode.c      | 66 ++++++++++++++++++++++++++++++++++++++++++++
 src/math/mode.h      | 46 ++++++++++++++++++++++++++++++
 3 files changed, 113 insertions(+)
 create mode 100644 src/math/mode.c
 create mode 100644 src/math/mode.h

diff --git a/src/math/automake.mk b/src/math/automake.mk
index 5237583b60..fa9fb82ae7 100644
--- a/src/math/automake.mk
+++ b/src/math/automake.mk
@@ -39,6 +39,7 @@ src_math_libpspp_math_la_SOURCES = \
 	src/math/levene.c src/math/levene.h \
 	src/math/linreg.c src/math/linreg.h \
 	src/math/merge.c  src/math/merge.h \
+	src/math/mode.c src/math/mode.h \
 	src/math/moments.c  src/math/moments.h \
 	src/math/np.c src/math/np.h \
 	src/math/order-stats.c src/math/order-stats.h \
diff --git a/src/math/mode.c b/src/math/mode.c
new file mode 100644
index 0000000000..6c9f0c4563
--- /dev/null
+++ b/src/math/mode.c
@@ -0,0 +1,66 @@
+/* PSPP - a program for statistical analysis.
+   Copyright (C) 2008, 2009, 2011, 2022 Free Software Foundation, Inc.
+
+   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/>. */
+
+#include <config.h>
+
+#include "math/mode.h"
+
+#include "data/casereader.h"
+#include "data/val-type.h"
+#include "data/variable.h"
+#include "libpspp/assertion.h"
+#include "libpspp/cast.h"
+
+#include "gl/xalloc.h"
+
+
+static void
+mode_destroy (struct statistic *stat)
+{
+  struct mode *mode = UP_CAST (stat, struct mode, parent.parent);
+  free (mode);
+}
+
+static void
+mode_accumulate (struct statistic *stat, const struct ccase *cx UNUSED,
+                 double c, double cc UNUSED, double y)
+{
+  struct mode *mode = UP_CAST (stat, struct mode, parent.parent);
+  if (c > mode->mode_weight)
+    {
+      mode->mode = y;
+      mode->mode_weight = c;
+      mode->n_modes = 1;
+    }
+  else if (c == mode->mode_weight)
+    mode->n_modes++;
+}
+
+struct mode *
+mode_create (void)
+{
+  struct mode *mode = xmalloc (sizeof *mode);
+  *mode = (struct mode) {
+    .parent = {
+      .parent = {
+        mode_destroy,
+      },
+      .accumulate = mode_accumulate,
+    },
+    .mode = SYSMIS,
+  };
+  return mode;
+}
diff --git a/src/math/mode.h b/src/math/mode.h
new file mode 100644
index 0000000000..b2d1441ac7
--- /dev/null
+++ b/src/math/mode.h
@@ -0,0 +1,46 @@
+/* PSPP - a program for statistical analysis.
+   Copyright (C) 2004, 2008, 2009, 2022 Free Software Foundation, Inc.
+
+   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/>. */
+
+#ifndef MATH_MODE_H
+#define MATH_MODE_H 1
+
+#include <stddef.h>
+
+#include "order-stats.h"
+
+/* To calculate the mode:
+
+   - Create a "struct mode" with mode_create().
+   - Feed in the data with order_stats_accumulate() or
+     order_stats_accumulate_idx().  The data must be in sorted order: if
+     necessary, use one of the sorting functions from sort.h to sort them.
+   - The members of "struct mode" then designate the mode.
+   - Destroy the data structure with statistic_destroy().
+*/
+
+struct mode
+{
+  struct order_stats parent;
+
+  /* These are initialized by order_stats_accumulate{_idx}(). */
+  double mode;          /* The value of the smallest mode, if 'n_modes' > 0. */
+  double mode_weight;   /* The weight of each mode, if 'n_modes' > 0. */
+  size_t n_modes;       /* The number of modes. */
+};
+
+struct mode *mode_create (void);
+
+#endif /* mode.h */
-- 
2.30.2