4 files changed, 155 insertions, 0 deletions

diff --git a/libnetdata/circular_buffer/Makefile.am b/libnetdata/circular_buffer/Makefile.am
new file mode 100644
index 00000000..161784b8
--- /dev/null
+++ b/libnetdata/circular_buffer/Makefile.am
@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+AUTOMAKE_OPTIONS = subdir-objects
+MAINTAINERCLEANFILES = $(srcdir)/Makefile.in
+
+dist_noinst_DATA = \
+    README.md \
+    $(NULL)
diff --git a/libnetdata/circular_buffer/README.md b/libnetdata/circular_buffer/README.md
new file mode 100644
index 00000000..c4738651
--- /dev/null
+++ b/libnetdata/circular_buffer/README.md
@@ -0,0 +1,14 @@
+<!--
+title: "Circular Buffer"
+custom_edit_url: https://github.com/netdata/netdata/edit/master/libnetdata/circular_buffer/README.md
+sidebar_label: "Circular Buffer"
+learn_status: "Published"
+learn_topic_type: "Tasks"
+learn_rel_path: "Developers/libnetdata"
+-->
+
+# Circular Buffer
+
+`struct circular_buffer` is an adaptive circular buffer. It will start at an initial size
+and grow up to a maximum size as it fills. Two indices within the structure track the current
+`read` and `write` position for data.
diff --git a/libnetdata/circular_buffer/circular_buffer.c b/libnetdata/circular_buffer/circular_buffer.c
new file mode 100644
index 00000000..7ffe6b8b
--- /dev/null
+++ b/libnetdata/circular_buffer/circular_buffer.c
@@ -0,0 +1,113 @@
+#include "../libnetdata.h"
+
+struct circular_buffer *cbuffer_new(size_t initial, size_t max, size_t *statistics) {
+    struct circular_buffer *buf = mallocz(sizeof(struct circular_buffer));
+    buf->size = initial;
+    buf->data = mallocz(initial);
+    buf->write = 0;
+    buf->read = 0;
+    buf->max_size = max;
+    buf->statistics = statistics;
+
+    if(buf->statistics)
+        __atomic_add_fetch(buf->statistics, sizeof(struct circular_buffer) + buf->size, __ATOMIC_RELAXED);
+
+    return buf;
+}
+
+void cbuffer_free(struct circular_buffer *buf) {
+    if (unlikely(!buf))
+        return;
+
+    if(buf->statistics)
+        __atomic_sub_fetch(buf->statistics, sizeof(struct circular_buffer) + buf->size, __ATOMIC_RELAXED);
+
+    freez(buf->data);
+    freez(buf);
+}
+
+static int cbuffer_realloc_unsafe(struct circular_buffer *buf) {
+    // Check that we can grow
+    if (buf->size >= buf->max_size)
+        return 1;
+
+    size_t old_size = buf->size;
+    size_t new_size = buf->size * 2;
+    if (new_size > buf->max_size)
+        new_size = buf->max_size;
+
+    // We know that: size < new_size <= max_size
+    // For simplicity align the current data at the bottom of the new buffer
+    char *new_data = mallocz(new_size);
+    if (buf->read == buf->write)
+        buf->write = 0; // buffer is empty
+    else if (buf->read < buf->write) {
+        memcpy(new_data, buf->data + buf->read, buf->write - buf->read);
+        buf->write -= buf->read;
+    } else {
+        size_t top_part = buf->size - buf->read;
+        memcpy(new_data, buf->data + buf->read, top_part);
+        memcpy(new_data + top_part, buf->data, buf->write);
+        buf->write = top_part + buf->write;
+    }
+    buf->read = 0;
+
+    // Switch buffers
+    freez(buf->data);
+    buf->data = new_data;
+    buf->size = new_size;
+
+    if(buf->statistics)
+        __atomic_add_fetch(buf->statistics, new_size - old_size, __ATOMIC_RELAXED);
+
+    return 0;
+}
+
+size_t cbuffer_available_size_unsafe(struct circular_buffer *buf) {
+    size_t len = (buf->write >= buf->read) ? (buf->write - buf->read) : (buf->size - buf->read + buf->write);
+    return buf->max_size - len;
+}
+
+int cbuffer_add_unsafe(struct circular_buffer *buf, const char *d, size_t d_len) {
+    size_t len = (buf->write >= buf->read) ? (buf->write - buf->read) : (buf->size - buf->read + buf->write);
+    while (d_len + len >= buf->size) {
+        if (cbuffer_realloc_unsafe(buf)) {
+            return 1;
+        }
+    }
+    // Guarantee: write + d_len cannot hit read
+    if (buf->write + d_len < buf->size) {
+        memcpy(buf->data + buf->write, d, d_len);
+        buf->write += d_len;
+    }
+    else {
+        size_t top_part = buf->size - buf->write;
+        memcpy(buf->data + buf->write, d, top_part);
+        memcpy(buf->data, d + top_part, d_len - top_part); 
+        buf->write = d_len - top_part;
+    }
+    return 0;
+}
+
+// Assume caller does not remove too many bytes (i.e. read will jump over write)
+void cbuffer_remove_unsafe(struct circular_buffer *buf, size_t num) {
+    buf->read += num;
+    // Assume num < size (i.e. caller cannot remove more bytes than are in the buffer)
+    if (buf->read >= buf->size)
+        buf->read -= buf->size;
+}
+
+size_t cbuffer_next_unsafe(struct circular_buffer *buf, char **start) {
+    if (start != NULL)
+      *start = buf->data + buf->read;
+
+    if (buf->read <= buf->write) {
+        return buf->write - buf->read;      // Includes empty case
+    }
+    return buf->size - buf->read;
+}
+
+void cbuffer_flush(struct circular_buffer*buf) {
+    buf->write = 0;
+    buf->read = 0;
+}
\ No newline at end of file
diff --git a/libnetdata/circular_buffer/circular_buffer.h b/libnetdata/circular_buffer/circular_buffer.h
new file mode 100644
index 00000000..9d29a84d
--- /dev/null
+++ b/libnetdata/circular_buffer/circular_buffer.h
@@ -0,0 +1,20 @@
+#ifndef CIRCULAR_BUFFER_H
+#define CIRCULAR_BUFFER_H 1
+
+#include <string.h>
+
+struct circular_buffer {
+    size_t size, write, read, max_size;
+    size_t *statistics;
+    char *data;
+};
+
+struct circular_buffer *cbuffer_new(size_t initial, size_t max, size_t *statistics);
+void cbuffer_free(struct circular_buffer *buf);
+int cbuffer_add_unsafe(struct circular_buffer *buf, const char *d, size_t d_len);
+void cbuffer_remove_unsafe(struct circular_buffer *buf, size_t num);
+size_t cbuffer_next_unsafe(struct circular_buffer *buf, char **start);
+size_t cbuffer_available_size_unsafe(struct circular_buffer *buf);
+void cbuffer_flush(struct circular_buffer*buf);
+
+#endif