kern/cbuf: new module

3 files changed, 180 insertions, 0 deletions

diff --git a/Makefrag.am b/Makefrag.am
index 7f74690b..243c2245 100644
--- a/Makefrag.am
+++ b/Makefrag.am
@@ -15,6 +15,8 @@ x15_SOURCES +=                                              \
         kern/bitmap.c                                       \
         kern/bitmap.h                                       \
         kern/bitmap_i.h                                     \
+        kern/cbuf.c                                         \
+        kern/cbuf.h                                         \
         kern/condition.c                                    \
         kern/condition.h                                    \
         kern/condition_types.h                              \
diff --git a/kern/cbuf.c b/kern/cbuf.c
new file mode 100644
index 00000000..17369afe
--- /dev/null
+++ b/kern/cbuf.c
@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2015 Richard Braun.
+ *
+ * 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 <kern/assert.h>
+#include <kern/cbuf.h>
+#include <kern/error.h>
+#include <kern/macros.h>
+
+/* Negative close to 0 so that an overflow occurs early */
+#define CBUF_INIT_INDEX ((unsigned long)-500)
+
+void
+cbuf_init(struct cbuf *cbuf, char *buf, unsigned long capacity)
+{
+    assert(ISP2(capacity));
+
+    cbuf->buf = buf;
+    cbuf->capacity = capacity;
+    cbuf->start = CBUF_INIT_INDEX;
+    cbuf->end = cbuf->start;
+}
+
+static unsigned long
+cbuf_index(const struct cbuf *cbuf, unsigned long abs_index)
+{
+    return abs_index & (cbuf->capacity - 1);
+}
+
+void
+cbuf_push(struct cbuf *cbuf, char byte)
+{
+    cbuf->buf[cbuf_index(cbuf, cbuf->end)] = byte;
+    cbuf->end++;
+
+    /* Mind integer overflows */
+    if (cbuf_size(cbuf) > cbuf->capacity) {
+        cbuf->start = cbuf->end - cbuf->capacity;
+    }
+}
+
+int
+cbuf_pop(struct cbuf *cbuf, char *bytep)
+{
+    if (cbuf_size(cbuf) == 0) {
+        return ERROR_AGAIN;
+    }
+
+    *bytep = cbuf->buf[cbuf_index(cbuf, cbuf->start)];
+    cbuf->start++;
+    return 0;
+}
+
+int
+cbuf_read(const struct cbuf *cbuf, unsigned long index, char *bytep)
+{
+    /* Mind integer overflows */
+    if ((cbuf->end - index - 1) >= cbuf_size(cbuf)) {
+        return ERROR_INVAL;
+    }
+
+    *bytep = cbuf->buf[cbuf_index(cbuf, index)];
+    return 0;
+}
diff --git a/kern/cbuf.h b/kern/cbuf.h
new file mode 100644
index 00000000..3ea38418
--- /dev/null
+++ b/kern/cbuf.h
@@ -0,0 +1,101 @@
+/*
+ * Copyright (c) 2015 Richard Braun.
+ *
+ * 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/>.
+ *
+ *
+ * Circular character buffer.
+ */
+
+#ifndef _KERN_CBUF_H
+#define _KERN_CBUF_H
+
+/*
+ * Circular buffer descriptor.
+ *
+ * The buffer capacity must be a power-of-two. Indexes are absolute values
+ * which can overflow. Their difference cannot exceed the capacity.
+ */
+struct cbuf {
+    char *buf;
+    unsigned long capacity;
+    unsigned long start;
+    unsigned long end;
+};
+
+static inline unsigned long
+cbuf_capacity(const struct cbuf *cbuf)
+{
+    return cbuf->capacity;
+}
+
+static inline unsigned long
+cbuf_start(const struct cbuf *cbuf)
+{
+    return cbuf->start;
+}
+
+static inline unsigned long
+cbuf_end(const struct cbuf *cbuf)
+{
+    return cbuf->end;
+}
+
+static inline unsigned long
+cbuf_size(const struct cbuf *cbuf)
+{
+    return cbuf->end - cbuf->start;
+}
+
+static inline void
+cbuf_clear(struct cbuf *cbuf)
+{
+    cbuf->start = cbuf->end;
+}
+
+/*
+ * Initialize a circular buffer.
+ *
+ * The descriptor is set to use the given buffer for storage. Capacity
+ * must be a power-of-two.
+ */
+void cbuf_init(struct cbuf *cbuf, char *buf, unsigned long capacity);
+
+/*
+ * Append a byte to a circular buffer.
+ *
+ * The end index is incremented. If the buffer is full, the oldest byte
+ * is overwritten and the start index is updated accordingly.
+ */
+void cbuf_push(struct cbuf *cbuf, char byte);
+
+/*
+ * Read a byte from a circular buffer.
+ *
+ * If the buffer is empty, ERROR_AGAIN is returned. Otherwise, the oldest
+ * byte is stored at the bytep address, the start index is incremented,
+ * and 0 is returned.
+ */
+int cbuf_pop(struct cbuf *cbuf, char *bytep);
+
+/*
+ * Read a byte at a specific location.
+ *
+ * If the given index is outside buffer boundaries, ERROR_INVAL is returned.
+ * Otherwise the byte is stored at the bytep address and 0 is returned.
+ * The buffer isn't changed by this operation.
+ */
+int cbuf_read(const struct cbuf *cbuf, unsigned long index, char *bytep);
+
+#endif /* _KERN_CBUF_H */