Add basic Read interface with a simple BufReader

Author: micahswitzer

kernel/lib/io.hpp

@@ -0,0 +1,78 @@
+#pragma once
+
+#include <stddef.h>
+#include <stdbool.h>
+#include <sys/types.h>
+
+namespace io {
+
+enum whence {
+    SEEK_SET,
+    SEEK_CUR,
+    SEEK_END,
+};
+
+class Read {
+public:
+    virtual ~Read() { }
+    virtual bool isSeekable(void) = 0;
+    virtual size_t tell(void) = 0;
+    virtual bool seek(off_t offset, whence whence) = 0;
+    virtual size_t read(void *ptr, size_t size, size_t nmem) = 0;
+    virtual int error(void) = 0;
+    virtual bool eof(void) = 0;
+};
+
+class BufReader :
+    public Read
+{
+public:
+    BufReader(const void *buf, off_t size)
+        : _buffer((const char *)buf)
+        , _size(size > 0 ? size : 0)
+        , _pos(0) {
+    }
+    virtual ~BufReader() { }
+    virtual bool isSeekable(void) { return true; }
+    virtual size_t tell(void) { return _pos; }
+    virtual bool seek(off_t offset, whence whence) {
+        off_t target;
+        switch (whence) {
+        case SEEK_SET:
+            target = offset;
+            break;
+        case SEEK_CUR:
+            target = _pos + offset;
+            break;
+        case SEEK_END:
+            target = _size + offset;
+            break;
+        default:
+            return false;
+        }
+        if (target < 0 || target > _size)
+            return false;
+        _pos = target;
+        return true;
+    }
+    virtual size_t read(void *ptr, size_t size, size_t nmem) {
+        off_t count = (off_t)(size * nmem);
+        off_t final = _pos + count;
+        if (count <= 0 || final < _pos)
+            return 0;
+        if (final > _size)
+            count = _size - _pos;
+        memcpy(ptr, _buffer + _pos, count);
+        _pos += count;
+        return count;
+    }
+    virtual int error(void) { return 0; }
+    virtual bool eof(void) { return _pos == _size; }
+
+private:
+    const char *_buffer;
+    const off_t _size;
+    off_t _pos;
+};
+
+}