lib: Add buffer truncate

src/lib/buffer.c

@@ -358,3 +358,28 @@ void buffer_verify_pool(buffer_t *_buf)
 		i_assert(ret == buf->w_buffer);
 	}
 }
+
+void buffer_truncate_rshift_bits(buffer_t *buf, size_t bits)
+{
+	/* no-op if it's shorten than bits in any case.. */
+	if (buf->used * 8 < bits) return;
+
+	if (bits > 0) {
+		/* truncate it to closest byte boundary */
+		size_t bytes = ((bits + 7) & -8U)/8;
+		/* remainding bits */
+		bits = bits % 8;
+		buffer_set_used_size(buf, I_MIN(bytes, buf->used));
+		unsigned char *ptr = buffer_get_modifiable_data(buf, &bytes);
+		/* right shift over byte array */
+		if (bits > 0) {
+			for(size_t i=bytes-1;i>0;i--)
+				ptr[i] = (ptr[i]>>(8-bits)) +
+					 ((ptr[i-1]&(0xff>>(bits)))<<bits);
+			ptr[0] = ptr[0]>>(8-bits);
+		}
+	} else {
+		buffer_set_used_size(buf, 0);
+	}
+}
+

src/lib/buffer.h

@@ -121,4 +121,29 @@ buffer_get_used_size(const buffer_t *buf)
    buffer needs to be increased. */
 void buffer_verify_pool(buffer_t *buf);
 
+/* This will truncate your byte buffer to contain at most
+   given number of bits. 
+
+ 1 bits:    01 00000001
+ 2 bits:    03 00000011
+ 3 bits:    07 00000111
+ 4 bits:    0f 00001111
+ 5 bits:    1f 00011111
+ 6 bits:    3f 00111111
+ 7 bits:    7f 01111111
+ 8 bits:    ff 11111111
+ 9 bits:  01ff 0000000111111111
+10 bits:  03ff 0000001111111111
+11 bits:  07ff 0000011111111111
+12 bits:  0fff 0000111111111111
+13 bits:  1fff 0001111111111111
+14 bits:  3fff 0011111111111111
+15 bits:  7fff 0111111111111111
+16 bits:  ffff 1111111111111111
+
+ and so forth
+
+*/
+void buffer_truncate_rshift_bits(buffer_t *buf, size_t bits);
+
 #endif