---
title: 2021q1 Homework3 (quiz3)
tags: Linux Kernel Course
---
# 2021q1 Homework3 (quiz3)
contributed by < `Cancer-of-Japan` >
:::danger
Instead of listing the source, you should address the motivation, requirements, and initial analysis.
:notes: jserv
:::
> OFF = `4`
> LLL = `32 - __builtin_clz(n) - 1`
> NNN = `16`
> CCC = `mask[(uint8_t) byte / 8] & 1 << (uint8_t) byte % 8`
> SSS = `mask[(uint8_t) byte / 8] |= 1 << (uint8_t) byte % 8`
---
```cpp
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#define MAX_STR_LEN_BITS (54)
#define MAX_STR_LEN ((1UL << MAX_STR_LEN_BITS) - 1)
#define LARGE_STRING_LEN 256
```
Codes for what to include and preparation.
```c=
typedef union {
char data[16];
struct {
uint8_t filler[15],
/* how many free bytes in this stack allocated string
* same idea as fbstring
*/
space_left : 4,
/* if it is on heap, set to 1 */
is_ptr : 1, is_large_string : 1, flag2 : 1, flag3 : 1;
};
/* heap allocated */
struct {
char *ptr;
/* supports strings up to 2^MAX_STR_LEN_BITS - 1 bytes */
size_t size : MAX_STR_LEN_BITS,
/* capacity is always a power of 2 (unsigned)-1 */
capacity : 6;
/* the last 4 bits are important flags */
};
} xs;
```
This union can hold up to 15 chars, stack with in its 16-byte size header, and max of (2^53^-1) chars on heap.
- ***MAX_STR_LEN_BITS***...defines how many bits that are used to represent the size of *xs_string*
- ***MAX_STR_LEN***...the max length of *xs_string* can be calculated with following expression of ((1UL << MAX_STR_LEN_BITS)-1).
- ***LARGE_STRING_LEN***...defiens the min size of string.
```c=
static inline bool xs_is_ptr(const xs *x) { return x->is_ptr; }
static inline bool xs_is_large_string(const xs *x)
{
return x->is_large_string;
}
static inline size_t xs_size(const xs *x)
{
return xs_is_ptr(x) ? x->size : 15 - x->space_left;
}
static inline char *xs_data(const xs *x)
{
if (!xs_is_ptr(x))
return (char *) x->data;
if (xs_is_large_string(x))
return (char *) (x->ptr + 4);
return (char *) x->ptr;
}
static inline size_t xs_capacity(const xs *x)
{
return xs_is_ptr(x) ? ((size_t) 1 << x->capacity) - 1 : 15;
}
static inline void xs_set_refcnt(const xs *x, int val)
{
*((int *) ((size_t) x->ptr)) = val;
}
static inline void xs_inc_refcnt(const xs *x)
{
if (xs_is_large_string(x))
++(*(int *) ((size_t) x->ptr));
}
static inline int xs_dec_refcnt(const xs *x)
{
if (!xs_is_large_string(x))
return 0;
return --(*(int *) ((size_t) x->ptr));
}
static inline int xs_get_refcnt(const xs *x)
{
if (!xs_is_large_string(x))
return 0;
return *(int *) ((size_t) x->ptr);
}
```
```c=
#define xs_literal_empty() \
(xs) { .space_left = 15 }
```
initializer for *xs_string*
```c=
/* lowerbound (floor log2) */
static inline int ilog2(uint32_t n) {
return 32 - __builrin_clz(n) - 1;
}
static void xs_allocate_data(xs *x, size_t len, bool reallocate)
{
/* Medium string */
if (len < LARGE_STRING_LEN) {
x->ptr = reallocate ? realloc(x->ptr, (size_t) 1 << x->capacity)
: malloc((size_t) 1 << x->capacity);
return;
}
/* Large string */
x->is_large_string = 1;
/* The extra 4 bytes are used to store the reference count */
x->ptr = reallocate ? realloc(x->ptr, (size_t)(1 << x->capacity) + 4)
: malloc((size_t)(1 << x->capacity) + 4);
xs_set_refcnt(x, 1);
}
```
```c=
xs *xs_new(xs *x, const void *p)
{
*x = xs_literal_empty();
size_t len = strlen(p) + 1;
if (len > 16) {
x->capacity = ilog2(len) + 1;
x->size = len - 1;
x->is_ptr = true;
xs_allocate_data(x, x->size, 0);
memcpy(xs_data(x), p, len);
} else {
memcpy(x->data, p, len);
x->space_left = 15 - (len - 1);
}
return x;
}
```
***xs_new***... funciton aims to initialize a *xs_string* with given str content.
```c=
/* Memory leaks happen if the string is too long but it is still useful for
* short strings.
*/
#define xs_tmp(x) \
((void) ((struct { \
_Static_assert(sizeof(x) <= MAX_STR_LEN, "it is too big"); \
int dummy; \
}){1}), \
xs_new(&xs_literal_empty(), x))
/* grow up to specified size */
xs *xs_grow(xs *x, size_t len)
{
char buf[16];
if (len <= xs_capacity(x))
return x;
/* Backup first */
if (!xs_is_ptr(x))
memcpy(buf, x->data, 16);
x->is_ptr = true;
x->capacity = ilog2(len) + 1;
if (xs_is_ptr(x)) {
xs_allocate_data(x, len, 1);
} else {
xs_allocate_data(x, len, 0);
memcpy(xs_data(x), buf, 16);
}
return x;
}
static inline xs *xs_newempty(xs *x)
{
*x = xs_literal_empty();
return x;
}
static inline xs *xs_free(xs *x)
{
if (xs_is_ptr(x) && xs_dec_refcnt(x) <= 0)
free(x->ptr);
return xs_newempty(x);
}
static bool xs_cow_lazy_copy(xs *x, char **data)
{
if (xs_get_refcnt(x) <= 1)
return false;
/* Lazy copy */
xs_dec_refcnt(x);
xs_allocate_data(x, x->size, 0);
if (data) {
memcpy(xs_data(x), *data, x->size);
/* Update the newly allocated pointer */
*data = xs_data(x);
}
return true;
}
```
- xs_concat
```c=
xs *xs_concat(xs *string, const xs *prefix, const xs *suffix)
{
size_t pres = xs_size(prefix), sufs = xs_size(suffix),
size = xs_size(string), capacity = xs_capacity(string);
char *pre = xs_data(prefix), *suf = xs_data(suffix),
*data = xs_data(string);
xs_cow_lazy_copy(string, &data);
if (size + pres + sufs <= capacity) {
memmove(data + pres, data, size);
memcpy(data, pre, pres);
memcpy(data + pres + size, suf, sufs + 1);
if (xs_is_ptr(string))
string->size = size + pres + sufs;
else
string->space_left = 15 - (size + pres + sufs);
} else {
xs tmps = xs_literal_empty();
xs_grow(&tmps, size + pres + sufs);
char *tmpdata = xs_data(&tmps);
memcpy(tmpdata + pres, data, size);
memcpy(tmpdata, pre, pres);
memcpy(tmpdata + pres + size, suf, sufs + 1);
xs_free(string);
*string = tmps;
string->size = size + pres + sufs;
}
return string;
}
```
```c=
xs *xs_trim(xs *x, const char *trimset)
{
if (!trimset[0])
return x;
char *dataptr = xs_data(x), *orig = dataptr;
if (xs_cow_lazy_copy(x, &dataptr))
orig = dataptr;
/* similar to strspn/strpbrk but it operates on binary data */
uint8_t mask[32] = {0};
#define check_bit(byte) (mask[(uint8_t) byte / 8] & 1 << (uint8_t) byte % 8)
#define set_bit(byte) (mask[(uint8_t) byte / 8] |= 1 << (uint8_t) byte % 8)
size_t i, slen = xs_size(x), trimlen = strlen(trimset);
for (i = 0; i < trimlen; i++)
set_bit(trimset[i]);
for (i = 0; i < slen; i++)
if (!check_bit(dataptr[i]))
break;
for (; slen > 0; slen--)
if (!check_bit(dataptr[slen - 1]))
break;
dataptr += i;
slen -= i;
/* reserved space as a buffer on the heap.
* Do not reallocate immediately. Instead, reuse it as possible.
* Do not shrink to in place if < 16 bytes.
*/
memmove(orig, dataptr, slen);
/* do not dirty memory unless it is needed */
if (orig[slen])
orig[slen] = 0;
if (xs_is_ptr(x))
x->size = slen;
else
x->space_left = 15 - slen;
return x;
#undef check_bit
#undef set_bit
}
int main(int argc, char *argv[])
{
xs string = *xs_tmp("\n foobarbar \n\n\n");
xs_trim(&string, "\n ");
printf("[%s] : %2zu\n", xs_data(&string), xs_size(&string));
xs prefix = *xs_tmp("((("), suffix = *xs_tmp(")))");
xs_concat(&string, &prefix, &suffix);
printf("[%s] : %2zu\n", xs_data(&string), xs_size(&string));
return 0;
}
```
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