sds.c

/* SDSLib, A C dynamic strings library */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <assert.h>
#include "sds.h"

/*
 * 根据给定的初始化字符串 init 和字符串长度 initlen
 * 创建一个新的 sds
 *
 * 参数
 *  init ：初始化字符串指针
 *  initlen ：初始化字符串的长度
 *
 * 返回值
 *  sds ：创建成功返回 sdshdr 相对应的 sds
 *        创建失败返回 NULL
 *
 * 复杂度
 *  T = O(N)
 */
sds sdsnewlen(const void *init, size_t initlen) {

	struct sdshdr *sh;

	// 根据是否有初始化内容，选择适当的内存分配方式
	// T = O(N)
	if (init) {
		// zmalloc 不初始化所分配的内存
		sh = zmalloc(sizeof(struct sdshdr) + initlen + 1);
	}
	else {
		// zcalloc 将分配的内存全部初始化为 0
		sh = zcalloc(sizeof(struct sdshdr) + initlen + 1);
	}

	// 内存分配失败，返回
	if (sh == NULL) return NULL;

	// 设置初始化长度
	sh->len = initlen;
	// 新 sds 不预留任何空间
	sh->free = 0;
	// 如果有指定初始化内容，将它们复制到 sdshdr 的 buf 中
	// T = O(N)
	if (initlen && init)
		memcpy(sh->buf, init, initlen);
	// 以 \0 结尾
	sh->buf[initlen] = '\0';

	// 返回 buf 部分，而不是整个 sdshdr
	return (char*)sh->buf;
}

/*
 * 创建并返回一个只保存了空字符串 "" 的 sds
 *
 * 返回值
 *  sds ：创建成功返回 sdshdr 相对应的 sds
 *        创建失败返回 NULL
 *
 * 复杂度
 *  T = O(1)
 */
sds sdsempty(void) {
	return sdsnewlen("", 0);
}

/*
 * 根据给定字符串 init ，创建一个包含同样字符串的 sds
 *
 * 参数
 *  init ：如果输入为 NULL ，那么创建一个空白 sds
 *         否则，新创建的 sds 中包含和 init 内容相同字符串
 *
 * 返回值
 *  sds ：创建成功返回 sdshdr 相对应的 sds
 *        创建失败返回 NULL
 *
 * 复杂度
 *  T = O(N)
 */
sds sdsnew(const char *init) {
	size_t initlen = (init == NULL) ? 0 : strlen(init);
	return sdsnewlen(init, initlen);
}

/*
 * 复制给定 sds 的副本
 *
 * 返回值
 *  sds ：创建成功返回输入 sds 的副本
 *        创建失败返回 NULL
 *
 * 复杂度
 *  T = O(N)
 */
sds sdsdup(const sds s) {
	return sdsnewlen(s, sdslen(s));
}

/*
 * 释放给定的 sds
 *
 * 复杂度
 *  T = O(N)
 */
void sdsfree(sds s) {
	if (s == NULL) return;
	zfree(s - sizeof(struct sdshdr));
}



/*
 * 在不释放 SDS 的字符串空间的情况下，
 * 重置 SDS 所保存的字符串为空字符串。
 *
 * 复杂度
 *  T = O(1)
 */
void sdsclear(sds s) {

	// 取出 sdshdr
	struct sdshdr *sh = (void*)(s - (sizeof(struct sdshdr)));

	// 重新计算属性
	sh->free += sh->len;
	sh->len = 0;

	// 将结束符放到最前面（相当于惰性地删除 buf 中的内容）
	sh->buf[0] = '\0';
}


/*
 * 对 sds 中 buf 的长度进行扩展，确保在函数执行之后，
 * buf 至少会有 addlen + 1 长度的空余空间
 * （额外的 1 字节是为 \0 准备的）
 *
 * 返回值
 *  sds ：扩展成功返回扩展后的 sds
 *        扩展失败返回 NULL
 *
 * 复杂度
 *  T = O(N)
 */
sds sdsMakeRoomFor(sds s, size_t addlen) {

	struct sdshdr *sh, *newsh;

	// 获取 s 目前的空余空间长度
	size_t free = sdsavail(s);

	size_t len, newlen;

	// s 目前的空余空间已经足够，无须再进行扩展，直接返回
	if (free >= addlen) return s;

	// 获取 s 目前已占用空间的长度
	len = sdslen(s);
	sh = (void*)(s - (sizeof(struct sdshdr)));

	// s 最少需要的长度
	newlen = (len + addlen);

	// 根据新长度，为 s 分配新空间所需的大小
	if (newlen < SDS_MAX_PREALLOC)
		// 如果新长度小于 SDS_MAX_PREALLOC 
		// 那么为它分配两倍于所需长度的空间
		newlen *= 2;
	else
		// 否则，分配长度为目前长度加上 SDS_MAX_PREALLOC
		newlen += SDS_MAX_PREALLOC;
	// T = O(N)
	newsh = zrealloc(sh, sizeof(struct sdshdr) + newlen + 1);

	// 内存不足，分配失败，返回
	if (newsh == NULL) return NULL;

	// 更新 sds 的空余长度
	newsh->free = newlen - len;

	// 返回 sds
	return newsh->buf;
}

/*
 * 回收 sds 中的空闲空间，
 * 回收不会对 sds 中保存的字符串内容做任何修改。
 *
 * 返回值
 *  sds ：内存调整后的 sds
 *
 * 复杂度
 *  T = O(N)
 */
sds sdsRemoveFreeSpace(sds s) {
	struct sdshdr *sh;

	sh = (void*)(s - (sizeof(struct sdshdr)));

	// 进行内存重分配，让 buf 的长度仅仅足够保存字符串内容
	// T = O(N)
	sh = zrealloc(sh, sizeof(struct sdshdr) + sh->len + 1);

	// 空余空间为 0
	sh->free = 0;

	return sh->buf;
}

/*
 * 返回给定 sds 分配的内存字节数
 *
 * 复杂度
 *  T = O(1)
 */
size_t sdsAllocSize(sds s) {
	struct sdshdr *sh = (void*)(s - (sizeof(struct sdshdr)));

	return sizeof(*sh) + sh->len + sh->free + 1;
}

/*
 * 根据 incr 参数，增加 sds 的长度，缩减空余空间，
 * 并将 \0 放到新字符串的尾端
 *
 * 这个函数是在调用 sdsMakeRoomFor() 对字符串进行扩展，
 * 然后用户在字符串尾部写入了某些内容之后，
 * 用来正确更新 free 和 len 属性的。
 *
 * 如果 incr 参数为负数，那么对字符串进行右截断操作。
 *
 * Usage example:
 *
 * Using sdsIncrLen() and sdsMakeRoomFor() it is possible to mount the
 * following schema, to cat bytes coming from the kernel to the end of an
 * sds string without copying into an intermediate buffer:
 *
 * 以下是 sdsIncrLen 的用例：
 *
 * oldlen = sdslen(s);
 * s = sdsMakeRoomFor(s, BUFFER_SIZE);
 * nread = read(fd, s+oldlen, BUFFER_SIZE);
 * ... check for nread <= 0 and handle it ...
 * sdsIncrLen(s, nread);
 *
 * 复杂度
 *  T = O(1)
 */
void sdsIncrLen(sds s, int incr) {
	struct sdshdr *sh = (void*)(s - (sizeof(struct sdshdr)));

	// 确保 sds 空间足够
	assert(sh->free >= incr);

	// 更新属性
	sh->len += incr;
	sh->free -= incr;

	// 这个 assert 其实可以忽略
	// 因为前一个 assert 已经确保 sh->free - incr >= 0 了
	assert(sh->free >= 0);

	// 放置新的结尾符号
	s[sh->len] = '\0';
}

/*
 * 将 sds 扩充至指定长度，未使用的空间以 0 字节填充。
 *
 * 返回值
 *  sds ：扩充成功返回新 sds ，失败返回 NULL
 *
 * 复杂度：
 *  T = O(N)
 */
sds sdsgrowzero(sds s, size_t len) {
	struct sdshdr *sh = (void*)(s - (sizeof(struct sdshdr)));
	size_t totlen, curlen = sh->len;

	// 如果 len 比字符串的现有长度小，
	// 那么直接返回，不做动作
	if (len <= curlen) return s;

	// 扩展 sds
	// T = O(N)
	s = sdsMakeRoomFor(s, len - curlen);
	// 如果内存不足，直接返回
	if (s == NULL) return NULL;

	// 将新分配的空间用 0 填充，防止出现垃圾内容
	// T = O(N)
	sh = (void*)(s - (sizeof(struct sdshdr)));
	memset(s + curlen, 0, (len - curlen + 1)); // also set trailing \0 byte

	// 更新属性
	totlen = sh->len + sh->free;
	sh->len = len;
	sh->free = totlen - sh->len;

	// 返回新的 sds
	return s;
}

/*
 * 将长度为 len 的字符串 t 追加到 sds 的字符串末尾
 *
 * 返回值
 *  sds ：追加成功返回新 sds ，失败返回 NULL
 *
 * 复杂度
 *  T = O(N)
 */
sds sdscatlen(sds s, const void *t, size_t len) {

	struct sdshdr *sh;

	// 原有字符串长度
	size_t curlen = sdslen(s);

	// 扩展 sds 空间
	// T = O(N)
	s = sdsMakeRoomFor(s, len);

	// 内存不足？直接返回
	if (s == NULL) return NULL;

	// 复制 t 中的内容到字符串后部
	// T = O(N)
	sh = (void*)(s - (sizeof(struct sdshdr)));
	memcpy(s + curlen, t, len);

	// 更新属性
	sh->len = curlen + len;
	sh->free = sh->free - len;

	// 添加新结尾符号
	s[curlen + len] = '\0';

	// 返回新 sds
	return s;
}

/*
 * 将给定字符串 t 追加到 sds 的末尾
 *
 * 返回值
 *  sds ：追加成功返回新 sds ，失败返回 NULL
 *
 * 复杂度
 *  T = O(N)
 */
sds sdscat(sds s, const char *t) {
	return sdscatlen(s, t, strlen(t));
}

/*
 * 将另一个 sds 追加到一个 sds 的末尾
 *
 * 返回值
 *  sds ：追加成功返回新 sds ，失败返回 NULL
 *
 * 复杂度
 *  T = O(N)
 */

sds sdscatsds(sds s, const sds t) {
	return sdscatlen(s, t, sdslen(t));
}

/*
 * 将字符串 t 的前 len 个字符复制到 sds s 当中，
 * 并在字符串的最后添加终结符。
 *
 * 如果 sds 的长度少于 len 个字符，那么扩展 sds
 *
 * 复杂度
 *  T = O(N)
 * 
 * 返回值
 *  sds ：复制成功返回新的 sds ，否则返回 NULL
 */

sds sdscpylen(sds s, const char *t, size_t len) {

	struct sdshdr *sh = (void*)(s - (sizeof(struct sdshdr)));

	// sds 现有 buf 的长度
	size_t totlen = sh->free + sh->len;

	// 如果 s 的 buf 长度不满足 len ，那么扩展它
	if (totlen < len) {
		// T = O(N)
		s = sdsMakeRoomFor(s, len - sh->len);
		if (s == NULL) return NULL;
		sh = (void*)(s - (sizeof(struct sdshdr)));
		totlen = sh->free + sh->len;
	}

	// 复制内容
	// T = O(N)
	memcpy(s, t, len);

	// 添加终结符号
	s[len] = '\0';

	// 更新属性
	sh->len = len;
	sh->free = totlen - len;

	// 返回新的 sds
	return s;
}

/*
 * 将字符串复制到 sds 当中，
 * 覆盖原有的字符。
 *
 * 如果 sds 的长度少于字符串的长度，那么扩展 sds 。
 *
 * 复杂度
 *  T = O(N)
 *
 * 返回值
 *  sds ：复制成功返回新的 sds ，否则返回 NULL
 */

sds sdscpy(sds s, const char *t) {
	return sdscpylen(s, t, strlen(t));
}

/* Helper for sdscatlonglong() doing the actual number -> string
* conversion. 's' must point to a string with room for at least
* SDS_LLSTR_SIZE bytes.
*
* The function returns the lenght of the null-terminated string
* representation stored at 's'. */
#define SDS_LLSTR_SIZE 21
int sdsll2str(char *s, long long value) {
	char *p, aux;
	unsigned long long v;
	size_t l;

	/* Generate the string representation, this method produces
	* an reversed string. */
	v = (value < 0) ? -value : value;
	p = s;
	do {
		*p++ = '0' + (v % 10);
		v /= 10;
	} while (v);
	if (value < 0) *p++ = '-';

	/* Compute length and add null term. */
	l = p - s;
	*p = '\0';

	/* Reverse the string. */
	p--;
	while (s < p) {
		aux = *s;
		*s = *p;
		*p = aux;
		s++;
		p--;
	}
	return l;
}

/* Identical sdsll2str(), but for unsigned long long type. */
int sdsull2str(char *s, unsigned long long v) {
	char *p, aux;
	size_t l;

	/* Generate the string representation, this method produces
	* an reversed string. */
	p = s;
	do {
		*p++ = '0' + (v % 10);
		v /= 10;
	} while (v);

	/* Compute length and add null term. */
	l = p - s;
	*p = '\0';

	/* Reverse the string. */
	p--;
	while (s < p) {
		aux = *s;
		*s = *p;
		*p = aux;
		s++;
		p--;
	}
	return l;
}

/* Create an sds string from a long long value. It is much faster than:
*
* sdscatprintf(sdsempty(),"%lld\n", value);
*/
/*
 * 根据输入的 long long 值 value ，创建一个 SDS
 */
sds sdsfromlonglong(long long value) {
	char buf[SDS_LLSTR_SIZE];
	int len = sdsll2str(buf, value);

	return sdsnewlen(buf, len);
}

/*
 * 打印函数，被 sdscatprintf 所调用
 *
 * T = O(N^2)
 */
/* Like sdscatpritf() but gets va_list instead of being variadic. */
sds sdscatvprintf(sds s, const char *fmt, va_list ap) {
	va_list cpy;
	char staticbuf[1024], *buf = staticbuf, *t;
	size_t buflen = strlen(fmt) * 2;

	/* We try to start using a static buffer for speed.
	* If not possible we revert to heap allocation. */
	if (buflen > sizeof(staticbuf)) {
		buf = zmalloc(buflen);
		if (buf == NULL) return NULL;
	}
	else {
		buflen = sizeof(staticbuf);
	}

	/* Try with buffers two times bigger every time we fail to
	* fit the string in the current buffer size. */
	while (1) {
		buf[buflen - 2] = '\0';
		va_copy(cpy, ap);
		// T = O(N)
		vsnprintf(buf, buflen, fmt, cpy);
		if (buf[buflen - 2] != '\0') {
			if (buf != staticbuf) zfree(buf);
			buflen *= 2;
			buf = zmalloc(buflen);
			if (buf == NULL) return NULL;
			continue;
		}
		break;
	}

	/* Finally concat the obtained string to the SDS string and return it. */
	t = sdscat(s, buf);
	if (buf != staticbuf) zfree(buf);
	return t;
}

/*
 * 打印任意数量个字符串，并将这些字符串追加到给定 sds 的末尾
 *
 * T = O(N^2)
 */
/* Append to the sds string 's' a string obtained using printf-alike format
* specifier.
*
* After the call, the modified sds string is no longer valid and all the
* references must be substituted with the new pointer returned by the call.
*
* Example:
*
* s = sdsempty("Sum is: ");
* s = sdscatprintf(s,"%d+%d = %d",a,b,a+b).
*
* Often you need to create a string from scratch with the printf-alike
* format. When this is the need, just use sdsempty() as the target string:
*
* s = sdscatprintf(sdsempty(), "... your format ...", args);
*/
sds sdscatprintf(sds s, const char *fmt, ...) {
	va_list ap;
	char *t;
	va_start(ap, fmt);
	// T = O(N^2)
	t = sdscatvprintf(s, fmt, ap);
	va_end(ap);
	return t;
}

/* This function is similar to sdscatprintf, but much faster as it does
* not rely on sprintf() family functions implemented by the libc that
* are often very slow. Moreover directly handling the sds string as
* new data is concatenated provides a performance improvement.
*
* However this function only handles an incompatible subset of printf-alike
* format specifiers:
*
* %s - C String
* %S - SDS string
* %i - signed int
* %I - 64 bit signed integer (long long, int64_t)
* %u - unsigned int
* %U - 64 bit unsigned integer (unsigned long long, uint64_t)
* %% - Verbatim "%" character.
*/
sds sdscatfmt(sds s, char const *fmt, ...) {
	struct sdshdr *sh = (void*)(s - (sizeof(struct sdshdr)));
	size_t initlen = sdslen(s);
	const char *f = fmt;
	int i;
	va_list ap;

	va_start(ap, fmt);
	f = fmt;    /* Next format specifier byte to process. */
	i = initlen; /* Position of the next byte to write to dest str. */
	while (*f) {
		char next, *str;
		size_t l;
		long long num;
		unsigned long long unum;

		/* Make sure there is always space for at least 1 char. */
		if (sh->free == 0) {
			s = sdsMakeRoomFor(s, 1);
			sh = (void*)(s - (sizeof(struct sdshdr)));
		}

		switch (*f) {
		case '%':
			next = *(f + 1);
			f++;
			switch (next) {
			case 's':
			case 'S':
				str = va_arg(ap, char*);
				l = (next == 's') ? strlen(str) : sdslen(str);
				if (sh->free < l) {
					s = sdsMakeRoomFor(s, l);
					sh = (void*)(s - (sizeof(struct sdshdr)));
				}
				memcpy(s + i, str, l);
				sh->len += l;
				sh->free -= l;
				i += l;
				break;
			case 'i':
			case 'I':
				if (next == 'i')
					num = va_arg(ap, int);
				else
					num = va_arg(ap, long long);
				{
					char buf[SDS_LLSTR_SIZE];
					l = sdsll2str(buf, num);
					if (sh->free < l) {
						s = sdsMakeRoomFor(s, l);
						sh = (void*)(s - (sizeof(struct sdshdr)));
					}
					memcpy(s + i, buf, l);
					sh->len += l;
					sh->free -= l;
					i += l;
				}
				break;
			case 'u':
			case 'U':
				if (next == 'u')
					unum = va_arg(ap, unsigned int);
				else
					unum = va_arg(ap, unsigned long long);
				{
					char buf[SDS_LLSTR_SIZE];
					l = sdsull2str(buf, unum);
					if (sh->free < l) {
						s = sdsMakeRoomFor(s, l);
						sh = (void*)(s - (sizeof(struct sdshdr)));
					}
					memcpy(s + i, buf, l);
					sh->len += l;
					sh->free -= l;
					i += l;
				}
				break;
			default: /* Handle %% and generally %<unknown>. */
				s[i++] = next;
				sh->len += 1;
				sh->free -= 1;
				break;
			}
			break;
		default:
			s[i++] = *f;
			sh->len += 1;
			sh->free -= 1;
			break;
		}
		f++;
	}
	va_end(ap);

	/* Add null-term */
	s[i] = '\0';
	return s;
}

/*
 * 对 sds 左右两端进行修剪，清除其中 cset 指定的所有字符
 *
 * 比如 sdsstrim(xxyyabcyyxy, "xy") 将返回 "abc"
 *
 * 复杂性：
 *  T = O(M*N)，M 为 SDS 长度， N 为 cset 长度。
 */
/* Remove the part of the string from left and from right composed just of
* contiguous characters found in 'cset', that is a null terminted C string.
*
* After the call, the modified sds string is no longer valid and all the
* references must be substituted with the new pointer returned by the call.
*
* Example:
*
* s = sdsnew("AA...AA.a.aa.aHelloWorld     :::");
* s = sdstrim(s,"A. :");
* printf("%s\n", s);
*
* Output will be just "Hello World".
*/
sds sdstrim(sds s, const char *cset) {
	struct sdshdr *sh = (void*)(s - (sizeof(struct sdshdr)));
	char *start, *end, *sp, *ep;
	size_t len;

	// 设置和记录指针
	sp = start = s;
	ep = end = s + sdslen(s) - 1;

	// 修剪, T = O(N^2)
	while (sp <= end && strchr(cset, *sp)) sp++;
	while (ep > start && strchr(cset, *ep)) ep--;

	// 计算 trim 完毕之后剩余的字符串长度
	len = (sp > ep) ? 0 : ((ep - sp) + 1);

	// 如果有需要，前移字符串内容
	// T = O(N)
	if (sh->buf != sp) memmove(sh->buf, sp, len);

	// 添加终结符
	sh->buf[len] = '\0';

	// 更新属性
	sh->free = sh->free + (sh->len - len);
	sh->len = len;

	// 返回修剪后的 sds
	return s;
}

/*
 * 按索引对截取 sds 字符串的其中一段
 * start 和 end 都是闭区间（包含在内）
 *
 * 索引从 0 开始，最大为 sdslen(s) - 1
 * 索引可以是负数， sdslen(s) - 1 == -1
 *
 * 复杂度
 *  T = O(N)
 */
/* Turn the string into a smaller (or equal) string containing only the
* substring specified by the 'start' and 'end' indexes.
*
* start and end can be negative, where -1 means the last character of the
* string, -2 the penultimate character, and so forth.
*
* The interval is inclusive, so the start and end characters will be part
* of the resulting string.
*
* The string is modified in-place.
*
* Example:
*
* s = sdsnew("Hello World");
* sdsrange(s,1,-1); => "ello World"
*/
void sdsrange(sds s, int start, int end) {
	struct sdshdr *sh = (void*)(s - (sizeof(struct sdshdr)));
	size_t newlen, len = sdslen(s);

	if (len == 0) return;
	if (start < 0) {
		start = len + start;
		if (start < 0) start = 0;
	}
	if (end < 0) {
		end = len + end;
		if (end < 0) end = 0;
	}
	newlen = (start > end) ? 0 : (end - start) + 1;
	if (newlen != 0) {
		if (start >= (signed)len) {
			newlen = 0;
		}
		else if (end >= (signed)len) {
			end = len - 1;
			newlen = (start > end) ? 0 : (end - start) + 1;
		}
	}
	else {
		start = 0;
	}

	// 如果有需要，对字符串进行移动
	// T = O(N)
	if (start && newlen) memmove(sh->buf, sh->buf + start, newlen);

	// 添加终结符
	sh->buf[newlen] = 0;

	// 更新属性
	sh->free = sh->free + (sh->len - newlen);
	sh->len = newlen;
}

/*
 * 将 sds 字符串中的所有字符转换为小写
 *
 * T = O(N)
 */
void sdstolower(sds s) {
	int len = sdslen(s), j;

	for (j = 0; j < len; j++) s[j] = tolower(s[j]);
}

/*
 * 将 sds 字符串中的所有字符转换为大写
 *
 * T = O(N)
 */
void sdstoupper(sds s) {
	int len = sdslen(s), j;

	for (j = 0; j < len; j++) s[j] = toupper(s[j]);
}

/*
 * 对比两个 sds ， strcmp 的 sds 版本
 *
 * 返回值
 *  int ：相等返回 0 ，s1 较大返回正数， s2 较大返回负数
 *
 * T = O(N)
 */
int sdscmp(const sds s1, const sds s2) {
	size_t l1, l2, minlen;
	int cmp;

	l1 = sdslen(s1);
	l2 = sdslen(s2);
	minlen = (l1 < l2) ? l1 : l2;
	cmp = memcmp(s1, s2, minlen);

	if (cmp == 0) return l1 - l2;

	return cmp;
}

/* Split 's' with separator in 'sep'. An array
 * of sds strings is returned. *count will be set
 * by reference to the number of tokens returned.
 *
 * 使用分隔符 sep 对 s 进行分割，返回一个 sds 字符串的数组。
 * *count 会被设置为返回数组元素的数量。
 *
 * 如果出现内存不足、字符串长度为 0 或分隔符长度为 0
 * 的情况，返回 NULL
 *
 * Note that 'sep' is able to split a string using
 * a multi-character separator. For example
 * sdssplit("foo_-_bar","_-_"); will return two
 * elements "foo" and "bar".
 *
 * 注意分隔符可以的是包含多个字符的字符串
 *
 * This version of the function is binary-safe but
 * requires length arguments. sdssplit() is just the
 * same function but for zero-terminated strings.
 *
 * 这个函数接受 len 参数，因此它是二进制安全的。
 * （文档中提到的 sdssplit() 已废弃）
 *
 * T = O(N^2)
 */
sds *sdssplitlen(const char *s, int len, const char *sep, int seplen, int *count) {
	int elements = 0, slots = 5, start = 0, j;
	sds *tokens;

	if (seplen < 1 || len < 0) return NULL;

	tokens = zmalloc(sizeof(sds)*slots);
	if (tokens == NULL) return NULL;

	if (len == 0) {
		*count = 0;
		return tokens;
	}

	// T = O(N^2)
	for (j = 0; j < (len - (seplen - 1)); j++) {
		/* make sure there is room for the next element and the final one */
		if (slots < elements + 2) {
			sds *newtokens;

			slots *= 2;
			newtokens = zrealloc(tokens, sizeof(sds)*slots);
			if (newtokens == NULL) goto cleanup;
			tokens = newtokens;
		}
		/* search the separator */
		// T = O(N)
		if ((seplen == 1 && *(s + j) == sep[0]) || (memcmp(s + j, sep, seplen) == 0)) {
			tokens[elements] = sdsnewlen(s + start, j - start);
			if (tokens[elements] == NULL) goto cleanup;
			elements++;
			start = j + seplen;
			j = j + seplen - 1; /* skip the separator */
		}
	}
	/* Add the final element. We are sure there is room in the tokens array. */
	tokens[elements] = sdsnewlen(s + start, len - start);
	if (tokens[elements] == NULL) goto cleanup;
	elements++;
	*count = elements;
	return tokens;

cleanup:
	{
		int i;
		for (i = 0; i < elements; i++) sdsfree(tokens[i]);
		zfree(tokens);
		*count = 0;
		return NULL;
	}
}

/*
 * 释放 tokens 数组中 count 个 sds
 *
 * T = O(N^2)
 */
void sdsfreesplitres(sds *tokens, int count) {
	if (!tokens) return;
	while (count--)
		sdsfree(tokens[count]);
	zfree(tokens);
}

/*
 * 将长度为 len 的字符串 p 以带引号（quoted）的格式
 * 追加到给定 sds 的末尾
 *
 * T = O(N)
 */
sds sdscatrepr(sds s, const char *p, size_t len) {

	s = sdscatlen(s, "\"", 1);

	while (len--) {
		switch (*p) {
		case '\\':
		case '"':
			s = sdscatprintf(s, "\\%c", *p);
			break;
		case '\n': s = sdscatlen(s, "\\n", 2); break;
		case '\r': s = sdscatlen(s, "\\r", 2); break;
		case '\t': s = sdscatlen(s, "\\t", 2); break;
		case '\a': s = sdscatlen(s, "\\a", 2); break;
		case '\b': s = sdscatlen(s, "\\b", 2); break;
		default:
			if (isprint(*p))
				s = sdscatprintf(s, "%c", *p);
			else
				s = sdscatprintf(s, "\\x%02x", (unsigned char)*p);
			break;
		}
		p++;
	}

	return sdscatlen(s, "\"", 1);
}

/*
 * 如果 c 为十六进制符号的其中一个，返回正数
 *
 * T = O(1)
 */
int is_hex_digit(char c) {
	return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') ||
		(c >= 'A' && c <= 'F');
}

/*
 * 将十六进制符号转换为 10 进制
 *
 * T = O(1)
 */
int hex_digit_to_int(char c) {
	switch (c) {
	case '0': return 0;
	case '1': return 1;
	case '2': return 2;
	case '3': return 3;
	case '4': return 4;
	case '5': return 5;
	case '6': return 6;
	case '7': return 7;
	case '8': return 8;
	case '9': return 9;
	case 'a': case 'A': return 10;
	case 'b': case 'B': return 11;
	case 'c': case 'C': return 12;
	case 'd': case 'D': return 13;
	case 'e': case 'E': return 14;
	case 'f': case 'F': return 15;
	default: return 0;
	}
}

/* Split a line into arguments, where every argument can be in the
 * following programming-language REPL-alike form:
 *
 * 将一行文本分割成多个参数，每个参数可以有以下的类编程语言 REPL 格式：
 *
 * foo bar "newline are supported\n" and "\xff\x00otherstuff"
 *
 * The number of arguments is stored into *argc, and an array
 * of sds is returned.
 *
 * 参数的个数会保存在 *argc 中，函数返回一个 sds 数组。
 *
 * The caller should free the resulting array of sds strings with
 * sdsfreesplitres().
 *
 * 调用者应该使用 sdsfreesplitres() 来释放函数返回的 sds 数组。
 *
 * Note that sdscatrepr() is able to convert back a string into
 * a quoted string in the same format sdssplitargs() is able to parse.
 *
 * sdscatrepr() 可以将一个字符串转换为一个带引号（quoted）的字符串，
 * 这个带引号的字符串可以被 sdssplitargs() 分析。
 *
 * The function returns the allocated tokens on success, even when the
 * input string is empty, or NULL if the input contains unbalanced
 * quotes or closed quotes followed by non space characters
 * as in: "foo"bar or "foo'
 *
 * 即使输入出现空字符串， NULL ，或者输入带有未对应的括号，
 * 函数都会将已成功处理的字符串先返回。
 *
 * 这个函数主要用于 config.c 中对配置文件进行分析。
 * 例子：
 *  sds *arr = sdssplitargs("timeout 10086\r\nport 123321\r\n");
 * 会得出
 *  arr[0] = "timeout"
 *  arr[1] = "10086"
 *  arr[2] = "port"
 *  arr[3] = "123321"
 *
 * T = O(N^2)
 */
sds *sdssplitargs(const char *line, int *argc) {
	const char *p = line;
	char *current = NULL;
	char **vector = NULL;

	*argc = 0;
	while (1) {

		// 跳过空白
		// T = O(N)
		while (*p && isspace(*p)) p++;

		if (*p) {
			/* get a token */
			int inq = 0;  /* set to 1 if we are in "quotes" */
			int insq = 0; /* set to 1 if we are in 'single quotes' */
			int done = 0;

			if (current == NULL) current = sdsempty();

			// T = O(N)
			while (!done) {
				if (inq) {
					if (*p == '\\' && *(p + 1) == 'x' &&
						is_hex_digit(*(p + 2)) &&
						is_hex_digit(*(p + 3)))
					{
						unsigned char byte;

						byte = (hex_digit_to_int(*(p + 2)) * 16) +
							hex_digit_to_int(*(p + 3));
						current = sdscatlen(current, (char*)&byte, 1);
						p += 3;
					}
					else if (*p == '\\' && *(p + 1)) {
						char c;

						p++;
						switch (*p) {
						case 'n': c = '\n'; break;
						case 'r': c = '\r'; break;
						case 't': c = '\t'; break;
						case 'b': c = '\b'; break;
						case 'a': c = '\a'; break;
						default: c = *p; break;
						}
						current = sdscatlen(current, &c, 1);
					}
					else if (*p == '"') {
						/* closing quote must be followed by a space or
						* nothing at all. */
						if (*(p + 1) && !isspace(*(p + 1))) goto err;
						done = 1;
					}
					else if (!*p) {
						/* unterminated quotes */
						goto err;
					}
					else {
						current = sdscatlen(current, p, 1);
					}
				}
				else if (insq) {
					if (*p == '\\' && *(p + 1) == '\'') {
						p++;
						current = sdscatlen(current, "'", 1);
					}
					else if (*p == '\'') {
						/* closing quote must be followed by a space or
						* nothing at all. */
						if (*(p + 1) && !isspace(*(p + 1))) goto err;
						done = 1;
					}
					else if (!*p) {
						/* unterminated quotes */
						goto err;
					}
					else {
						current = sdscatlen(current, p, 1);
					}
				}
				else {
					switch (*p) {
					case ' ':
					case '\n':
					case '\r':
					case '\t':
					case '\0':
						done = 1;
						break;
					case '"':
						inq = 1;
						break;
					case '\'':
						insq = 1;
						break;
					default:
						current = sdscatlen(current, p, 1);
						break;
					}
				}
				if (*p) p++;
			}
			/* add the token to the vector */
			// T = O(N)
			vector = zrealloc(vector, ((*argc) + 1) * sizeof(char*));
			vector[*argc] = current;
			(*argc)++;
			current = NULL;
		}
		else {
			/* Even on empty input string return something not NULL. */
			if (vector == NULL) vector = zmalloc(sizeof(void*));
			return vector;
		}
	}

err:
	while ((*argc)--)
		sdsfree(vector[*argc]);
	zfree(vector);
	if (current) sdsfree(current);
	*argc = 0;
	return NULL;
}

/* Modify the string substituting all the occurrences of the set of
 * characters specified in the 'from' string to the corresponding character
 * in the 'to' array.
 *
 * 将字符串 s 中，
 * 所有在 from 中出现的字符，替换成 to 中的字符
 *
 * 比如调用 sdsmapchars(mystring, "ho", "01", 2)
 * 就会将 "hello" 转换为 "0ell1"
 *
 * 因为无须对 sds 进行大小调整，
 * 所以返回的 sds 输入的 sds 一样
 *
 * T = O(N^2)
 */
sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen) {
	size_t j, i, l = sdslen(s);

	// 遍历输入字符串
	for (j = 0; j < l; j++) {
		// 遍历映射
		for (i = 0; i < setlen; i++) {
			// 替换字符串
			if (s[j] == from[i]) {
				s[j] = to[i];
				break;
			}
		}
	}
	return s;
}

/* Join an array of C strings using the specified separator (also a C string).
* Returns the result as an sds string. */
sds sdsjoin(char **argv, int argc, char *sep) {
	sds join = sdsempty();
	int j;

	for (j = 0; j < argc; j++) {
		join = sdscat(join, argv[j]);
		if (j != argc - 1) join = sdscat(join, sep);
	}
	return join;
}