text: /* SDSLib 2.0 -- A C dynamic strings library
   *
   * Copyright (c) 2006-2015, Salvatore Sanfilippo <antirez at gmail dot com>
   * Copyright (c) 2015, Oran Agra
   * Copyright (c) 2015, Redis Labs, Inc
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are met:
   *
   *   * Redistributions of source code must retain the above copyright notice,
   *     this list of conditions and the following disclaimer.
   *   * Redistributions in binary form must reproduce the above copyright
   *     notice, this list of conditions and the following disclaimer in the
   *     documentation and/or other materials provided with the distribution.
   *   * Neither the name of Redis nor the names of its contributors may be used
   *     to endorse or promote products derived from this software without
   *     specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
   * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
   * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
   * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
   * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
   * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
   * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   * POSSIBILITY OF SUCH DAMAGE.
   */
  
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <ctype.h>
  #include <assert.h>
  #include <limits.h>
  #include "sds.h"
  #include "sdsalloc.h"
  
  const char *SDS_NOINIT = "SDS_NOINIT";  // 不对sds进行初始化的关键字
  
  // sds存储不同长度的字符串，结构sds结构体所占用的内存大小也不一样
  // 获取存储不同长度字符串，sds结构体所占用的内存大小
  static inline int sdsHdrSize(char type) {
      switch(type&SDS_TYPE_MASK) {
          case SDS_TYPE_5:
              return sizeof(struct sdshdr5);
          case SDS_TYPE_8:
              return sizeof(struct sdshdr8);
          case SDS_TYPE_16:
              return sizeof(struct sdshdr16);
          case SDS_TYPE_32:
              return sizeof(struct sdshdr32);
          case SDS_TYPE_64:
              return sizeof(struct sdshdr64);
      }
      return 0;
  }
  
  // 根据目标字符串的长度string_size，确定用来存储该字符串的sds结构体类型
  static inline char sdsReqType(size_t string_size) {
      if (string_size < 1<<5)
          return SDS_TYPE_5;
      if (string_size < 1<<8)
          return SDS_TYPE_8;
      if (string_size < 1<<16)
          return SDS_TYPE_16;
  #if (LONG_MAX == LLONG_MAX)
      if (string_size < 1ll<<32)
          return SDS_TYPE_32;
      return SDS_TYPE_64;
  #else
      return SDS_TYPE_32;
  #endif
  }
  
  /* Create a new sds string with the content specified by the 'init' pointer
   * and 'initlen'.
   * If NULL is used for 'init' the string is initialized with zero bytes.
   * If SDS_NOINIT is used, the buffer is left uninitialized;
   *
   * The string is always null-termined (all the sds strings are, always) so
   * even if you create an sds string with:
   *
   * mystring = sdsnewlen("abc",3);
   *
   * You can print the string with printf() as there is an implicit \0 at the
   * end of the string. However the string is binary safe and can contain
   * \0 characters in the middle, as the length is stored in the sds header. */
  sds sdsnewlen(const void *init, size_t initlen) {
      void *sh;
      sds s;
      char type = sdsReqType(initlen);    // 根据目标字符串的长度initlen，得到该sds结构的类型
      /* Empty strings are usually created in order to append. Use type 8
       * since type 5 is not good at this. */
      if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8;
      int hdrlen = sdsHdrSize(type);
      unsigned char *fp; /* flags pointer. */
  
      // 申请的内存是，sds结构体，也就是header的大小（hdrlen）+ initlen + 1
      sh = s_malloc(hdrlen+initlen+1);
      if (init==SDS_NOINIT)
          init = NULL;
      else if (!init)
          memset(sh, 0, hdrlen+initlen+1);
      if (sh == NULL) return NULL;    // 此处应该放在104行之前，在 deps/hiredis/sds.c 中fix了
      s = (char*)sh+hdrlen;   // sh是指向sds的头的，这里加上hdrlen的偏移，就获取真正存储字符串的地址
      fp = ((unsigned char*)s)-1;
      switch(type) {
          case SDS_TYPE_5: {
              *fp = type | (initlen << SDS_TYPE_BITS);
              break;
          }
          case SDS_TYPE_8: {
              SDS_HDR_VAR(8,s);   // 这里重新定义了一个局部变量sh，并将其指向sds的t部。没get到这样做的好处。。
              sh->len = initlen;
              sh->alloc = initlen;
              *fp = type;
              break;
          }
          case SDS_TYPE_16: {
              SDS_HDR_VAR(16,s);
              sh->len = initlen;
              sh->alloc = initlen;
              *fp = type;
              break;
          }
          case SDS_TYPE_32: {
              SDS_HDR_VAR(32,s);
              sh->len = initlen;
              sh->alloc = initlen;
              *fp = type;
              break;
          }
          case SDS_TYPE_64: {
              SDS_HDR_VAR(64,s);
              sh->len = initlen;
              sh->alloc = initlen;
              *fp = type;
              break;
          }
      }
      if (initlen && init)
          memcpy(s, init, initlen);
      s[initlen] = '\0';
      return s;
  }
  
  /* Create an empty (zero length) sds string. Even in this case the string
   * always has an implicit null term. */
  // 创建一个空sds
  sds sdsempty(void) {
      return sdsnewlen("",0);
  }
  
  /* Create a new sds string starting from a null terminated C string. */
  // 用c字符串去创建sds
  sds sdsnew(const char *init) {
      size_t initlen = (init == NULL) ? 0 : strlen(init);
      return sdsnewlen(init, initlen);
  }
  
  /* Duplicate an sds string. */
  // 复制一个sds
  sds sdsdup(const sds s) {
      return sdsnewlen(s, sdslen(s));
  }
  
  /* Free an sds string. No operation is performed if 's' is NULL. */
  // 释放sds占用的空间
  void sdsfree(sds s) {
      if (s == NULL) return;
      s_free((char*)s-sdsHdrSize(s[-1])); // 指针要指向sds结构体的头部
  }
  
  /* Set the sds string length to the length as obtained with strlen(), so
   * considering as content only up to the first null term character.
   *
   * This function is useful when the sds string is hacked manually in some
   * way, like in the following example:
   *
   * s = sdsnew("foobar");
   * s[2] = '\0';
   * sdsupdatelen(s);
   * printf("%d\n", sdslen(s));
   *
   * The output will be "2", but if we comment out the call to sdsupdatelen()
   * the output will be "6" as the string was modified but the logical length
   * remains 6 bytes. */
  // 用strlen()获取并更新sds的长度
  void sdsupdatelen(sds s) {
      size_t reallen = strlen(s);
      sdssetlen(s, reallen);
  }
  
  /* Modify an sds string in-place to make it empty (zero length).
   * However all the existing buffer is not discarded but set as free space
   * so that next append operations will not require allocations up to the
   * number of bytes previously available. */
  // 将sds的存储的字符串长度置为0，内存空间并没有释放。
  void sdsclear(sds s) {
      sdssetlen(s, 0);
      s[0] = '\0';
  }
  
  /* Enlarge the free space at the end of the sds string so that the caller
   * is sure that after calling this function can overwrite up to addlen
   * bytes after the end of the string, plus one more byte for nul term.
   *
   * Note: this does not change the *length* of the sds string as returned
   * by sdslen(), but only the free buffer space we have. */
  sds sdsMakeRoomFor(sds s, size_t addlen) {
      void *sh, *newsh;
      size_t avail = sdsavail(s);
      size_t len, newlen;
      char type, oldtype = s[-1] & SDS_TYPE_MASK; // s[-1]就是flags，是因为buf指向存储的字符串，其值是flags的地址+1.
      int hdrlen;
  
      /* Return ASAP if there is enough space left. */
      if (avail >= addlen) return s;
  
      len = sdslen(s);
      sh = (char*)s-sdsHdrSize(oldtype);
      newlen = (len+addlen);
      if (newlen < SDS_MAX_PREALLOC)
          newlen *= 2;
      else
          newlen += SDS_MAX_PREALLOC;
  
      type = sdsReqType(newlen);
  
      /* Don't use type 5: the user is appending to the string and type 5 is
       * not able to remember empty space, so sdsMakeRoomFor() must be called
       * at every appending operation. */
      if (type == SDS_TYPE_5) type = SDS_TYPE_8;
  
      hdrlen = sdsHdrSize(type);
      if (oldtype==type) {
          newsh = s_realloc(sh, hdrlen+newlen+1);
          if (newsh == NULL) return NULL;
          s = (char*)newsh+hdrlen;
      } else {
          /* Since the header size changes, need to move the string forward,
           * and can't use realloc */
          newsh = s_malloc(hdrlen+newlen+1);
          if (newsh == NULL) return NULL;
          memcpy((char*)newsh+hdrlen, s, len+1);
          s_free(sh);
          s = (char*)newsh+hdrlen;
          s[-1] = type;
          sdssetlen(s, len);
      }
      sdssetalloc(s, newlen);
      return s;
  }
  
  /* Reallocate the sds string so that it has no free space at the end. The
   * contained string remains not altered, but next concatenation operations
   * will require a reallocation.
   *
   * After the call, the passed sds string is no longer valid and all the
   * references must be substituted with the new pointer returned by the call. */
  sds sdsRemoveFreeSpace(sds s) {
      void *sh, *newsh;
      char type, oldtype = s[-1] & SDS_TYPE_MASK;
      int hdrlen, oldhdrlen = sdsHdrSize(oldtype);
      size_t len = sdslen(s);
      size_t avail = sdsavail(s);
      sh = (char*)s-oldhdrlen;
  
      /* Return ASAP if there is no space left. */
      if (avail == 0) return s;
  
      /* Check what would be the minimum SDS header that is just good enough to
       * fit this string. */
      type = sdsReqType(len);
      hdrlen = sdsHdrSize(type);
  
      /* If the type is the same, or at least a large enough type is still
       * required, we just realloc(), letting the allocator to do the copy
       * only if really needed. Otherwise if the change is huge, we manually
       * reallocate the string to use the different header type. */
      if (oldtype==type || type > SDS_TYPE_8) {
          newsh = s_realloc(sh, oldhdrlen+len+1);
          if (newsh == NULL) return NULL;
          s = (char*)newsh+oldhdrlen;
      } else {
          newsh = s_malloc(hdrlen+len+1);
          if (newsh == NULL) return NULL;
          memcpy((char*)newsh+hdrlen, s, len+1);
          s_free(sh);
          s = (char*)newsh+hdrlen;
          s[-1] = type;
          sdssetlen(s, len);
      }
      sdssetalloc(s, len);
      return s;
  }
  
  /* Return the total size of the allocation of the specified sds string,
   * including:
   * 1) The sds header before the pointer.
   * 2) The string.
   * 3) The free buffer at the end if any.
   * 4) The implicit null term.
   */
  size_t sdsAllocSize(sds s) {
      size_t alloc = sdsalloc(s);
      return sdsHdrSize(s[-1])+alloc+1;
  }
  
  /* Return the pointer of the actual SDS allocation (normally SDS strings
   * are referenced by the start of the string buffer). */
  void *sdsAllocPtr(sds s) {
      return (void*) (s-sdsHdrSize(s[-1]));
  }
  
  /* Increment the sds length and decrements the left free space at the
   * end of the string according to 'incr'. Also set the null term
   * in the new end of the string.
   *
   * This function is used in order to fix the string length after the
   * user calls sdsMakeRoomFor(), writes something after the end of
   * the current string, and finally needs to set the new length.
   *
   * Note: it is possible to use a negative increment in order to
   * right-trim the string.
   *
   * 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:
   *
   * 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);
   */
  void sdsIncrLen(sds s, ssize_t incr) {
      unsigned char flags = s[-1];
      size_t len;
      switch(flags&SDS_TYPE_MASK) {
          case SDS_TYPE_5: {
              unsigned char *fp = ((unsigned char*)s)-1;
              unsigned char oldlen = SDS_TYPE_5_LEN(flags);
              assert((incr > 0 && oldlen+incr < 32) || (incr < 0 && oldlen >= (unsigned int)(-incr)));
              *fp = SDS_TYPE_5 | ((oldlen+incr) << SDS_TYPE_BITS);
              len = oldlen+incr;
              break;
          }
          case SDS_TYPE_8: {
              SDS_HDR_VAR(8,s);
              assert((incr >= 0 && sh->alloc-sh->len >= incr) || (incr < 0 && sh->len >= (unsigned int)(-incr)));
              len = (sh->len += incr);
              break;
          }
          case SDS_TYPE_16: {
              SDS_HDR_VAR(16,s);
              assert((incr >= 0 && sh->alloc-sh->len >= incr) || (incr < 0 && sh->len >= (unsigned int)(-incr)));
              len = (sh->len += incr);
              break;
          }
          case SDS_TYPE_32: {
              SDS_HDR_VAR(32,s);
              assert((incr >= 0 && sh->alloc-sh->len >= (unsigned int)incr) || (incr < 0 && sh->len >= (unsigned int)(-incr)));
              len = (sh->len += incr);
              break;
          }
          case SDS_TYPE_64: {
              SDS_HDR_VAR(64,s);
              assert((incr >= 0 && sh->alloc-sh->len >= (uint64_t)incr) || (incr < 0 && sh->len >= (uint64_t)(-incr)));
              len = (sh->len += incr);
              break;
          }
          default: len = 0; /* Just to avoid compilation warnings. */
      }
      s[len] = '\0';
  }
  
  /* Grow the sds to have the specified length. Bytes that were not part of
   * the original length of the sds will be set to zero.
   *
   * if the specified length is smaller than the current length, no operation
   * is performed. */
  sds sdsgrowzero(sds s, size_t len) {
      size_t curlen = sdslen(s);
  
      if (len <= curlen) return s;
      s = sdsMakeRoomFor(s,len-curlen);
      if (s == NULL) return NULL;
  
      /* Make sure added region doesn't contain garbage */
      memset(s+curlen,0,(len-curlen+1)); /* also set trailing \0 byte */
      sdssetlen(s, len);
      return s;
  }
  
  /* Append the specified binary-safe string pointed by 't' of 'len' bytes to the
   * end of the specified sds string 's'.
   *
   * After the call, the passed sds string is no longer valid and all the
   * references must be substituted with the new pointer returned by the call. */
  sds sdscatlen(sds s, const void *t, size_t len) {
      size_t curlen = sdslen(s);
  
      s = sdsMakeRoomFor(s,len);
      if (s == NULL) return NULL;
      memcpy(s+curlen, t, len);
      sdssetlen(s, curlen+len);
      s[curlen+len] = '\0';
      return s;
  }
  
  /* Append the specified null termianted C string to the sds string 's'.
   *
   * After the call, the passed sds string is no longer valid and all the
   * references must be substituted with the new pointer returned by the call. */
  sds sdscat(sds s, const char *t) {
      return sdscatlen(s, t, strlen(t));
  }
  
  /* Append the specified sds 't' to the existing sds 's'.
   *
   * 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. */
  sds sdscatsds(sds s, const sds t) {
      return sdscatlen(s, t, sdslen(t));
  }
  
  /* Destructively modify the sds string 's' to hold the specified binary
   * safe string pointed by 't' of length 'len' bytes. */
  sds sdscpylen(sds s, const char *t, size_t len) {
      if (sdsalloc(s) < len) {
          s = sdsMakeRoomFor(s,len-sdslen(s));
          if (s == NULL) return NULL;
      }
      memcpy(s, t, len);
      s[len] = '\0';
      sdssetlen(s, len);
      return s;
  }
  
  /* Like sdscpylen() but 't' must be a null-termined string so that the length
   * of the string is obtained with strlen(). */
  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 length of the null-terminated string
   * representation stored at 's'. */
  #define SDS_LLSTR_SIZE 21
  // 把long long类型的值转换成
  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++ = '-';  // 字符串从左往右是数字的低位跟高位，所以最后还要反转以下字符串。
  
      /* 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);
   */
  // 将long long转换成sds
  sds sdsfromlonglong(long long value) {
      char buf[SDS_LLSTR_SIZE];
      int len = sdsll2str(buf,value);
  
      return sdsnewlen(buf,len);
  }
  
  /* Like sdscatprintf() but gets va_list instead of being variadic. */
  // 打印sds字符串， fmt是格式化字符串，ap是格式化字符串中的参数。
  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 = s_malloc(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);
          vsnprintf(buf, buflen, fmt, cpy);
          va_end(cpy);
          if (buf[buflen-2] != '\0') {
              if (buf != staticbuf) s_free(buf);
              buflen *= 2;
              buf = s_malloc(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) s_free(buf);
      return t;
  }
  
  /* 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 = sdsnew("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);
   */
  // 将格式化字符串拼接到sds字符串上。
  sds sdscatprintf(sds s, const char *fmt, ...) {
      va_list ap;
      char *t;
      va_start(ap, fmt);
      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, ...) {
      size_t initlen = sdslen(s);
      const char *f = fmt;
      long 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 (sdsavail(s)==0) {
              s = sdsMakeRoomFor(s,1);
          }
  
          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 (sdsavail(s) < l) {
                      s = sdsMakeRoomFor(s,l);
                  }
                  memcpy(s+i,str,l);
                  sdsinclen(s,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 (sdsavail(s) < l) {
                          s = sdsMakeRoomFor(s,l);
                      }
                      memcpy(s+i,buf,l);
                      sdsinclen(s,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 (sdsavail(s) < l) {
                          s = sdsMakeRoomFor(s,l);
                      }
                      memcpy(s+i,buf,l);
                      sdsinclen(s,l);
                      i += l;
                  }
                  break;
              default: /* Handle %% and generally %<unknown>. */
                  s[i++] = next;
                  sdsinclen(s,1);
                  break;
              }
              break;
          default:
              s[i++] = *f;
              sdsinclen(s,1);
              break;
          }
          f++;
      }
      va_end(ap);
  
      /* Add null-term */
      s[i] = '\0';
      return s;
  }
  
  /* 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,"Aa. :");
   * printf("%s\n", s);
   *
   * Output will be just "HelloWorld".
   */
  sds sdstrim(sds s, const char *cset) {
      char *start, *end, *sp, *ep;
      size_t len;
  
      sp = start = s;
      ep = end = s+sdslen(s)-1;
      while(sp <= end && strchr(cset, *sp)) sp++;
      while(ep > sp && strchr(cset, *ep)) ep--;
      len = (sp > ep) ? 0 : ((ep-sp)+1);
      if (s != sp) memmove(s, sp, len);   // 因为源地址，目标地址有重叠。所以要用memmov
      s[len] = '\0';
      sdssetlen(s,len);
      return s;
  }
  
  /* 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"
   */
  // 截取sds中的一段，截取的范围为[start, end]
  void sdsrange(sds s, ssize_t start, ssize_t end) {
      size_t newlen, len = sdslen(s);
  
      if (len == 0) return;
      if (start < 0) {    // start 可能是负的，例如[-5,-1]
          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 >= (ssize_t)len) {    // line754没有校验start是否超出len - 1
              newlen = 0;
          } else if (end >= (ssize_t)len) {
              end = len-1;
              newlen = (start > end) ? 0 : (end-start)+1;
          }
      } else {
          start = 0;
      }
      if (start && newlen) memmove(s, s+start, newlen);
      s[newlen] = 0;
      sdssetlen(s,newlen);
  }
  
  /* Apply tolower() to every character of the sds string 's'. */
  void sdstolower(sds s) {
      size_t len = sdslen(s), j;
  
      for (j = 0; j < len; j++) s[j] = tolower(s[j]);
  }
  
  /* Apply toupper() to every character of the sds string 's'. */
  void sdstoupper(sds s) {
      size_t len = sdslen(s), j;
  
      for (j = 0; j < len; j++) s[j] = toupper(s[j]);
  }
  
  /* Compare two sds strings s1 and s2 with memcmp().
   *
   * Return value:
   *
   *     positive if s1 > s2.
   *     negative if s1 < s2.
   *     0 if s1 and s2 are exactly the same binary string.
   *
   * If two strings share exactly the same prefix, but one of the two has
   * additional characters, the longer string is considered to be greater than
   * the smaller one. */
  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? 1: (l1<l2? -1: 0);
      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.
   *
   * On out of memory, zero length string, zero length
   * separator, NULL is returned.
   *
   * 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".
   *
   * This version of the function is binary-safe but
   * requires length arguments. sdssplit() is just the
   * same function but for zero-terminated strings.
   */
  // 将字符串按照sep进行分割，sep可以是多个字符的分隔符。返回sds的数组，数组长度为*count
  sds *sdssplitlen(const char *s, ssize_t len, const char *sep, int seplen, int *count) {
      int elements = 0, slots = 5;
      long start = 0, j;
      sds *tokens;
  
      if (seplen < 1 || len < 0) return NULL;
  
      tokens = s_malloc(sizeof(sds)*slots);   // 预分配内存
      if (tokens == NULL) return NULL;
  
      if (len == 0) {
          *count = 0;
          return tokens;
      }
      for (j = 0; j < (len-(seplen-1)); j++) {    // 将整个sep作为分割符
          /* make sure there is room for the next element and the final one */
          if (slots < elements+2) {
              sds *newtokens;
  
              slots *= 2;
              newtokens = s_realloc(tokens,sizeof(sds)*slots);    // 用s_realloc省去了扩展空间时人为释放已开辟空间的操作。
              if (newtokens == NULL) goto cleanup;
              tokens = newtokens;
          }
          /* search the separator */
          if ((seplen == 1 && *(s+j) == sep[0]) || (memcmp(s+j,sep,seplen) == 0)) {
              tokens[elements] = sdsnewlen(s+start,j-start); // 提取子串，并将其转换成sds
              if (tokens[elements] == NULL) goto cleanup;
              elements++;
              start = j+seplen;
              j = j+seplen-1; /* skip the separator */ // -1的原因是line 846会+1
          }
      }
      /* 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]);
          s_free(tokens);
          *count = 0;
          return NULL;
      }
  }
  
  /* Free the result returned by sdssplitlen(), or do nothing if 'tokens' is NULL. */
  void sdsfreesplitres(sds *tokens, int count) {
      if (!tokens) return;
      while(count--)
          sdsfree(tokens[count]);
      s_free(tokens);
  }
  
  /* Append to the sds string "s" an escaped string representation where
   * all the non-printable characters (tested with isprint()) are turned into
   * escapes in the form "\n\r\a...." or "\x<hex-number>".
   *
   * 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. */
  // 将p以格式添加到s后面，如果是无法打印的字符会输出16进制值。
  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; // 特殊字符要加上转移符
          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);
  }
  
  /* Helper function for sdssplitargs() that returns non zero if 'c'
   * is a valid hex digit. */
  int is_hex_digit(char c) {
      return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') ||
             (c >= 'A' && c <= 'F');
  }
  
  /* Helper function for sdssplitargs() that converts a hex digit into an
   * integer from 0 to 15 */
  // 将16进制字符转换成10进制数
  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:
   *
   * foo bar "newline are supported\n" and "\xff\x00otherstuff"
   *
   * The number of arguments is stored into *argc, and an array
   * of sds is returned.
   *
   * The caller should free the resulting array of sds strings with
   * sdsfreesplitres().
   *
   * Note that sdscatrepr() is able to convert back a string into
   * a quoted string in the same format sdssplitargs() is able to parse.
   *
   * 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'
   */
  sds *sdssplitargs(const char *line, int *argc) {
      const char *p = line;
      char *current = NULL;
      char **vector = NULL;
  
      *argc = 0;
      while(1) {
          /* skip blanks */
          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();
              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 */
              vector = s_realloc(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 = s_malloc(sizeof(void*));
              return vector;
          }
      }
  
  err:
      while((*argc)--)
          sdsfree(vector[*argc]);
      s_free(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.
   *
   * For instance: sdsmapchars(mystring, "ho", "01", 2)
   * will have the effect of turning the string "hello" into "0ell1".
   *
   * The function returns the sds string pointer, that is always the same
   * as the input pointer since no resize is needed. */
  sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen) {
      size_t j, i, l = sdslen(s);
  
      for (j = 0; j < l; j++) {
          for (i = 0; i < setlen; i++) {
              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. */
  // 将字符串数组argv中的字符用sep中的字符join起来
  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;
  }
  
  /* Like sdsjoin, but joins an array of SDS strings. */
  // 将sds数组argv中的字符用sep中的字符join起来
  sds sdsjoinsds(sds *argv, int argc, const char *sep, size_t seplen) {
      sds join = sdsempty();
      int j;
  
      for (j = 0; j < argc; j++) {
          join = sdscatsds(join, argv[j]);
          if (j != argc-1) join = sdscatlen(join,sep,seplen);
      }
      return join;
  }
  
  /* Wrappers to the allocators used by SDS. Note that SDS will actually
   * just use the macros defined into sdsalloc.h in order to avoid to pay
   * the overhead of function calls. Here we define these wrappers only for
   * the programs SDS is linked to, if they want to touch the SDS internals
   * even if they use a different allocator. */
  void *sds_malloc(size_t size) { return s_malloc(size); }
  void *sds_realloc(void *ptr, size_t size) { return s_realloc(ptr,size); }
  void sds_free(void *ptr) { s_free(ptr); }
  
  #if defined(SDS_TEST_MAIN)
  #include <stdio.h>
  #include "testhelp.h"
  #include "limits.h"
  
  #define UNUSED(x) (void)(x)
  int sdsTest(void) {
      {
          sds x = sdsnew("foo"), y;
  
          test_cond("Create a string and obtain the length",
              sdslen(x) == 3 && memcmp(x,"foo\0",4) == 0)
  
          sdsfree(x);
          x = sdsnewlen("foo",2);
          test_cond("Create a string with specified length",
              sdslen(x) == 2 && memcmp(x,"fo\0",3) == 0)
  
          x = sdscat(x,"bar");
          test_cond("Strings concatenation",
              sdslen(x) == 5 && memcmp(x,"fobar\0",6) == 0);
  
          x = sdscpy(x,"a");
          test_cond("sdscpy() against an originally longer string",
              sdslen(x) == 1 && memcmp(x,"a\0",2) == 0)
  
          x = sdscpy(x,"xyzxxxxxxxxxxyyyyyyyyyykkkkkkkkkk");
          test_cond("sdscpy() against an originally shorter string",
              sdslen(x) == 33 &&
              memcmp(x,"xyzxxxxxxxxxxyyyyyyyyyykkkkkkkkkk\0",33) == 0)
  
          sdsfree(x);
          x = sdscatprintf(sdsempty(),"%d",123);
          test_cond("sdscatprintf() seems working in the base case",
              sdslen(x) == 3 && memcmp(x,"123\0",4) == 0)
  
          sdsfree(x);
          x = sdsnew("--");
          x = sdscatfmt(x, "Hello %s World %I,%I--", "Hi!", LLONG_MIN,LLONG_MAX);
          test_cond("sdscatfmt() seems working in the base case",
              sdslen(x) == 60 &&
              memcmp(x,"--Hello Hi! World -9223372036854775808,"
                       "9223372036854775807--",60) == 0)
          printf("[%s]\n",x);
  
          sdsfree(x);
          x = sdsnew("--");
          x = sdscatfmt(x, "%u,%U--", UINT_MAX, ULLONG_MAX);
          test_cond("sdscatfmt() seems working with unsigned numbers",
              sdslen(x) == 35 &&
              memcmp(x,"--4294967295,18446744073709551615--",35) == 0)
  
          sdsfree(x);
          x = sdsnew(" x ");
          sdstrim(x," x");
          test_cond("sdstrim() works when all chars match",
              sdslen(x) == 0)
  
          sdsfree(x);
          x = sdsnew(" x ");
          sdstrim(x," ");
          test_cond("sdstrim() works when a single char remains",
              sdslen(x) == 1 && x[0] == 'x')
  
          sdsfree(x);
          x = sdsnew("xxciaoyyy");
          sdstrim(x,"xy");
          test_cond("sdstrim() correctly trims characters",
              sdslen(x) == 4 && memcmp(x,"ciao\0",5) == 0)
  
          y = sdsdup(x);
          sdsrange(y,1,1);
          test_cond("sdsrange(...,1,1)",
              sdslen(y) == 1 && memcmp(y,"i\0",2) == 0)
  
          sdsfree(y);
          y = sdsdup(x);
          sdsrange(y,1,-1);
          test_cond("sdsrange(...,1,-1)",
              sdslen(y) == 3 && memcmp(y,"iao\0",4) == 0)
  
          sdsfree(y);
          y = sdsdup(x);
          sdsrange(y,-2,-1);
          test_cond("sdsrange(...,-2,-1)",
              sdslen(y) == 2 && memcmp(y,"ao\0",3) == 0)
  
          sdsfree(y);
          y = sdsdup(x);
          sdsrange(y,2,1);
          test_cond("sdsrange(...,2,1)",
              sdslen(y) == 0 && memcmp(y,"\0",1) == 0)
  
          sdsfree(y);
          y = sdsdup(x);
          sdsrange(y,1,100);
          test_cond("sdsrange(...,1,100)",
              sdslen(y) == 3 && memcmp(y,"iao\0",4) == 0)
  
          sdsfree(y);
          y = sdsdup(x);
          sdsrange(y,100,100);
          test_cond("sdsrange(...,100,100)",
              sdslen(y) == 0 && memcmp(y,"\0",1) == 0)
  
          sdsfree(y);
          sdsfree(x);
          x = sdsnew("foo");
          y = sdsnew("foa");
          test_cond("sdscmp(foo,foa)", sdscmp(x,y) > 0)
  
          sdsfree(y);
          sdsfree(x);
          x = sdsnew("bar");
          y = sdsnew("bar");
          test_cond("sdscmp(bar,bar)", sdscmp(x,y) == 0)
  
          sdsfree(y);
          sdsfree(x);
          x = sdsnew("aar");
          y = sdsnew("bar");
          test_cond("sdscmp(bar,bar)", sdscmp(x,y) < 0)
  
          sdsfree(y);
          sdsfree(x);
          x = sdsnewlen("\a\n\0foo\r",7);
          y = sdscatrepr(sdsempty(),x,sdslen(x));
          test_cond("sdscatrepr(...data...)",
              memcmp(y,"\"\\a\\n\\x00foo\\r\"",15) == 0)
  
          {
              unsigned int oldfree;
              char *p;
              int step = 10, j, i;
  
              sdsfree(x);
              sdsfree(y);
              x = sdsnew("0");
              test_cond("sdsnew() free/len buffers", sdslen(x) == 1 && sdsavail(x) == 0);
  
              /* Run the test a few times in order to hit the first two
               * SDS header types. */
              for (i = 0; i < 10; i++) {
                  int oldlen = sdslen(x);
                  x = sdsMakeRoomFor(x,step);
                  int type = x[-1]&SDS_TYPE_MASK;
  
                  test_cond("sdsMakeRoomFor() len", sdslen(x) == oldlen);
                  if (type != SDS_TYPE_5) {
                      test_cond("sdsMakeRoomFor() free", sdsavail(x) >= step);
                      oldfree = sdsavail(x);
                  }
                  p = x+oldlen;
                  for (j = 0; j < step; j++) {
                      p[j] = 'A'+j;
                  }
                  sdsIncrLen(x,step);
              }
              test_cond("sdsMakeRoomFor() content",
                  memcmp("0ABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJABCDEFGHIJ",x,101) == 0);
              test_cond("sdsMakeRoomFor() final length",sdslen(x)==101);
  
              sdsfree(x);
          }
      }
      test_report()
      return 0;
  }
  #endif
  
  #ifdef SDS_TEST_MAIN
  int main(void) {
      return sdsTest();
  }
  #endif
  