From 4660d01fc129d1a00896ccc7ec9e9ecb10e04a4f Mon Sep 17 00:00:00 2001
From: zeekling <lingzhaohui@zeekling.cn>
Date: Fri, 11 Sep 2020 20:45:47 +0800
Subject: [PATCH] =?UTF-8?q?=E5=A2=9E=E5=8A=A0sds=E6=B3=A8=E9=87=8A?=
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---
 src/sds.c | 23 +++++++++++------------
 src/sds.h |  4 ++--
 2 files changed, 13 insertions(+), 14 deletions(-)

diff --git a/src/sds.c b/src/sds.c
index a1b6189..867ca70 100644
--- a/src/sds.c
+++ b/src/sds.c
@@ -215,34 +215,33 @@ sds sdsMakeRoomFor(sds s, size_t addlen) {
     sh = (char*)s-sdsHdrSize(oldtype);
     newlen = (len+addlen);
     if (newlen < SDS_MAX_PREALLOC)
+		// SDS_MAX_PREALLOC这个宏的值是1MB
         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. */
+    /* 不使用sds5：向字符串添加内容，sds5无法记住空白，因此需要转换成sds8 */
     if (type == SDS_TYPE_5) type = SDS_TYPE_8;
 
     hdrlen = sdsHdrSize(type);
     if (oldtype==type) {
+		// 无需更改类型，通过realloc扩大柔性数组即可，注意这里指向buf的指针s被跟新了
         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);
+        /* 扩容后数据类型和头部长度发生了变化，此时不再进行realloc操作，而是直接重新开 */
+        newsh = s_malloc(hdrlen+newlen+1);// 按照长度进行开辟内存
         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);
+        memcpy((char*)newsh+hdrlen, s, len+1);// 将原buf内容移动到新位置
+        s_free(sh); // 释放旧指针
+        s = (char*)newsh+hdrlen; // 偏移sds结构的起始地址，得到字符串起始地址
+        s[-1] = type; // 为flags赋值
+        sdssetlen(s, len); // 为len属性赋值
     }
-    sdssetalloc(s, newlen);
+    sdssetalloc(s, newlen);// 为alloc属性赋值
     return s;
 }
 
diff --git a/src/sds.h b/src/sds.h
index 8f8a742..e3186ef 100644
--- a/src/sds.h
+++ b/src/sds.h
@@ -42,8 +42,8 @@ extern const char *SDS_NOINIT;
 
 typedef char *sds;
 
-/* Note: sdshdr5 is never used, we just access the flags byte directly.
- * However is here to document the layout of type 5 SDS strings. */
+/* 注意: 不使用sdshdr5，我们只直接访问标志字节。
+ * 不过，这里是为了记录sdshdr5的布局。 */
 struct __attribute__ ((__packed__)) sdshdr5 {
     unsigned char flags; /* 低三位存储类型，高5为存储长度 */
     char buf[]; /* 柔性数组，存放实际内容  */