边工作边刷题：70天一遍leetcode: day 73

Read N Characters Given Read4 I/II

要点：这题的要点就是搞清楚几个变量的内在逻辑：只有buffer是整4 bytes的。而client要读的bytes（需求）和实际上disk上有的bytes（供给）都是不整的。所以，

• 循环的条件就是either 供给不足 or 需求不足：供给不足的判定是上一轮数据不够4 bytes的mark，而需求不足是toRead的计数<=0
• 所以循环体内，就是min(读进来的bytes, toRead)来把数据copy到buffer里，同时更新toRead和结束条件
• II就是加了个buffer来存上一轮的leftover和offset，在下一次读的时候，把剩余数据假装作为一个read4来处理。
  • 为什么要用bufSize而不是offset本身？offset表示的是数据的起始位置（当前位置是还没读的），可能有数据，也可能没有。
  • 所以逻辑是只有bufSize==0才读4 bytes，global buffer做缓冲区，而bufSize永远标示待读区间的大小
  • offset不断从0向右，然后再回到0：4 bytes肯定一次读进来

I: https://repl.it/Cjjw/1

II: https://repl.it/CjkR/2

错误点：

• self.offset>=4，不是>4
• 别忘了eof（在bufsize==0里面）

# The API: int read4(char *buf) reads 4 characters at a time from a file.
# The return value is the actual number of characters read. For example, it returns 3 if there is only 3 characters left in the file.
# By using the read4 API, implement the function int read(char *buf, int n) that reads n characters from the file.
# Note:
# The read function will only be called once for each test case.
# Hide Company Tags Facebook
# Hide Tags String
# Hide Similar Problems (H) Read N Characters Given Read4 II - Call multiple times
# The read4 API is already defined for you.
# @param buf, a list of characters
# @return an integer
# def read4(buf):
class Solution(object):
    def read(self, buf, n):
        """
        :type buf: Destination buffer (List[str])
        :type n: Maximum number of characters to read (int)
        :rtype: The number of characters read (int)
        """
        eof, nbytes = False, 0
        while not eof and nbytes<n:
            buf4 = [""]*4
            nread = read4(buf4)
            if nread<4:
                eof = True
            nnext = min(nread, n-nbytes)
            for i in xrange(nnext):
                buf[nbytes+i]=buf4[i]
            nbytes+=nnext
        return nbytes

# The API: int read4(char *buf) reads 4 characters at a time from a file.
# The return value is the actual number of characters read. For example, it returns 3 if there is only 3 characters left in the file.
# By using the read4 API, implement the function int read(char *buf, int n) that reads n characters from the file.
# Note:
# The read function may be called multiple times.
# Hide Company Tags Bloomberg Google Facebook
# Hide Tags String
# Hide Similar Problems (E) Read N Characters Given Read4
# The read4 API is already defined for you.
# @param buf, a list of characters
# @return an integer
# def read4(buf):
class Solution(object):
    def __init__(self):
        self.bufbytes, self.offset = 0,0
        self.buf4 = [""]*4
    def read(self, buf, n):
        """
        :type buf: Destination buffer (List[str])
        :type n: Maximum number of characters to read (int)
        :rtype: The number of characters read (int)
        """
        eof, nbytes = False, 0
        while not eof and nbytes<n:
            if self.bufbytes==0:
                self.bufbytes = read4(self.buf4)
                if self.bufbytes<4: # error: don't forget
                    eof = True
            toread = min(n-nbytes, self.bufbytes)
            #print toread,self.offset
            for i in xrange(toread):
                buf[nbytes+i]=self.buf4[self.offset+i]
            self.offset+=toread
            if self.offset >= 4: # error: >=4, last index is 3
                self.offset-=4
            self.bufbytes-=toread
            nbytes+=toread
        return nbytes