You are given a file-like object that can only read in chunks of 4 characters at a time via a read4
function. This function is already implemented and returns the number of characters actually read (0-4). Your task is to implement a read(buf, n)
function that reads up to n
characters from the file into the provided buffer buf
. The read
function may be called multiple times.
The challenge is that read4
might return fewer than 4 characters, especially at the end of the file. Furthermore, if read(buf, n)
is called multiple times, any leftover characters from the previous read4
call should be used before calling read4
again.
For example, suppose the file contains the string "abcdefghijk".
read(buf, 5)
should read "abcde" into buf
and return 5.read(buf, 5)
should read "fghij" into buf
and return 5.read(buf, 5)
should read "k" into buf
and return 1.read(buf, 5)
should return 0 because there are no more characters to read.Important Constraints:
read4(char[] buf4)
is a pre-defined function that you cannot modify.int read(char[] buf, int n)
.read(buf, n)
is called repeatedly.When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:
The goal is to read a specific number of characters, but the reading function only reads a limited amount at a time. The brute force strategy involves repeatedly calling the reading function and handling any leftover characters from previous calls until we have enough characters.
Here's how the algorithm would work step-by-step:
class Solution:
def __init__(self):
self.remaining_characters = ""
def read(self, buffer, number_of_characters):
characters_read = 0
# Use leftover characters if available
while self.remaining_characters and characters_read < number_of_characters:
buffer[characters_read] = self.remaining_characters[0]
self.remaining_characters = self.remaining_characters[1:]
characters_read += 1
# Keep reading until enough chars are read
while characters_read < number_of_characters:
read_buffer = [''] * 4
chars_from_read4 = read4(read_buffer)
# Reached end of file
if chars_from_read4 == 0:
break
read_string = ''.join(read_buffer[:chars_from_read4])
# Use chars to fill buffer
characters_to_copy = min(number_of_characters - characters_read, chars_from_read4)
for i in range(characters_to_copy):
buffer[characters_read] = read_string[i]
characters_read += 1
# Store unused characters for next read
if characters_to_copy < chars_from_read4:
self.remaining_characters = read_string[characters_to_copy:]
return characters_read
This problem is about reading data in chunks, but the catch is that we might not always use up the whole chunk. We need to remember the leftover data from previous calls to make sure we don't lose any information when the function is called again.
Here's how the algorithm would work step-by-step:
class Solution:
def __init__(self):
self.remaining_characters = []
def read(self, buffer, number_of_characters):
characters_read = 0
# Use up remaining characters from the last read, if any.
while self.remaining_characters and characters_read < number_of_characters:
buffer[characters_read] = self.remaining_characters.pop(0)
characters_read += 1
# Read more characters until enough is read or end of file is reached.
while characters_read < number_of_characters:
read_buffer = [''] * 4
characters_from_read4 = read4(read_buffer)
# If read4 returns less than 4, it's the end of the file.
if characters_from_read4 == 0:
break
# Copy characters into the output buffer and save any leftover.
for i in range(characters_from_read4):
if characters_read < number_of_characters:
buffer[characters_read] = read_buffer[i]
characters_read += 1
else:
# Store the remaining characters for the next call
self.remaining_characters.append(read_buffer[i])
return characters_read
Case | How to Handle |
---|---|
n is 0 | Return 0 immediately, as no characters need to be read. |
read4 returns 0 on the first call | Return 0 immediately, as there are no characters to read from the source. |
n is larger than total characters available in the file | Return the number of characters actually read, which may be less than n. |
read4 returns less than 4 characters, but n is not yet reached | Return the number of characters read so far; do not keep trying to read more if read4 returns less than 4. |
Internal buffer from previous calls still has data | Prioritize reading from the internal buffer before calling read4 again. |
read4 returns 4 consistently, but the total number of characters is not a multiple of 4 | Ensure the algorithm correctly handles the last chunk of characters when fewer than 4 are read. |
Repeated calls with small n values, depleting the buffer slowly | Algorithm should efficiently manage the internal buffer and not unnecessarily call read4. |
Integer overflow when tracking total characters read. | If applicable, prevent integer overflow by using a sufficiently large data type or adding check constraints if the number of chars to read is bounded. |