#157 - Read N Characters Given Read4

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Problem Description

Given a file and assume that you can only read the file using a given method read4, implement a method to read n characters.

Method read4:

The API read4 reads four consecutive characters from file, then writes those characters into the buffer array buf4.

The return value is the number of actual characters read.

Note that read4() has its own file pointer, much like FILE *fp in C.

Solution

/**
 * @param {function} read4()
 * @return {function}
 */
var solution = function(read4) {
  let cache = [];
  let cacheIndex = 0;
  let cacheSize = 0;

  /**
    * @param {character[]} buf Destination buffer
    * @param {number} n Number of characters to read
    * @return {number} The number of actual characters read
    */
  return function(buf, n) {
    let charsRead = 0;

    while (charsRead < n) {
      if (cacheIndex >= cacheSize) {
        cache = new Array(4);
        cacheSize = read4(cache);
        cacheIndex = 0;
        if (cacheSize === 0) break;
      }

      buf[charsRead++] = cache[cacheIndex++];
    }

    return charsRead;
  };
};