Human-readable solution for the NailingPlanks puzzle at Codility (https://codility.com/programmers/lessons/14-binary_search_algorithm/nailing_planks/)

NailingPlanks.js

function solution(A,B,C) {
  return nailsBinarySearch(A,B,C);
}

function nailsBinarySearch(boardStarts, boardEnds, nailPositions) {
  var boards = boardStarts.map((cur, curIndex) => [cur, boardEnds[curIndex]],[]);
  var lowerBorderNailsNumber = 1;
  var upperBorderNailsNumber = nailPositions.length;
  var minimalNailsNumber = -1;

  while (lowerBorderNailsNumber <= upperBorderNailsNumber) {
    var tentativeNailsNumber = Math.floor((lowerBorderNailsNumber+upperBorderNailsNumber)/2);
    if (allBoardsNailed(boards, nailPositions.slice(0, tentativeNailsNumber))) {
      // good result
      minimalNailsNumber = tentativeNailsNumber;
      // maybe tentativeNailsNumber is too large...
      upperBorderNailsNumber = tentativeNailsNumber -1;
    } else {
      // gotta try with a larger tentative
      lowerBorderNailsNumber = tentativeNailsNumber + 1;
    }
  }
  return minimalNailsNumber;
}

function allBoardsNailed(boards, nailPositions) {
  return boards.every(board => nailPositions.some(nailPos => board[0]<=nailPos && board[1]>=nailPos));
}

This one appears to be 100% correct, but still too slow and, as a total, it gets only 62% of the votes. Problems are most likely inside row 27 (which is in any case far too elegant). Every improvement suggestion is welcome.