Matrix and algorithm “spiral”

by Igor Cuckovic

JavaScript

// http://stackoverflow.com/questions/8757514/matrix-and-algorithm-spiral?rq=1

// return an array representing a matrix of size MxN COLxROW
function spiralMatrix(M, N) {
var result = new Array(M * N);
var counter = M * N;
// start position
var curCol = Math.floor((M - 1) / 2);
var curRow = Math.floor(N / 2);
// set the center
result[(curRow * M) + curCol] = counter--;
// your possible moves RIGHT, UP, LEFT, DOWN * y axis is flipped
var allMoves = [[1,0], [0,-1], [-1,0], [0,1]];
var curMove = 0;
var moves = 1; // how many times to make current Move, 1,1,2,2,3,3,4,4 etc
// spiral
while(true) {
 for(var i = 0; i < moves; i++) {
  // move in a spiral outward counter clock-wise direction
  curCol += allMoves[curMove][0];
  curRow += allMoves[curMove][1];
  // naively skips locations that are outside of the matrix bounds
  if(curCol >= 0 && curCol < M && curRow >= 0 && curRow < N) {
   // set the value and decrement the counter
   result[(curRow * M) + curCol] = counter--;
   // if we reached the end return the result
      if(counter == 0) {
          console.log(result)
          return result;
      };
  }
 }
 // increment the number of times to move if necessary UP->LEFT and DOWN->RIGHT
 if(curMove == 1 || curMove == 3) moves++;
 // go to the next move in a circular array fashion
 curMove = (curMove + 1) % allMoves.length;
}
}

spiralMatrix(3,2);