Zig-zag matrix

# title: Zig-zag matrix
# source: https://rosettacode.org/wiki/Zig-zag_matrix
# category: Rosetta Code
# difficulty: Medium
# summary: Fill a small square matrix with zig-zag diagonal ordering.
# tags: popular, arrays, matrices, indexing
# vibe: 0.2

def blank_matrix(size)
  matrix = []
  row = 0

  while row < size
    current = []
    column = 0
    while column < size
      current = current.push(0)
      column = column + 1
    end
    matrix = matrix.push(current)
    row = row + 1
  end

  matrix
end

def set_cell(matrix, row, column, value)
  current = matrix[row]
  current[column] = value
  matrix[row] = current
  matrix
end

def zig_zag_matrix(size)
  matrix = blank_matrix(size)
  diagonal = 0
  value = 0

  while diagonal <= (size - 1) * 2
    if diagonal.even?
      row = diagonal
      if row >= size
        row = size - 1
      end
      column = diagonal - row

      while row >= 0 && column < size
        matrix = set_cell(matrix, row, column, value)
        value = value + 1
        row = row - 1
        column = column + 1
      end
    else
      column = diagonal
      if column >= size
        column = size - 1
      end
      row = diagonal - column

      while column >= 0 && row < size
        matrix = set_cell(matrix, row, column, value)
        value = value + 1
        row = row + 1
        column = column - 1
      end
    end

    diagonal = diagonal + 1
  end

  matrix
end

def run
  zig_zag_matrix(5)
end