https://habr.com/en/articles/543618/

func spiralOrder(matrix [][]int) (out []int){ if len(matrix) == 0 { return out }

n, m := len(matrix), len(matrix[0])

// processed cells

seen := make([][]bool, n) for row := 0; row < n; row++ { seen[row] = make([]bool, m) }

moves := [][]int{ {0, 1}, // move to the left column {1, 0}, // move down to the lower row {0, -1}, // move to the right column {-1, 0}, // move to the upper row }

row, col := 0, 0 move := 0

applyMove := func() {
  seen[row][col] = true
    newRow := row + moves[move][0]
    newCol := col + moves[move][1]
    if newRow == -1 || newRow == n || newCol == -1 || newCol == m || seen[newRow][newCol] {
       // change the direction
       move = (move + 1) % len(moves)      
         row = row + moves[move][0]
       col = col + moves[move][1] 
    } else {
   row, col = newRow, newCol
}

}

for i := 0; i < n * m; i++ {
    value := matrix[row][col]
out = append(out, value)
row, col = applyMove()
}

return out

}