Tricks about Coordinates

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Tricks when manipulating coordinates in algorithms.

Coordinates Mapping

For a 2-D coordinate (x,y), 0 <= x < M, 0 <= y < N. \(N,M \in \mathbb{N}\).

We can map it into 1-D coordinate: \[ \text{Let} \ S = \{ (x, y)| \ 0 \le x < M, 0 \le y < N. \ x, y \in \mathbb{N}\}, \\ \forall (x,y) \in S, \ f(x,y) = Nx + y \]

Code:

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int linearMap(int x, int y, int M, int N)
{
	int t = x * N + y;
	return t;
}

Theorem: Bijective Function

\(f(x,y)\) is a bijective function.

Proof:

We know that \(S = \mathrm{Dom}(f)\). Let \(T = \mathrm{Ran}(f)\). So \(f: S \rightarrow T\).

According to XX, if \(X\) and \(Y\) are finite sets with the same cardinality, and \(f: X \rightarrow Y\), then the following are equivalent:

  1. f is a bijection.
  2. f is a surjection.
  3. f is an injection.

The proof follows this.

Let \(t = f(x, y) \in \mathbb{N}, t \in T\).

  1. Prove the equipotential(等势) of \(S\) and \(T\).

    We have: \[ \begin{align} \max(t) &= f(\max (x), \max (y)) = N(M-1) + (N-1) = MN - 1 \\ \min(t) &= f(\min (x), \min (y)) = 0 \end{align} \] So \(T = \{0,1,\cdots,MN-1\}\), and \(|T| = MN\).

    We can easily know that \(|S| = MN\).

    So \[ |T| = |S| \]

  2. Prove \(f\) is an injective(单射) function.

    We use contradiction, if \(\exists (x_1,y_1), (x_2,y_2) \in S, (x_1,y_1) \ne (x_2,y_2)\) \[ t = x_1 N + y_1 = x_2 N + y_2 \] Then \(N = \frac {y_2 - y_1}{x_1 - x_2}\).

    While \(|y_2 - y_1| \le N-1\), \(1 \le |x_1 - x_2| \le M-1\). We have \(\max(\frac {y_2 - y_1}{x_1 - x_2}) = N-1 < N\).

    Thus \(N < N\). This is impossible. So two elements in \(S\) can't map to the same element in \(t\). Thus \(f\) is an injective function.

  3. Because (1) and (2), \(f\) is a bijective function. QED.

Move Points in a Linear Space

以二维平面为例, 二维平面上的点\((x,y)\) 张成了线性空间\(S = \{ (x, y)| x, y \in \mathbb{R}\}\).

在二维平面中移动一个单位长度 = 通过给对应维度的值加/减 1. 可以用数组实现:

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class DirectionMatrix{
    private int[] left = new int[]{-1,0};
    private int[] right = new int[]{1,0};
    private int[] down = new int[]{0,-1};
    private int[] up = new int[]{0,1};

    private int[][] directions = { left, right, down, up };

    public int[] moveLeft(int x, int y)
    {
        x += left[0];
        y += left[1];
        return new int[]{ x, y };
    }


    public int[] moveRight(int x, int y)
    {
        x += right[0];
        y += right[1];
        return new int[]{ x, y };
    }
    public int[] moveDown(int x, int y)
    {
        x += down[0];
        y += down[1];
        return new int[]{ x, y };
    }
    public int[] moveUp(int x, int y)
    {
        x += up[0];
        y += up[1];
        return new int[]{ x, y };
    }

}