Function stbi__shiftsigned

lite/example/cpp_example/mge/cv/stb_image.h:5944–5967

extract an arbitrarily-aligned N-bit value (N=bits) from v, and then make it 8-bits long and fractionally extend it to full full range.

// from v, and then make it 8-bits long and fractionally
// extend it to full full range.
static int stbi__shiftsigned(unsigned int v, int shift, int bits) {
    static unsigned int mul_table[9] = {
            0,
            0xff /*0b11111111*/,
            0x55 /*0b01010101*/,
            0x49 /*0b01001001*/,
            0x11 /*0b00010001*/,
            0x21 /*0b00100001*/,
            0x41 /*0b01000001*/,
            0x81 /*0b10000001*/,
            0x01 /*0b00000001*/,
    };
    static unsigned int shift_table[9] = {
            0, 0, 0, 1, 0, 2, 4, 6, 0,
    };
    if (shift < 0)
        v <<= -shift;
    else
        v >>= shift;
    STBI_ASSERT(v < 256);
    v >>= (8 - bits);
    STBI_ASSERT(bits >= 0 && bits <= 8);
    return (int)((unsigned)v * mul_table[bits]) >> shift_table[bits];
}

typedef struct {
    int bpp, offset, hsz;