test rounding to 16 and 32 bit integer boundaries - output on os x 10.7: https://gist.github.com/4f5eea309fc4656b5261

test-bit-rounding.cpp

#include <iostream>

/*

clang++ -o test-bit-rounding test-bit-rounding.cpp -O3 -g -Wconversion -Wsign-conversion;./test-bit-rounding

https://github.com/mapnik/mapnik/commit/e7ae5121a2551763e81aa19fe8a4db66226e829e

*/

/*
 warning: operand of ? changes signedness: 'int' to 'unsigned int' [-Wsign-conversion]
    return width > 7 ? (int(0.125*width) + 1)&~1 : 1;
    ~~~~~~             ~~~~~~~~~~~~~~~~~~~~~~^~~
*/

unsigned old_16(unsigned width) {
    return width > 7 ? (int(0.125*width) + 1)&~1 : 1;
}


unsigned new_16(unsigned width) {
    return ((width + 15) >> 3) & ~1U;
}

unsigned old_32(unsigned width) {
    return width > 3 ? (int(0.5*width) + 3)&~3 : 4;
}


unsigned new_32(unsigned width) {
    return ((width + 7) >> 1) & ~3U;
}


int main() {

    for (unsigned i=0;i<640;++i) {
        if (new_16(i) != old_16(i)) {
            std::clog << new_16(i) << " != " << old_16(i) << " for (" << i << " width will crash)\n";
        } else {
            std::clog << new_16(i) << " == " << old_16(i) << " for (" << i << " width is okay)\n";
        }
    }
    return 0;
}