41 lines
1.1 KiB
JavaScript
41 lines
1.1 KiB
JavaScript
|
// ax = (a + x)2/2 - a2/2 - x2/2
|
||
|
|
|
||
|
|
function half_square(x) {
|
||
|
|
return Math.round(x * x / 2) & 0xffff >>> 0;
|
||
|
|
}
|
||
|
|
|
||
|
|
function mul8(a, b) {
|
||
|
|
let result = half_square(a + b) & 0xffff;
|
||
|
|
result = (result - half_square(a)) & 0xffff;
|
||
|
|
result = (result - half_square(b)) & 0xffff;
|
||
|
|
result = (result + (b & a & 1)) & 0xffff;
|
||
|
|
return result >>> 0;
|
||
|
|
}
|
||
|
|
|
||
|
|
function mul16(a, b) {
|
||
|
|
let ah = (a & 0xff00) >>> 8;
|
||
|
|
let al = (a & 0x00ff) >>> 0;
|
||
|
|
let bh = (b & 0xff00) >>> 8;
|
||
|
|
let bl = (b & 0x00ff) >>> 0;
|
||
|
|
let result = (mul8(al, bl) & 0xffff) >>> 0;
|
||
|
|
result = ((result + (mul8(ah, bl) << 8)) & 0x00ffffff) >>> 0;
|
||
|
|
result = ((result + (mul8(al, bh) << 8)) & 0x01ffffff) >>> 0;
|
||
|
|
result = ((result + (mul8(ah, bh) << 16)) & 0xffffffff) >>> 0;
|
||
|
|
return result;
|
||
|
|
}
|
||
|
|
|
||
|
|
let max = 65536;
|
||
|
|
//let max = 256;
|
||
|
|
//let max = 128;
|
||
|
|
//let max = 8;
|
||
|
|
|
||
|
|
for (let a = 0; a < max; a++) {
|
||
|
|
for (let b = 0; b < max; b++) {
|
||
|
|
let expected = Math.imul(a, b) >>> 0;
|
||
|
|
//let actual = mul8(a, b);
|
||
|
|
let actual = mul16(a, b);
|
||
|
|
if (expected !== actual) {
|
||
|
|
console.log(`wrong! ${a} * ${b} expected ${expected} got ${actual}`);
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|