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Brooke Vibber 2023-01-07 18:30:32 -08:00
parent a6c49a2ef9
commit 7da3993bfd
1 changed files with 39 additions and 19 deletions
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58
silly.js
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@ -4,18 +4,15 @@
let logBits = 8;
let logEntries = 2 ** logBits;
let powBits = 19;
let powBits = 16;
let powEntries = 2 ** powBits;
// Room to hold powers up to 4
let shift = 3;
// Room to hold exponents up to 15.9 for 16-bit output
let shift = 4;
let base = 2 ** (powBits - shift);
// 16-bit lookup table
let reduction = 7;
// 12-bit lookup table
//let reduction = 4;
// total of 12-bit lookup table
let reduction = 0;
let powCount = powEntries >> reduction;
@ -32,6 +29,9 @@ function toFloat(fixed) {
let enloggen = new Uint32Array(logEntries);
for (let i = 0; i < logEntries; i++) {
enloggen[i] = toFixed(Math.log2(i));
if (enloggen[i] > 65535) {
throw new Error('enloggen entry out of range')
}
}
// 64k entries
@ -40,8 +40,10 @@ for (let i = 0; i < logEntries; i++) {
// or splitting into high & low vals
let empower = new Uint16Array(powCount);
for (let i = 0; i < powCount; i++) {
let fixed = i << reduction;
empower[i] = Math.round(2 ** toFloat(fixed));
empower[i] = Math.round(2 ** toFloat(i << reduction));
if (empower[i] > 65535) {
throw new Error('empower entry out of range')
}
}
@ -52,27 +54,44 @@ function log2(val) {
// returns rounded integer
function pow2(fixed) {
n = (fixed >> reduction);
let n = fixed >> reduction;
if (empower[n] == undefined) {
console.log(`float ${toFloat(fixed)} fixed ${fixed} n ${n} max ${empower.length}`);
//throw new Error('whoops');
}
return empower[n];
}
function mul(a, b) {
if (a == 0) return 0;
if (b == 0) return 0;
let la = log2(a);
let lb = log2(b);
let la = log2(a)|0;
let lb = log2(b)|0;
let sum = la + lb;
if (sum >= (2 ** powBits)) {
// overflow
throw new Error('overflow on mul');
}
return pow2(la + lb);
}
for (let i = 0; i < logEntries; i++) {
let l = log2(i);
let p = pow2(l);
//console.log(`${i} ${l} ${p}`)
console.log(`${i} ${l} ${p}`)
if (i !== p) {
//console.log(`mismatch ${i} expected, got ${p} via log value ${l} (${toFloat(l)})`);
console.log(`mismatch ${i} expected, got ${p} via log value ${l} (${toFloat(l)})`);
}
}
//process.exit(1);
console.log('empower');
for (let i = 0; i < powEntries; i++) {
let fixed = i << reduction;
let float = toFloat(fixed);
let val = pow2(fixed);
console.log(`${i} ${fixed} ${float} ${val}`)
}
// now just try multipling numbers
@ -96,17 +115,18 @@ for (let i = 0; i < 65536; i++) {
let b = (i >> 8) & 0xff;
let expected = a * b;
let result = mul(a, b);
console.log(`a ${a} b ${b} expected ${expected} result ${result} loga ${log2(a)} logb ${log2(b)} pow ${pow2(log2(a)+log2(b))}`);
let delta = Math.abs(result - expected);
let epsilon = 1;
if (delta >= epsilon) {
if (delta >= epsilon || result === undefined) {
let percent = 100 * (delta / expected);
//console.log(`${a} * ${b} = ${expected}, but got ${result} delta ${delta} ${Math.round(percent * 100) / 100}%`);
console.log(`${a} * ${b} = ${expected}, but got ${result} delta ${delta} ${Math.round(percent * 100) / 100}%`);
deltas += delta;
deltaCount++;
} else {
//console.log(`${a} * ${b} = ${result}`);
console.log(`${a} * ${b} = ${result}`);
}
count++;
}