From 49831dc04846831649fdc1e059e38b79bdfcce93 Mon Sep 17 00:00:00 2001
From: Brion Vibber <brion@pobox.com>
Date: Sat, 7 Jan 2023 17:20:18 -0800
Subject: [PATCH] wip logs

---
 silly.js | 94 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 94 insertions(+)
 create mode 100644 silly.js

diff --git a/silly.js b/silly.js
new file mode 100644
index 0000000..defd097
--- /dev/null
+++ b/silly.js
@@ -0,0 +1,94 @@
+// input range: 0..255 * 0..255
+// output range: 16 bits
+
+let logBits = 8;
+let logEntries = 2 ** logBits;
+
+let powBits = 16;
+let powEntries = (2 ** powBits);
+
+// Room to hold powers up to 4
+let shift = 4;
+let base = 2 ** (powBits - shift);
+
+// 16-bit lookup table
+// 12-bit lookup table
+let reduction = 4;
+
+function toFixed(float) {
+    return Math.round(float * base);
+}
+
+function toFloat(fixed) {
+    return fixed / base;
+}
+
+// 256x2 = 512 bytes
+let enloggen = new Uint16Array(logEntries);
+for (let i = 0; i < logEntries; i++) {
+    enloggen[i] = toFixed(Math.log2(i));
+}
+
+// 64k entries
+// 64kx2 = 128 KiB
+// can reduce number by reducing precision
+// or splitting into high & low vals
+let powCount = powEntries >> reduction;
+let empower = new Uint16Array(powCount);
+for (let i = 0; i < powCount; i++) {
+    let fixed = i << reduction;
+    empower[i] = Math.round(2 ** toFloat(fixed));
+}
+
+
+// returns fixed
+function log2(byte) {
+    return enloggen[byte];
+}
+
+// returns 16-bit number
+function pow2(fixed) {
+    n = (fixed >> reduction);
+    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);
+    return pow2(la + lb);
+}
+
+/*
+for (let i = 0; i < logEntries; i++) {
+    let l = log2(i);
+    let p = pow2(l);
+    console.log(`${i} ${l} ${p}`)
+    if (i !== p) {
+        //console.log(`mismatch ${i} expected, got ${p} via log value ${l} (${toFloat(l)})`);
+    }
+}
+process.exit(1);
+*/
+
+// now just try multipling numbers
+while (true) {
+    let a = Math.trunc(Math.random() * logEntries);
+    let b = Math.trunc(Math.random() * logEntries);
+
+    let pExpected = a * b;
+    let p = mul(a, b);
+
+    let delta = Math.abs(p - pExpected);
+    let epsilon = 1;
+
+    if (delta >= epsilon) {
+        let percent = 100 * (delta / pExpected);
+        console.log(`${a} * ${b} = ${pExpected}, but got ${p} delta ${delta} ${Math.round(percent * 100) / 100}%`);
+    } else {
+        console.log(`${a} * ${b} = ${p}`);
+    }
+}
+
+console.log('done');