Produce NDX files, various improvements to spec, etc.

core/mbf.js

@@ -0,0 +1,59 @@
+const { Errors } = require('./enig_error');
+
+//
+//  Utils for dealing with Microsoft Binary Format (MBF) used
+//  in various BASIC languages, etc.
+//
+//  - https://en.wikipedia.org/wiki/Microsoft_Binary_Format
+//  - https://stackoverflow.com/questions/2268191/how-to-convert-from-ieee-python-float-to-microsoft-basic-float
+//
+
+//  Number to 32bit MBF
+numToMbf32 = (v) => {
+    const mbf = Buffer.alloc(4);
+
+	if (0 === v) {
+		return mbf;
+	}
+
+	const ieee = Buffer.alloc(4);
+	ieee.writeFloatLE(v, 0);
+
+	const sign  = ieee[3] & 0x80;
+	let exp     = (ieee[3] << 1) | (ieee[2] >> 7);
+
+    if (exp === 0xfe) {
+		throw Errors.Invalid(`${v} cannot be converted to mbf`);
+	}
+
+	exp += 2;
+
+    mbf[3] = exp;
+	mbf[2] = sign | (ieee[2] & 0x7f);
+	mbf[1] = ieee[1];
+	mbf[0] = ieee[0];
+
+	return mbf;
+}
+
+mbf32ToNum = (mbf) => {
+	if (0 === mbf[3]) {
+		return 0.0;
+	}
+
+	const ieee	= Buffer.alloc(4);
+	const sign	= mbf[2] & 0x80;
+	const exp	= mbf[3] - 2;
+
+	ieee[3] = sign | (exp >> 1);
+	ieee[2] = (exp << 7) | (mbf[2] & 0x7f);
+	ieee[1] = mbf[1];
+	ieee[0] = mbf[0];
+
+	return ieee.readFloatLE(0);
+}
+
+module.exports = {
+	numToMbf32,
+	mbf32ToNum,
+}