add gx decode utility

js/util-gx.js

@@ -0,0 +1,53 @@
+class GXReader {
+    constructor(buffer) {
+        this.buf = buffer;
+        this.pos = 16;
+        this.magic = buffer.slice(0,16).toString().trim();
+
+        console.log({
+            magic: this.magic,
+            bmpoff: this.u32(),
+            gc1off: this.u32(),
+            gc2off: this.u32(),
+            prsecs: this.u32(),
+            prfila: this.u32(),
+            unk01:  this.u32(),
+            unk02:  this.u16(),
+            unk03:  this.u16(),
+            unk04:  this.u16(),
+            unk05:  this.u16(),
+            unk06:  this.u16(),
+            unk07:  this.u16(),
+            unk08:  this.u16(),
+            unk09:  this.u16(),
+            unk10:  this.u8(),
+            unk11:  this.u8(),
+            index:  this.pos
+        });
+    }
+
+    inc(inc) {
+        const ret = this.pos;
+        this.pos += inc;
+        return ret;
+    }
+
+    u8() {
+        return this.buf.readUInt8(this.inc(1));
+    }
+
+    u16() {
+        return this.buf.readUInt16LE(this.inc(2));
+    }
+
+    u32() {
+        return this.buf.readUInt32LE(this.inc(4));
+    }
+
+    string() {
+
+    }
+}
+
+const fs = require('fs');
+const gx = new GXReader(fs.readFileSync(process.argv[2]));