Add Map Exchange String parser

Add readMapExchangeString to lib/factorio which parses and decodes the
data in Map Exchange into a format suitable for giving to the Factorio
server command line.

test/lib/factorio/exchange_string.js

@@ -0,0 +1,102 @@
+"use strict";
+const assert = require("assert").strict;
+const zlib = require("zlib");
+
+const libFactorio = require("@clusterio/lib/factorio");
+
+
+let defaultString = `
+>>>eNpjZGBkUGQAgwZ7EOZgSc5PzIHxQJgrOb+gILVIN78oFVmYM
+7moNCVVNz8TVXFqXmpupW5SYjGKYo7Movw8dBNYi0vy81BFSopSU
+4uRRbhLixLzMktz0fUyMF5imubZ0CLHAML/6xkU/v8HYSDrAdAvI
+MzA2ABRCRSDAdbknMy0NAYGBUcgdgJJMzIyVousc39YNcWeEaJGz
+wHK+AAVOZAEE/GEMfwccEqpwBgmSOYYg8FnJAbE0hKgFVBVHA4IB
+kSyBSTJyNj7duuC78cu2DH+Wfnxkm9Sgj2joavIuw9G6+yAkuwgf
+zLBiVkzQWAnzCsMMDMf2EOlbtoznj0DAm/sGVlBOkRAhIMFkDjgz
+czAKMAHZC3oARIKMgwwp9nBjBFxYEwDg28wnzyGMS7bo/sDGBA2I
+MPlQMQJEAG2EO4yRgjTod+B0UEeJiuJUALUb8SA7IYUhA9Pwqw9j
+GQ/mkMwIwLZH2giKg5YooELZGEKnHjBDHcNMDwvsMN4DvMdGJlBD
+JCqL0AxCA8kAzMKQgs4gIObmQEBgGmjrO/lHgDxa6G4<<<
+`;
+
+let testString = `
+>>>eNp1UzFs00AUvZ8mbRIEypCBSlCCyFAhOQqBKULxlQUxMCCxgnCc
+S2rh2On5LAgMZOjAgISEurQLXSmCBTGwRWKhEkgIJraiMgBiaGmFOiC
+FO5/PsUz6pfv+9/6/99+/kwEBOo2k6c83NhrZtOkaNkIDXa286fZ6hG
+ouJaJIwTmT+i2iuZYdQ3GeOKTb15qGJ4oxQsMglbWo60iGYcSQ8Zjrh
+GUhwighHpcRSBHIEZ8ajuV35dmBwIPK1DRxmMX6ADnqmre1Rb/D86UF
+hNYX1lZncZpZNkGpmQ41PE87J6RFTe4YjND4hPDq943NwfIcEmv0AJV
+GI7F4tMXViYVgECgCjoWWPm66DqOurXmEMcvp1A3/br1pGV5Oq1ZqVW
+Hzk0ralCz5xDH79a5vM6tnW4Rma5XgQPVk8kTXtTzmU5Jg1g6tm0hfr
+ZwPLGPaVrstL6p0SYwEAPeLLy9/vbeigxysgsPgIESGTYVcCYMnL/Bh
+qbRKoQsq2NFBdt+LBbIp4y3CqiweBzK5LJIAu4vbD18f7Dfg77PdT1e
+bt3S4/q2w5J39IbQfFW+TitzaqrA3ahSkOLf0MPVFhw/vhf3SYUqcOC
+PczkoKQfXaNILCMb5df8Rd6QRS0hqKpoihHdgfNcm2Cj7ryTnKGC4K8
+jnh3gmXQRElVwYyxI8x4FMqOzsu4edrKK6hNZ5wU7V9G+ufEFL+7yHi
+cySQMp7wDHnRsBW571ORGn6fH2fUDj/FwV0iUbXPMbmTP7akkt8ChiL
+/BPceEs3j1M3iz71/ObT9tA==<<<`;
+
+// Simple linear congruential random number generator
+class Random {
+	constructor(seed) {
+		this.prev = seed;
+	}
+
+	next() {
+		let curr = (1103515245 * this.prev + 12345) & 0x7fffffff;
+		this.prev = curr;
+		return curr >> 16;
+	}
+}
+
+describe("lib/factorio/exchange_string", function() {
+	describe("readMapExchangeString", function() {
+		it("should parse a valid string", function() {
+			let result = libFactorio.readMapExchangeString(defaultString);
+			assert.equal(result.map_gen_settings.seed, 1234567890);
+
+			result = libFactorio.readMapExchangeString(testString);
+			assert.equal(result.checksum, 4092204126);
+		});
+
+		it("should handle malformed strings", function() {
+			assert.throws(
+				() => libFactorio.readMapExchangeString("<<blah>>"),
+				new Error("Not a map exchange string")
+			);
+
+			assert.throws(
+				() => libFactorio.readMapExchangeString(defaultString.slice(0, 100)),
+				new Error("Not a map exchange string")
+			);
+
+			assert.throws(
+				() => libFactorio.readMapExchangeString(`>>>${Buffer.from("abk430ia404ah3b4").toString("base64")}<<<`),
+				new Error("Malformed map exchange string: zlib inflate failed")
+			);
+
+			for (let i = 0; i < 100; i++) {
+				let gen = new Random(i);
+				let size = gen.next() % 200 + 100;
+				let data = Buffer.alloc(size);
+				for (let j = 0; j < size; j++) {
+					data[j] = gen.next() % 256;
+				}
+				// eslint-disable-next-line node/no-sync
+				data = zlib.deflateSync(data);
+				assert.throws(
+					() => libFactorio.readMapExchangeString(`>>>${data.toString("base64")}<<<`),
+					new Error("Malformed map exchange string: reached end before finishing parsing")
+				);
+			}
+
+			let pastEnd = Buffer.from(defaultString.replace(/><\n/g, ""), "base64");
+			// eslint-disable-next-line node/no-sync
+			pastEnd = zlib.deflateSync(Buffer.concat([zlib.inflateSync(pastEnd), Buffer.from("junk")]));
+			assert.throws(
+				() => libFactorio.readMapExchangeString(`>>>${pastEnd.toString("base64")}<<<`),
+				new Error("Malformed map exchange string: data after end")
+			);
+		});
+	});
+});