Add decode subcommand

Accepts decimal or 0x-hex GID input. gid=0 is rendered as the
literal 'empty'; non-zero values print the three components in a
parse-stable space-separated key=value form.

src/cli.js

@@ -31,6 +31,7 @@ function run(argv, { out, err }) {
   }
   const handlers = {
     encode: require('./commands/encode'),
+    decode: require('./commands/decode'),
   };
   const handler = handlers[head];
   if (!handler) {

src/commands/decode.js

@@ -0,0 +1,39 @@
+'use strict';
+
+const { decodeGid } = require('../gid');
+
+const USAGE = 'usage: sporel-mapper decode <gid>\n';
+
+function parseGid(s) {
+  let n;
+  if (/^0x[0-9a-fA-F]+$/.test(s)) {
+    n = parseInt(s, 16);
+  } else if (/^\d+$/.test(s)) {
+    n = parseInt(s, 10);
+  } else {
+    throw new RangeError(`gid '${s}' is not a decimal or 0x-hex integer`);
+  }
+  return n;
+}
+
+function run(args, { out, err }) {
+  if (args.length !== 1) {
+    err.write(USAGE);
+    return 1;
+  }
+  try {
+    const gid = parseGid(args[0]);
+    if (gid === 0) {
+      out.write('empty\n');
+      return 0;
+    }
+    const { atlas, tile, rotation } = decodeGid(gid);
+    out.write(`atlas=${atlas} tile=${tile} rotation=${rotation}\n`);
+    return 0;
+  } catch (e) {
+    err.write(`error: ${e.message}\n`);
+    return 1;
+  }
+}
+
+module.exports = { run };