Allow single-digit zone when parsing MGRS grid reference

Single-digit zones needn't have a leading zero, as in e.g. Widipedia article.

Also add tests from Defense Mapping Agency Technical Manual 8358.1.

Author: chrisveness

CHANGELOG.md

@@ -8,6 +8,7 @@
 - Fix UTM constructor northing range check
 - Fix Mgrs.toUtm() edge case at zone boundaries (e.g. @ 64°S,0°E)
 - Fix rounding error in Utm.toMgrs() which caused UTM for 80°S,0°E to fail
+- Allow single-digit zone when parsing MGRS grid reference [#104]
 
 ## [2.4.0] - 2022-03-16
 

mgrs.js

@@ -148,18 +148,24 @@ class Mgrs {
      *
      * @example
      *   const mgrsRef = Mgrs.parse('31U DQ 48251 11932');
-     *   const mgrsRef = Mgrs.parse('31UDQ4825111932');
+     *   const mgrsRef = Mgrs.parse('31UDQ4825111932'); // military style no separators
      *   //  mgrsRef: { zone:31, band:'U', e100k:'D', n100k:'Q', easting:48251, northing:11932 }
      */
     static parse(mgrsGridRef) {
         if (!mgrsGridRef) throw new Error(`invalid MGRS grid reference ‘${mgrsGridRef}’`);
 
         // check for military-style grid reference with no separators
-        if (!mgrsGridRef.trim().match(/\s/)) {
-            if (!Number(mgrsGridRef.slice(0, 2))) throw new Error(`invalid MGRS grid reference ‘${mgrsGridRef}’`);
-            let en = mgrsGridRef.trim().slice(5); // get easting/northing following zone/band/100ksq
-            en = en.slice(0, en.length/2)+' '+en.slice(-en.length/2); // separate easting/northing
-            mgrsGridRef = mgrsGridRef.slice(0, 3)+' '+mgrsGridRef.slice(3, 5)+' '+en; // insert spaces
+        if (!mgrsGridRef.trim().match(/\s/)) { // replace with standard space-separated format
+            const milref = mgrsGridRef.match(/(\d\d?[A-Z])([A-Z]{2})([0-9]{2,10})/);
+            if (!milref) throw new Error(`invalid MGRS grid reference ‘${mgrsGridRef}’`);
+            const mil = {
+                gzd:    milref[1],
+                en100k: milref[2],
+                en:     milref[3],
+            };
+            mil.e = mil.en.slice(0, mil.en.length/2);
+            mil.n = mil.en.slice(-mil.en.length/2);
+            mgrsGridRef = `${mil.gzd} ${mil.en100k} ${mil.e} ${mil.n}`;
         }
 
         // match separate elements (separated by whitespace)
@@ -168,9 +174,9 @@ class Mgrs {
         if (ref==null || ref.length!=4) throw new Error(`invalid MGRS grid reference ‘${mgrsGridRef}’`);
 
         // split gzd into zone/band
-        const gzd = ref[0];
-        const zone = gzd.slice(0, 2);
-        const band = gzd.slice(2, 3);
+        const gzd = ref[0].match(/(\d\d?)([A-Z])/i);
+        const zone = gzd[1];
+        const band = gzd[2];
 
         // split 100km letter-pair into e/n
         const en100k = ref[1];

test/utm-mgrs-tests.js

@@ -70,6 +70,16 @@ describe('utm/mgrs', function() {
     });
 
     describe('MGRS parse', function() {
+        // note Wikipedia considers 4Q & 4Q FJ to be valid MGRS values; this library expects easting & northing;
+        test('Wikipedia 4Q FJ 1 6', () => Mgrs.parse('4Q FJ 1 6').toString(2).should.equal('04Q FJ 1 6'));
+        test('Wikipedia 4Q FJ 12 67', () => Mgrs.parse('4Q FJ 12 67').toString(4).should.equal('04Q FJ 12 67'));
+        test('Wikipedia 4Q FJ 123 678', () => Mgrs.parse('4Q FJ 123 678').toString(6).should.equal('04Q FJ 123 678'));
+        test('Wikipedia 4Q FJ 1234 6789', () => Mgrs.parse('4Q FJ 1234 6789').toString(8).should.equal('04Q FJ 1234 6789'));
+        test('Wikipedia 4Q FJ 12345 67890', () => Mgrs.parse('4Q FJ 12345 67890').toString(10).should.equal('04Q FJ 12345 67890'));
+        // Defense Mapping Agency Technical Manual 8358.1: Datums, Ellipsoids, Grids, and Grid Reference Systems 3-4
+        test('DMA 18SUU80', () => Mgrs.parse('18SUU80').toString(2).should.equal('18S UU 8 0'));
+        test('DMA 18SUU8401', () => Mgrs.parse('18SUU8401').toString(4).should.equal('18S UU 84 01'));
+        test('DMA 18SUU8360140', () => Mgrs.parse('18SUU836014').toString(6).should.equal('18S UU 836 014'));
         test('parse fail 1', () => should.Throw(function() { Mgrs.parse(null); }, Error, 'invalid MGRS grid reference ‘null’'));
         test('parse fail 2', () => should.Throw(function() { Mgrs.parse('Cambridge'); }, Error, 'invalid MGRS grid reference ‘Cambridge’'));
         test('parse fail 3', () => should.Throw(function() { Mgrs.parse('New York'); }, Error, 'invalid MGRS grid reference ‘New York’'));