4 files changed, 68 insertions, 1 deletions
diff --git a/bin/reduceOverlap.js b/bin/reduceOverlap.js
index d302cbb..ab4f6e7 100755
--- a/bin/reduceOverlap.js
+++ b/bin/reduceOverlap.js
@@ -4,6 +4,7 @@ const fs = require('fs')
 const { Readable, Transform, pipeline } = require('stream')
 const ndjson = require('ndjson')
 const cloneDeep = require('clone-deep')
+const unitsToRanges = require('../lib/unitsToRanges.js')
 
 const argv = require('yargs/yargs')(process.argv.slice(2))
   .option('debug', {
diff --git a/lib/unitsToRanges.js b/lib/unitsToRanges.js
new file mode 100644
index 0000000..8ffd7b0
--- /dev/null
+++ b/lib/unitsToRanges.js
@@ -0,0 +1,23 @@
+/**
+ * Convert a list of unit numbers into an addr:flats list. eg. converts 1,2,3,5 into 1-3;5
+ *
+ * @param {Array} units
+ *
+ * @returns {string} addr:flats list
+ */
+module.exports = (units) => {
+  // adapted from https://stackoverflow.com/a/54973116/6702659
+  const unitRanges = units
+    .slice()
+    .sort((a, b) => a - b)
+    .reduce((acc, cur, idx, src) => {
+      if ((idx > 0) && ((cur - src[idx - 1]) === 1)) {
+        acc[acc.length - 1][1] = cur
+      } else {
+        acc.push([cur])
+      }
+      return acc
+    }, [])
+    .map(range => range.join('-'))
+  return unitRanges.length ? unitRanges.join(';') : null
+}
diff --git a/package.json b/package.json
index 13dc6ab..a3a45f0 100644
--- a/package.json
+++ b/package.json
@@ -6,7 +6,7 @@
   "author": "Andrew Harvey <andrew@alantgeo.com.au>",
   "license": "MIT",
   "scripts": {
-    "test": "./node_modules/.bin/tape test/toOSM.js test/cluster.js"
+    "test": "./node_modules/.bin/tape test/toOSM.js test/cluster.js test/unitsToRanges.js"
   },
   "dependencies": {
     "capital-case": "^1.0.4",
diff --git a/test/unitsToRanges.js b/test/unitsToRanges.js
new file mode 100644
index 0000000..53d08aa
--- /dev/null
+++ b/test/unitsToRanges.js
@@ -0,0 +1,43 @@
+const test = require('tape')
+
+const unitsToRanges = require('../lib/unitsToRanges.js')
+
+test('units list to addr:flats', t => {
+  t.same(
+    unitsToRanges([]),
+    null,
+    'empty input returns empty output'
+  )
+
+  t.same(
+    unitsToRanges(['1'], 100),
+    '1',
+    'single unit'
+  )
+
+  t.same(
+    unitsToRanges(['1', '3']),
+    '1;3',
+    'two units without a range'
+  )
+
+  t.same(
+    unitsToRanges(['1', '2']),
+    '1-2',
+    'two consecutive units form a range'
+  )
+
+  t.same(
+    unitsToRanges(['1', '2', '3']),
+    '1-3',
+    'three consecutive units form a range'
+  )
+
+  t.same(
+    unitsToRanges(['1', '2', '4']),
+    '1-2;4',
+    'range and singular'
+  )
+
+  t.end()
+})