Refactor, units, tests

src/operation.js

@@ -0,0 +1,19 @@
+const op_prefix = exports.op_prefix = str => {
+    const suffix = str.slice(1)
+    if (str.startsWith('+')) return {name: suffix, operation: 'add'}
+    if (str.startsWith('-')) return {name: suffix, operation: 'subtract'}
+    if (str.startsWith('~')) return {name: suffix, operation: 'intersect'}
+    if (str.startsWith('^')) return {name: suffix, operation: 'stack'}
+    return {name: str, operation: 'add'}
+}
+
+exports.operation = (str, choices={}, order=Object.keys(choices)) => {
+    let res = op_prefix(str)
+    for (const key of order) {
+        if (choices[key].includes(res.name)) {
+            res.type = key
+            break
+        }
+    }
+    return res
+}