Sanitize discover parameters

lib/index.js

@@ -12,10 +12,60 @@ async function discover(params, options = {}) {
         artistImageFormat: await _parseImageFormatArg(options.artistImageFormat, 21)
     };
 
-    const url = utils.getDiscoverUrl(params);
+    let resultParams;
-    return fetch(url)
+    return sanitizeDiscoverParams(params)
+        .then( sanitizedParams => {
+            resultParams = sanitizedParams;
+            return utils.getDiscoverUrl(sanitizedParams);
+        })
+        .then( url => fetch(url) )
         .then( res => res.json() )
-        .then( json => parser.parseDiscoverResults(json, opts) );
+        .then( json => {
+            const result = parser.parseDiscoverResults(json, opts);
+            result.params = resultParams;
+            return result;
+        });
+}
+
+async function sanitizeDiscoverParams(params) {
+    return getDiscoverOptions().then( options => {
+        const getOptionValue = (optArr, value) => {
+            if (value !== undefined && optArr) {
+                const opt = optArr.find( o => o.value === value );
+                if (opt) {
+                    return opt.value;
+                }
+            }
+            if (optArr) {
+                return optArr[0].value;
+            }
+            else {
+                return null;
+            }
+        }
+        const sanitized = {
+            genre: getOptionValue(options.genres, params.genre),
+            sortBy: getOptionValue(options.sortBys, params.sortBy),
+            page: params.page || 0
+        };
+        if (sanitized.sortBy !== 'rec') {
+            // following only valid when sortBy is not 'rec' (artist-recommend)
+            if (params.subgenre !== undefined && getOptionValue(options.subgenres[sanitized.genre], params.subgenre) !== null) {
+                sanitized.subgenre = params.subgenre;                
+            }
+            if (sanitized.subgenre === undefined) {
+                // 'time' only valid when 'subgenre' is not set
+                sanitized.time =  getOptionValue(options.times, params.time);
+            }
+            sanitized.location = getOptionValue(options.locations, params.location);
+            sanitized.format = getOptionValue(options.formats, params.format);
+        }
+        else {
+            sanitized.artistRecommendationType = getOptionValue(options.artistRecommendationTypes, params.artistRecommendationType);
+        }
+
+        return sanitized;
+    });
 }
 
 async function getDiscoverOptions() {
@@ -173,6 +223,7 @@ async function getTags() {
 module.exports = {
     discover,
     getDiscoverOptions,
+    sanitizeDiscoverParams,
     getImageFormats,
     getImageFormat,
     getAlbumInfo,