[enh] Add Springer Nature engine

Springer Nature is a global publisher dedicated to providing service to research
community [1] with official API [2].

To test this PR, first get your API key following this page:

   https://dev.springernature.com/signup

In searx/engines/springer.py at line 24, add this API key.  I left my own key,
commented out in the line aboce.  Feel free to use it, if needed.

[1] https://www.springernature.com/
[2] https://dev.springernature.com/

Makefile

@@ -194,6 +194,7 @@ PYLINT_FILES=\
 	searx/engines/meilisearch.py \
 	searx/engines/solidtorrents.py \
 	searx/engines/solr.py \
+	searx/engines/springer.py \
 	searx/engines/google_scholar.py \
 	searx/engines/yahoo_news.py \
 	searx/engines/apkmirror.py \

searx/engines/springer.py

@@ -0,0 +1,74 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Springer Nature (science)
+
+"""
+
+# pylint: disable=missing-function-docstring
+
+from datetime import datetime
+from json import loads
+from urllib.parse import urlencode
+
+from searx import logger
+from searx.exceptions import SearxEngineAPIException
+
+logger = logger.getChild('Springer Nature engine')
+
+about = {
+    "website": 'https://www.springernature.com/',
+    "wikidata_id": 'Q21096327',
+    "official_api_documentation": 'https://dev.springernature.com/',
+    "use_official_api": True,
+    "require_api_key": True,
+    "results": 'JSON',
+}
+
+categories = ['science']
+paging = True
+nb_per_page = 10
+api_key = 'unset'
+
+base_url = 'https://api.springernature.com/metadata/json?'
+
+def request(query, params):
+    if api_key == 'unset':
+        raise SearxEngineAPIException('missing Springer-Nature API key')
+    args = urlencode({
+        'q' : query,
+        's' : nb_per_page * (params['pageno'] - 1),
+        'p' : nb_per_page,
+        'api_key' : api_key
+        })
+    params['url'] = base_url + args
+    logger.debug("query_url --> %s", params['url'])
+    return params
+
+
+def response(resp):
+    results = []
+    json_data = loads(resp.text)
+
+    for record in json_data['records']:
+        content = record['abstract'][0:500]
+        if len(record['abstract']) > len(content):
+            content += "..."
+        published = datetime.strptime(record['publicationDate'], '%Y-%m-%d')
+
+        metadata = [record[x] for x in [
+            'publicationName',
+            'identifier',
+            'contentType',
+        ] if record.get(x) is not None]
+
+        metadata = ' / '.join(metadata)
+        if record.get('startingPage') and record.get('endingPage') is not None:
+            metadata += " (%(startingPage)s-%(endingPage)s)" % record
+
+        results.append({
+            'title': record['title'],
+            'url': record['url'][0]['value'].replace('http://', 'https://', 1),
+            'content' : content,
+            'publishedDate' : published,
+            'metadata' : metadata
+        })
+    return results

searx/settings.yml

@@ -961,6 +961,16 @@ engines:
 #    query_fields : '' # query fields
 #    enable_http : True
 
+  - name : springer nature
+    engine : springer
+    # get your API key from: https://dev.springernature.com/signup
+    # api_key : "a69685087d07eca9f13db62f65b8f601" # working API key, for test & debug
+    # set api_key and comment out disabled ..
+    disabled: True
+    shortcut : springer
+    categories : science
+    timeout : 6.0
+
   - name : startpage
     engine : startpage
     shortcut : sp