lib/json: introduces json store a simple json data storage system.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --git a/lib/json/store.nit b/lib/json/store.nit
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+# This file is part of NIT ( http://www.nitlanguage.org ).
+#
+# This file is free software, which comes along with NIT.  This software is
+# distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+# without  even  the implied warranty of  MERCHANTABILITY or  FITNESS FOR A
+# PARTICULAR PURPOSE.  You can modify it is you want,  provided this header
+# is kept unaltered, and a notification of the changes is added.
+# You  are  allowed  to  redistribute it and sell it, alone or is a part of
+# another product.
+
+# Store and load json data.
+#
+# This simple system can be used to store and retrieve json data.
+#
+# ## Usage
+#
+# Create a new JsonStore or reuse an existing one.
+# ~~~
+# var store = new JsonStore("store_dir")
+# ~~~
+#
+# JsonStore can store json of type JsonObject and JsonArray.
+# ~~~
+# var red = "red"
+# var obj = new JsonObject
+# obj["color"] = red
+# obj["code"] = "FF0000"
+# ~~~
+#
+# Data are stored under a key.
+# ~~~
+# var key = "colors/{red}"
+# ~~~
+#
+# Store the object.
+# ~~~
+# store.store_object(key, obj)
+# assert store.has_key(key)
+# ~~~
+#
+# Load the object.
+# ~~~
+# var res = store.load_object(key)
+# assert res["name"] == red
+# ~~~
+#
+# Clear all stored data.
+# ~~~
+# store.clear
+# ~~~
+module store
+
+import static
+
+# A JsonStore can save and load json data from file system.
+#
+# Json files are stored under a `key`.
+# This key represents the path to the json file from `store_dir`
+# without the `.json` extension.
+#
+# Examples:
+#
+# * key `document` will store data under `store_dir / "document.json"`
+# * key `collection/data` will store data under `store_dir / "collection/data.json"`
+class JsonStore
+
+       # Directory where data are stored.
+       #
+       # Directory is created lazilly at the first write.
+       var store_dir: String
+
+       # Delete all stored data.
+       #
+       # Warning: all `JsonStore` instances sharing the same `store_dir` will
+       # be cleared.
+       fun clear do
+               if not store_dir.file_exists then return
+               store_dir.rmdir
+       end
+
+       # Is there data are stored under `key`.
+       fun has_key(key: String): Bool do
+               return (store_dir / "{key}.json").file_exists
+       end
+
+       # Save `json` object under `key`.
+       fun store_object(key: String, json: JsonObject) do
+               store_json(key, json)
+       end
+
+       # Save `json` array under `key`.
+       fun store_array(key: String, json: JsonArray) do
+               store_json(key, json)
+       end
+
+       # Save `json` data under `key`.
+       #
+       # Only `JsonObject` and `JsonArray` are allowed in a json file.
+       # Use `store_object` or `store_array` instead.
+       private fun store_json(key: String, json: Jsonable) do
+               var path = store_dir / "{key}.json"
+               path.dirname.mkdir
+               var file = new OFStream.open(path)
+               file.write(json.to_json)
+               file.close
+       end
+
+       # Load a JsonObject associated to `key` from store.
+       fun load_object(key: String): JsonObject do
+               return load_json(key).as(JsonObject)
+       end
+
+       # Load a JsonArray associated to `key` from store.
+       fun load_array(key: String): JsonArray do
+               return load_json(key).as(JsonArray)
+       end
+
+       # Load a JsonObject associated to `key` from store.
+       #
+       # Ensure `has_data(key)`
+       private fun load_json(key: String): nullable Jsonable do
+               assert has_key(key)
+               var path = store_dir / "{key}.json"
+               var file = new IFStream.open(path)
+               var text = file.read_all
+               file.close
+               return text.parse_json
+       end
+end