Achievements: add support for time constraints

data/json/achievements.json

@@ -0,0 +1,15 @@
+[
+  {
+    "id": "achievement_kill_zombie",
+    "type": "achievement",
+    "description": "One down, billions to go\u2026",
+    "requirements": [ { "event_statistic": "num_avatar_zombie_kills", "is": ">=", "target": 1 } ]
+  },
+  {
+    "id": "achievement_kill_in_first_minute",
+    "type": "achievement",
+    "description": "Rude awakening",
+    "time_constraint": { "since": "game_start", "is": "<=", "target": "1 minute" },
+    "requirements": [ { "event_statistic": "num_avatar_kills", "is": ">=", "target": 1 } ]
+  }
+]

data/json/scores.json

@@ -30,7 +30,8 @@
     "id": "num_avatar_kills",
     "type": "event_statistic",
     "stat_type": "count",
-    "event_transformation": "avatar_kills"
+    "event_transformation": "avatar_kills",
+    "description": "Number of monsters killed"
   },
   {
     "id": "num_avatar_zombie_kills",
@@ -42,15 +43,8 @@
   {
     "id": "score_kills",
     "type": "score",
-    "description": "Number of monsters killed: %s",
     "statistic": "num_avatar_kills"
   },
-  {
-    "id": "achievement_kill_zombie",
-    "type": "achievement",
-    "description": "One down, billions to go\u2026",
-    "requirements": [ { "event_statistic": "num_avatar_zombie_kills", "is": ">=", "target": 1 } ]
-  },
   {
     "id": "moves_not_mounted",
     "type": "event_transformation",

doc/JSON_INFO.md

@@ -1181,7 +1181,8 @@ given field for that unique event:
 
 Regardless of `stat_type`, each `event_statistic` can also have:
 ```C++
-"description": "Number of things" // Intended for use in describing achievement requirements.
+// Intended for use in describing scores and achievement requirements.
+"description": "Number of things"
 ```
 
 #### `score`
@@ -1193,6 +1194,9 @@ of scores.  The `description` specifies a string which is expected to contain a
 Note that even though most statistics yield an integer, you should still use
 `%s`.
 
+If the underlying statistic has a description, then the score description is
+optional.  It defaults to "<statistic description>: <value>".
+
 ```C++
 "id": "score_headshots",
 "type": "score",
@@ -1222,6 +1226,26 @@ an `event_statistic`.  For example:
 },
 ```
 
+Currently the `"is"` field must be `">="` or `"<="` and the `"target"` must be
+an integer, but these restrictions might loosen in the future.
+
+Another optional field is
+
+```C++
+"time_constraint": { "since": "game_start", "is": "<=", "target": "1 minute" }
+```
+
+This allows putting a time limit (either a lower or upper bound) on when the
+achievement can be claimed.  The `"since"` field can be either `"game_start"`
+or `"cataclysm"`.  The `"target"` describes an amount of time since that
+reference point.
+
+Note that achievements can only be captured when a statistic listed in their
+requirements changes.  So, if you want an achievement such as "survived a
+certain amount of time" which effectively only has a time constraint then you
+must still place some requirement alongside it; pick some statistic which is
+likely to change often, and a vacuous or weak constraint on it.
+
 ### Skills
 
 ```C++