Add nextCastTime() to DssExec (#37)

Author: brianmcmichael

src/DssExec.sol

@@ -53,6 +53,33 @@ contract DssExec {
         return SpellAction(action).officeHours();
     }
 
+    function nextCastTime() external view returns (uint256 castTime) {
+        require(eta != 0, "DssExec/spell-not-scheduled");
+        castTime = block.timestamp > eta ? block.timestamp : eta; // Any day at XX:YY
+
+        if (SpellAction(action).officeHours()) {
+            uint256 day    = (castTime / 1 days + 3) % 7;
+            uint256 hour   = castTime / 1 hours % 24;
+            uint256 minute = castTime / 1 minutes % 60;
+            uint256 second = castTime % 60;
+
+            if (day >= 5) {
+                castTime += (6 - day) * 1 days;                 // Go to Sunday XX:YY
+                castTime += (24 - hour + 14) * 1 hours;         // Go to 14:YY UTC Monday
+                castTime -= minute * 1 minutes + second;        // Go to 14:00 UTC
+            } else {
+                if (hour >= 21) {
+                    if (day == 4) castTime += 2 days;           // If Friday, fast forward to Sunday XX:YY
+                    castTime += (24 - hour + 14) * 1 hours;     // Go to 14:YY UTC next day
+                    castTime -= minute * 1 minutes + second;    // Go to 14:00 UTC
+                } else if (hour < 14) {
+                    castTime += (14 - hour) * 1 hours;          // Go to 14:YY UTC same day
+                    castTime -= minute * 1 minutes + second;    // Go to 14:00 UTC
+                }
+            }
+        }
+    }
+
     // @param _description  A string description of the spell
     // @param _expiration   The timestamp this spell will expire. (Ex. now + 30 days)
     // @param _spellAction  The address of the spell action