IDs

This is the full list of what the API gives you, in the order a mod meets it: ids, items, icons, models, recipes, then what happens to saves. Text in brackets at the end of a note is extra detail you can skip on a first read.

Every item and recipe your mod adds gets a number, its id. You never choose the number. The framework works it out from your mod id and the key you give the item, so the same pair always gives the same number.

• IdFor(modGuid, key) gives the number for a mod id and a key, the same in every session and in any load order. Items and recipes have separate lists of numbers, so using the same key for an item and a recipe gives the same number in each, which does no harm. (The number is 1000000 + (FNV-1a 32-bit of the UTF-8 bytes of modGuid + "/" + key) % 1999000000.)
• Capital letters matter in ids. Define("MyMod", key) and Define("mymod", key) give two different numbers. Use one spelling for your mod id everywhere.
• Saves and log files do not care about capitals, so the same mod written two ways shares its save data and its log but not its item numbers. (IdFor hashes the exact text. SaveData and CoreLog compare names without regard to case. The hash is not changed, because that would break saved ids.)
• modGuid and key are 1 to 64 characters from a-z A-Z 0-9 . _ -. Never rename a key after you have released the mod: a new key is a new item.