update parameter passing protocol with presence byte (by @TheAwiteb).

1 files changed, 26 insertions, 12 deletions

diff --git a/5C.md b/5C.md
index dcf0d2c..37197e1 100644
--- a/5C.md
+++ b/5C.md
@@ -117,24 +117,38 @@ For example, this scroll could be displayed as an option to be clicked on any us
 
 ### Parameter Passing
 
-When the host calls `run()`, it passes a single pointer to a buffer containing all parameters in the order they appear in the tags. Each type has a specific encoding:
+When the host calls `run()`, it passes a single pointer to a buffer containing all parameters in the order they appear in the tags. Each type has a specific length or a way to determine its length (for strings, as described in "String Convention", this is a 4-byte prefix).
 
-| Type         | Encoding                                                              | Size          |
-| ------------ | --------------------------------------------------------------------- | ------------- |
-| `public_key` | 32 bytes (should all be set to zero if the parameter is not provided) | 32 bytes      |
-| `event`      | i32 handle                                                            | 4 bytes       |
-| `string`     | u32_be length followed by UTF-8 bytes                                 | 4 + len bytes |
-| `number`     | i32                                                                   | 4 bytes       |
-| `timestamp`  | unix timestamp as u32_be                                              | 4 bytes       |
-| `relay`      | relay URL, same as string                                             | 4 + len bytes |
+The host must prefix every parameter with a single presence byte: `1` indicates the parameter is provided, and `0` indicates it is omitted. If a required parameter is prefixed with `0`, the program must panic.
 
-For example: in a scroll with parameters `[me: public_key, target_event: event, target_relay: relay]` the buffer layout would be:
+| Type         | Encoding                                                              | Size (if provided) |
+| ------------ | --------------------------------------------------------------------- | ------------------ |
+| `public_key` | 32 bytes                                                              | 32 bytes           |
+| `event`      | i32 handle                                                            | 4 bytes            |
+| `string`     | u32_be length followed by UTF-8 bytes                                 | 4 + len bytes      |
+| `number`     | i32                                                                   | 4 bytes            |
+| `timestamp`  | Unix timestamp as u32_be                                              | 4 bytes            |
+| `relay`      | Relay URL, same encoding as string                                    | 4 + len bytes      |
 
+
+For example, in a scroll with these parameters:
+
+```json
+[
+    ["param", "me", "", "public_key", "required"],
+    ["param", "note", "", "event", "required", "1,1111"],
+    ["param", "author", "", "public_key", ""],
+    ["param", "place", "", "relay", "required"]
+]
 ```
-[ 32-byte current user pubkey ][ 4-byte i32 handle ][ 4-byte len for relay URL ][ UTF-8 relay URL ]
+
+The buffer layout would be:
+
+```text
+[ 1 ][ 32-byte pubkey ][ 1 ][ 4-byte handle ][ 0 ][ 1 ][ 4-byte len ][ UTF-8 relay URL ]
 ```
 
-The WASM program reads this by iterating through the declared parameters in order, reading the appropriate number of bytes for each type.
+Because `author` is optional and not provided, its position contains only the `0` presence byte.
 
 ---