Add spec document for event trigger defintions

docs/event.md

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+# EventTriggerDefinition Format Specification
+
+This specification provides the format and encoding rules necessary to implement
+tooling that generates `EventTriggerDefinition` instances programmatically.
+These
+[`triggerDefinition`](https://github.com/shutter-network/contracts/blob/8b0051b13875a7450811c4634e72f2edad7f5016/src/shutter-service/ShutterEventTriggerRegistry.sol#L38)s
+are used to register event based decryption triggers. Keypers that take part in
+a keyper set that has these enabled, will watch the blockchain for events
+matching these definitions. The registration is done through a contract specific
+to the keyperset. An example
+[can be seen here](https://github.com/shutter-network/contracts/blob/8b0051b13875a7450811c4634e72f2edad7f5016/src/shutter-service/ShutterEventTriggerRegistry.sol).
+
+## Elements / Types
+
+### EventTriggerDefinition
+
+An `EventTriggerDefinition` is a structured data object that combines:
+
+- A reference to a specific smart contract (by address)
+- A set of log predicates that filter which events from that contract should
+  trigger the release of the decryption key.
+
+Reference:
+[EventTriggerDefinition Type](https://github.com/shutter-network/rolling-shutter/blob/42f562532acfc4f89f630d3de809fc4451636ab2/rolling-shutter/keyperimpl/shutterservice/eventtrigger.go#L21-L25)
+
+### LogPredicate
+
+A `LogPredicate` pairs a reference to a specific value within an event log with
+a predicate that must be satisfied for the trigger to fire.
+
+Reference:
+[LogPredicate Type](https://github.com/shutter-network/rolling-shutter/blob/42f562532acfc4f89f630d3de809fc4451636ab2/rolling-shutter/keyperimpl/shutterservice/eventtrigger.go#L27-L32)
+
+### LogValueRef
+
+A `LogValueRef` identifies which value within an event log should be evaluated.
+It can reference:
+
+- **Topics (Offsets 0-3)**: The indexed parameters of the event log. A topic is
+  referenced by its index (0-3), where offset 0 is topic[0], etc.
+- **Data (Offsets 4+)**: The non-indexed data section of the log. Offset
+  values >= 4 refer to 32-byte words in the log data, where offset 4 is the
+  first word (bytes 0-31), offset 5 is the second word (bytes 32-63), etc.
+
+Reference:
+[LogValueRef Type and Documentation](https://github.com/shutter-network/rolling-shutter/blob/42f562532acfc4f89f630d3de809fc4451636ab2/rolling-shutter/keyperimpl/shutterservice/eventtrigger.go#L34-L43)
+
+### ValuePredicate
+
+A `ValuePredicate` defines the condition that must be satisfied on a referenced
+log value. It consists of an operation (`Op`) and a set of arguments. The type
+and number of arguments required depend on the operation.
+
+Reference:
+[ValuePredicate Type](https://github.com/shutter-network/rolling-shutter/blob/42f562532acfc4f89f630d3de809fc4451636ab2/rolling-shutter/keyperimpl/shutterservice/eventtrigger.go#L45-L53)
+
+## Encoding Format
+
+### Binary Encoding
+
+`EventTriggerDefinition` values are encoded using RLP (Recursive Length Prefix)
+encoding with a version byte prefix:
+
+```
+EventTriggerDefinitionBytes := {
+    version: uint8                    // Current version is 0x01
+    rlp_encoded_content: []byte       // RLP-encoded EventTriggerDefinition
+}
+```
+
+The version byte (`0x01`) allows for future format changes. After the version
+byte, the remaining bytes are RLP-encoded content representing the
+`EventTriggerDefinition` structure.
+
+Reference:
+[MarshalBytes and UnmarshalBytes Implementation](https://github.com/shutter-network/rolling-shutter/blob/42f562532acfc4f89f630d3de809fc4451636ab2/rolling-shutter/keyperimpl/shutterservice/eventtrigger.go#L58-L86)
+
+### Versioning
+
+The current version is **0x01** as defined by the `Version` constant.
+
+### RLP Encoding Details
+
+The RLP encoding format for the core structures is:
+
+```
+EventTriggerDefinition := {
+    contract: Address,
+    logPredicates: [LogPredicate, ...]
+}
+
+LogPredicate := {
+    logValueRef: LogValueRef,
+    valuePredicate: ValuePredicate
+}
+
+LogValueRef := Offset                    // if Length == 1
+LogValueRef := [Offset, Length]          // if Length > 1
+
+ValuePredicate := {
+    op: uint64,
+    intArgs: [BigInt, ...],
+    byteArgs: [Bytes, ...]
+}
+```
+
+Reference:
+[RLP Encoding Implementation](https://github.com/shutter-network/rolling-shutter/blob/42f562532acfc4f89f630d3de809fc4451636ab2/rolling-shutter/keyperimpl/shutterservice/eventtrigger.go#L211-L267)
+
+## Operators
+
+The `Op` type defines the operations that can be performed when evaluating a
+value predicate.
+
+### Supported Operators
+
+- **`Uint...` (0-4) Operators**: Unsigned integer (`uint256` / `big.Int`)
+  comparisons
+  - Argument: 1 unsigned integer
+  - **`UintLt` (0)**
+    - Returns true if `value < argument`
+  - **`UintLte` (1)**
+    - Returns true if `value <= argument`
+  - **`UintEq` (2)**
+    - Returns true if `value == argument`
+  - **`UintGt` (3)**
+    - Returns true if `value > argument`
+  - **`UintGte` (4)**
+    - Returns true if `value >= argument`
+- **`BytesEq` (5)**: Byte sequence equality comparison
+  - Arguments: 1 byte sequence (exactly matching the value size)
+  - Returns true if value == argument (byte-by-byte comparison)
+
+Reference:
+[Operator Constants](https://github.com/shutter-network/rolling-shutter/blob/42f562532acfc4f89f630d3de809fc4451636ab2/rolling-shutter/keyperimpl/shutterservice/eventtrigger.go#L298-L305)
+
+Note, that different operators require different numbers and types of arguments:
+
+- **Uint...-operators**: 1 integer argument, 0 byte arguments
+- **BytesEq**: 0 integer arguments, 1 byte argument
+
+## Matching Logs Against Definitions
+
+In the keyper implementation, the `Match` method is used to check if a given
+Ethereum log satisfies the `EventTriggerDefinition`:
+
+1. The log's contract address must match the definition's contract address
+2. The log must satisfy **all** log predicates (logical `AND` of all conditions)
+
+Reference:
+[Match Method](https://github.com/shutter-network/rolling-shutter/blob/42f562532acfc4f89f630d3de809fc4451636ab2/rolling-shutter/keyperimpl/shutterservice/eventtrigger.go#L159-L176)
+
+### LogValueRef.GetValue
+
+To retrieve a value from a log:
+
+- **Topics (Offset < 4)**: Returns the log's topic at index Offset as a 32-byte
+  value
+- **Data (Offset >= 4)**: Extracts a slice of 32-byte words from the log's data,
+  starting at word index (Offset - 4). If the slice extends beyond the log's
+  data, it is zero-padded on the right to the expected length.
+
+Reference:
+[GetValue Method](https://github.com/shutter-network/rolling-shutter/blob/main/rolling-shutter/keyperimpl/shutterservice/eventtrigger.go#L210-L228)
+
+## Generating EventTriggerDefinition from EVM Contract ABI
+
+To generate an `EventTriggerDefinition` from an EVM contract ABI and a set of
+predicates:
+
+### Input Requirements
+
+1. **Contract Address**: The Ethereum address where the contract is deployed
+2. **Contract ABI**: Standard Solidity JSON ABI format
+3. **Target Event Name**: The name of the event to trigger on
+4. **Predicate Specifications**: List of conditions, each specifying:
+   - Parameter name (e.g., "to", "amount") or parameter index
+   - Whether it's indexed or not (helps determine offset calculation)
+   - Operation to apply (UintLt, UintEq, BytesEq, etc.)
+   - Argument value(s)
+
+### Algorithm Overview
+
+```
+function generateEventTriggerDefinition(
+    contractAddress: Address,
+    abi: ContractABI,
+    eventName: string,
+    predicateSpecs: PredicateSpec[]
+): EventTriggerDefinition {
+
+    // 1. Find the event in the ABI
+    event = findEventInABI(abi, eventName)
+    if (!event) {
+        throw EventNotFoundError(eventName)
+    }
+
+    // 2. Build log predicates from predicate specifications
+    logPredicates = []
+
+    for spec in predicateSpecs {
+        // Find the parameter in the event
+        param = findParameterInEvent(event, spec.parameterName)
+        if (!param) {
+            throw ParameterNotFoundError(spec.parameterName)
+        }
+
+        // Calculate the LogValueRef offset
+        if (param.indexed) {
+            // Topics are at offsets 0-3
+            offset = calculateTopicIndex(event, param)
+            length = 1
+        } else {
+            // Data starts at offset 4
+            offset = 4 + calculateDataWordOffset(event, param)
+            length = calculateDataWordLength(param.type)
+        }
+
+        logValueRef = LogValueRef{
+            Offset: offset,
+            Length: length
+        }
+
+        // Create the ValuePredicate
+        valuePredicate = createValuePredicate(
+            spec.operation,
+            spec.value,
+            param.type
+        )
+
+        logPredicates.append(LogPredicate{
+            LogValueRef: logValueRef,
+            ValuePredicate: valuePredicate
+        })
+    }
+
+    // 3. Assemble and return
+    return EventTriggerDefinition{
+        Contract: contractAddress,
+        LogPredicates: logPredicates
+    }
+}
+```
+
+### Calculating Offsets
+
+**For Indexed Parameters (Topics)**:
+
+Topics are indexed parameters in Solidity events and are filtered by topic
+values in the EVM log. The offset corresponds to the topic index:
+
+- Topic 0 is always the event signature hash and is implicit (offset 0 if
+  included)
+- Additional topics (offset 1, 2, 3) correspond to indexed parameters
+
+**For Non-Indexed Parameters (Data)**:
+
+Non-indexed parameters are stored in the log's data field. They are packed as
+32-byte words:
+
+- Word 0 (bytes 0-31) corresponds to offset 4
+- Word 1 (bytes 32-63) corresponds to offset 5
+- And so on...
+
+For types smaller than 32 bytes, they are right-padded (for fixed-size types) or
+stored in a single word.
+
+## Examples
+
+### Example 1: ERC20 Transfer Trigger
+
+Trigger when a specific address transfers tokens:
+
+**Event ABI:**
+
+```json
+{
+  "name": "Transfer",
+  "inputs": [
+    { "name": "from", "type": "address", "indexed": true },
+    { "name": "to", "type": "address", "indexed": true },
+    { "name": "value", "type": "uint256", "indexed": false }
+  ]
+}
+```
+
+**Predicate Specification:**
+
+- Parameter "from" equals 0x742d35Cc6634C0532925a3b844Bc9e7595f1bEb
+
+**Generated EventTriggerDefinition (JSON):**
+
+```json
+{
+  "version": 1,
+  "contract": "0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48",
+  "logPredicates": [
+    {
+      "logValueRef": {
+        "offset": 1,
+        "length": 1
+      },
+      "valuePredicate": {
+        "op": 5,
+        "intArgs": [],
+        "byteArgs": [
+          "0x000000000000000000000000742d35cc6634c0532925a3b844bc9e7595f1beb"
+        ]
+      }
+    }
+  ]
+}
+```
+
+### Example 2: Multi-Condition Trigger
+
+Trigger when a swap occurs with minimum token amount:
+
+**Event ABI (Uniswap V3 SwapRouter):**
+
+```json
+{
+  "name": "Swap",
+  "inputs": [
+    { "name": "sender", "type": "address", "indexed": true },
+    { "name": "recipient", "type": "address", "indexed": true },
+    { "name": "amount0Delta", "type": "int256", "indexed": false },
+    { "name": "amount1Delta", "type": "int256", "indexed": false }
+  ]
+}
+```
+
+**Predicate Specifications:**
+
+- Parameter "amount1Delta" >= 1000000000000000000 (1 token with 18 decimals)
+- Parameter "sender" equals 0x742d35Cc6634C0532925a3b844Bc9e7595f1bEb
+
+**Generated EventTriggerDefinition (JSON):**
+
+```json
+{
+  "version": 1,
+  "contract": "0x1111111254fb6d44bac0bed2854e76f90643097d",
+  "logPredicates": [
+    {
+      "logValueRef": {
+        "offset": 1,
+        "length": 1
+      },
+      "valuePredicate": {
+        "op": 5,
+        "intArgs": [],
+        "byteArgs": [
+          "0x000000000000000000000000742d35cc6634c0532925a3b844bc9e7595f1beb"
+        ]
+      }
+    },
+    {
+      "logValueRef": {
+        "offset": 5,
+        "length": 1
+      },
+      "valuePredicate": {
+        "op": 4,
+        "intArgs": ["1000000000000000000"],
+        "byteArgs": []
+      }
+    }
+  ]
+}
+```