interop/engine: update documentation

pkg/interop/engine/engine.go

@@ -1,34 +1,56 @@
+/*
+Package engine provides access to VM execution metadata and allows to make contract calls.
+It's roughly similar in function to ExecutionEngine class in the Neo .net
+framework.
+*/
 package engine
 
 import "github.com/nspcc-dev/neo-go/pkg/interop/transaction"
 
-// Package engine provides function signatures that can be used inside
-// smart contracts that are written in the neo-go framework.
-
-// GetScriptContainer returns the transaction that is in the execution context.
+// GetScriptContainer returns the transaction that initially triggered current
+// execution context. It never changes in a single execution, no matter how deep
+// this execution goes. See `transaction` package for details on how to use the
+// returned value. This function uses `System.ExecutionEngine.GetScriptContainer`
+// syscall.
 func GetScriptContainer() transaction.Transaction {
 	return transaction.Transaction{}
 }
 
-// GetExecutingScriptHash returns the script hash of the contract that is
-// currently being executed.
+// GetExecutingScriptHash returns script hash (160 bit in BE form represented
+// as 20-byte slice) of the contract that is currently being executed. Any
+// AppCall can change the value returned by this function if it calls a
+// different contract. This function uses
+// `System.ExecutionEngine.GetExecutingScriptHash` syscall.
 func GetExecutingScriptHash() []byte {
 	return nil
 }
 
-// GetCallingScriptHash returns the script hash of the contract that started
-// the execution of the current script.
+// GetCallingScriptHash returns script hash (160 bit in BE form represented
+// as 20-byte slice) of the contract that started the execution of the currently
+// running context (caller of current contract or function), so it's one level
+// above the GetExecutingScriptHash in the call stack. It uses
+// `System.ExecutionEngine.GetCallingScriptHash` syscall.
 func GetCallingScriptHash() []byte {
 	return nil
 }
 
-// GetEntryScriptHash returns the script hash of the contract that started the
-// execution from the start.
+// GetEntryScriptHash returns script hash (160 bit in BE form represented
+// as 20-byte slice) of the contract that initially started current execution
+// (this is a script that is contained in a transaction returned by
+// GetScriptContainer) execution from the start. This function uses
+// `System.ExecutionEngine.GetEntryScriptHash` syscall.
 func GetEntryScriptHash() []byte {
 	return nil
 }
 
-// AppCall executes script with specified hash using provided arguments.
+// AppCall executes previously deployed blockchain contract with specified hash
+// (160 bit in BE form represented as 20-byte slice) using provided arguments.
+// It returns whatever this contract returns. Even though this function accepts
+// a slice for scriptHash you can only use it for contracts known at
+// compile time, because there is a significant difference between static and
+// dynamic calls in Neo (contracts should have a special property declared
+// and paid for to be able to use dynamic calls). This function uses `APPCALL`
+// opcode.
 func AppCall(scriptHash []byte, args ...interface{}) interface{} {
 	return nil
 }