Add guest example for greet

Signed-off-by: Rita Zhang <rita.z.zhang@gmail.com>

greet.go

@@ -0,0 +1,87 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"reflect"
+	"unsafe"
+)
+
+// main is required for TinyGo to compile to Wasm.
+func main() {}
+
+// greet prints a greeting to the console.
+func greet() {
+	objectToTest := os.Args[1]
+	parameters := os.Args[2]
+	decision := true
+	fmt.Println(decision)
+	log(fmt.Sprint("wasm guest objectToTest >> ", string(objectToTest), ", parameters >> ", string(parameters)))
+}
+
+// log a message to the console using _log.
+func log(message string) {
+	ptr, size := stringToPtr(message)
+	_log(ptr, size)
+}
+
+// _log is a WebAssembly import which prints a string (linear memory offset,
+// byteCount) to the console.
+//
+// Note: In TinyGo "//export" on a func is actually an import!
+//
+//go:wasm-module env
+//export log
+func _log(ptr uint32, size uint32)
+
+// greeting gets a greeting for the name.
+func greeting(name string) string {
+	return fmt.Sprint("Hello, ", name, "!")
+}
+
+// _greet is a WebAssembly export that accepts a string pointer (linear memory
+// offset) and calls greet.
+//
+//export greet
+func _greet() {
+	greet()
+}
+// func _greet(ptr, size uint32) {
+// 	name := ptrToString(ptr, size)
+// 	greet(name)
+// }
+
+// _greeting is a WebAssembly export that accepts a string pointer (linear memory
+// offset) and returns a pointer/size pair packed into a uint64.
+//
+// Note: This uses a uint64 instead of two result values for compatibility with
+// WebAssembly 1.0.
+//
+////export greeting
+// func _greeting(ptr, size uint32) (ptrSize uint64) {
+// 	name := ptrToString(ptr, size)
+// 	g := greeting(name)
+// 	ptr, size = stringToPtr(g)
+// 	return (uint64(ptr) << uint64(32)) | uint64(size)
+// }
+
+// ptrToString returns a string from WebAssembly compatible numeric types
+// representing its pointer and length.
+func ptrToString(ptr uint32, size uint32) string {
+	// Get a slice view of the underlying bytes in the stream. We use SliceHeader, not StringHeader
+	// as it allows us to fix the capacity to what was allocated.
+	return *(*string)(unsafe.Pointer(&reflect.SliceHeader{
+		Data: uintptr(ptr),
+		Len:  uintptr(size), // Tinygo requires these as uintptrs even if they are int fields.
+		Cap:  uintptr(size), // ^^ See https://github.com/tinygo-org/tinygo/issues/1284
+	}))
+}
+
+// stringToPtr returns a pointer and size pair for the given string in a way
+// compatible with WebAssembly numeric types.
+func stringToPtr(s string) (uint32, uint32) {
+	buf := []byte(s)
+	ptr := &buf[0]
+	unsafePtr := uintptr(unsafe.Pointer(ptr))
+	return uint32(unsafePtr), uint32(len(buf))
+}