// Package session holds what the two front ends — the terminal REPL
// (internal/agent) and the editor front end (internal/acp) — agree a
// conversation starts from, and how it is reset by the /new command.
//
// A session, for both of them, is a plain []*ai.Message whose first element is
// the system prompt. "Starting a new session" means going back to exactly that:
// the system prompt alone, every question, answer and tool turn forgotten.
// The package knows nothing about the screen or the protocol: printing the
// report line, or sending the session/update, stays each front end's job.
package session

import (
	"strings"

	"github.com/firebase/genkit/go/ai"
)

// NewCommand is the slash command that clears the history and starts a new
// session. The same spelling is used at the terminal prompt and in an ACP
// prompt; ACP advertises it without the slash (see NewCommandName).
const NewCommand = "/new"

// NewCommandName is NewCommand as ACP names it: clients list commands by bare
// name and send them back as prompt text with the leading slash.
const NewCommandName = "new"

// IsNewCommand reports whether the user's input is the /new command. The
// input is trimmed first: an editor sends the line as typed, spaces and
// trailing newline included, and "/new " must not be mistaken for a question.
//
//	session.IsNewCommand("/new")     // true
//	session.IsNewCommand("  /new\n") // true
//	session.IsNewCommand("/new one") // false — not the command
func IsNewCommand(input string) bool {
	return strings.TrimSpace(input) == NewCommand
}

// Fresh returns the history of a brand-new session: the system prompt and
// nothing else. Both front ends start from it, and /new returns to it.
//
//	messages := session.Fresh(config.Cfg.System)
//	// len(messages) == 1, messages[0].Role == ai.RoleSystem
func Fresh(system string) []*ai.Message {
	return []*ai.Message{ai.NewSystemTextMessage(system)}
}

// Forgotten counts the messages a reset throws away: everything except the
// system prompt. It is the figure the report line shows ("12 message(s)
// forgotten"), so the user sees that the command did something — or that
// there was nothing to forget.
//
//	session.Forgotten(session.Fresh("you are bob")) // 0
func Forgotten(msgs []*ai.Message) int {
	n := 0
	for _, m := range msgs {
		if m.Role != ai.RoleSystem {
			n++
		}
	}
	return n
}