//! Events: what Koh learns about saves that already exist.
//!
//! Save records are write-once, so a flag is not stored on the save. It is
//! appended as a small event that syncs alongside saves, and a save's flag
//! state is always derived from the newest event about it — never stored twice.
//! A strip event is what makes `strip` visible to other machines.

module events

import strings
import time
import src.machine
import src.storage

pub const kind_flag = 'flag'
pub const kind_clear = 'clear'
pub const kind_strip = 'strip'

// An event about one save.
pub struct Event {
pub mut:
	id         string
	kind       string
	save_id    string
	path       string
	reason     string
	created_at i64
	machine    string
}

// Flag is the derived answer to "is this save flagged, and why".
pub struct Flag {
pub:
	save_id string
	flagged bool
	reason  string
	at      i64
}

const select_event = 'SELECT id, kind, save_id, path, reason, created_at, machine FROM events'

fn read_event(stmt &storage.Stmt) Event {
	return Event{
		id:         stmt.text(0)
		kind:       stmt.text(1)
		save_id:    stmt.text_or_none(2) or { '' }
		path:       stmt.text_or_none(3) or { '' }
		reason:     stmt.text_or_none(4) or { '' }
		created_at: stmt.int(5)
		machine:    stmt.text(6)
	}
}

// write appends an event. The ID is the first 12 hex characters of the Blake3
// hash of the record, so two machines that record the same fact agree.
pub fn write(store &storage.Store, event Event) !Event {
	mut record := event
	if record.created_at == 0 {
		// Events are ordered by time, and flag/clear can land in the same
		// second. Bumping past the newest event keeps "newest wins" true, and
		// keeps the ordering the same on every machine.
		record.created_at = next_timestamp(store)!
	}
	if record.machine == '' {
		record.machine = machine.name()
	}
	record.id = event_id(record)
	insert(store, record)!
	return record
}

// receive stores an event exactly as another machine recorded it, ID and all.
// An ID re-derived here that disagreed with the sender's would make the event
// look new on every steal. Reports whether it was new.
pub fn receive(store &storage.Store, event Event) !bool {
	insert(store, event)!
	return store.db.changes() > 0
}

// next_timestamp is now, or one second past the newest event when that is not earlier.
fn next_timestamp(store &storage.Store) !i64 {
	now := time.now().unix()
	mut stmt := store.db.prepare('SELECT MAX(created_at) FROM events')!
	defer {
		stmt.finalize()
	}
	if !stmt.step()! {
		return now
	}
	newest := stmt.int_or_none(0) or { return now }
	if newest >= now {
		return newest + 1
	}
	return now
}

// insert ignores an ID already stored, which is how receive tells a repeat from a new event.
fn insert(store &storage.Store, record Event) ! {
	mut stmt := store.db.prepare('INSERT OR IGNORE INTO events
		(id, kind, save_id, path, reason, created_at, machine) VALUES (?, ?, ?, ?, ?, ?, ?)')!
	defer {
		stmt.finalize()
	}
	stmt.bind_text(1, record.id)!
	stmt.bind_text(2, record.kind)!
	bind_optional(stmt, 3, record.save_id)!
	bind_optional(stmt, 4, record.path)!
	bind_optional(stmt, 5, record.reason)!
	stmt.bind_int(6, record.created_at)!
	stmt.bind_text(7, record.machine)!
	stmt.step()!
}

pub fn flag(store &storage.Store, save_id string, reason string) !Event {
	return write(store, Event{
		kind:    kind_flag
		save_id: save_id
		reason:  reason
	})!
}

pub fn clear_flag(store &storage.Store, save_id string) !Event {
	return write(store, Event{
		kind:    kind_clear
		save_id: save_id
	})!
}

// strip records that a path was removed from a save (or from every save, when
// save_id is empty). Each strip is its own event, even of a path stripped
// before: a machine that applied the first must still hear about the second.
pub fn strip(store &storage.Store, save_id string, path string) !Event {
	return write(store, Event{
		kind:    kind_strip
		save_id: save_id
		path:    path
	})!
}

// valid_strip is a strip event Doctor may act on. Doctor reports an invalid one
// and leaves the saves alone (spec §7.6.2).
pub fn valid_strip(event Event) bool {
	return event.kind == kind_strip && event.path != '' && event.created_at >= 0
}

// event_id hashes each field with its length, so two different facts cannot spell the same input.
pub fn event_id(event Event) string {
	mut sb := strings.Builder{}
	put(mut sb, 'kind', event.kind)
	put(mut sb, 'save', event.save_id)
	put(mut sb, 'path', event.path)
	put(mut sb, 'reason', event.reason)
	put(mut sb, 'created', event.created_at.str())
	put(mut sb, 'machine', event.machine)
	return storage.hash_hex(sb.str().bytes())[..12]
}

fn put(mut sb strings.Builder, name string, value string) {
	sb.write_string('${name}:${value.len}:')
	sb.write_string(value)
	sb.write_string('\n')
}

fn bind_optional(stmt &storage.Stmt, index int, value string) ! {
	if value == '' {
		stmt.bind_null(index)!
	} else {
		stmt.bind_text(index, value)!
	}
}

// all returns every event, oldest first.
pub fn all(store &storage.Store) ![]Event {
	mut stmt := store.db.prepare('${select_event} ORDER BY created_at, id')!
	defer {
		stmt.finalize()
	}
	mut out := []Event{}
	for stmt.step()! {
		out << read_event(stmt)
	}
	return out
}

// latest_flags derives the flag state of every save that has ever been flagged
// or cleared. Newest wins, ties broken by ID so the answer is the same on every
// machine.
pub fn latest_flags(store &storage.Store) !map[string]Flag {
	mut stmt := store.db.prepare('${select_event} WHERE kind IN (?, ?) AND save_id IS NOT NULL
		ORDER BY created_at, id')!
	defer {
		stmt.finalize()
	}
	stmt.bind_text(1, kind_flag)!
	stmt.bind_text(2, kind_clear)!
	mut flags := map[string]Flag{}
	for stmt.step()! {
		event := read_event(stmt)
		flags[event.save_id] = Flag{
			save_id: event.save_id
			flagged: event.kind == kind_flag
			reason:  event.reason
			at:      event.created_at
		}
	}
	return flags
}

// flag_state is latest_flags for one save; a save never flagged reads as not flagged.
pub fn flag_state(store &storage.Store, save_id string) !Flag {
	flags := latest_flags(store)!
	return flags[save_id] or {
		Flag{
			save_id: save_id
		}
	}
}

// strip_events lists every strip event, whole-history or one save, oldest first.
pub fn strip_events(store &storage.Store) ![]Event {
	mut stmt := store.db.prepare('${select_event} WHERE kind = ? ORDER BY created_at, id')!
	defer {
		stmt.finalize()
	}
	stmt.bind_text(1, kind_strip)!
	mut out := []Event{}
	for stmt.step()! {
		out << read_event(stmt)
	}
	return out
}