//! Sync: offer and steal.
//!
//! All the cleverness sits here, above the five-operation remote interface, so
//! the backends stay thin and this logic is written once and tested offline.
//!
//! The remote is a line of saves, the same shape as the local one. `offer`
//! pushes local saves the remote lacks; `steal` pulls remote saves and, when
//! both sides moved, replays the local work on top of the remote's. Divergence
//! is resolved per file, never per line.

module sync

import os
import src.events
import src.history
import src.remote
import src.storage

// Where the repo's name on the remote is remembered.
const repo_state_key = 'remote_repo'

pub const no_saves = 'this repo has no saves yet — nothing to offer'

pub struct OfferReport {
pub mut:
	repo    string
	saves   int
	objects int
	events  int
	head    string
}

// remember_repo binds a repo to its name on the remote. A clone does it at
// once: its folder may be renamed before it ever offers.
pub fn remember_repo(store &storage.Store, name string) ! {
	store.set_state(repo_state_key, name)!
}

// repo_name is the folder's name at first offer, remembered in the repo so
// renaming the folder later changes nothing.
pub fn repo_name(store &storage.Store, mut backend remote.Backend) !string {
	if remembered := store.state(repo_state_key) {
		return remembered
	}
	name := os.base(store.root)
	existing := backend.get(head_key(name))!
	if existing.found {
		theirs := existing.body.bytestr().trim_space()
		if history.by_id(store, theirs) == none {
			return error('the remote already has "${name}" with a history this repo does not share — rename the folder, or use another remote')
		}
	}
	store.set_state(repo_state_key, name)!
	return name
}

// offer sends what the remote lacks and then swaps `head`. Nothing is ever
// queued: the next offer simply sends everything the remote does not have.
pub fn offer(store &storage.Store, mut backend remote.Backend) !OfferReport {
	name := repo_name(store, mut backend)!
	local := history.head(store) or { return error(no_saves) }
	mut report := OfferReport{
		repo: name
		head: local.id
	}
	remote_head := backend.get(head_key(name))!
	// Records offered by a machine that has not applied a strip would put the
	// stripped path back on the remote, where nothing would rewrite it again.
	// ponytail: an offer racing a strip on another machine inside the same few
	// seconds can still slip through; closing that needs a remote lock.
	if unapplied_strip(store, mut backend, name)! {
		return error('the remote has a strip this repo has not applied — run koh steal')
	}
	if remote_head.found {
		theirs := remote_head.body.bytestr().trim_space()
		newer := saves_after(store, local.id, theirs) or {
			return error('remote has new saves — run koh steal')
		}
		for save in newer {
			upload_save(store, mut backend, name, save, mut report)!
		}
	} else {
		oldest_first := history.line(store)!
		for index := oldest_first.len - 1; index >= 0; index-- {
			upload_save(store, mut backend, name, oldest_first[index], mut report)!
		}
	}
	upload_events(store, mut backend, name, mut report)!
	if !backend.put_if_match(head_key(name), local.id.bytes(), remote_head.tag)! {
		return error('remote has new saves — run koh steal')
	}
	return report
}

// saves_after lists the local saves newer than `theirs`, oldest first. It fails
// when `theirs` is not on this line, which is the honest answer: we cannot
// offer on top of a save we do not have.
fn saves_after(store &storage.Store, ours string, theirs string) ![]history.Save {
	mut newer := []history.Save{}
	mut cursor := ours
	for cursor != '' && cursor != theirs {
		save := history.by_id(store, cursor) or { return error('save ${cursor} is missing') }
		newer << save
		cursor = save.parent
	}
	if cursor != theirs {
		return error('${theirs} is not on this line')
	}
	newer.reverse_in_place()
	return newer
}

fn upload_save(store &storage.Store, mut backend remote.Backend, repo string, save history.Save, mut report OfferReport) ! {
	files := history.files_of(store, save.id)!
	for file in files {
		if backend.put_if_absent(object_key(repo, file.hash), store.object_blob(file.hash)!)! {
			report.objects++
		}
	}
	record := save_record(store, save)!
	if backend.put_if_absent(save_key(repo, save.id), encode_save(record).bytes())! {
		report.saves++
	}
}

// upload_events sends every event the remote does not have. Events are tiny, so
// listing them costs less than tracking which ones were sent.
fn upload_events(store &storage.Store, mut backend remote.Backend, repo string, mut report OfferReport) ! {
	already := backend.list(events_prefix(repo))!
	mut already_sent := map[string]bool{}
	for key in already {
		already_sent[os.base(key)] = true
	}
	for event in events.all(store)! {
		if event.id in already_sent {
			continue
		}
		if backend.put_if_absent(event_key(repo, event.id), encode_event(event).bytes())! {
			report.events++
		}
	}
}

// fetched holds everything a steal read from the remote, before any of it is
// written locally. Network work happens first; the database gets one
// transaction, so a failure leaves nothing half-applied.
pub struct Fetched {
pub mut:
	repo_objects map[string][]u8
	saves        []SaveRecord // oldest first
	events_out   []events.Event
	head         string
	base         string // the newest save both sides already shared, '' when none
}

pub fn fetch_back_to_known(store &storage.Store, mut backend remote.Backend, repo string) !Fetched {
	remote_head := backend.get(head_key(repo))!
	if !remote_head.found {
		return error('the remote has no "${repo}" yet — run koh offer here first')
	}
	mut fetched := Fetched{
		head: remote_head.body.bytestr().trim_space()
	}
	mut cursor := fetched.head
	for cursor != '' && history.by_id(store, cursor) == none {
		record_object := backend.get(save_key(repo, cursor))!
		if !record_object.found {
			return error('the remote is missing save ${cursor} — run koh offer there')
		}
		record := decode_save(record_object.body)!
		for file in record.files {
			if !store.has_object(file.hash)! && file.hash !in fetched.repo_objects {
				blob := backend.get(object_key(repo, file.hash))!
				if !blob.found {
					return error('the remote is missing object ${file.hash} that save ${record.id} needs')
				}
				fetched.repo_objects[file.hash] = blob.body
			}
		}
		fetched.saves << record
		cursor = record.parent
	}
	fetched.saves.reverse_in_place()
	fetched.base = cursor
	fetched.events_out = fetch_events(store, mut backend, repo)!
	return fetched
}

fn fetch_events(store &storage.Store, mut backend remote.Backend, repo string) ![]events.Event {
	have := events.all(store)!
	mut known := map[string]bool{}
	for event in have {
		known[event.id] = true
	}
	mut incoming := []events.Event{}
	for key in backend.list(events_prefix(repo))! {
		id := os.base(key)
		if id in known {
			continue
		}
		body := backend.get(event_key(repo, id))!
		if !body.found {
			continue
		}
		incoming << decode_event(body.body)!
	}
	return incoming
}

// write_fetched lands a fetch in one transaction: objects first (verified), then
// the saves they belong to, then the events.
pub fn write_fetched(store &storage.Store, fetched Fetched) ! {
	for hash, blob in fetched.repo_objects {
		store.put_object_blob(hash, blob)!
	}
	for record in fetched.saves {
		history.import_save(store, to_save(record), to_files(record))!
	}
	for event in fetched.events_out {
		if !events.receive(store, event)! {
			continue
		}
		if event.kind == events.kind_strip {
			// Somebody stripped a path: remove it here too, so the two
			// histories agree from now on.
			apply_strip(store, event)!
		}
	}
}