//! Going back: `koh undo`, `koh revert` and `koh load`.
//!
//! Nothing here rewinds history. A going-back command restores an earlier tree
//! and records that as a new save whose `restores` field names the save it
//! brought back, so the move itself can be undone.
//!
//! The order matters and is the same everywhere: show what will change, ask,
//! record the new save, then write the files. Recording before writing means a
//! crash mid-restore shows up in `status` and `revert` finishes it.

module cli

import json
import src.history
import src.snapshot
import src.storage

pub fn cmd_undo(raw []string) int {
	args := parse('undo', raw, Options{
		flags: ['session', 'yes', 'json']
	}) or { return usage_error(err.msg()) }
	store := open_store() or { return refuse(args.json(), err.msg()) }
	defer {
		store.close()
	}
	effective := history.effective(store) or { return refuse(args.json(), err.msg()) }
	mut target := history.Save{}
	if args.flag('session') {
		start := history.session_start(store, effective.id) or {
			return refuse(args.json(), err.msg())
		}
		if start.parent == '' {
			return refuse(args.json(), 'there is nothing before session "${start.session}"')
		}
		target = history.by_id(store, start.parent) or { return refuse(args.json(), err.msg()) }
	} else {
		mut steps := 1
		if args.positional.len > 0 {
			steps = args.positional_at(0).int()
			if steps < 1 {
				return usage_error('undo takes a positive number of saves')
			}
		}
		target = history.step_back(store, effective.id, steps) or {
			return refuse(args.json(), err.msg())
		}
	}
	return going_back(store, target, 'undo', args, false)
}

pub fn cmd_revert(raw []string) int {
	args := parse('revert', raw, Options{
		flags: ['yes', 'json']
	}) or { return usage_error(err.msg()) }
	store := open_store() or { return refuse(args.json(), err.msg()) }
	defer {
		store.close()
	}
	head := history.head(store) or {
		return refuse(args.json(), 'this repo has no saves yet — nothing to revert to')
	}
	changes := compare_with_head(store, head) or { return refuse(args.json(), err.msg()) }
	if !changes.dirty {
		if args.json() {
			emit_json(json.encode(RevertJson{
				reverted: false
				head_id:  head.id
			}))
			return 0
		}
		ok('working tree already matches [${head.id}]')
		return 0
	}
	// The state about to be discarded is recorded as a checkpoint, so a revert
	// is itself undoable. `record_discard` does that only after the answer.
	return going_back(store, head, 'revert', args, true)
}

pub fn cmd_load(raw []string) int {
	args := parse('load', raw, Options{
		flags: ['checkpoint', 'yes', 'json']
	}) or { return usage_error(err.msg()) }
	store := open_store() or { return refuse(args.json(), err.msg()) }
	defer {
		store.close()
	}
	mut target := history.Save{}
	if args.flag('checkpoint') {
		target = history.latest_checkpoint(store) or {
			return refuse(args.json(), 'this repo has no checkpoints')
		}
	} else {
		id := args.positional_at(0)
		if id == '' {
			return usage_error('load needs a save ID, or --checkpoint')
		}
		target = history.resolve(store, id) or { return refuse(args.json(), err.msg()) }
	}
	return going_back(store, target, 'load', args, false)
}

// going_back is the shared path. `discard_first` is revert's extra step: record
// the changes being thrown away before restoring.
fn going_back(store &storage.Store, target history.Save, verb string, args Args, discard_first bool) int {
	as_json := args.json()
	if as_json && !args.flag('yes') {
		return refuse(true, 'needs --yes when not interactive')
	}
	plan := plan_restore(store, target, discard_first) or { return refuse(as_json, err.msg()) }
	if !as_json {
		ok('${verb}: [${target.id}]  ${target.message}')
		for delta in plan.deltas {
			println('  ${change_mark(delta.kind)} ${delta.path}')
		}
		note('${plural(plan.deltas.len, 'file')} change  ·  ${plural(plan.remove.len, 'file')} removed')
	}
	approved := confirm(args.flag('yes')) or { return refuse(as_json, err.msg()) }
	if !approved {
		return 1
	}
	if discard_first {
		history.commit_worktree(store, store.root, snapshot.load(store.root), history.SaveInput{
			message:       'revert: discarded changes'
			is_checkpoint: true
		}) or { return refuse(as_json, err.msg()) }
	}
	save := history.commit_files(store, '', plan.files, history.SaveInput{
		message:  '${verb}: ${target.id}'
		restores: target.id
	}) or { return refuse(as_json, err.msg()) }
	report := snapshot.restore_files(store, store.root, plan.files, plan.remove) or {
		return refuse(as_json, err.msg())
	}
	if as_json {
		emit_json(json.encode(BackJson{
			id:        save.id
			restores:  target.id
			written:   report.written
			removed:   report.removed
			discarded: discard_first
		}))
		return 0
	}
	ok('restored [${save.id}]  ${save.message}')
	return 0
}

struct RestorePlan {
	files  []snapshot.FileEntry
	remove []string
	deltas []snapshot.Delta
}

// plan_restore works out what writing `target` back onto disk involves.
//
// `prune_untracked` is revert's rule: revert promises to discard unsaved
// changes, so a file that was never saved goes too. undo and load are more
// conservative and only remove paths history already knows about; a file the
// user never saved is not theirs to delete.
fn plan_restore(store &storage.Store, target history.Save, prune_untracked bool) !RestorePlan {
	files := history.files_of(store, target.id)!
	mut wanted := map[string]bool{}
	for file in files {
		wanted[file.path] = true
	}
	from := working_tree(store)!
	mut remove := []string{}
	if prune_untracked {
		for file in from.files {
			if file.path !in wanted {
				remove << file.path
			}
		}
	} else if head := history.head(store) {
		for file in history.files_of(store, head.id) or { []snapshot.FileEntry{} } {
			if file.path !in wanted {
				remove << file.path
			}
		}
	}
	deltas := snapshot.diff_trees(store, from, snapshot.Tree{
		files: files
	})!
	return RestorePlan{
		files:  files
		remove: remove
		deltas: deltas
	}
}

fn change_mark(kind string) string {
	if kind == 'added' {
		return green('+')
	}
	if kind == 'removed' {
		return red('-')
	}
	return amber('~')
}

struct BackJson {
	id        string
	restores  string
	written   int
	removed   int
	discarded bool
}

struct RevertJson {
	reverted bool
	head_id  string
}