// Copyright (c) 2020 Shivaram Lingamneni <slingamn@cs.stanford.edu>
// released under the MIT license

package history

import (
	"time"
)

// Selector represents a parameter to a CHATHISTORY command;
// at most one of Msgid or Time may be nonzero
type Selector struct {
	Msgid string
	Time  time.Time
}

// Sequence is an abstract sequence of history entries that can be queried;
// it encapsulates restrictions such as registration time cutoffs, or
// only looking at a single "query buffer" (DMs with a particular correspondent)
type Sequence interface {
	Between(start, end Selector, limit int) (results []Item, complete bool, err error)
	Around(start Selector, limit int) (results []Item, err error)
}

// This is a bad, slow implementation of CHATHISTORY AROUND using the BETWEEN semantics
func GenericAround(seq Sequence, start Selector, limit int) (results []Item, err error) {
	var halfLimit int
	halfLimit = (limit + 1) / 2
	initialResults, _, err := seq.Between(Selector{}, start, halfLimit)
	if err != nil {
		return
	} else if len(initialResults) == 0 {
		// TODO: this fails if we're doing an AROUND on the first message in the buffer
		// would be nice to fix this but whatever
		return
	}
	newStart := Selector{Time: initialResults[0].Message.Time}
	results, _, err = seq.Between(newStart, Selector{}, limit)
	return
}

// MinMaxAsc converts CHATHISTORY arguments into time intervals, handling the most
// general case (BETWEEN going forwards or backwards) natively and the other ordering
// queries (AFTER, BEFORE, LATEST) as special cases.
func MinMaxAsc(after, before, cutoff time.Time) (min, max time.Time, ascending bool) {
	startIsZero, endIsZero := after.IsZero(), before.IsZero()
	if !startIsZero && endIsZero {
		// AFTER
		ascending = true
	} else if startIsZero && !endIsZero {
		// BEFORE
		ascending = false
	} else if !startIsZero && !endIsZero {
		if before.Before(after) {
			// BETWEEN going backwards
			before, after = after, before
			ascending = false
		} else {
			// BETWEEN going forwards
			ascending = true
		}
	} else if startIsZero && endIsZero {
		// LATEST
		ascending = false
	}
	if after.IsZero() || after.Before(cutoff) {
		// this may result in an impossible query, which is fine
		after = cutoff
	}
	return after, before, ascending
}