-- Retention Cohort -- of users whose first `initial_event` lands on day D,
-- what share triggered `return_event` on day D+k for k in 1..Periods.
--
-- Required parameters (clickhouse.Named):
--   workspace_id  : String
--   from          : DateTime64(3,'UTC')
--   to            : DateTime64(3,'UTC')
--   initial_event : String
--   return_event  : String
--
-- Template inputs:
--   .Outer : []{ RIndex int; OffsetDay int; Last bool }
--     One entry per follow-up day. RIndex is the position in the retention()
--     output array; OffsetDay is the day delta from the cohort day.
SELECT
    cohort_day,
    countIf(arrayElement(r, 1)) AS cohort_size,
{{- range $p := .Outer }}
    countIf(arrayElement(r, {{ $p.RIndex }})) AS retained_d{{ $p.OffsetDay }}{{ if not $p.Last }},{{ end }}
{{- end }}
FROM (
    SELECT
        user_id,
        toDate(min(if(event = {initial_event:String}, timestamp, NULL))) AS cohort_day,
        retention(
            event = {initial_event:String} AND toDate(timestamp) = cohort_day,
{{- range $p := .Outer }}
            event = {return_event:String} AND toDate(timestamp) = addDays(cohort_day, {{ $p.OffsetDay }}){{ if not $p.Last }},{{ end }}
{{- end }}
        ) AS r
    FROM events_track
    WHERE workspace_id = {workspace_id:String}
      AND received_at >= {from:DateTime64(3,'UTC')}
      AND received_at <  {to:DateTime64(3,'UTC')}
      AND user_id != ''
      AND event IN ({initial_event:String}, {return_event:String})
    GROUP BY user_id
    HAVING cohort_day IS NOT NULL
)
GROUP BY cohort_day
ORDER BY cohort_day