package adminapi

import (
	"context"
	"encoding/csv"
	"fmt"
	"io"
	"net/http"
	"strconv"
	"strings"
	"time"

	"gitea.mynodes.xyz/ilker/llmgw/internal/store"
)

// Exporter totals usage for an export.
type Exporter interface {
	UsageExport(ctx context.Context, from, to time.Time, period string, by []string) ([]store.ExportRow, error)
}

// usageExport answers GET /api/v1/usage/export?from=2026-09-01&to=2026-10-01
// &period=day&by=project,model as CSV, or JSON with format=json.
func (a *API) usageExport(w http.ResponseWriter, r *http.Request) {
	ex, ok := a.usage.(Exporter)
	if !ok {
		writeError(w, http.StatusNotImplemented, "this control plane keeps no usage records")
		return
	}
	q := r.URL.Query()
	day := func(s string, def time.Time) (time.Time, error) {
		if s == "" {
			return def, nil
		}
		if t, err := time.Parse("2006-01-02", s); err == nil {
			return t, nil
		}
		return time.Parse(time.RFC3339, s)
	}
	now := time.Now().UTC()
	from, err := day(q.Get("from"), time.Date(now.Year(), now.Month(), 1, 0, 0, 0, 0, time.UTC))
	if err != nil {
		writeError(w, http.StatusBadRequest, "from: a date, 2026-09-01")
		return
	}
	to, err := day(q.Get("to"), now)
	if err != nil {
		writeError(w, http.StatusBadRequest, "to: a date, 2026-10-01")
		return
	}
	period := q.Get("period")
	if period == "" {
		period = "day"
	}
	var by []string
	if b := q.Get("by"); b != "" {
		by = strings.Split(b, ",")
	}
	rows, err := ex.UsageExport(r.Context(), from, to, period, by)
	if err != nil {
		writeError(w, http.StatusBadRequest, err.Error())
		return
	}
	if q.Get("format") == "json" {
		writeJSON(w, http.StatusOK, rows)
		return
	}
	w.Header().Set("Content-Type", "text/csv; charset=utf-8")
	w.Header().Set("Content-Disposition", fmt.Sprintf(`attachment; filename="llmgw-usage-%s-%s.csv"`,
		from.Format("20060102"), to.Format("20060102")))
	WriteExportCSV(w, rows)
}

// WriteExportCSV writes rows with a header; cost in units as well as micros,
// so a spreadsheet needs no arithmetic.
func WriteExportCSV(w io.Writer, rows []store.ExportRow) {
	c := csv.NewWriter(w)
	_ = c.Write([]string{"period_start", "org", "team", "project", "user", "key", "model",
		"requests", "errors", "limited", "prompt_tokens", "completion_tokens", "cost_micros", "cost", "images"})
	for _, r := range rows {
		_ = c.Write([]string{r.Period.Format(time.RFC3339), safe(r.Org), safe(r.Team), safe(r.Project), safe(r.User),
			r.Key, safe(r.Model), i(r.Requests), i(r.Errors), i(r.Limited), i(r.Prompt), i(r.Completion),
			i(r.CostMicros), strconv.FormatFloat(float64(r.CostMicros)/1e6, 'f', 6, 64), i(r.Images)})
	}
	c.Flush()
}

func i(n int64) string { return strconv.FormatInt(n, 10) }

// safe keeps a name from being read as a formula by a spreadsheet: a cell
// starting with = + - @ is one in Excel and LibreOffice.
func safe(s string) string {
	if s != "" && strings.ContainsRune("=+-@\t\r", rune(s[0])) {
		return "'" + s
	}
	return s
}