// labels.jsonl read/write and URL normalization.
//
// labels.jsonl is the canonical label store: one JSON LabelRow per line,
// append-only. ReadLabels is strict (per-line errors with line numbers);
// AppendLabels is the only writer — rows are never rewritten in place.
package core

import (
	"bufio"
	"encoding/json"
	"errors"
	"fmt"
	"net/url"
	"os"
	"sort"
	"strings"
)

// NormalizeURL collapses trivially-equivalent article links into one dedup
// key: scheme/host lowercased, leading "www." and trailing "/" dropped.
// Unparseable input degrades to a plain lowercase-trim.
func NormalizeURL(u string) string {
	u = strings.TrimSpace(u)
	pr, err := url.Parse(u)
	if err != nil || pr.Host == "" {
		return strings.TrimSuffix(strings.ToLower(u), "/")
	}
	pr.Scheme = strings.ToLower(pr.Scheme)
	pr.Host = strings.TrimPrefix(strings.ToLower(pr.Host), "www.")
	return strings.TrimSuffix(pr.String(), "/")
}

// ReadLabels parses path strictly. A missing file is not an error — it just
// means no labels yet (first export run).
func ReadLabels(path string) ([]LabelRow, error) {
	f, err := os.Open(path)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return nil, nil
		}
		return nil, fmt.Errorf("opening labels: %w", err)
	}
	defer func() { _ = f.Close() }()

	var rows []LabelRow
	scanner := bufio.NewScanner(f)
	scanner.Buffer(make([]byte, 64*1024), 4*1024*1024)
	line := 0
	for scanner.Scan() {
		line++
		text := strings.TrimSpace(scanner.Text())
		if text == "" {
			continue
		}
		var row LabelRow
		if err := json.Unmarshal([]byte(text), &row); err != nil {
			return nil, fmt.Errorf("%s:%d: invalid JSON: %w", path, line, err)
		}
		if row.EntryID <= 0 {
			return nil, fmt.Errorf("%s:%d: entry_id must be positive", path, line)
		}
		if row.URL == "" {
			return nil, fmt.Errorf("%s:%d: url is required", path, line)
		}
		if row.Label != LabelPos && row.Label != LabelNeg {
			return nil, fmt.Errorf("%s:%d: label must be %q or %q, got %q", path, line, LabelPos, LabelNeg, row.Label)
		}
		if row.CapturedAt.IsZero() {
			return nil, fmt.Errorf("%s:%d: captured_at is required", path, line)
		}
		rows = append(rows, row)
	}
	if err := scanner.Err(); err != nil {
		return nil, fmt.Errorf("reading %s: %w", path, err)
	}
	return rows, nil
}

// SplitByTime partitions rows into train (older) and val (newest valFrac)
// by CapturedAt, the time-split validation strategy: train on older labels,
// evaluate on the most recent window, which is what serve actually faces.
// Sort is stable; ties on CapturedAt break by EntryID for determinism.
// valFrac is clamped: <=0 → all train, >=1 → all val. nVal is computed by
// truncation, so a tiny set may yield an empty val side (callers guard).
func SplitByTime(rows []LabelRow, valFrac float64) (train, val []LabelRow) {
	if len(rows) == 0 {
		return nil, nil
	}
	if valFrac <= 0 {
		return rows, nil
	}
	if valFrac >= 1 {
		return nil, rows
	}
	sorted := make([]LabelRow, len(rows))
	copy(sorted, rows)
	sort.SliceStable(sorted, func(i, j int) bool {
		if sorted[i].CapturedAt.Equal(sorted[j].CapturedAt) {
			return sorted[i].EntryID < sorted[j].EntryID
		}
		return sorted[i].CapturedAt.Before(sorted[j].CapturedAt)
	})
	nVal := int(float64(len(sorted)) * valFrac)
	split := len(sorted) - nVal
	return sorted[:split], sorted[split:]
}

// AppendLabels appends rows to path (created if needed). Rows already in
// the file are never touched.
func AppendLabels(path string, rows []LabelRow) error {
	if len(rows) == 0 {
		return nil
	}
	f, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o644)
	if err != nil {
		return fmt.Errorf("opening labels for append: %w", err)
	}
	defer func() { _ = f.Close() }()

	enc := json.NewEncoder(f)
	for _, row := range rows {
		if err := enc.Encode(row); err != nil {
			return fmt.Errorf("writing label row: %w", err)
		}
	}
	return nil
}