package main

import (
	"bufio"
	"context"
	"fmt"
	"os"
	"slices"
	"strings"
)

// ioBufSize is the buffer size for the buffered stdout writers.
const ioBufSize = 1 << 20

// minGroupSize is the smallest number of members that makes a
// duplicate group.
const minGroupSize = 2

// scanRec is one file record from the database. The signature (size,
// head, tail) is the duplicate key; mtime is informational only and
// used by scan for change detection.
type scanRec struct {
	size  int64
	mtime int64
	head  string
	tail  string
	path  string
}

// loadRecords opens the database and reads every file record for the
// report and trees subcommands. Any database problem — including a
// missing database — is fatal. The error is returned rather than
// exiting, so that the deferred close — which checkpoints the SQLite
// WAL — always runs; the database is closed before the caller formats
// its output, so it stays closed even if that output fails.
func loadRecords(ctx context.Context) ([]scanRec, error) {
	dbPath := databasePath()

	db, err := openReportDatabase(ctx, dbPath)
	if err != nil {
		return nil, err
	}

	defer func() { _ = db.Close() }()

	recs, err := loadFileRows(ctx, db)
	if err != nil {
		return nil, fmt.Errorf("database %s: %w", dbPath, err)
	}

	return recs, nil
}

// dupeGroup is one set of candidate-duplicate files: identical size,
// head hash, and tail hash. paths is sorted lexicographically; the
// first entry is the group's "first", the rest are dupes.
type dupeGroup struct {
	size  int64
	paths []string
}

// runReport implements the report subcommand: it reads every record
// from the database and prints the file-level duplicates report as TSV
// on stdout. It never touches the scanned filesystem; its only I/O is
// the database, stdout, and stderr.
func runReport(ctx context.Context) error {
	recs, err := loadRecords(ctx)
	if err != nil {
		return err
	}

	dupes := collectDupeGroups(recs)

	out := bufio.NewWriterSize(os.Stdout, ioBufSize)

	_, err = fmt.Fprintln(out, "first\tdupe\tsize")
	if err != nil {
		return fmt.Errorf("write stdout: %w", err)
	}

	dupeFiles := 0

	var reclaimable int64

	for _, g := range dupes {
		for _, p := range g.paths[1:] {
			_, err = fmt.Fprintf(out, "%s\t%s\t%d\n",
				g.paths[0], p, g.size)
			if err != nil {
				return fmt.Errorf("write stdout: %w", err)
			}

			dupeFiles++
			reclaimable += g.size
		}
	}

	err = out.Flush()
	if err != nil {
		return fmt.Errorf("write stdout: %w", err)
	}

	fmt.Fprintf(os.Stderr,
		"report: %d records read, %d duplicate groups, %d dupe files, "+
			"%s reclaimable\n",
		len(recs), len(dupes), dupeFiles, humanBytes(reclaimable))

	return nil
}

// collectDupeGroups groups records by signature and returns every group
// with two or more paths, each group's paths sorted lexicographically,
// groups ordered by size descending then by first path ascending.
func collectDupeGroups(recs []scanRec) []dupeGroup {
	groups := make(map[fileSig][]string)

	for _, r := range recs {
		// A record without hashes (its size was unique when last
		// scanned) has unknown content and is never reported as a
		// duplicate.
		if r.head == "" {
			continue
		}

		k := fileSig{size: r.size, head: r.head, tail: r.tail}
		groups[k] = append(groups[k], r.path)
	}

	var dupes []dupeGroup

	for k, paths := range groups {
		if len(paths) < minGroupSize {
			continue
		}

		slices.Sort(paths)
		dupes = append(dupes, dupeGroup{size: k.size, paths: paths})
	}

	// Biggest reclaimable space first; ties broken by first path.
	slices.SortFunc(dupes, func(a, b dupeGroup) int {
		if a.size != b.size {
			if a.size > b.size {
				return -1
			}

			return 1
		}

		return strings.Compare(a.paths[0], b.paths[0])
	})

	return dupes
}

// humanBytes formats a byte count in human units (binary prefixes).
func humanBytes(n int64) string {
	const unit = 1024
	if n < unit {
		return fmt.Sprintf("%d B", n)
	}

	div, exp := int64(unit), 0
	for m := n / unit; m >= unit; m /= unit {
		div *= unit
		exp++
	}

	return fmt.Sprintf("%.1f %ciB", float64(n)/float64(div), "KMGTPE"[exp])
}