// ingestion/internal/pipeline/backfill.go
package pipeline

import (
	"context"
	"fmt"
	"os"
	"path/filepath"
	"strings"

	"github.com/mathiasbq/hyperguild/ingestion/internal/wiki"
)

// BackfillRefs walks wiki/sources/ and injects source back-references into every
// concept and entity page that each source links to.
// Changes for all sources are accumulated in memory before writing, so multiple
// sources referencing the same concept are merged in one pass.
// Deduplication is handled by wiki.Merge — running this multiple times is safe.
// Returns the number of concept/entity pages written.
func BackfillRefs(ctx context.Context, brainDir string) (int, error) {
	inventory, err := wiki.LoadInventory(brainDir)
	if err != nil {
		return 0, fmt.Errorf("load inventory: %w", err)
	}

	sourcesDir := filepath.Join(brainDir, "wiki", "sources")
	entries, err := os.ReadDir(sourcesDir)
	if err != nil {
		if os.IsNotExist(err) {
			return 0, nil
		}
		return 0, fmt.Errorf("read sources dir: %w", err)
	}

	// Accumulate all changes before writing: relPath → updated Page.
	// Collecting first means two sources that both link the same concept
	// get both refs merged before a single write.
	pending := make(map[string]wiki.Page)

	for _, e := range entries {
		if ctx.Err() != nil {
			return 0, ctx.Err()
		}
		if e.IsDir() || !strings.HasSuffix(e.Name(), ".md") {
			continue
		}

		b, err := os.ReadFile(filepath.Join(sourcesDir, e.Name()))
		if err != nil {
			continue
		}
		sourceContent := string(b)
		sourceSlug := strings.TrimSuffix(e.Name(), ".md")
		sourceTitle := extractTitle(sourceContent)
		if sourceTitle == "" {
			sourceTitle = sourceSlug
		}
		sourceRef := "- [[" + sourceSlug + "|" + sourceTitle + "]]"

		for slug := range extractWikilinks(sourceContent) {
			if slug == sourceSlug {
				continue
			}
			pt, ok := findInInventory(slug, inventory)
			if !ok {
				continue
			}
			relPath := "wiki/" + string(pt) + "/" + slug + ".md"

			// Start from already-accumulated version if we've seen this page.
			page, seen := pending[relPath]
			if !seen {
				raw, err := os.ReadFile(filepath.Join(brainDir, filepath.FromSlash(relPath)))
				if err != nil {
					continue
				}
				page = wiki.Page{Path: relPath, Content: string(raw)}
			}
			pending[relPath] = addSourceRef(page, sourceRef)
		}
	}

	for relPath, page := range pending {
		dest := filepath.Join(brainDir, filepath.FromSlash(relPath))
		if err := os.WriteFile(dest, []byte(page.Content), 0o644); err != nil {
			return 0, fmt.Errorf("write %s: %w", relPath, err)
		}
	}

	return len(pending), nil
}