package source

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
)

// DigestDocument returns a deterministic digest of the canonical source
// document content. The existing Digest field is excluded from its own digest.
func DigestDocument(doc *SourceDocument) (string, error) {
	if doc == nil {
		return "", fmt.Errorf("source document must not be nil")
	}
	payload := struct {
		ID       string         `json:"id"`
		Kind     string         `json:"kind"`
		Format   string         `json:"format"`
		Units    []SourceUnit   `json:"units"`
		Metadata map[string]any `json:"metadata,omitempty"`
	}{
		ID:       doc.ID,
		Kind:     doc.Kind,
		Format:   doc.Format,
		Units:    doc.Units,
		Metadata: doc.Metadata,
	}
	encoded, err := json.Marshal(payload)
	if err != nil {
		return "", fmt.Errorf("encode source document for digest: %w", err)
	}
	sum := sha256.Sum256(encoded)
	return "sha256:" + hex.EncodeToString(sum[:]), nil
}