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
}

// DigestChunk returns a deterministic digest of a chunk, including its source
// provenance, content, units, and metadata.
func DigestChunk(chunk Chunk) (string, error) {
	payload := struct {
		ID        string         `json:"id"`
		SourceID  string         `json:"source_id"`
		Index     int            `json:"index"`
		Ref       SourceRef      `json:"ref"`
		Content   []byte         `json:"content"`
		MediaType string         `json:"media_type"`
		Units     []SourceUnit   `json:"units"`
		Metadata  map[string]any `json:"metadata,omitempty"`
	}{
		ID:        chunk.ID,
		SourceID:  chunk.SourceID,
		Index:     chunk.Index,
		Ref:       chunk.Ref,
		Content:   chunk.Content,
		MediaType: chunk.MediaType,
		Units:     chunk.Units,
		Metadata:  chunk.Metadata,
	}
	encoded, err := json.Marshal(payload)
	if err != nil {
		return "", fmt.Errorf("encode source chunk for digest: %w", err)
	}
	sum := sha256.Sum256(encoded)
	return "sha256:" + hex.EncodeToString(sum[:]), nil
}