package backend

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

// EnvelopeContentHash returns the SHA-256 of the canonical JSON encoding
// of env, prefixed with "sha256:". The algorithm MUST match the one
// admin-api/internal/attest uses on the client side so that a client-sent
// content_hash is bit-identical to one we recompute here.
func EnvelopeContentHash(env Envelope) (string, error) {
	b, err := canonicalJSON(env)
	if err != nil {
		return "", err
	}
	sum := sha256.Sum256(b)
	return "sha256:" + hex.EncodeToString(sum[:]), nil
}

// canonicalJSON is the stable JSON encoding we use for hashing. It round-trips
// through generic unmarshal and then re-emits with sorted object keys. This
// is NOT a full RFC 8785 implementation; it is the subset that is sufficient
// for the admin-api <-> transparency-log boundary (envelopes contain strings,
// nested string maps, and arrays of those).
func canonicalJSON(v any) ([]byte, error) {
	first, err := json.Marshal(v)
	if err != nil {
		return nil, err
	}
	var any_ any
	if err := json.Unmarshal(first, &any_); err != nil {
		return nil, err
	}
	var buf bytes.Buffer
	if err := writeSorted(&buf, any_); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

func writeSorted(buf *bytes.Buffer, v any) error {
	switch x := v.(type) {
	case nil:
		buf.WriteString("null")
	case bool:
		if x {
			buf.WriteString("true")
		} else {
			buf.WriteString("false")
		}
	case float64:
		b, _ := json.Marshal(x)
		buf.Write(b)
	case string:
		b, _ := json.Marshal(x)
		buf.Write(b)
	case []any:
		buf.WriteByte('[')
		for i, el := range x {
			if i > 0 {
				buf.WriteByte(',')
			}
			if err := writeSorted(buf, el); err != nil {
				return err
			}
		}
		buf.WriteByte(']')
	case map[string]any:
		keys := make([]string, 0, len(x))
		for k := range x {
			keys = append(keys, k)
		}
		sort.Strings(keys)
		buf.WriteByte('{')
		for i, k := range keys {
			if i > 0 {
				buf.WriteByte(',')
			}
			kb, _ := json.Marshal(k)
			buf.Write(kb)
			buf.WriteByte(':')
			if err := writeSorted(buf, x[k]); err != nil {
				return err
			}
		}
		buf.WriteByte('}')
	default:
		return fmt.Errorf("writeSorted: unsupported type %T", v)
	}
	return nil
}