package operatorask

import (
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"sort"
	"strconv"
	"strings"
)

// An Act is what an option of an ask binds (novox/hq ADR 0286 §10, §11): named fields, each a string — the verb, the
// machine, the level, and each argument under a name of its own ("arg.delivery"). Flat on purpose: its digest is over
// these names and values alone, never over how a language happens to encode a struct. It is mesh-sdk's (design 47 G
// lead answers A-198, A-219), so the seat server's gate in go/process digests an act as every asker does;
// operatorchannelclient re-exports it.
type Act map[string]string

// ActDigest is the digest an asker puts in an option's binds: SHA-256 over the act's canonical encoding, written
// "sha256:<hex>". The same names and values give the same digest in any language, whatever order they were set in; a
// field renamed, added or emptied gives another.
func ActDigest(act Act) (string, error) {
	if len(act) == 0 {
		return "", fmt.Errorf("the act cannot be digested: it names nothing")
	}
	names := make([]string, 0, len(act))
	for k := range act {
		if k == "" {
			return "", fmt.Errorf("the act cannot be digested: a field has no name")
		}
		names = append(names, k)
	}
	sort.Strings(names)
	var b strings.Builder
	b.WriteString("novox.act.v1\n")
	for _, k := range names {
		canonical(&b, k)
		canonical(&b, act[k])
	}
	sum := sha256.Sum256([]byte(b.String()))
	return "sha256:" + hex.EncodeToString(sum[:]), nil
}

// canonical writes one value as its length in bytes, a colon, the bytes and a newline: no value can be read as
// another's end or start, so two different sequences of values never encode the same.
func canonical(b *strings.Builder, v string) {
	b.WriteString(strconv.Itoa(len(v)))
	b.WriteByte(':')
	b.WriteString(v)
	b.WriteByte('\n')
}