// Package approvalfact provides data-only Approval claim correspondence.
// It neither authenticates a provider nor decides permission or consumes objects.
package approvalfact

import (
	"bytes"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"io"
	"math"
	"math/big"
	"sort"
	"strconv"
	"strings"
	"unicode/utf8"
)

const MaxJSONBytes = 1 << 20

var errJSON = errors.New("invalid Approval JSON")

// DecodeJSON retains numeric lexical types, refuses duplicates and invalid
// Unicode, and bounds work. A Go map previously decoded into float64 cannot
// recover the producer's integer-versus-floating identity; pass source bytes.
func DecodeJSON(raw []byte) (any, error) {
	if len(raw) > MaxJSONBytes || !utf8.Valid(raw) || !validSurrogates(raw) {
		return nil, errJSON
	}
	d := json.NewDecoder(bytes.NewReader(raw))
	d.UseNumber()
	var read func(int) (any, error)
	read = func(depth int) (any, error) {
		if depth > 128 {
			return nil, errJSON
		}
		t, e := d.Token()
		if e != nil {
			return nil, errJSON
		}
		switch x := t.(type) {
		case json.Delim:
			if x == '{' {
				m := map[string]any{}
				for d.More() {
					k, e := d.Token()
					if e != nil {
						return nil, errJSON
					}
					s, ok := k.(string)
					if !ok {
						return nil, errJSON
					}
					if _, ok = m[s]; ok {
						return nil, errJSON
					}
					v, e := read(depth + 1)
					if e != nil {
						return nil, e
					}
					m[s] = v
				}
				end, e := d.Token()
				if e != nil || end != json.Delim('}') {
					return nil, errJSON
				}
				return m, nil
			}
			if x == '[' {
				a := []any{}
				for d.More() {
					v, e := read(depth + 1)
					if e != nil {
						return nil, e
					}
					a = append(a, v)
				}
				end, e := d.Token()
				if e != nil || end != json.Delim(']') {
					return nil, errJSON
				}
				return a, nil
			}
			return nil, errJSON
		case json.Number:
			if _, e := pythonNumber(x); e != nil {
				return nil, e
			}
			return x, nil
		case string, bool, nil:
			return x, nil
		default:
			return nil, errJSON
		}
	}
	v, e := read(0)
	if e != nil {
		return nil, e
	}
	if _, e = d.Token(); e != io.EOF {
		return nil, errJSON
	}
	return v, nil
}

// encoding/json replaces lone UTF-16 surrogates, whereas the Python producer
// cannot encode them as UTF-8. Reject before the decoder can erase that fact.
func validSurrogates(raw []byte) bool {
	in := false
	for i := 0; i < len(raw); i++ {
		if raw[i] == '"' {
			in = !in
			continue
		}
		if !in || raw[i] != '\\' {
			continue
		}
		i++
		if i >= len(raw) {
			return false
		}
		if raw[i] != 'u' {
			continue
		}
		if i+4 >= len(raw) {
			return false
		}
		n, e := strconv.ParseUint(string(raw[i+1:i+5]), 16, 16)
		if e != nil {
			return false
		}
		i += 4
		if n >= 0xdc00 && n <= 0xdfff {
			return false
		}
		if n < 0xd800 || n > 0xdbff {
			continue
		}
		if i+6 >= len(raw) || raw[i+1] != '\\' || raw[i+2] != 'u' {
			return false
		}
		low, e := strconv.ParseUint(string(raw[i+3:i+7]), 16, 16)
		if e != nil || low < 0xdc00 || low > 0xdfff {
			return false
		}
		i += 6
	}
	return !in
}

func pythonNumber(n json.Number) (string, error) {
	s := string(n)
	if !strings.ContainsAny(s, ".eE") {
		i, ok := new(big.Int).SetString(s, 10)
		if !ok {
			return "", errJSON
		}
		return i.String(), nil
	}
	f, e := strconv.ParseFloat(s, 64)
	if e != nil || math.IsInf(f, 0) || math.IsNaN(f) {
		return "", errJSON
	}
	// Python repr uses shortest round-trip digits, fixed form for exponents
	// -4..15, a mandatory .0 for integral floats, and two exponent digits.
	sci := strconv.FormatFloat(f, 'e', -1, 64)
	at := strings.IndexByte(sci, 'e')
	exp, _ := strconv.Atoi(sci[at+1:])
	if exp >= -4 && exp < 16 {
		s = strconv.FormatFloat(f, 'f', -1, 64)
		if !strings.Contains(s, ".") {
			s += ".0"
		}
		return s, nil
	}
	sign := "+"
	if exp < 0 {
		sign = "-"
		exp = -exp
	}
	digits := strconv.Itoa(exp)
	if len(digits) < 2 {
		digits = "0" + digits
	}
	return sci[:at] + "e" + sign + digits, nil
}

func quote(s string) string {
	var b strings.Builder
	b.WriteByte('"')
	for _, r := range s {
		switch r {
		case '"', '\\':
			b.WriteByte('\\')
			b.WriteRune(r)
		case '\b':
			b.WriteString(`\b`)
		case '\f':
			b.WriteString(`\f`)
		case '\n':
			b.WriteString(`\n`)
		case '\r':
			b.WriteString(`\r`)
		case '\t':
			b.WriteString(`\t`)
		default:
			if r < 32 {
				b.WriteString(`\u00`)
				const h = "0123456789abcdef"
				b.WriteByte(h[byte(r)>>4])
				b.WriteByte(h[byte(r)&15])
			} else {
				b.WriteRune(r)
			}
		}
	}
	b.WriteByte('"')
	return b.String()
}

func canonical(v any) (string, error) {
	switch x := v.(type) {
	case nil:
		return "null", nil
	case bool:
		if x {
			return "true", nil
		}
		return "false", nil
	case string:
		return quote(x), nil
	case json.Number:
		return pythonNumber(x)
	case []any:
		a := make([]string, len(x))
		for i, v := range x {
			s, e := canonical(v)
			if e != nil {
				return "", e
			}
			a[i] = s
		}
		return "[" + strings.Join(a, ",") + "]", nil
	case map[string]any:
		keys := make([]string, 0, len(x))
		for k := range x {
			keys = append(keys, k)
		}
		sort.Strings(keys)
		a := make([]string, 0, len(keys))
		for _, k := range keys {
			s, e := canonical(x[k])
			if e != nil {
				return "", e
			}
			a = append(a, quote(k)+":"+s)
		}
		return "{" + strings.Join(a, ",") + "}", nil
	default:
		return "", errJSON
	}
}

// CanonicalBinding reproduces approval_engine.binding.binding_digest over the
// five native act fields. Extra binding metadata is excluded by that contract.
func CanonicalBinding(raw []byte) ([]byte, string, error) {
	v, e := DecodeJSON(raw)
	if e != nil {
		return nil, "", e
	}
	m, ok := v.(map[string]any)
	if !ok {
		return nil, "", errJSON
	}
	act := map[string]any{}
	for _, k := range []string{"action", "actor", "principal", "purpose"} {
		s, ok := m[k].(string)
		if !ok || s == "" {
			return nil, "", errJSON
		}
		act[k] = s
	}
	target, ok := m["target"].(map[string]any)
	if !ok {
		return nil, "", errJSON
	}
	act["target"] = target
	s, e := canonical(act)
	if e != nil {
		return nil, "", e
	}
	sum := sha256.Sum256([]byte(s))
	return []byte(s), "sha256:" + hex.EncodeToString(sum[:]), nil
}