package captcha

import (
	"fmt"
	"strings"
)

const prngMask = 0xFFFFFFFF

// prng generates a deterministic lowercase-hex string of length chars from
// seed, matching the Cap.js widget PRNG exactly: FNV-1a seeds a 32-bit state,
// then xorshift32 produces the hex output. Must stay byte-for-byte compatible
// with the widget or solutions will never verify.
func prng(seed string, length int) string {
	state := fnv1a(seed)
	var b strings.Builder
	for b.Len() < length {
		state ^= (state << 13) & prngMask
		state ^= state >> 17
		state ^= (state << 5) & prngMask
		state &= prngMask
		fmt.Fprintf(&b, "%08x", state)
	}
	return b.String()[:length]
}

// fnv1a returns the 32-bit FNV-1a hash of s. Iterates over runes (code points)
// to match the JS/Python reference (which use charCodeAt / ord).
func fnv1a(s string) uint32 {
	var h uint32 = 2166136261
	for _, ch := range s {
		h ^= uint32(ch)
		h = (h + (h << 1) + (h << 4) + (h << 7) + (h << 8) + (h << 24)) & prngMask
	}
	return h
}