// Package abogus computes the a_bogus signature required by 抖店/抖音 risk-controlled endpoints.
//
// Implementation: embeds a JavaScript file with the a_bogus algorithm and runs it via goja
// (pure-Go JS engine, no CGO). The JS itself does not depend on any browser globals — it
// takes the URL query string and User-Agent as inputs and returns the signature string.
package abogus

import (
	_ "embed"
	"fmt"
	"sync"

	"github.com/dop251/goja"
)

//go:embed a_bogus.js
var aBogusJS string

// Generator is reusable. NewGenerator compiles the JS once; Sign() is goroutine-safe.
type Generator struct {
	program *goja.Program
	mu      sync.Mutex
	pool    []*goja.Runtime
}

// NewGenerator compiles the embedded a_bogus.js and returns a ready-to-use generator.
func NewGenerator() (*Generator, error) {
	// strict=false: a_bogus.js 原文有 `n = ...`（未声明）等浏览器宽松模式特性，goja 严格模式会拒绝
	prog, err := goja.Compile("a_bogus.js", aBogusJS, false)
	if err != nil {
		return nil, fmt.Errorf("compile a_bogus.js: %w", err)
	}
	return &Generator{program: prog}, nil
}

// Sign computes the a_bogus signature for a given URL query string and User-Agent.
// urlSearchParams is the raw query string WITHOUT a_bogus itself (everything else included).
func (g *Generator) Sign(urlSearchParams, userAgent string) (string, error) {
	rt := g.acquire()
	defer g.release(rt)

	if _, err := rt.RunProgram(g.program); err != nil {
		return "", fmt.Errorf("run a_bogus.js: %w", err)
	}
	fn, ok := goja.AssertFunction(rt.Get("generate_a_bogus"))
	if !ok {
		return "", fmt.Errorf("generate_a_bogus is not a function")
	}
	v, err := fn(goja.Undefined(), rt.ToValue(urlSearchParams), rt.ToValue(userAgent))
	if err != nil {
		return "", fmt.Errorf("call generate_a_bogus: %w", err)
	}
	return v.String(), nil
}

func (g *Generator) acquire() *goja.Runtime {
	g.mu.Lock()
	defer g.mu.Unlock()
	if n := len(g.pool); n > 0 {
		rt := g.pool[n-1]
		g.pool = g.pool[:n-1]
		return rt
	}
	return goja.New()
}

func (g *Generator) release(rt *goja.Runtime) {
	g.mu.Lock()
	defer g.mu.Unlock()
	g.pool = append(g.pool, rt)
}