// Package healthcheck 提供针对 Spring Boot Actuator 健康端点的采样与等待能力，
// 供更新执行器在启动容器后确认工作负载是否就绪。
package healthcheck

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"time"
)

// maximumResponseBytes 限制 Actuator 响应体的最大读取字节数，防止异常大响应。
const maximumResponseBytes = 1 << 20

// ErrWorkloadStopped 表示工作负载在变为健康之前已经停止。
var ErrWorkloadStopped = errors.New("workload stopped before becoming healthy")

// RunningProbe 在每次健康采样前核对容器是否仍处于运行状态。
type RunningProbe func(context.Context) (bool, error)

// ActuatorReport 一次成功健康采样的非敏感摘要。
type ActuatorReport struct {
	// Status 表示顶层健康状态，例如 UP、DOWN。
	Status string
	// Components 表示各子组件的健康状态映射。
	Components map[string]string
}

// ActuatorChecker 等待 Spring Boot Actuator 顶层状态进入 UP。
// 它只采样健康状态，不负责重启容器。
type ActuatorChecker struct {
	// client 执行健康检查 HTTP 请求的客户端。
	client *http.Client
	// interval 相邻两次采样之间的间隔。
	interval time.Duration
}

// Check 执行一次健康采样，供事务恢复时核对已经记录意图的健康步骤。
func (c *ActuatorChecker) Check(ctx context.Context, endpoint string, running RunningProbe) (ActuatorReport, bool, error) {
	if running == nil {
		return ActuatorReport{}, false, errors.New("running probe is required")
	}
	if err := validateEndpoint(endpoint); err != nil {
		return ActuatorReport{}, false, err
	}
	return c.sample(ctx, endpoint, running)
}

// NewActuatorChecker 创建固定间隔的 Actuator 检查器。
func NewActuatorChecker(client *http.Client, interval time.Duration) (*ActuatorChecker, error) {
	if client == nil {
		return nil, errors.New("HTTP client is required")
	}
	if interval <= 0 {
		return nil, errors.New("health check interval must be positive")
	}
	return &ActuatorChecker{client: client, interval: interval}, nil
}

// Wait 在 timeout 范围内持续采样。容器停止时立即失败；其他未就绪结果保留到截止时间。
func (c *ActuatorChecker) Wait(ctx context.Context, endpoint string, timeout time.Duration, running RunningProbe) (ActuatorReport, error) {
	if timeout <= 0 {
		return ActuatorReport{}, errors.New("health check timeout must be positive")
	}
	if running == nil {
		return ActuatorReport{}, errors.New("running probe is required")
	}
	if err := validateEndpoint(endpoint); err != nil {
		return ActuatorReport{}, err
	}

	waitContext, cancel := context.WithTimeout(ctx, timeout)
	defer cancel()
	var lastErr error
	for {
		report, ready, err := c.sample(waitContext, endpoint, running)
		if ready {
			return report, nil
		}
		if errors.Is(err, ErrWorkloadStopped) {
			return ActuatorReport{}, err
		}
		lastErr = err

		timer := time.NewTimer(c.interval)
		select {
		case <-waitContext.Done():
			timer.Stop()
			return ActuatorReport{}, fmt.Errorf("Actuator did not become UP within %s: %w", timeout, errors.Join(waitContext.Err(), lastErr))
		case <-timer.C:
		}
	}
}

// validateEndpoint 校验 Actuator 端点必须是合法的 http URL 且包含主机部分。
func validateEndpoint(endpoint string) error {
	parsed, err := url.ParseRequestURI(endpoint)
	if err != nil || parsed.Scheme != "http" || parsed.Host == "" {
		return fmt.Errorf("invalid Actuator HTTP endpoint: %q", endpoint)
	}
	return nil
}

// sample 执行一次完整健康采样：先核对容器仍在运行，
// 再向 Actuator 端点发起 GET 请求并解析返回的 JSON 状态。
// 返回的报告在状态非 UP 时仍会携带实际采样到的状态信息。
func (c *ActuatorChecker) sample(ctx context.Context, endpoint string, running RunningProbe) (ActuatorReport, bool, error) {
	isRunning, err := running(ctx)
	if err != nil {
		return ActuatorReport{}, false, fmt.Errorf("inspect workload before health check: %w", err)
	}
	if !isRunning {
		return ActuatorReport{}, false, ErrWorkloadStopped
	}

	request, err := http.NewRequestWithContext(ctx, http.MethodGet, endpoint, nil)
	if err != nil {
		return ActuatorReport{}, false, fmt.Errorf("create Actuator request: %w", err)
	}
	request.Header.Set("Accept", "application/json")
	response, err := c.client.Do(request)
	if err != nil {
		return ActuatorReport{}, false, fmt.Errorf("request Actuator health: %w", err)
	}
	defer response.Body.Close()
	body, err := io.ReadAll(io.LimitReader(response.Body, maximumResponseBytes+1))
	if err != nil {
		return ActuatorReport{}, false, fmt.Errorf("read Actuator response: %w", err)
	}
	if len(body) > maximumResponseBytes {
		return ActuatorReport{}, false, errors.New("Actuator response exceeds size limit")
	}
	if response.StatusCode != http.StatusOK {
		return ActuatorReport{}, false, fmt.Errorf("Actuator returned HTTP status %d", response.StatusCode)
	}

	var payload actuatorPayload
	if err := json.Unmarshal(body, &payload); err != nil {
		return ActuatorReport{}, false, fmt.Errorf("decode Actuator response: %w", err)
	}
	report := ActuatorReport{Status: payload.Status, Components: make(map[string]string, len(payload.Components))}
	for name, component := range payload.Components {
		report.Components[name] = component.Status
	}
	if payload.Status != "UP" {
		return report, false, fmt.Errorf("Actuator status is %q", payload.Status)
	}
	return report, true, nil
}

// actuatorPayload Actuator health 端点的顶层 JSON 结构。
type actuatorPayload struct {
	Status     string                       `json:"status"`
	Components map[string]actuatorComponent `json:"components"`
}

// actuatorComponent 健康检查组件的 JSON 结构，这里只关心其状态。
type actuatorComponent struct {
	Status string `json:"status"`
}