package agent

import (
	"github.com/sipeed/picoclaw/pkg/bus"
)

// computeContextUsage estimates current context window consumption for the
// given agent and session. Includes history, system prompt (with dynamic context,
// summary, and skills — mirroring BuildMessages composition), and tool definitions.
// The output reserve (MaxTokens) is not counted as "used" but reduces the
// effective budget, matching isOverContextBudget's compression trigger:
//
//	compress when: history + system + tools + maxTokens > contextWindow
//	equivalent to: history + system + tools > contextWindow - maxTokens
//
// Returns nil when the agent or session is unavailable.
func computeContextUsage(agent *AgentInstance, sessionKey string) *bus.ContextUsage {
	if agent == nil || agent.Sessions == nil {
		return nil
	}
	contextWindow := agent.ContextWindow
	if contextWindow <= 0 {
		return nil
	}

	// History tokens
	history := agent.Sessions.GetHistory(sessionKey)
	historyTokens := 0
	for _, m := range history {
		historyTokens += EstimateMessageTokens(m)
	}

	// System message tokens: uses EstimateSystemTokens which mirrors
	// the full system message composition in BuildMessages (static prompt,
	// dynamic context, active skills, summary with wrapping prefix).
	systemTokens := 0
	if agent.ContextBuilder != nil {
		summary := agent.Sessions.GetSummary(sessionKey)
		// Pass nil for active skills: skills are only injected when the user
		// explicitly activates them via /use, which is rare. Using nil matches
		// the common case and avoids over-counting all installed skills.
		systemTokens = agent.ContextBuilder.EstimateSystemTokens(summary, nil)
	}

	// Tool definition tokens
	toolTokens := 0
	if agent.Tools != nil {
		toolTokens = EstimateToolDefsTokens(agent.Tools.ToProviderDefs())
	}

	// Used = history + system (includes summary) + tools
	usedTokens := historyTokens + systemTokens + toolTokens

	// Effective budget = contextWindow minus output reserve (maxTokens)
	effectiveWindow := contextWindow - agent.MaxTokens
	if effectiveWindow < 0 {
		effectiveWindow = contextWindow
	}

	// compressAt = effectiveWindow: aligns with isOverContextBudget's
	// proactive trigger (msgTokens + toolTokens + maxTokens > contextWindow).
	compressAt := effectiveWindow

	usedPercent := 0
	if compressAt > 0 {
		usedPercent = usedTokens * 100 / compressAt
	}
	if usedPercent > 100 {
		usedPercent = 100
	}

	return &bus.ContextUsage{
		UsedTokens:       usedTokens,
		TotalTokens:      contextWindow,
		CompressAtTokens: compressAt,
		UsedPercent:      usedPercent,
	}
}